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EK-VT100-TM-003

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Document:	VT100 Series Technical Manual
Order Number:	EK-VT100-TM
Revision:	003
Pages:	460
Original Filename:	http://www.mirrorservice.org/sites/www.bitsavers.org/www.computer.museum.uq.edu.au/pdf/EK-VT100-TM-003%20VT100%20Series%20Video%20Terminal%20Technical%20Manual.pdf

OCR Text

EK· VT1 00-TM-003

SERIES
VIDEO TERMINAL·
TECHNICAL MANUAL

,. . l

Prepared by Educational Services
.

Digital Equlp~nt Corporation

First Edition, August 1979
Second· Edition, September 1980
Third Edition, July 1982

The reproduction of this material, in part or whole,
is strictly prohibited. For copy information, contact
the Educational Services Department, Digital
Equipment Corporation, Maynard, Massachusetts
01754.
The information in this document is subject to
change without notice. Digital Equipment Corporation assumes no responsibility for any errors that
may appear in this document.

Printed in U.S.A.

The following are trademarks of Digital Equipment
Corporation, Maynard, Massachusetts.
DEC
DECUS
DIGITAL
Digital Logo
PDP
UNIBUS
VAX

OMNIBUS
DECnet
DECsystem-IO
OS/8
DECSYSTEM-20 PDT
DECwriter
RSTS
RSX
DIBOL
VMS
EduSystem
VT
lAS
MASSBUS

CONTENTS

CHAPTER 1

}

. .

VT100 Specifications .......................................................... , . . . . . . .
Ordering Documentation .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Related Documentation ................................................

)

1-2
1-4
1-5

CHAPTER 2

)

INTRODUCTION AND SPECIFICATIONS

OPERATOR INFORMATION

2-1
Part 1 - Keyboard Controls and Indicators ............................................ .
2-3
Numeric Keypad ........................... '.......................... .
2-5
Monitor Control ..................................................... .
2-5
Audible Indicators (Tones) ............................................ .
2-6
Part 2 - SET- UP Mode .............................................................. .
2-7
SET-UP Features ".' .................................................. .
2-7
SET-UP A .......................................................... .
2-8
SET-UP B .......................................................... .
2-8
Determining What a SET-UP Feature Does ............................. .
How to Change a SET-UP Feature ..................................... . 2-10
Setting the Answerback Message ....................................... . 2-11
Saving SET-UP Features ............................................. . 2-12
Recalling SET-UP Features ..... '" ................ ; .................. . 2-12
Resetting the Terminal ................................................ . 2-13
Part 3 - Definitions of SET- UP Features .............................................. . 2-13
ANSI/VT52 Mode ................................................... . 2-13
Answerback Message ................................................. . 2-13
Auto Repeat ........................................................ . 2-13
Auto XON/XOFF ................................................... . 2-14
Bits per Character .............................................. ; ..... . 2-14
Characters per Line .................................................. . 2-14
Cursor .......................................... '.................... . 2-14
Interlace ............................................................. . 2-14
Keyclick Tone ............. '" ....................................... . 2-15
LINE/LOCAL ...................................................... . 2-15
Margin Bell ......................................................... . 2-15
New Line ........................................................... . 2-15
Parity ".............................................................. . 2-15
Parity Sense ......................................................... . 2-15
Power .............................................................. ,. 2-16
Receive Speed ..... '.................................................. . 2-16
Screen Background ................................................... . , 2-16

iii

Screen Brightness .....................................................
Scroll ...............................................................
Tabs ............................................... '. . . . . . . . . .. . ... ..
Transmit Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Wraparound.. . . .... . . . . . . .... ..... . . . ... ... ... . .. . . . . . . . . .. .. . . .. .. . ..
Part 4 .... Self-Testing the VT100 .......................................................
Self-Test Error Codes .................................................
Part 5 - What To Do in the Event of a Problem. . ... ..... . .. . .. . . . . . . .... . . . . . ... . .. . ...

2-16
2-16
2-16
2-16
2-17
2-17
2-17
2-19

CHAPTER 3, INSTALLATION AND INTERFACE INFORMATION
Site Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Unpacking and Installation .............................................................
User Maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Interface Information ..................................................................
EIA Interface ................................................ ;.......
Electrical Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VT100 Output Voltages ...........................................
VT100 Input Voltages ............................................
Optional 20 rnA Current Loop Interface .............................
External Video Connections ............................................
Composite Video Output (J9) . . . . . . . . . . .. . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Video Input (J8) ......................................................
CHAPTER 4
4.1
4.1.1
4.1.2
4.1.3
4.1.4
4.1.5
4.1.5.1
4.1.5.2
4.1.5.3
4.1.5.4
4.1.5.5
4.1.5.6
4.1.5.7
4.1.5.8
4.1.5.9
4.1.5.10
4.1.5.11
4.1.5.12
4.1.5.13
4.1.5.14
4.1.5.15
4.1.5.16
4.1.5.17
4.1.5.18
4.1.5.19
4.1.5.20
4.1.6

3-1
3-1
3-3
3-4
3-4
3-5
3-5
3-5
3-5
3-6
3-6
3-8

TECHNICAL DESCRIPTION
4-1
Introduction to VTl 00 Technical Description .......................... . . .
Scope of Chapter . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-1
Order of Presentation .............................................
4-1
Definition of Terms and Abbreviations ..............................
4-1
Hardware Introduction ............................................
4-1
Block Diagram Description ........................................
4-3
Microprocessor. . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-3
Program ROM ..............................................
4-3
Scratch RAM ...............................................
4-3
Nonvolatile RAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-3
Advanced Video Option ......................................
4-3
Keyboard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-5
LEDs ......................................................
4-5
Keyboard Translator ......................................... ,4-5
Transmit Buffer. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-5
Communication Transmitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-5
Communication Receiver .....................................
4-5
SILO ................................. ;....................
4-5
Control Function Parser ...................................... . 4-5
Screen RAM ................................................ , :,>4-6
Video Processor ..............................................
4-6
CRT Monitor .......................................... ,....
4-6
Power Supply ...............................................
4-7
Standard Terminal Port ....................................... ' 4-7
EIA Interface ............................................... "4-7
Current Loop Interface Adapter ............................... , ~ ;,,4-7
·
I ntroductlon
.
1'~4 7
F lrmware
................. ; ..........................
, j:..~ -

4.2

Microprocessor .......................................................
4-9
... :-~-.--.-.::-.- <t~9~-----Data Bus and System Controller.... .. ............. ........ ... ... . .. 4-11
Clock Generator ................................................. 4-13
Bus Timing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-13
Microprocessor Memory .......................................... 4-13
Memory Map ............................................... 4-16
Memory Devices ............................................ 4-16
ROM Decoding .............................. ',." . .. .. ... . .. 4-16
RAM Decoding............................................. 4-16
Memory Disable.. . .... . ....... .. ..... .. . ..... . ... .. ...... ... 4-16
I/O Decoding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-17
PUSART Read and Write.................................... 4-17
I/O Read and Write. . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-17
Interrupt Vector ................................ :..... .... ........ 4..:18
Power:"Up and Self-Test ...................... "..................... 4-19
Communication Transceiver. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-20
PUSART Principles.............................................. 4-20
PUSART Operation .............................................. 4.:23
PUSART Addressing............................................. 4-23
PUSART Programming........................................... 4-23
Data Transmission ............................................... 4-23
Data Reception ................................ ~ . . . . . . . . . . . . . . . .. 4-27
Baud Rate Generator ...................... :...................... 4-27
Serial Interface ........................................... ~ . . . . . .. 4-28
Modem Control .................................................. 4-28
Data Types .................. , ....... , ... , .. ..... .. .. . . .. . ..... .. 4-28
SILO ............. '" ....................................... , ... 4-28
XON/XOFF .................................................... 4-29
Control Function Parser. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-29
Local. .. ... ... . . .. . . ... . .. .. . . . .. . .. ..... ... . .. . . . .. . ... .. .... .. 4-29
Standard Terminal Port ........................................... 4-29
Communication Self-Test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-30
Keyboard ............................................................ 4-30
Keyboard Block Diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-30
Keyboard UARTs ............................................... , 4-31
Keyboard Status Byte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-31
Key Address Counter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-32
Key Scanning and Address Formation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-34
Bidirectional Interface Operation ................................... 4-34
Interface Line ............................................... 4-34
Receiving Side. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-34
Terminal Data Encoding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-37
Combined Interface Signal .................................... 4-38
Decoding of Data from Terminal. . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. 4-38
Keyboard Output ............................................ 4-38
Bell .................................................. , . . . . . . . . .. 4-38
Keyboard Interrupt Routine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-41
Logical Keyboard Processor ....................... ,............... 4-41
Key Recognition. .... . . . ... . .. ... ........ . .. ... .. . . .. . .. ... .. 4-42
Key Rollover ............................................... , 4-42
Generation of Codes ......................................... 4-43
Keyboard Transmit Buffer .................................... 4-44
Auto Repeat ................................................ 4-44

~------~"r-----4~2-;-1---~~--------8080-Microprocessor- -;-;--;-~~-~ ;-;-;-;-:-;-;-;-;-~-.-;-:-.;-;-.-::-:-:-.-. -:~~

/
4.2.2
"4.2.3
4.2.4
4.2.5
4.2.5.1
4.2.5.2
4.2.5.3
4.2.5.4
4.2.5.5
4.2.6
4.2.6.1
4.2.6.2
4.2.7
4.2.8
4.3
4.3.1
)
4.3.2
4.3.3
4.3.4
4.3.5
4.3.6
4.3.7
4.3.8
4.3.9
4.3.10
\
;-~ 4.3.11
: " 4.3.12
)
4.3.13
4.3.14
4.3.15
4.3.16
4.4
4.4.1
4.4.2
4.4.3
)
4.4.4
4.4.5
4.4.6
4.4.6.1
4.4.6.2
4.4.6.3
4.4.6.4
4.4.6.5
4.4.6.6
4.4.7
4.4.8
4.4.9
4.4.9.1
4.4.9.2
4.4.9.3
". 4.4.9.4
)
\ 4.4.9.5

4.5
4.5.1
4.5.2
4.5.3
4.5.4
4.5.5
4.5.6
4.6
4.6.1
4.6.1.1
4.6.1.2
4.6.1.3
4.6.1.4
4.6.1.5
4.6.1.6
4.6.2
4.6.2.1
4.6.2.2
4.6.2.3
4.6.2.4
4.6.2.5
4.6.2.6
4.6.2.7
4.6.2.8
4.6.2.9
4.6.2.10
4.6.2.11
4.6.2.12
4.6.3
4.6.3.1
4.6.3.2
4.6.3.3
4.6.3.4
4.6.3.5
4.6.3.6
4.6.3.7
4.6.3.8
4.6.3.9
4.6.4
4.6.5
4.6.6
4.6.7
4.6.8
4.6.9
4.6.10
4.6.11
4.6.12
4.6.12.1
4.6.12.2
4.6.12.3
4.6.13
4.7
4.7.1
4.7.2

Nonvolatile RAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-44
Principles ....................................................... 4-44
NVR Device . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-45
NVR Control .................................................... 4-45
NVR Support Circuits ..................................... -. . . . . .. 4-46
Microprocessor Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-47
NVR Timing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-47
Video Processor ...................................................... 4-48
Introduction ...................................................... 4-48
The Raster . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-51
Character Formation ......................................... 4-51
Video Processor Data .............................
4-52
Video Processor Character Generation. . . . . . . . . . . . . . . . . . . . . . . . . . 4-52
Attributes ...................... '. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-52
Advanced Video Option (AVO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-54
Timing Chip Description .......................................... 4-55
Input Decoder ....................... ' ..................
4-55
80/132 Column Selection .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-57
Dot Counter ................................................ 4-57
Double-Width MUltiplexer .................................... 4-58
Horizontal Counter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-60
Horizontal Drive and Horizontal Blank ......................... 4-60
Line Buffer Addressing ....................................... 4-60
Vertical Operation ........................................... 4-64
Vertical Counter ..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-64
Vertical Outputs ............................................. 4-64
Composite Sync ...... ,...................................... 4-67
Hold Request, Address Load, and Double-Width. . . . . . . . . . . . . . . .. 4-67
Control Chip Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-67
Input Decoder Functions ..................................... , 4-70
Attribute Latches ............................................ 4- 71
Scroll Counter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4- 71
Scan Count Math. . . . ... . . .. .. . . .... . . . .... . . . . . . . . .. . . .. . . .. 4-72
Generation of HOLD REQUEST ............................. 4-72
Horizontal Blank and Terminate ............................... 4-73
Double-Width and Hold Request . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-74
Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-74
Dot Stretcher . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .. 4-74
Address Counter and Data Structure in RAM ........................ 4-74
Address Latch Buffer ............................................. 4-77
Line Buffer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-77
Character Generator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-77
Video Shift Register .............................................. 4-80
Terminator ...................................................... 4-80
DMA Cycle Timing Diagram .................................. '.' . . 4-80
Video Blinking " . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . .. . . . .. 4-82
Video Input and Output ........................................... 4-83
Direct Drive Video. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-83
Composite Video Out ........................................ 4-84
Video In . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-84
Intensity Control ................................................. 4-84
Microprocessor - Video Processor Interface .............................. 4-85
Screen Memory Organization ...................................... 4-86
Fill Line Operation ..................•... ~ . . . . . . . . . . . . . . . . . . . . . . .. . a-87
i

• •

•

• •

•

• • ••

•

))

)

\
)

I'·

4.7.3
4.7.3.1
4.7.3.2
4.7.4
4.7.5
4.7.6
4.7.7
4.7.8
4.7.9
4.7.10
4.7.11
4.8
4.8.1
4.8.1.1
4.8.1.2
4.8.1.3
4.8.1.4
4.8.1.5
4.8.1.6
4.8.1.7
4.8.1.8
4.8.1.9
4.8.1.10
4. 8.2
4.8.2.1
4.8.2.2
4.8.2.3
4.9
4.9.1
4.9.2
4.9.3
4.9.4 .
4.9.5
4.9.5.1
4.9.5.2
4.9.5.3
CHAPTER 5

5.1
5.2
5.2.1
5.2.2
5.2.3
5.2.4
5.2.5
5.2.5.1
5.2.6
5.2.7
5.2.7.1
5.2.7.2
5.2.7.3
5.2.7.4

)

", . ~.2.7.5

Line Organization ................................................ 4-88
Physical Screen. . . . . . . . . . . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-92
Logical Screen .............................................. 4-92
Address Shuffling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4'-92
Shuffle Timing ................................................... 4-94
Scrolling Region ...............................'. . . . . . . . . . . . . . . . . .. 4-95
Split Screen Jump Scrolling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-95
Smooth Scroll. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-95
Split Screen Smooth Scrolling. . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-97
Cursor ............................... '.' . . . . . . . . . . . . . .. . . . . . . . . .. 4-98
SET-UP ........................................................ 4-99
Monitor ............................................................. 4-99
Monitor Description: 30-16080 (Elston) ............................. 4-100
Video Driver ................................................. 4-100
Brightness .................................................. 4-100
Vertical Oscillator ........................................... 4-100
Self-Oscillation ..... ~ ........................................ 4-100
Vertical Output .............................................. 4-100
Linearization ................................................ 4-10 l
Horizontal Driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-101
Horizontal Deflection Operation ................................ 4-101
Horizontal Output Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-103
High Voltage and Focus ....................................... 4-104
Monitor Description: 30-14590 (Ball) ............................... 4-104
Video Amplifier ............................................. 4-104
Vertical Deflection ........................................... 4-104
Horizontal Deflection ........................................ 4-105
Power Supply ........................................................ 4-107
Power Input ..................................................... 4-107
Start-Up Circuit .............................................. : ... 4-107
Control Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-108
Outputs ......................................................... 4-110
Power Supply Specifications ....................................... 4-1 i 2
Input Specifications .......................................... 4-112
Cooling ................................. , .................... 4-115
Base Product Power Requirements ........ '..................... 4-115
VT100 SERIES SERVICE
Introduction ......................................................... .
Troubleshooting ...................................................... .
Troubleshooting the Basic VT100 .................................. .
Troubleshooting Basic VT100 Variations Without Self-Test ........... .
Troubleshooting the VTl05 ....................................... .
Troubleshooting the VTl32 ....................................... .
Troubleshooting the VTl25 ....................................... .
VTl25 Self-Test Error Codes ................................ .
Troubleshooting the Options ...................................... .
VT100 Internal Self-Tests ........................................ .
Error Codes ....... '.' ........ , .............................. .
Power-Up Test ..... , ....................................... .
Data Loopback Test ........................................ .
EIA Test .................................................. .
Keyboard Tests ............................................. .

vii

5-1
5-1
5-1
5-3
5-8
5-8
5-9
5-10
5-13
5-13
5-13
5-13
5..:16
5-19
5-19

5.2.7.6
5.2.7.7
5.2.8
5.2.8.1
5.2.8.2
5.2.8.3
5.2.8.4
5.2.8.5
5.2.8.6
5.3
5.3.1
5.3:1.1
5.3.1.2

5.3.2
5.3.2.1
5.3.2.2
5.3.2.3
5.3.2.4
5.3.2.5
5.3.2.6
5.3.2.7
5.3.2.8
5.3.2.9
5.3.3
5.3.3.1
5.3.3.2
5.3.3.3
5.3.3.4
5.3.3.5
5.3.3.6
5.3.3.7
5.4
5.4.1
5.4.2
5.4.3
5.4.4
5.4.5
5.4.6
5.4.7
5.4.8
5.4.9
5.4.10
5.4.11
5.4.12
5.4.13
5.4.14
5.4.15
5.4.16
5.4.17
5.4.18
5.4.19
5.4.20
5.4.21
5.4.22

SET-UP Screen Test ........................................ .
Video Adjust Test ........................................... .
VT125 Tests ....................... : ................ '.' ......... .
Loopback Connector Installation .............................. .
VT125 Power-Up Test ................................ -...... .
VT125 Computer Data Port Loopback Test .................... .
VTI25 Auxiliary Port Loopback Test ......................... .
VT125 Display Test ........................................ .
VT125 Video Bit Map Memory Test .......................... .
Video Alignment ..................................................... .
Alignment Template ............................................. .
Making a Paper Template .................................... .
Attaching the Template ...................................... .
Monitor Adjustments (Ball Brothers) ............................... .
Brightness .... ............................................. .
Yoke Rotation .............................................. .
Vertical Height ........................................ '..... .
Horizontal Width ........................................... .
Centering .................................................. .
Vertical Linearity .................... , ...................... .
Horizontal Linearity ......................................... .
Vertical Hold ............................................... .
Focus ..................................................... .
Monitor Adjustments (Elston and DIGITAL) ....................... .
Brightness ................................................. .
Yoke Rotation .............................................. .
Vertical Height ............. -................................ .
Horizontal Width ........................................... .
Centering .................................................. .
Vertical Linearity ........................................... .
Focus ..................................................... .
Module Removal and Replacement ..................................... .
Access Cover ................................................... .
Terminal Controller Board ........................................ .
Advanced Video Option Board .................................... .
ROMs ......................................................... .
Printer Port/VTl25 STP Board ................................... .
VTl 05 Waveform Generator Board ................................ .
VTl25 Graphics Board ........................................... .
20 rnA Current Loop Board ....................................... .
Keyboard Top Cover ........................ : ................... .
Keyboard ...................................................... .
Keyboard Cable ................................................. .
Keyboard Speaker ............................................... .
Keycap ........................................................ .
Top Cover .................. '.................................... .
Video Monitor Board (Ball Monitor) ............................... .
Flyback Transformer (Ball Monitor) ............................... .
Video Monitor Board (Elston or DIGITAL) ........................ .
Flyback Transformer (Elston or DIGITAL) ......................... .
Bottom Cover ................................................... .
Power Supply .......... " ........................................ .
RF Shield ...................................................... .
DC Power Harness .............................................. .

5-19

5-20
5-20
5-21
5-21
5-22
5-23
5-23
5-24
5-24
5-25
5-25
5-25
5-25
5-28
5-29
5-30
5-30
5-30
5-30
5-30
5-31
5-31
5-31
5-31
5-32
5-32
5-32
5-33
5-33
5-34
5-34
5-34
5-37
5-38
5-39
5-39
5-43
5-43
5-47
5-47
5-47
5-47
5-49
5-49
5-51
5-51
5-53
5-54
5-55
5-55
5-56
5-58
5-58
.. (.

})
/

viii

)

5.4.23
5.4.24
5.4.25
5.4.26
5.5
. 5.5.1
5.5.2
5.5.3
5.5.4
5.5.5
5.5.6
5.6
5.6.1
5.6.1.1
5.6.1.2
5.6.1.3
5.6.1.4
5.6.2
5.6.3
5.6.4
5.6.5
CHAPTER 6

)

6.1
6.1.1
6.1.2
6.1:3
6.2
6.2.1
6.2.2
6.2.3
6.2.4
6.2.4.1
6.2.4.2
6.2.5
6.2.5.1
6.2.5.2
6.2.5.3
6.2.6
6.3
6.3.1
6.3.2
6.3.3
6.3.4
6.3.4.1
6.3.5
6.3.6
6.3.7
6.4
6.4.1
6.4.1~1

6.4.1.2
6.4.1.3

VT105/VT125 Expansion Backplane ...............................
CRT and Yoke Assembly..........................................
CRT Discharge and Anode Cap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Field Handling of CRTs . . .. . .... . ....... . .. ..... . ........ ... . .. . ..
Board Configurations ..................................................
Terminal Controller Board .......................... ; . . . . . . . . . . . . . .
Advanced Video Option Board with Jumpers .........................
Advanced Video Option Board with Switches ........................
VTIXX-AC Printer Port Option ...................................
VT105 Waveform Generator Board ...................... , ... . . .. . ..
VT125 Graphics Terminal STP Board ..............................
Component Level Troubleshooting ......................................
Troubleshooting the Terminal Controller.. . . . .. . . . . . . . . . . . . . . . . . . . . . .
Microprocessor ..............................................
Video Processor .............................................
Communications .............................................
Nonvolatile RAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . .
Troubleshooting the Keyboard. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
Troubleshooting the Power Supply ..................................
Troubleshooting the CRT Monitor ..................................
Troubleshooting the Options .......................................

5-62
5-62
5-63
5-66
5-68
5-68
5-68
5-68
5-68
5-68
5-68
5-73
5-73
5-73
5-75
5-76
5-77
5-77
5-77
5-78
5-78

OPTIONS
SET-UP Procedures .................................................. .
SET-UP A ..................................................... .
SET-UP B ..................................................... .
SET-UP C ..................................................... .
Advanced Video Option .............................................. .
Advanced Video Option Installation ................................ .
Advanced Video Option Checkout ................................. .
Program Memory Expansion ...................................... .
Alternate Character Set .......................................... .
Alternate ROM Description .................................. .
Character ROM Programming Instructions .................... '..
AVO Technical Description ...................................... .
Extended Character and Attribute Memory ..................... .
Character Attribute Latches .................................. .
Program ROM Decoding .................................... .
Troubleshooting the AVO, ........................................ .
20 rnA Current Loop Adapter ......................................... .
20 rnA Current Loop Option Installation ........................... .
Configurations ................................................... .
20 rnA Current Loop Option Checkout ............................. .
Current Loop Principles .......................................... .
Current Loop Adapter Description ............................ .
Interface Signals ................................................ .
Interface Specifications ........................................... .
Troubleshooting the Current Loop Adapter .......................... .
VT105 Graphics Processor ............................................ .
Enabling Graphic Information ..................................... .
Writing Data to the Waveform Generator ...................... .
Reading Data from the Waveform Generator ................... .
Decoding the Input .......................................... .

6-1
6-1
6-2
6-2
6-13
6-13
6-13
6-15
6-16
6-16
6-17
6-17
6-17
6-18
6-18
6-18
6-18
6-18
6-19
6-20
6-20
6-20
6-22
6-22
6-23
6-23
6-23
6-25
6-26
6-26

6.4.1.4
6.4.1.5
6.4.1.6
6.4.1.7
6.4.1.8
6.4.1.9
6.4.1.10
6.4.1.11
6.4.1.12
6.4.1.13
6.4.1.14
6.4.1.15
6.4.1.16
6.4.2
6.4.2.1
6.4.2.2
6.4.2.3
6.4.2.4
6.4.2.5
6.4.2.6
6.4.2.7
6.4.2.8
6.4.2.9
6.4.2.10 .
6.5
6.5.1
6.5.2
6.5.2.1
6.5.2.2
6.6
, 6.6.1
6.6.2
6.7
6.7.1
6.7.2
6.7.2.1
6.7.2.2
6.7.2.3
6.7.2.4
6.7.3
6.7.3.1
6.7.3.2
6.7.3.3
6.7.3.4
6.7.3.5
6.7.3.6
6.7.4
6.7.4.1
6.7.4.2
6.7.4.3
6.7.4.4
6.7.5
6.7.5.1
6.7.5.2

Selecting Mode of Operation .................................. 6-28
Decoding Field Selection ..................................... 6-28
Phase-Lock-Loop Timing ...................................... 6-30
Establishing Desired Display .................................... 6-30
Loading X-Address Information ............................... 6-34
Loading Graph Memories ...................................... 6-34
Generating Baselines (Shade Lines) ............................ 6-38
Enabling a Histogram (Shading a Graph) ....................... 6-38
Loading Vertical Lines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 6-38
Adding Graph Marker . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. 6-41
Generating Horizontal Lines .................................. 6-41
Generating Strip Charts ...................................... , 6-.:41
Combining Video Out and Timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-46
VT105 Graphic Test Procedure .................................... 6-48
Test Setup .................................................. 6-48
Test Graph 0, Histogram 0, and Graph 0 Markers ................ 6-.:48
Test Graph 1, Histogram 1, and Graph 1 Markers ............... 6-51
Test the Horizontal Lines ...................................... 6-51
Test the Vertical Lines ....................................... 6-51
Test Shade Line 0 (Baseline 0) ................................ 6-53
Test Shade Line 1 (Baseline 1) ................................ 6-54
Test Strip Chart 0 ....................................... ,... 6-54
Test Strip Chart 1 ............................................ 6-55
Exit the Graphic Test Mode. . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. 6-55
VTIXX-AC Printer Option. . . . . . . . . . . . . . . . .. .... . . . .. .. . . . . .. . . . . . . . .. 6-56
Option Installation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 6-56
Printer Port Option Checkout Procedure .. . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-57
Power-Up Test .............................................. 6-57
Printer Interface Data Loopback Test. . . . . . . . . . . . . . . . . . . . . . . . . .. 6-58
VTIXX-CB, -CL Kit .................................................. 6-59
Installation ...................................................... 6-60
Graphics Option Checkout Procedure ............................... 6-67
VT125 Graphics Processor Technical Description. . . . . . . . . . . . . . . . . . . . . . . .. 6-70'
Block Diagram Description ........................................ 6-70
Microprocessor and Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 6-72
Memory, I/O, and Bus Operation... . ... .. . ... . .. . . . . . .. . . . . ... 6-73
Refresh Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 6-75
DRAM Control Timing Sequencer Details ...................... 6-77
The Microprocessor DRAM Memory Cycle. . . .. . . . . . . ... . . . . . .. 6-79
Timing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 6-79
Clocks ..................................................... 6-80
Horizontal and Vertical Counters .............................. 6-82
Offsets ...................................................... 6-82
Scaling ...................................................... 6-84
State Sequencer ............................................. 6-84
Blanking and Clock Synchronization. . . . . . . . . . . . . . . . . . . . . . . . . . .. 6-85
Vector Generator ............................. '... , . . . . . . . . . . . . . . . .. 6-87
Circuit ...................... '. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-87
Vector Generator Timing ..................................... 6-89
Drawing Vectors ............................ ~....... .. . . .. . . ... 6-91
Vector Calculation Timing ..................................... '6-96
Bit Map Write Control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 6:-98
Block Diagram .............................................. 6-9'8
Character Writing ............................................ 6-100

))

6.7.6
6.7.7
6.7.8
6.7.9
6.7.9.1
6.7.10
6.7.10.1
6.7.10.2
6.7.10.3
6.7.10.4
6.8
6.8.1
6.8.2
6.8.2.1
6.8.2.2

)

CHAPTER 7

STANDARD TERMINAL PORT

7.1
7.2
7.3
7.4
7.4.1
7.4.2
7.4.2.1
7.4.2.2
7.4.2.3
7.4.2.4
7.4.2.5
7.5
7.5.1
7.5.1.1
7.5.1.2
7.5.2
7.5.3
7.6
7.6.1·
7.6.2

Introduction ......................................................... .
Definitions .......................................................... .
Overview ........................................................... .
Functional Specifications .............................................. .
Interface Signal Lines ............................................ .
Protocol Specifications ........................................... .
Terminal Operation with No Option Present .................... .
Standard Set-Up for Local Link ............................... .
Control Sequences for Terminal Parameters .................... .
Initialization ............................................... .
BREAK ................................................... .
Electrical Specifications ............................................... .
Signal Lines .................................................... .
Signal Levels ............................................... .
Signal Timing ...................... ,........................ .
Power Supply Lines ............................................. .
Connector 'Pinout ................................................ .
Mechanical Specifications ............................................. .
Shorting Connector .: ............................................ .
STP Connector Card ............................................ .
Guidelines for the Designer ............................................ .
Use with Receive Only Device .................................... .
Single UART Method ........................ .' .............. .
Two UART Method ........................................ .
'Use with Passive Device ........ '................................. .
Use with Active Device .......................................... .
Terminal Processor as Standalone CPU ........................ .
Terminal Processor Augmenting Basic Terminal Operation ....... .
Use with Communications Option ................................. .
Use with an External Processor ................................... .
Use with More than One Option ................................... .

7.7
7.7.1
7.7.1.1
7.7.1.2
7.7.2
7.7.3
7.7.3.1
7.7.3.2
7.7.4
7.7.5
7.7.6
CHAPTER 8

)

Bit Map Memory ......... ' . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 6-102
Output Map ..................................................... 6-104
Output Latches and DAC Converters ............................... 6-104
STP Board ...................................................... 6-106
UARTs .................................................... 6-106
Firmware ........................................................ 6-106
Memory and 110 Addresses ................................... ,6-106
Self-Test Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 6-106
Graphics Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 6-108
Communication Protocols ..................................... 6-109
VT125 Testing and Troubleshooting ..................................... 6-115
Preparing for Self-Tests ...................................•....... 6-115
Error Reporting .................................................. 6-117
VT100 Errors ............................................... 6-117
VT125 Errors ............................................... 6-117

8.1
8.2

7-1
7-1
7-2
7-3
7-3
7-6
7-6
7-6
7-7

7-9
7-9
7-9
7-9
7-9
7-9
7-10
7-10
7-10
7-10
7-10
7-11
7-11
7-11
7-13
7-13
7-13
7-13
7-14
7-14
7-14
7-14

GRAPHICS CONNECTOR
Introduction ........ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hardcopy Enable .....................................................

8-1
8-3

APPENDIX A

PROGRAMMING INFORMATION

APPENDIX B

RECOMMENDED SPARES LIST (RSL)

APPENDIX C

GLOSSARY OF TERMS AND ABBREVIATIONS

APPENDIX D

ANSI CODE EXTENSION TECHNIQUES

FIGURES
2-1
2-2
2-3
2-4
2-5
3-1
3-2
3-3
3-4
4-1-1
4-1-2
4-1-3
4-2-1
4-2-2
4-2-3
4-2-4
4-3-1
4-3-2

4-3-3
4-3-4
4-3-5
4-4-1
4-4-2
4-4-3
4-4-4
4-4·5 A
4-4-5B
4-4-5C
4-4-6

4-4-7
4-4-8
4-4-9
4-4-10
4-4-11
4-5-1
4-5-2
4-5-3
4-6-1
4-6-2
4-6-3
4-6-4
4-6-5
4-6-6
4-6-7

VT100 Keyboard .........................., .......................... .
2-2
2-6
VT100 Terminal (Rear View) .............................. '" ......... .
SET-UP A Mode Presentation .................. " ..................... .
2-7
SET-UP B Mode Presentation ......................................... .
2-8
SET-UP B Mode Summary ........................................... .
2-9
VT100 Terminal Dimensions .......................................... .
3-2
VT100 Rear View ................................................... .
3-3
20 rnA Current Loop Interface ......................................... .
3-7
Composite Video Output .............................................. .
3-8
VT100 Components .................................................. .
4-2
Functional Block Diagram .............................................. . 4-4
VTl 00 Firmware Block Diagram ........................................ . 4-8
8080 Block Diagram ....................................' ............. . 4-10
Microprocessor Block Diagram ........................................ . 4-12
Microprocessor Bus Timing ........................................... . 4-14
Memory Map ....................................................... . 4-15
8251 A PUSART Block Diagram ...................................... . 4-21
Asynchronous Data Format ........................................... . 4-22
Mode Instruction ..................................................... . 4-24
Command Instruction ................................................. . 4-25
Status Byte Format .................................................. . 4-26
Keyboard Block Diagram ............................................. . 4-31
Keyboard Status Byte ................................................ . 4-32
Key Address Transmission Delays ..................................... . 4-33
Keyboard Switch Array ............................................... . 4-35
Keyboard Interface Circuit ............................................ . 4-36
Bias Network - E8 High .............................................. . 4-36
Bias Network - E8 Low .............................................. . 4-36
Encoding of Terminal Data and Clock .................................. . 4-39
F our Keyboard Interface States ........................................ . 4-40
Keyboard Interface Signal ............................................. . 4-40
Bell Circuit ........ '................................................. . 4-41
Keys Flag Byte ...................................................... . 4-41
Sneak Path .................... '....... " ............................. . 4-43
ER1400 NVR Block Diagram ......................................... . 4-46
NVR Signals ........................................................ . 4-48
NVR Timing Diagrams ............................................... . 4-49
Video Processor Block Diagram ........................................ . 4-50
Dots, Scans, and Characters ........................................... . 4-51
Video Processor Functional Diagram ................................... . 4-53
Character Generator Example ......................................... . 4-54
DCO 11 Block Diagram ............................................... . 4-56
Video Latch Timing - 80 Column ..................... ; ................ . 4-57
Video Latch Timing - 132 Column ..................................... . 4-58

xii

))

)

4-6.:8
4-6-9
4-6-10
4-6-11
4-6-12
4-6-13
4-6-14
4-6-15
4-6-16
4-6-17
4-6-18
4-6-19
4-6-20
4-6-21
4-7-1
4-7-2
4-7-3
4-7-4
4-7-5
4-7-6
4-7-7
4-7-8
4-7-9
4-7-10
4-7-11
4-8-1
4-8-2
4-8-3
4-8-4
4-8-5
4-9-1
4-9-2
4-9-3
5-1
5-2
5-3
5-4
5-5
5-6
5-7
5-8
5-9
5-10
5-11
5-12
5-13
5-14
5-15
5-16
5-17
5-18
5-19
5-20
5-21

Address Counting Timing .......................... : .................. . 4-59
Horizontal Timing ................................................... . 4-61
Line Buffer Address Outputs - 80 Column .............................. . 4-62
Line Buffer Address Outputs - 132 Column ............................. . 4-63
Vertical Signals - 60 Hz .............................................. . 4-65
Vertical Signals - 50 Hz .............................................. . 4-66
Composite Sync Outpuf ............................................... . 4-68
DC012 Block Diagram ............................................... . 4-69
Dot Stretching ....................................................... . 4-76
Dot Stretcher Example ............................................... . 4-76
Character Generator ROM Patterns .................................... . 4-78
DMA Cycle Timing Diagram .......................................... . 4-81
Character Latch Timing ............................................... . 4-83
Video Blanking ...................................................... . 4-84
Line Organization .................................................... . 4-86
Terminator and Address Bytes ......................................... . 4-86
Screen RAM Organization - 80 Column, 60 Hz ......................... . 4-88
Need for Fill Lines ................................................... . 4-90
Fill Line Operation - 60 Hz " ......................................... . 4-90
Fill Line Operation - 50 Hz ........................................... . 4-91
End of Screen Fill Line Operation ...................................... . 4-91
Line Address Offset Table ............................................ . 4-93
Full Screen Address Shuffie ........................................... . 4-94
Split Screen Address Shuffie ........................................... . 4-96
Split Screen Smooth Scroll at Midpoint ................................. . 4-97
Horizontal Deflection Current - TO ..................................... . 4-102
Horizontal Deflection Current - Tl ..................................... . 4-102
Horizontal Deflection Current - T2 ..................................... . 4-102
Horizontal Deflection Current - T3 ..................................... . 4-103
Horizontal Deflection Waveforms ...................................... . 4-103
VT100 Power Supply Block Diagram ................................... . 4-107
3524 Regulator Block Diagram ........................................ . 4-108
Power Supply Timing Diagram ........................................ . 4-109
5-9
VT125 General Block Diagram ........................................ .
EIA Loopback Connector ............................................. . 5-17
20 rnA Loopback Connector ........................................... . 5-18
Video Alignment Templates ........................................... . 5-26
Paper Alignment Template ............................................ . 5-27
Ball Video Monitor Board Adjustments ................................. . 5-28
Ball Monitor CRT Adjustments ........................................ . 5-29
Elston Video Monitor Board Adjustments ............................... . 5-32
Elston Monitor CRT Adjustments ...................................... . 5-33
Removal Procedures Sequence ......................................... . 5-35
VT100 Terminal (Rear View) .......................................... . 5-36
Terminal Controller with Advanced Video Board ......................... . 5-38
Terminal Controller Board ROM Installation ............................ . 5-40
Installing Printer Port on VT125 STP Board ............................. . 5-41
VT125 24-Pin Flat Cable on STP Board ................................ . 5-42
VTI05 Interconnections .............................................. . 5-44
VT125 16-Pin Flat Cable on Terminal Controller ........................ . 5-45
VT125 Graphic Cable Connections ..................................... . 5-46
Keyboard Dissassembly ............................................... . 5-48
Keycap Removal ..................................................... . 5-50
Top Cover Removal .................................................. . 5-52

Xlll

5-22
5-23
5-24
5-25
5-26
5-27
5-28
5-29
5-30
5-31
5-32
5-33
5-34
5-35
5-36
5-37
6-1
6-2
6-3
6-4
6-5
6-6
6-7
6-8
6-9
6-10
6-11
6-12
6-13
6-14
6-15
6-16
6-17
6-18
6-19
6-20
6-21
6-22
6-23
6-24
6-25
6-26
6-27
6-28
6-29
6-30
6-31
6-32
6-33
6-34
6-35
6-36
6-37
6-38

Ball Video Monitor Board Removal ..................................... .
Elston/DIGITAL Video Monitor Board Removal ........................ .
Power Supply Removal ............................................... .
Power Supply Capacitator Discharging .................................. .
FCC Shield on VT100 Cage .......................................... .
VT100 DC Power Harness Removal ................................... .
Expansion Backplane ................................................. .
CRT Anode Discharging .............................................. .
Removing Anode Cap ................................................ .
CRT Disposal ....................................................... .
Terminal Controller Board ............................................ .
Advanced Video Option Board with Jumpers ............................ .
Advanced Video Option Board with Switches ............................ .
VTIXX-AC Printer Port Option Board ................................. .
VTI05 Waveform Generator Board .................................... .
VT125 STP Board ................................................... .
SET-UP A with Advanced Video Option (AVO) ......................... .
VT100 SET-UP A ................................................... .
VT100-WC, -WK, VTI32, VTIXX-AC SET-UP B ..................... .
VT100-AA, -AB, -WA, -WB SET-UP B Summary ...................... .
VT100-WC, -WK, VTI32, VTIXX-AC SET-UP B Summary ............ .
VTI05 SET-UP B Summary .......................................... .
VTI25 SET-UP B Summary .......................................... .
VT100-WC, -WK SET-UP C ......................................... .
VTIXX-AC SET-UP C .............................................. .
VT132 SET-UP C ................................................... .
VTIXX-AC SET-UP C Summary ..................................... .
VT132 SET-UP C Summary .......................................... .
Advanced Video Option Installation .................................... .
20 rnA Current Loop Option .......................................... .
Interface States .................................................. ' .... .
M7071 Waveform Generator Block Diagram ............................ .
Decoding the Control Character ........................................ .
Rectangular Aspect Ratio Graph Drawing Field .......................... .
Square Aspect Ratio Graph Drawing Field .............................. .
Selecting Mode of Operation ..................... " .................... .
Phase-Lock-Loop Timing ............................................. .
Loading the Registers ................................................. .
Register 0 (1 st Data Character) ........................................ .
Register 0 (2nd Data Character) ....................................... .
Register 1 (1 st Data Character) ........................................ .
Register 1 (2nd Data Character) ....................................... .
Loading Address Data ................................................ .
Graph Memory and Y-Address Monitor ................................. .
Baseline (Shade Line) Registers and Comparators ........................ .
Vertical Line and Graph Marker Memory ............................... .
Graph Marker Y-Address Comparator .................................. .
Horizontal Line Memory .............................................. .
Generating Strip Charts ............................................... .
Dual Strip Chart Timing Diagram ...................................... .
Combining Video Out ................................................ .
Graph Test Pattern .................................................... .
Histogram Test Pattern ............................................... .
Graph Marker Test Pattern ............................................ .

xiv

5-53
5-54
5-56
5-57
5-59
5-60
5-61
5-64
5-65
5-67
5-69
5-70
5-71
5-72
5-72
5-73
6-1
6-2
6-3
6-4
6-5
6-6
6-7
6-8
6-9
6-10
6-11
6-12
6-14
6-19
6-21
6-24
6-27
6-29
6-29
6-30
6-31
6-32
6-33
6-33
6-35
6-35
6-36
6-37
6-39
6-40
6-42
6-43
6-44
6-46
6-47
6-49
6-50
6-50

))
7

)

6-39
6-40
6-41
6-42
6-43
6-44
6-45
6-46
6-47
6-48
6-49
6-50
6-51
6-51A
6-52
6-53
6-54
6-55
6-56
6-57
6-58
6-59
6-60
6-61
6-62
6-63
6-64
6-65
6-66
6-67
6-68
6-69
6-70
6-71
6-72
6-73
6-74
6-75
6-76
6-77
6-78
6-79
6-80
6-81
6-82
6-83
7-1A
7-1B
7-1C
7-2
7-3
7-4
7-5
A-I

Horizontal Lines Test Pattern. . . . . . . . . . . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . .. 6-52
Vertical Lines Test Pattern ............................................. 6-52
Shade Line (Baseline) Test Pattern. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 6-53
Strip Chart Test Pattern ............................................... 6-55
Check ROM Numbers .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 6-60
Installing Expansion Backplane ......................................... 6-61
Installing Ground Clip .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 6-62
16-Pin Flat Cable on Terminal Controller. . . .. . . . . . . . . . . . . . . . . . . . . . . . . . .. 6-63
Installing STP Board .................................................. 6-64
24-Pin Flat Cable on STP Board........................................ 6-65
Graphic Cable Connections ............................................ 6-66
STP Board Switch Types .............................................. 6-68
Connecting BNC Bracket Ground Wire....... ... ... ... ... . . .... .. . .. .. .. 6-69
EIA Line Filter Connector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 6-69
VT125 Graphics Processor Block Diagram ............................. .. 6-71
Microprocessor Address and Data Paths ................................. 6-74
DRAM Block Diagram ....................... ~ .. .. . . . . . . . . . . . . . . .. .. .. 6-75
Microprocessor Memory Control Timing Sequencer ....................... 6-76
Address Multiplexer and Refresh Counter ................................ 6-78
Opcode Fetch from DRAM............................................ 6-80
VT125 Timing Block Diagram ........' ...................... , . . . . . . . . . .. 6-81
Smooth Scroll Offset Timing ........................................... 6-83
V STROBE Timing ................................................... 6-86
Vector Generator ..................................................... 6-88
Deriviation of DIL and DVM ........................................... 6-89
Vector Generator and Bit Map Address Timing ................. , . . . . . . . .. 6-90
Basic Vectors. .. ... . .. . .. . . . . . . . .. ...... . ... ... .. . . . . .. .. . . .. . . . . . . . .. 6-91
Calculation 1 ..................... ~ .......'. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 6-92
Calculation 2 . ~ ......... ; . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-93
Calculation 3 ......................................................... 6-94
Calculation 4 ..................................................... . . .. 6-95
Calculation 5 ......................................................... 6-96
Arbitrary Vector Timing ................................................ 6-97
Bit Map Write Control Block Diagram......... ..... . ... . .. . . . . .. . . .. .. .. 6-99
Draw a Character ....' . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 6-101
Pattern Memory and Multiplier ......................................... 6-102
Bit Map ............................................................. 6-103
Bit.Map Planes and the Output Map .............. ~ ...................... 6-105
VT125 General Block Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 6-109
VT125 Data Paths .................................................... 6-110
VT125 as a Text-Only Terminal ........................................ 6-111
VT125 in a Graphics Terminal ......................................... 6-112
VT125 in ReGIS Graphics with Commands on Screen ..................... 6-113
VT125 Printing from Screen ............................................ 6-114
VT 125 Printing from Computer ........... ,...... ;..................... .. 6-115
EIA Loopback Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 6-116
Serial Line Splice Connection .................... ;.....................
7-2
STP with No Option Present. .... . ........... ..... . .. . . . ... . .. . . . .. .. . .
7-2
STP with Option Present. . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7-2
STP Signal Lines .....................................................
7-3
Local Clock. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 7-10
STP Connector ....................................................... 7-11
STP Option Card ................................................... '.. 7-12
Terminal Data Flow... ... . .. . . . ... .... ... ... ... ... . . .. . .. .. . ... . ... . . . A-I

A-2
A-3
A-4
0-1
0-2
0-3

VT100 Keyboard .. ~ ............................... '. . . . . . . . . . . . . . . . . . .
Standard Key Codes . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Function Key Control Codes ...........................................
ASCII Table ........................................................ ~
8-Bit ASCII Chart. ... . . . .. . . . . . . .. .. ...... .......... ... . . . . . . . . . .. . ..
Shift Out and Shift In ............•. ; . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A-3
A-4
A-6
0-2
0-9
0-9

TABLES
2-1
2-2
2-3
2-4
2-5
3-1
4-2-1
4-2-2
4-2-3
4-3-1
4-3-2
4-4-1
4-6-1
4-6-2
4-6-3
4-6-4
5-1
5-2
5-3
5-4
5-5
5-6
5-7
5-8
5-9
5-10
6-1
6-2
6-3
6-4
6-5
6-6
6-7
A-I
A-2
A-3
A-4

A-5
A-6

A-7
A-8
B-1

Categories of SET-UP Features ........................................ .
2-9
SET-UP Feature Change Summary .................................... . 2-10
Fatal Displayed Error Codes .......................................... . 2-18
Nonfatal Displayed Error Codes ....................................... . 2-18
Problem Checklist .................................................... . 2-19
EIA Connector Signals ............................................... .
3-5
ROM Chip Select Addressing ......................................... . 4-17
List of Hex I/O Addresses ............................................ . 4-17
Interrupt Addresses .................................................. . 4-18
Baud Rate Generator Divisors ......................................... . 4-27
Loopback Test Connectors ............................................ . 4-30
Keyboard Receiver Signals ............................................ . 4-37
Video Mode Selection (Write Address = C2H) .......................... . 4-55
Control Chip Commands (Write Address = A2H) ........................ . 4-70
Scan Count Sequence ................................................ . 4-72
Character Attribute Combinations ...................................... . 4-75
Keyboard LED Error Codes .......................................... .
5-1
VT100 Display Error Codes ........................................... .
5-2
Basic VT100 Troubleshooting Procedure ................................ .
5-4
VT125 Displayed Error Codes ......................................... . 5-12
LED Error Codes ................ ; .....................•.............. 5-15
Displayed Error Codes ............................................... . 5-15
Terminal Controller Board ROMs .................... ; ................. . 5-68
Advanced Video Option (AVO) Jumpers ................................ . 5-69
Advanced Video Option (AVO) Board ROMs ........................... . 5-70
Advanced Video Option Switches ...................................... . 5-71
Control Characters ................................................... . 6-26
Keyboard Indicator Error Codes ....................................... . 6-58
Displayed Error Codes ............................................... . 6-58
1/0 Addresses and Data .............................................. . 6-107
VT125 Fatal Error Indication ......................................... . 6-117
ROM and RAM Failure Indications .................................... . 6-118
Error Output Signal Levels ............................................ . 6-118
SET-UP Features and Machine States .................................. . A-4
Function Key Codes .................................................. . A-5
Cursor Control Key Codes ............................................. . A-7
Auxiliary Keypad Codes .............................................. . A-8
Special Graphics Characters ........................................... . A-I0
Control Characters ................................................... . A-13
VT100 Escape Sequences Summary .................................... . A-14
Special Character and Line Drawing Set and Graphics Mode Comparison ... . A-31
B-1
VT100 Recommended Spares ......................................... .

xvi

)

CHAPTER 1
INTRODUCTION AND SPECIFICATIONS

This is the technical manual for the VT100 series video terminals. It contains information a service
technician or engineer needs to operate, test, and repair the VT100 series to a component level.

)

Chapter I

Contains VT100 specifications and documentation ordering information.

Chapter 2

Contains basic operator information, including use of the keyboard. use of SETUP modes for setting terminal characteristics, and simple trouble checking.

Chapter 3

Contains ·installation procedures and interface information.

Chapter 4

Contains a technical description of the basic VT100 video terminal. This
chapter assumes that the reader has the Field Engineering Print Set, MP00633.

Chapter 5

Contains servicinginformation for the VT100 series video terminals.

Chapter 6

Contains SET-UP information for the various VT100 series options, installation
information, a technical description, and service information for the Advanced
Video option, Current Loop option, VTI05 Graphics Processor, VTIXX-AC
Printer option, and VT125 Graphics Processor.

Chapter 7

Contairis a technical description and interfacing information for the standard
terminal port (STP).

Chapter X

Contains interfacing information for the graphics connector.

. Appendix A

Contains programming informationfor the VT100 series, including interface
timing considerations and descriptions of control functions the VT 100 responds
to, both in ANSI mode and inDEC VT52-compatible mode. Also contains copy
of VTI25 Programming Reference Card, EK-VTI2S-RC-001.

Appendix B

Contains a recommended spares list (RSL).

Appendix C

Contains a glossary of terms and abbreviations used in this manual.

Appendix D

Contains a description of the ANSI code extension techniques.

)
1-1

VT100 SPECIFICATIONS
DIMENSIONS
Monitor
Height
Width
Depth

36.83 cm (14.5 inch)
45.72 cm (18 inch)
36.20 cm (14.25 inch)

Keyboard
Height
Width
Depth
Minimum table depth

8.89 cm (3.5 inch)
45.72 cm (18 inch)
20.32 cm (8 inch)
51.4 cm (20.25 inch)

Weight
Monitor
Keyboard
Shipping weight

13.6 kg (30 lb)
2.0 kg (4.5 lb)
18.6 kg (41Ib)

ENVIRONMENT AL
Operating
Temperature
Relative humidity
Maximum wet bulb
Minimum dew point
Altitude

10 0 t040° C (50 0 to 104 0 F)
10% to 90%
28 0 C (82 0 F)
2 0 C (36 0 F)
2.4 km (8,000 ft)

Nonoperating
Temperature
Relative humidity
Altitude

-40 0 t066° C( -40 0 to 151 0 F)
Ot095%
9.1 km (30,000 ft)

Power
Line voltage

90-128 V rms single phase, 2 wire
180-256 V rms single phase, 2 wire (switch-selectable)

Line frequency

47-63 Hz

Current

3.0 A rms maximum at 115 V rms
1.5 A rms maximum at 230 V rms

Input power
Current limiting
Power cord

250 VA apparent 150 W max.
3 A normal blow fuse
Detacha bIe, 3 prong, 1.9 m (6 ft)

1-2

Display
CRT

30 cm (12 inch) diagonal measure, P4 phosphor

Format

24 lines X 80 characters or 141ines X 132 characters (selectable)

Character
Character size
80 column mode
132 column mode
Active display size
Character set

7 X 9 dot matrix with descenders

Cursor type

3.35 mm X 2.0 mm (0.132 inch X 0.078 inch)
3.35 mm X 1.3 mm (0.132 inch X 0.051 inch)
203 mm X 127 mm (8 inch X 5 inch)
96-character displayable ASCII subset (upper- and lowercase, numeric,
and punctuation)
Keyboard-selectable, blinking block character or blinking underline

Keyboard
General

83-key detachable unit with 1.9 m (6 ft) coiled cord attached

Key layout

65-key arrangement and sculpturing similar to standard typewri ter, wi t h
18-key numeric keypad.

Numeric keypad

18-key with period, comma, minus, enter, and four general purpose
function keys.

Visual indicators

7 LEDs; 3 dedicated to ON LINE, LOCAL, and KBD LOCKED: 4 are
user-programmable.

)

Audible signals
Keyclick
Bell

Multiple bell

Sound simulates typewriter.
1) sounds upon receipt of BEL code; 2) sounds 8 characters from right
margin (keyboard-selectable).
Sounds upon detection of error in SET-UP save or recall operation.

Communication
Type
Speeds

EIA
Full duplex: 50, 75, 11 0 (two stop bits), 134.5, 150, 200, 300. 600. 1200,
1800,2000,2400,3600,4800,9600,19,200

Code
Character format

ASCII
Asynchronous

Character size

7 or 8 bits; keyboard-selectable. (Note: if 8-bit character is selected, 8th
bit is always space.)

Parity

Even, odd, or none (keyboard-selectable)

Synchronization

Keyboard-selectable via automatic generation of XON and XOFF control
codes.

1-3

ORDERING DOCUMENTATION
You can purchase the following VT100 series video terminal documents from DIGITAL's Accessories
and Supplies Group.
Title

Part Number

VT100 User Guide
VT I 00 Series Pocket Service Guide
VT100 Illustrated Parts Breakdown (IPB)
VT100 Print Set (contains VTIXX-AC)

EK-VTIOO-UG
EK-VTIOO-JI
EK-VTIOO-IP
MP-00633

VTI05 Technical Manual
VTIOS Illustrated Parts Breakdown (lPB)
VTIOS Print Set

EK-VTlOS-TM
EK-VTI0S-IP
MP-00642

VTI2S User Guide
VT 125 Print Set
VTI XX-CB, -CL Print Set
VTI2S Illustrated Parts Breakdown (IPB)

EK-VTI2S-UG
MP-OI053
MP-0IOS2
EK-VTI25-IP

VTI32 User Guide
VTI32Print Set (contains VTI XX-AC)
VTI32 Illustrated Parts Breakdown (IPB)

EK-VTI32-UG
MP-00748
EK-VTI25-IP

VTI XX-AC User Guide
VTI XX-AC Print Set

EK-VTIAC-UG
MP-00901

))

You can order accessories and supplies (including documentation) by mail or phone.

Continental l:SA
Call 800-258-1710 or mail order to:
Digital Equipment Corporation
P.O. Box CS2008
Nashua,!\H 03061

))

New Hampshire
Call 602-H84-6660 or mail order to:
Digital Equipment Corporation
P.O. Box CS2008
Nashua, N H 03061

Alaska or Hawaii .
Call 408-734-4915 or mail order to:
Digital Equipment Corporation
632 Caribbean Drive
Sunnyvale, CA 94086

))
1-4

)

Canada
Call 800-267-6146 or mail order to:
Digital Equipment Corporation
P.O. Box 13000
Kanata, Ontario Canada K2K 2A6
Att: A&SG Business Manager
Telex: 610-562-8732

Related Documentation
Intel 8080 Microcomputer Systems User's Manual
From:
Intel Corporation
3065 Bowers Avenue
Santa Clara, California 95051

)

EIA Specifications RS-232-C and RS-170
From:
Electronic Industry Association
EIA Engineering Department
2001 Eye Street, N.W.
Washington, DC 20006
ANSI Standards X3.41-1974, X3.64-1977, 3.4-1977
From:
Sales Department
American National Standards Institute
1430 Broadway
New York, NY 10018

)

1-5

CHAPTER 2
OPERATOR INFORMATION

PART I

KEYBOARD CONTROLS AND INDICATORS

The VT100 terminal normally performs a two-part function. It is an input device to a computer ...:
information entered through the keyboard is sent to the computer. It is simultaneously an output
device for the computer - that is, data coming in from the computer is displayed on the video screen.
The following controls and indicators on the VT100 keyboard are illustrated in Figure 2-1.

SET-UPKey

Used in conjunction with other keys to perform specific functions such as setting tabs, scrolling, and
altering terminal characteristics.

)

ON LINE Indicator
Lights to show that the VT100 is on-line and ready to transmit or receive messages.
LOCAL Indicator
Lights to show that the terminal is off-line and cannot communicate with the host device. In local
mode the keyboard remains active and all characters typed are placed on the screen.
KBD LOCKED Indicator
Lights to show the keyboard has been turned off. The VT100 is still able to receive data from the host.
This condition can be cleared by entering and exiting SET-UP mode.

)

Ll-L4 Indicators
These indicators are turned on and off by the host. Consult your local operating procedures for the
meaning of each indicator. Ll-L4 also show self-test errors.

[+]088

Keys

Each of these keys causes the VT100 to transmit a code that has a special meaning to your system.
Consult your local operating procedures for the meaning of these keys. In SET-UP mode the Tand ~
keys increase or decrease display brightness. The +- and -+ keys move the cursor left and right.

BACKSPACE Key
Transmits a backspace code.

)
2-1

ON LINE

LOCAL K8DLDCKEU L1

L2,

08
rtl
0

GBBEJ
r:lDJ-ril~·m~r71mr.lmm8r+lI-l~EJ~ 0000
LJ
~G0000~~08G0[oDJnEJr 0G0D
~ ,B~0000ffi000~CJCJ[::JITJ
~000
~I
10000000CJowc:J@J lolDO
0
CLEAR All

SEI UP

SEll

0
LINE/

0
SEI.IP

0
o· 0 080/132
TOGGLE TRANSMIT RECEIVE

~L!J~~~L!JL!JL!U~LJL::JLJ SPACE

SHIFT

r
Figure 2-1 VT100 Keyboard

"---'

.~/

"',-,'

......---/

IE]
i

'BREAK

BREAK Key

Transmits a break signal.

GBBB

PFI-F4Keys

Each of these keys causes the VT100 to transmit a code that has a special meaning to your system.
Consult your local operating procedures for the meanings of these keys.
Numeric Keypad
The numeric keypad enables numbers to be entered in calculator fashion. Each key in the numeric
keypad generates the same character as the corresponding numeric key on the main keyboard. The
ENTER key corresponds to the RETURN key. These keys may also be interpreted by the host computer as special function keys. Consult your local operating procedures for the meanings of these keys.

)

I I
DELETE

DELETE Key

Causes the VT I 00 to transmit a delete character code to the host system. The deleted character mayor
may not be erased from the screen.

)

RETURN Key

Transmits either a carriage return (CR) code or a carriage return and line feed (LF) code. This is a
SET-UP selectable feature.

)

I ~~~~ I LIN EFEED Key
This key transmits a line feed code.

c::J

SHIFT Key

When pressed, this key enables the uppercase function of all keys. If a key does not have an uppercase
function the SHIFT key is disregarded.

RESETKey

In SET-UP mode this key starts the reset sequence. This has the same result as turning the terminal
power off and then on.

)
2-3

80/132 COLUMNS Key

In SET-UP A mode this key switches the display line size from 80 to 132 characters per line or from
132 to 80 characters per line.

RECEIVE SPEED Key

In SET-UP B mode this key steps the terminal through the receive baud rate settings in ascending
order.

TRANSMIT SPEED Key
In SET-UP B mode this key steps the terminal through the transmit baud rate settings in ascending
order.

) )

TOGGLE 1/0 Key
In SET-UP B mode this key turns the selecteci operational feature on or off.

BELL Key
When pressed in combination with the CTRL key this key causes a bell code to be sent to the host.

r%l
~

SET-UP A/B Key

In SET-UP mode this key switches the terminal from SET-UP A to SET-UP B or from SET-U P B to
SET-UP A.

))
LINE/LOCAL Key

In SET-UP mode this key switches the VT100 to communicate with your system (ON LINE) or stops
the VT100 from communicating with your system (LOCAL).

I#£l
L!J CLEAR ALL TABS Key
In SET-UP A this key clears all horizontal tabs set in the VT100.

I ~ I SET /CLEAR TAB Key
In SET-UP A this key sets or clears individual horizontal tabs.

\ )
}

2-4

CAPS LOCK Key

This key enables the transmission of uppercase alphabetic characters only. All numeric and special
symbol keys remain in lowercase.

§;]
o

SCROLL

NO SCROLL Key

When first pressed, this key stops the transmission of data from the computer to the VT I 00. When
pressed a second time, transmission resumes from where it was stopped. Check your local operating
procedures to ensure that your system recognizes this key.

_CTRLKey

When pressed in combination with another key, CTRL causes the VT100 to transmit a code that has a
special meaning to your system.

TABKey

This key transmits a tab code.

ESC Key

)

This key transmits a code that normally has a special meaning to your system. In many applications, it
tells your system to treat the next keys pressed as a command.

MONITOR CONTROL
The VT100 monitor contains only one control, the power switch, shown in Figure 2-2.
AUDIBLE INDICATORS (TONES)
There are three audible indicators associated with the VT100: a short tone (click), a long tone (beep),
and a series of long tones.
Short Tone (click)
The short tone sounds whenever a key is pressed, with the following exceptions:
•

SHIFT or CTRL keys do not generate any keyclick because these keys do not transmit any
codes but only modify the codes transmitted by other keys.

•

When the KBD LOCKED indicator is turned on; in which case, the characters typed are
lost.

•

The keyclick feature has been turned off in SET-UP mode.

2-5

Long Tone (beep)
The long tone sounds to indicate one of the following conditions:
•

A bell code was received from the computer.

•

The cursor is eight characters away from the right margin and the margin bell feature is
enabled.

Series of Long Tones
The terminal sounds the long tone several times in rapid succession to indicate that the nonvolatile
memory (NVR) had difficulty reading or writing SET-UP features. (When this occurs, check the SETUP features and then perform the recall or save operation again.)

KEYBOARD
CONNECTOR

OPTIONAL 20mA

POWER
SELECTOR
SWITCH

POWER CORD
CONNECTOR

COMMUNICATION
CONNECTOR

MA-1987

Figure 2-2 VT100 Terminal (Rear View)

PART 2

SET-UP MODE

Unlike most terminals, the VT100 does not use switches or jumpers to individually turn the built-in
terminal features on or off. Instead, the VT100 uses a nonvolatile memory (NVR) that always remembers what features have been selected, as if a switch had been set.

2-6

))

Selection and storage of built-in terminal features is performed in a special mode of operation called
SET-UP mode. When you enter SET-UP mode, the status' of features stored in temporary memory
shows on the screen. You can then change the features and store any new feature selections either
temporarily, by leaving SET-UP mode; or on a fixed basis, by performing a Save operation. In either
case, terminal operation reflects the new feature selection. If a recall operation is performed, or the
terminal is reset, or terminal power is turned off, all temporary feature settings are replaced by features
that have been stored on a fixed basis.
SET-UP Features
SET-UP mode provides two brief summaries of the current feature status. The first presentation SET-UP A - displays the location of tab stops set and a visual ruler that numbers each character
position on the line. The second presentation - SET-UP B - summarizes the status of the other terminal features.

)

SET-UP A - To enter SET-UP A, press the SET-UP key. The display has a presentation similar to
Figure 2-3. The bottom line of the display consists of a "ruler" that numbers each character position
available on a line. Each tab stop is shown by a "T" above the ruler. If the tab stop(s) set are those
desired, you may exit SET-UP mode by pressing the SET-UP key again or you may now change the
tabs to meet your requirements.

SET-UP
A
TO EXIT PRESS "SET-UP·
)

)

MA·2732

)

Figure 2-3 SET-UP A Mode Presentation

2-7

SET-UP B - SET-UP B mode may only be entered from SET-UP A mode. To enter SET-UP B from
SET-UP A press the 5 key on the main keyboard. The display looks like Figure 2-4. Figure 2-5
summarizes the SET-UP B presentation. This summary allows you to quickly determine what features
are enabled. For additional information on a feature refer to in Part 3, SET-UP Feature Definitions.

\
))

To exit SET-UP B press the SET-UP key.

SET-UP B

Tn EXIT PRESS

u~ET-UP"

))

I
i

lEI! - .;;111
. - j 'imXi' 4- )I;J,~~
~

MA2733

))

Figure 2-4 SET-UP B Mode Presentation
Determining What a SET-UP Feature Does
SET-UP features are basically a series of options in the VT100 that allow the terminal to be tailored to
its operating environment. Table 2-1 lists each feature and shows one of the following general categories.
Installation
Computer compatability
Operator comfort
The installation category includes the initial installation of the terminal and any special options that
may be attached to the terminal. If any options are added or removed, or the physical location of the
terminal is changed, verify the settings of these SET -UP features.

2-8

))

SCROLL [0 = JUMP
= SMOOTH

[";£l (SHIFTED)

AUTOREPEAT

L2J

(0 = OFF

WRAP AROUND

\.1 = ON

SCREEN' [0 = DARK BACKGROUND
= LIGHT BACKGROUND

NEW LINE

(0 = UNDERLINE
\.1 = BLOCK

INTERLACE

\.1

CURSOR

3
AUTO XON XOFF

{o
{o

1 =

£.

= OFF
1 = ON

(0 = OFF

\.1 = ON

(OPTIONAL
DISPLAY
FOR CUSTOM
FUNCTIONS)

[0 = OFF

\.1 = ON

:; cr:r:IJ

(0 = OFF

T SPEED c:::::J

R SPEEDc:=J

POWER [0 = 60 Hz
\.1 = 50 Hz

\.1 = ON

[0 = VT52
(.1 = ANSI
KEYCLICK [0 = OFF
\.1 = ON
MARGIN BELL (0 = OFF
(.1 = ON

[0 = 7 BITS
BITS PER CHAR. \.1 = 8 BITS

ANSINT52

PARITY

[0 = OFF

\.1 = ON

PARITY SENSE

[0 = ODD

\., = EVEN

MA-l98B

)
Figure 2-5 SET-UP B Mode Summary

Table 2-1 Categories of SET-UP Features

Feature

)

Ansi/VT52 mode
Answerback message
Auto Repeat
Auto XON/XOFF
Bits per Character
Characters per Line
Cursor
Interlace
Keyclick
Line/Local
Margin Bell
New Line
Parity
Parity Sense
Power
Receive Speed
Screen Background
Screen Brightness
Scroll
Tabs
Transmit Speed
Wraparound

Computer
Compatibility

Installation

X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X

[YJ.
3

Operator
Comfort

(shIfted)

2-9

X
X
X

Computer compatibility contains the features that must be set correctly so that the VT100 can communicate with the host computer. An error in these settings may cause incorrect data to be sent to or
received from the computer; or an error may prevent the VT100 from communicating with the computer. The settings for these features must be obtained from the host computer programmer, operator,
or system manager since there are many combinations of settings designed to work with particular
computers and special software. These feature settings normally change only when you need to communicate with a different computer or· a unique software package.

) )

The operator comfort category contains SET-UP features designed exclusively for the operator. These
features allow the operator to tailor the VT100 to fit individual preference. These features do not affect
any operations that occur between the terminal and the computer.
Part 3, SET-UP Feature Definitions, describes the specific function of each feature.
How to Change a SET-UP Feature
Changing any or all SET-UP features is a simple operation and is generally performed by following the
same basic steps.
l.

Enter SET-UP mode by pressing the SET-UP key.

Select the appropriate SET-UP mode by pressing the 5 key on the main keyboard each time
you want to switch from SET-UP A to SET-UP B or from SET-UP B to SET-UP A.

Position the cursor above the feature switch or tab stop to be changed. To position the
cursor, the SPACE bar, +-, -+, TAB, and RETURN keys may be used. Some features do not
use this step since a specific key is dedicated to changing the feature.

Change the feature setting by pressing either the 6 key on the main keyboard or the appropriate dedicated key. Each time the key is pressed the feature changes, generally to the
opposite state.

)\ )

Table 2-2 briefly summarizes SET-UP features, the SET-UP mode you must be in to change a feature,
and the key used to change the feature setting.
Table 2-2 SET-UP Feature Change Summary

Feature

Changed In
SET-UP Mode

Key to
Change
Feature

Answerback message

ANSIjVT52 mode

Auto Repeat

Auto XON jXOFF

Bits per Character

Brightness

\) I

W
W
W
W
6

[J[J

* A special sequence is required for this feature. See the detailed features description.
2-10

))

Table 2-2 SET-UP Feature Change Summary (Cont)

)

Feature

Changed In
SET-UP Mode

Characters per Line

Cursor

Interlace

Keyclick

Line/Local

Margin Bell

New Line

Parity

Parity Sense

Power

Receive Speed

Screen

Scroll

Tabs

Transmit Speed

Wraparound

[YJ

rn[l]
[l]
[l]

[l]
[l]
[l]
[l]
[l]
[l]
[l]
[l]
[IJ and [X]

·3

[l]
[l]

Key to
Change
Feature

(shifted)

Setting the Answerback Message
Setting the answerback message differs from setting any other terminal feature. An answerback message can be typed into the VT100, using the following steps.
I.

Place the terminal in SET-UP B mode.

Press the SHIFT and A keys at the same time. The terminal responds by placing A = on the
screen. (The SHIFT key is required. The CAPS LOCK key does not work here.)

2-11

Type the message delimiter character which may be any character not used in the actual
answerback message. The message delimiter character is not part of the answerback message. If a mistake is made when typing the answerback message, type the message delimiter
character again and go back to step 2. This is the only way to correct errors'in the answerback message.

Type the answerback message. The message may be up to 20 characters, including space and
control characters. Control characters are displayed as a character to indicate their presence
in the message.

Type the message delimiter character. Once the message delimiter character is typed the
answerback message disappears from the screen.

\
" ,)

Once the above steps have been completed the answerback message is temporarily stored in the VT100
and can be saved with the save operation.
Saving SET-UP Features
SET-UP features may be changed and stored on either a temporary or a fixed basis. To temporarily
store a feature. exit SET-UP mode after changing the feature; the terminal now reacts according to the
new setting. If a recall operation is performed, or the terminal is reset. or terminal power is turned off,
all temporary feature settings are replaced by the features that have been stored on a fixed basis.

) )

To store SET-UP feature settings on a fixed basis. perform a save operation. This is a simple operation
that is accomplished by performing the following steps.
I.

Place the terminal in SET-UP mode.

Press the SHIFT and S keys at the same time. The screen clears and the message "Wait" is
displayed in the upper-left corner. After a brief wait, the terminal returns to SET-UP A
mode.
NOTE
The save operation must be performed at the terminal keyboard. The computer cannot perform this operation, although it can temporarily modify the
setting of VT100 features.

Once these steps have been performed. SET-UP features which had been temporarily stored are now
stored on a fixed basis.
Recalling SET-UP Features
Temporarily stored SET-UP feature settings may differ from settings that are stored on a fixed basis.
To return to the fixed settings, perform the recall operation as follows.
1.

Place the terminal in SET-UP mode.

Press the SHIFT and R keys at the same time. The screen clears and the message "Wait"
appears in the upper-left corner. After a brief wait the terminal returns to SET-UP A mode.
NOTE
When a recall operation is performed the contents of
the screen are destroyed.

2-12

) )

)

Resetting the Terminal
The VT100 may be reset from the keyboard. When the terminal is reset, the terminal memory is cleared
and i.nt: self-test program runs as if the terminal power switch was turned off and then back on. To
reset the terminal:
1.

Place the terminal in SET-UP mode.

Press the 0 key on the main keyboard. The VT lOO is reset, the power on self-test runs, and
the terminal reacts according to the fixed SET-UP features.
NOTE
When a reset operation is performed the contents of
the screen are destroyed.

PART 3

)

DEFINITIONS SET-UP FEATURE

This section describes each SET-UP feature in detail (in alphabetical order) and states how each
feature affects the terminal.
NOTE
Unless otherwise stated, entering SET-UP mode and
changing features does not result in the loss of data .
displayed on the screen.

)

ANSIjVT52 Mode
The VT100 terminal follows two programming standards - American National Standards Institute
(ANSI) and VT52. In ANSI mode, the VT100 generates and responds to coded sequences per ANSI
standards X3.41-1974 and X3.64-1977. In VT52 mode, the VT100 terminal is compatible with previous
DIGITAL software using the VT52 video terminal. Both ANSI and VT52 modes are outlined in
Appendix A of this manual.
Answerback Message
Answerback is a question and answer sequence where the host computer asks the terminal to identify
itself. This feature allows the terminal to identify itself by sending a message to the host. The entire
answerback sequence takes place automatically without affecting the screen or requiring operator
action. The answerback message may also be transmitted by pressing the CTRL and BREAK Keys at
the same time.
Auto Repeat
Auto repeat allows a key to be automatically repeated at the rate of about 30 characters per second
when the key is held down for more than one-half second. The auto repeat feature affects all keyboard
keys except the following:
SET-UP
ESC
NO SCROLL
TAB
RETURN
CTRL and any key

2-13

Auto XONjXOFF
The VT100 can automatically generate synchronizing codes XON (DCI) and XOFF (DC3). XOFF
stops data transmission from the computer to the terminal; XON resumes transmission. With the
feature enabled, the VT100 generates the XOFF code when one of the following occurs:
I.
2.
3.
4.

)\ )

the internal buffer is nearly full
the NO SCROLL key is pressed
the terminal is placed in SET-UP mode
CTRL-S is pressed.

When the buffer empties, the NO SCROLL key is pressed again, the terminal is taken out of SET-UP
mode, or CTRL-Q is pressed, the VT100 transmits the XON code to resume transmission from computer to terminal.

If the host computer software does not support the XONjXOFF codes, data sent during buffer full
conditions, or when the terminal is in SET-UP mode, may be lost.
NOTE
The VT100 always stops transmission when an
XOFF (DC3) code is receh'ed and resumes transmission when an XON (DCl) code is received regardless
of the Auto XONjXOFF feature setting.
Bits per Character
This feature allows the terminal to transmit and receive either 7- or 8-bit characters. When set for 8-bit
operation, bit 8 is set to a space (or 0) for characters transmitted and is ignored for all characters
received.
Characters per Line
The VT100 can display either 80 or 132 characters perIine. In 80 character per line mode, the screen is
80 characters wide by 24 lines high. In 132 character per line mode, the screen is 132 characters wide by
14 lines high (24 lines if the VT 100 is equipped with the Advanced Video Option). In 132 character per
line mode, the displayed lines are physically the same width as 80 character per line mode but the
.
characters are more compact.
NOTE
When changing from 80 to 132 character per line
mode or vice versa, the current contents of the screen
are lost.
Cursor
The VT100 has two cursor displays to indicate the "active positions" or where the next character will
be placed on the screen. The cursor may be displayed as either a blinking underline (-) or a blinking
block (_). The cursor selection may perform an additional function; see the SGR escape sequence
definition in Appendix A.
Interlace
This feature is used for high resolution options. The interlace feature should be turned off if such an
option is not installed.

2-14

))

Keyclick Tone
The keyc1ick is a tone that sounds every time a key is pressed. The keyc1ick may be turned on or off to
suit the operator's needs. However, research and experience has shown that an operator is more accurate when there is audible feedback from the keyboard. Like the bell tone, the keyc1ick volume is not
adjustable.
LINE/LOCAL
The LINE/LOCAL feature allows the operator to easily place the terminal in either an ON-LINE or a
LOCAL (off-line) condition. When the terminal is on-line (ON-LINE indicator is lit) all characters
typed on the keyboard are sent directly to the computer and messages from the computer are displayed
on the screen. In the LOCAL condition (LOCAL indicator is lit), the terminal is electrically disconnected from the computer; messages are not sent to or received from the computer; and characters
typed on the keyboard are echoed on the screen directly.

)

Margin Bell
The margin bell feature is much the same as the bell in a typewriter. If the cursor is eight characters
from the end of the current line while typing, the VT100 sounds a tone to alert the operator.
New Line
The new line feature enables the RETURN key on the terminal to function like the return key on an
electric typewriter. When the new line feature is enabled. pressing the RETURN key generates the
carriage return (CR) and line feed (LF) codes. When a line feed code is received, the code is interpreted
as a carriage return and line feed.
When the new line feature is disabled, the RETURN key generates only the CR code; an LF code
causes the terminal to perform a line feed only.

)

NOTE
If double line feeds occur consistently, turn this feature off since the computer is already performing this
function.
Parity
When enabled, parity checks for correct data transmission. If a transmission error occurs, the VT100
detects it and indicates its presence by placing a checkerboard character ( ~) on the screen in place of
the character with the error. The parity sense feature determines if the parity is even or odd. When
parity is disabled, no parity bit is transmitted or received.
Parity Sense
The parity sense feature defines which of the two methods of parity checking, odd or even, is being
used by the VT100. If the parity feature is on, the terminal's parity sense must be matched to the parity
the computer is sending. If the parity sense features do not match, most characters sent to the computer are rejected even though the character was received correctly by the VT100. If a parity incompatibility occurs, the checkerboard character appears on the screen in place of the received
character.
NOTE
If the parity feature is turned off, the parity sense
selection is disregarded.

2-15

Power
During initial installation, the terminal display must be set to the power line frequency. In the U.S. this
is 60 hertz.
.

)
)

Receive Speed
Receive speed must be set to match computer transmit speed. The VT100 is capable of receiving at any
one of the following preselected speeds: 50,75, 110, 134.5, 150,200,300,600, 1200, 1800,2000,2400,
3600, 4800, 9600, and 19,200 baud.
Receive speed is independent of transmit speed; the terminal may receive data at one speed and transmit data at a different speed.
Screen Background
This feature allows the operator to select the background of the screen display. In the normal screen
mode, the display contains light characters on a dark background; in reverse screen mode, the display
contains dark characters on a light background.
Screen Brightness
Unlike most video terminals, the VT 100 does not contain switches or knobs to adjust screen brightness. Instead, the VT100 electronically controls screen brightness. This feature eliminates the high
failure rate of mechanical controls and still allows the operator to select the desired level of brightness
for maximum comfort under varied lighting conditions. This setting may be saved like any other
feature in the terminal.
Scroll
Scrolling is the upward or downward movement of existing lines on the screen to make room for new
lines at the bottom or top of the screen. It can be performed in two ways: jump scroll or smooth scroll.
In jump scroll mode, new lines appear on the screen as fast as the computer sends them to the terminal.
At the higher baud rates, the data is very difficult to read due to the rapid upward movement of the
lines. In smooth scroll mode, a limit is placed on the speed at which new lines of data may be sent to
the terminal. Upward movement of lines occurs at a smooth steady rate allowing data to be read as it
appears on the screen.
NOTE
Smooth scroll mode allows a maximum of six lines of
data per second to be added to the screen. The auto
XON/XOFF feature must be enabled and supported
by the host computer to ensure that data is not lost
when smooth scroll mode is enabled.

))

Tabs
The VT100 can jump or tab to preselected points on a line just like a typewriter. The tab stops may be
individually changed or totally cleared and then reset.
Transmit Speed
Transmit speed must be set to match the computer receive speed. The VT100 can transmit at anyone
of the following preselected transmit speeds: 50,75, 110, 134.5, 150,200,300,600, 1200, 1800,2000,
2400, 3600, 4800, 9600, and 19.200 baud.
Transmit speed is independent of receive speed; the terminal may transmit data at one speed and
receive data at a different speed.

))
2-16

\ )

Wraparound
When this feature is enabled, the 81 st or 133rd character (depending on line size selected) inserted on a
line is automatically placed in the first character position of t,he next line. If the wraparound feature is
not enabled, the 81 st or 133rd character and all following characters are overwritten into the last
character position of the current line.
1

£1 (shifted)

The VT100 contains character sets for the U.S. and the United Kingdom. The difference between the
two sets is one character, the # or i sym bol. When the standard U.S. character set is selected, the
uppercase 3 key on the main keyboard displays the # character. The i character is displayed when the
U.K. character set is selected.
PART 4

SELF-TESTING THE VT100

A self-test mode in the VT100 automatically, or on command, tests the condition of the terminal if a
fault is suspected. The self-test program checks the following items.

)

Internal memory
Advanced video memory (if option is installed)
Nonvolatile memory (NVR)
Keyboard
This test is performed automatically whenever the terminal is turned on.

)

Self-Test Error Codes
There are two broad categories of errors: fatal and nonfatal. Fatal errors cause the terminal to immediately stop all operations. No intelligible information is displayed on the screen, but the screen most
likely contains a random pattern of characters. The only error indication (in addition to random
characters) is a possible error code displayed on programmable keyboard LEOs LJ-L4. However. no
terminal function, including the lighting of LEOs, is guaranteed if a fatal error is found. See Table 2-3,
Nonfatal errors do not halt the terminal processor. Instead, the terminal is forced to LOCAL mode
and an error code character is displayed in the upper-left corner of the screen. There are five types of
nonfatal errors:

)

I.
2.
3.
4.
5.

Advanced Video Option data RAM (AVO)
Nonvolatile data RAM checksum error (NVR)
Keyboard missing or malfunction (KBD)
Data loop back error (Data)
EIA mode~ control lines loopback error (EIA).
NOTE
Loopback tests are not performed on power-up; they
must be invoked separately with the proper escape
sequence. See Appendix A for further information on
this test.

Table 2-4 shows the possible nonfatal error characters that may appear on the screen and the failure
represented by each character.

If the terminal passes all these tests, a final check of its operation is possible in SET-UP mode. Examine the display and compare it with the printed examples. Double height, double width, bold, blinking,
reverse, etc., are all represented in SET-UP mode.

2-17

Table 2-3 Fatal Displayed Error Codes

Keyboard LEDs
Ll
L2
L3

Error

Replace FRU

OFF
OFF
OFF
OFF
OFF

ON
OFF
ON
OFF
ON

ROM 1
ROM2
ROM3
ROM4
Main Data RAM

Terminal controller
Terminal controller
Terminal controller
Terminal controller
Terminal controller

OFF
OFF
OFF
ON
ON

OFF
ON
ON
OFF
OFF

Table 2-4 Nonfatal Displayed Error Codes
Char
I
2
3
4

5
6
7
8
9

Faulty Module
AVO
NVR

X
X

X
X
X
X

X
X

X
X
X
X

C
D

X
X
X
X
X
X
X
X
X

F
G
H
I
J
K
L
M
N

EIA

))

Data

<
=

KBD

X
X
X
X

X
X

X
X
X
X

X
X
X
X
X
X
X
X

))

X
X
X
X
X
X
X
X
X
X
X
X
X
X
X

))

)
2-18

)

PART 5

WHAT TO DO IN THE EVENT OF A PROBLEM

If it appears that there is a problem in the terminal, you should initiate the self-test procedure. This test
will help to determine if the problem lies in your terminal or in some other part of the computer
system. If the terminal appears to be faulty, refer to Table 2-5. This table describes the items an
operator can check prior to making a service call.
Table 2-5

Problem Checklist

Symptom

Possible Cause and Corrective Action

VT 100 will not turn on
when power switch
is set to ON.

AC power cord not plugged into wall
outlet; plug in cord.
AC power cord not plugged into terminal;
plug in cord.

)

Power not coming from wall outlet; check
outlet with known working electrical
device (a lamp). If no power, call your
electrician.
AC line fuse blown. Turn terminal OFF and
have the fuse replaced. (See Figure 1-2
for location.)

)

No keyboard response

Keyboard cable not plugged into monitor;
plug in keyboard cable.
KBD LOCKED indicator on; computer has
turned keyboard off. If condition
persists, check with the host computer
software people for a possible operating
error.
Perform the self-test operation.

Garbled characters

Transmit and/or receive speeds, parity
sense, or parity enable may be wrong;
check settings.
Perform the self-test operation.

2-19

)
\.

CHAPTER 3
INSTALLATION AND INTERFACE INFORMATION

)

SITE CONSIDERATIONS
The design of the VT100 (Figure 3-1) normally poses few constraints on selecting a place in which to
install the terminal. In most ca.ses, any environment suitable to the operator is a satisfactory environment in which to operate the terminal. Extremes of temperature and humidity should be avoided. A
summary of VT100 guaranteed operating conditions may be found in Chapter I.
UNPACKING AND INSTALLATION
The VT100 shipping carton contains the following items:
VT100 monitor
VT100 detached keyboard
VT100 power cord
VT100 SET-UP label
VT100 User Guide.

)

To install the VT100 perform the following steps:
l.

Remove the VT 100 from the shipping carton and place it in the desired work area.

Place the keyboard in front of the terminal and plug the keyboard coiled cord into the
keyboard receptacle at the rear of the terminal (Figure 3-2).

Verify that the power selector switch shows the correct wall outlet voltage (115 V is standard
in the U.S.) and the power switch is off.

Connect the power cord to the power cord receptacle at the rear of the terminal and plug the
other end of the power cord into a nearby wall outlet.

Connect the communications cable to the appropriate communications receptacle.

Turn the power switch on. The terminal automatically performs the power-up self test and
either the ON LINE or LOCAL indicator on the keyboard will light. After approximately
one minute the cursor will be visible in the upper-left corner of the screen.

Set the desired SET-UP features outlined in the operator section of this manual.

Once the installation procedure is complete, record the SET-UP features selected on the
VT100 SET-UP label and attach the label to the underside of the keyboard.

)

)
3-1

"'
36.8 CM
(14 112 IN.)

L (43/8
11 lCM...J~
IN)
2032 CM

24.5 CM (9 5/8 IN.)~
30.5 CM (12 IN.)

(8 IN )

36.2 CM (14 1/4 IN.)
51.4 ·CM (20 1/4 IN.')

41.3 CM (16 1/4 IN.)

45.7CM (18 IN.)
• MEASUREMENT TAKEN WITH THE KEY80ARD PLACED FLUSH TO FRONT OF TERMINAL UNDER UNDERCUT.
MA-1991

Figure 3-1 VT100 Terminal Dimensions

3-2

)

CAPTIVE
SCREWS (4)

ACCESS
COVER.

EIA

LINE PRINTER
CONNECTOR

EXTERNAL VIDEO
INPUT CONNECTOR

PRINTER
OPTION
INDICATOR

)

R SWITCH
MODEL + SERIAL
NUMBER LABEL

)

KEY

CONNECTOR

SELECTOR
SWITCH

OPTIONAL 20mA
COMMUNICATION
CONNECTOR

MA-1987

Figure 3-2 VT100 Rear View

)

USER MAINTENANCE
The keyboard keys are the only moving parts of the terminal and require no preventive maintenance
by the owner. The VT100 surfaces may be cleaned with soap and water or any mild detergent. Cleaners
with solvents should not be used.
The VT I 00 packaging is not meant to be weatherproof; there are several openings in the case through
which liquids, coins, paper clips, and other objects can fall. Such objects would disturb the electronic
operation of the terminal if they came into contact with the circuitry. For this reason, avoid putting
drinks and metal objects on the top of the terminal, or using excessive water to clean the terminal.
Rubbing the keys with a dry or barely moist cloth should clean them. Do not remove the keycaps to
clean them more thoroughly; damage may result to the switch contacts if they are replaced incorrectly.
Keep the ventilation slots clear. Blocking these slots by placing objects on top of or under the VT100
. may cause the terminal to overheat.

3-3

INTERFACE INFORMATION
EIA Interface
The basic VT100 operates on full duplex, asynchronous communication lines. The terminal interfaces
to the line with a 25-pin connector mounted on the back of the terminal which meets the requirements
of EIA specification RS-232-C. Table 3-1 summarizes the EIA connector signals. The following paragraphs explain each signal as used in the basic VT 100.
Protective Ground - Pin 1
This conductor is electrically bonded to the VT100 chassis. Use of this conductor for reference potential purposes is not allowed.
Transmitted Data (from VT100) - Pin 2
The VT100 transmits serially encoded characters and break signals on this circuit, which is held in the
mark state when neither characters nor break signals are being transmitted.
Received Data (to VT100) - Pin 3
The VT100 receives serially encoded characters generated by the user's equipment on this circuit.

) )

Request to Send (from VT100) - Pin 4
Asserted at all times when terminal is powered up.
Carrier Detect (to VT100) - Pin 8
Ignored at all times.
Speed Select (from VT100) - Pins 11, 19, and 23
This signal is alternately called secondary request to send. The basic VT100 maintains this line in the
asserted state at all times.
Speed Indicator (to VT100) - Pin 12
This signal is alternately called secondary carrier detect and is ignored at all times.
Transmission Clock (to VT100) - Pin 15
Ignored at all times.
Clear to Send (to VT100) - Pin 5
Ignored at all times.

Data Set Ready (to VT100) - Pin 6
Ignored at all times.
Signal Ground _. Pin 7
This conductor establishes the common ground reference potential for all voltages on the interface. It
is permanently connected to the VT100 chassis.
Receive Clock (to VT100) -Pin 17
Ignored at all times.
Data Terminal Ready (From VT100) - Pin 20
Data terminal ready is asserted at all times.
Ring Indicator (to VT100) - Pin 22
Ignored at all times.

))
3-4

Table 3-1 EfA Connector Signals

)
Pin
No.
I

2
3
4
5
6

10
II
12
13
14
15

Same as pin 19
(Secondary carrier detect) speed indicator
(Not used)
(Not used)
Transmit clock

16
17
18
19
20

(Not used)
Receive clock
(Not used)
(Secondary request to send) speed select
Data terminal ready

(Not used)
Ring indicator
Same as pin 19
(Not used)
(Not used)

)

Protective ground
Transmitted data
Received data
Request to send
Clear to send
Data set ready
Signal ground (common return)
Carrier detect
(Not used)
(Not used)

7
8

)

Description

22
23

24
25

'}
i

Electrical Characteristics
VT100 Output Voltages - On all signals designated "from VT100," the mark or unasserted state is -6.0
V to -12.0 V; the space or asserted state is +6.0 V to + 12.0 V.
VT100 Input Voltages - On signals designated "to VT100,"--25.0 V to +0.75 V or an open circuit is
interpreted as a mark or un asserted state, and +25.0 V to +2.25 V is interpreted as a space or asserted
state. Voltages greater in magnitude than 25 V are not allowed. These levels are compatible with ETA
STD RS-232-C and CCITT Recommendation V.28.
Optional 20 rnA Current Loop Interface
In most current loop applications, the VT100 is connected in a passive configuration - that is, current
is supplied to the VT100. In this mode, the transmitter and receiver are both passive, both optically
isolated, and the transmitter goes to the mark state when power is turned off.

)

Conversion from active to passive (or vice versa) requires moving a slide switch.

3-5

In active mode either the transmitter or the receiver or both may be connected so that the VT100
sources the 20 rnA of current. In active mode isolation is not present and the transmitter goes to the
space state when power to the VT100 is turned off.

'
))

Figure 3-3 shows the 20 rnA current loop interface connector mounted to the access cover and lists the
individual pin assignments.
The electrical characteristics of the 20 rnA current loop interface are shown below.
Transmitter
Open circuit voltage
Voltage drop marking
Spacing current
Marking current

Min
5.0V
20mA

Max
50V
2.0V
2.0mA
SOmA

Receifer
Min
Voltage drop marking
Spacing current
Marking current

15 rnA

Max
2.5 V
3.0mA
SOmA

))

In addition to the above specifications for passive operation, active mode places the transmitter or
receiver in series with a source of 17 V ± 5 percent and 660 ohms.
External Video Connections
In addition to the EIA interface, the VT100 can easily interface to external video devices. The video
devices may act either as a slave to the VT100 when connected to the composite video output or
provide synchronized video to the VT 100 video section when connected to the video input. The external video connectors are the two female BNC connectors located on the back of the terminal just
below the EIA connector. The upper connector, J8, is the video input while the lower connector, J9, is
the video output.
Composite Video Output (J9)
The composite video output provides RS 170 output generated by combining the video signal with a
composite sync signal. The output contains all video data appearing on the VT100 screen except that
video which comes from J8. The output has the following nominal characteristics (Figure 3-4):
I.

Output impedance = 75 ohms, dc-coupled

Sync level

= 0V

Black level

= approximately 0.3 V when loaded with 75 ohms

White level

= approximately 1.0 V with a 75 ohm load

3-6

,'))
"

))

)

The composite sync waveform conforms to EIA RS 170 standards. The vertical interval is
composed of six equalizing pulses, six vertical sync pulses, and six more equalizing pulses.
The timing is as iollows:
Equalizing pulse width
Vertical pulse width
Horizontal pulse width
Horizontal blank width

=
=
=
=
=
=

Front porch

2.33 J.LS ± 50 ns
27.28 J.LS ± 200 ns
4.71J.Ls±50ns
11.84 J.LS ± 50 nsj80 column
mode
12.34J.Ls±50nsjI32column
mode
1.54 J.LS ± 50 ns.

o
)

___

.... ~

TERMINAL
ACCESS
COVER

)
PIN ASSIGNMENTS
1 - TEST NEGATIVE
2 - TRANSMIT 3 - RECEIVE 5 - TRANSMIT +
7 - RECEIVE +
8 - PROTECTIVE GROUND

o
I

00000000

1235

20mA CURRENT
LOOP CONNECTOR

MA-1992

Figure 3-3 20 rnA Current Loop Interface

3-7

EVEN FIELD:

60 Hz NON·INT .. 22 SCANS}
= 22 SCANS
{ 60 Hz INT
50 Hz NON.INT c'75 SCANS ------I
50 Hz INT
= 74 SCANS

VERTICAL BLANKING INTERVAL

LAST DISPLAYED SCAN OF
PREVIOUS FIELD

1----------------

FIRST OF 240 DISPLAYED

60 Hz NON·INT = 2 SCANS
= 2 SCANS
{ 60 Hz INT
50 Hz NON·INT = 25 SCANS
50 Hz INT
= 24 SCANS

1.0 V

!\/\

TIME_

START OF
EVEN FIELD

ODD FIELD:

----+

FIRST OF 240 DISPLAYED

LAST DISPLAYED SCAN OF ......._ _ _---4-:_V_E_RT_IC_A_L_B_LA_N_K_IN_G_IN_T_ER_V_AL_ _ _
EVEN FIELD
...
{

1.0V

60 Hz I NT = 2.5 SCANS
50 Hz INT = 24.5 SCANS

!V\

TlME_

START OF
000 FIELD

H PERIOO = 63.56

~s ± .0'"

NOTES
IN NON·INTERLACED OPERATION THE
EVEN FIELD IS REPEATED CONTIr..·
UOUSLY. AND THE ODD FIELD IS NOT
USED
IN THE INTERLACED OPERATION TI"E
EVEN FIELD ALTERNATES WITH THE ODD
FIELD

--l F

H BLANK {11.S4 ~s ± 50 ns I SO COLUMN
12.34 ~s ± 50 ns ! 132 COLUMN

BRIGHT = 1 . 0 0 V _ = = j
NORMAL
.79 V_
VDIM =
= .64
BLACK = .29 V -

~=~_

FRONT PORCH =

=====11

t-.
t-

1.54~. ± 5.0 n. ~I

H SYNC" 4.71 ~$ ± 50 n.

ALL FIELDS CONTAIN 240 DISPLAYED
SCANS

r---if

U
U
~Q = 2.33 ~. ± 50 ns ..j j.
H PERIOD = 63.56

I! -----!n

..j

~V SYNC = 27.28 uS

± 200 ns

,,$ ± .01%

Figure 3-4 Composite Video Output

Video Input (J8)
An analog signal applied to the video input is "ORed" with the internal video signal such that the
beam intensity at any point on the screen corresponds to the intensity of that signal which tends to
make the beam brighter at that point. A video signal on this input affects only the internal screen and
does not appear on the composite video output. This input has the following nominal characteristics:
1.
2.
3.
4.

Input impedance = 75 ohms, dc-coupled
Black level = 0 V
White level = 1.0 V
Maximum continuous input = ±2.0 V.

The external video source must be synchronized to the VT100; it may do this by referencing the
composite sync on the composite video output. This means that the VT100 video i!!~'-!t 'viii not synchronize with any composite video source including the composite video output of another VT100.

3-8

))

CHAPTER 4
TECHNICAL DESCRIPTION

4.1 INTRODUCTION TO VT100 TECHNICAL DESCRIPTION
This section describes the contents of the technical description chapter and provides a technical overview of the terminal.
4.1.1 Scope of Chapter
This technical description is intended to provide an understanding of the terminal operating principles
for Field Service, depot repair, and engineering personnel. The description considers the terminal to be
several functional or replaceable subunits. Each subunit is described in its own section. Several subunits interact. Those interactions are discussed in the section about the subunit most involved in the
particular process. For example, power-up involves the nonvolatile RAM but is described in the microprocessor section. Scrolling has its own section because it is a complex process that involves the
microprocessor and video processor equally.

)

4.1.2 Order of Presentation
Each section describes the hardware first and the associated firmware operations second. This section
contains a block diagram level discussion of the terminal hardware and firmware. More detail is
provided in the functional block descriptions in later sections. Refer to the VT100 series Field Maintenance Print Set, No. MP00633, for circuit details while studying this manual.
4.1.3 Definition of Terms and Abbreviations
Many terms are written out the first time they appear, with an abbreviation or mnemonic in parentheses after it. The abbreviation is generally used after that. There is a glossary of abbreviations and
signal names in the appendices of this manual. Numbers may be given in binary, octal, decimal, or
hexadecimal (hex). The normal form will be decimal for scalar or ordinal values, and hex for data and
addresses. Numbers are SUbscripted B for binary, Q for octal. H for hex, and no subscript for decimal.
4.1.4 Hardware Introduction
The VT100 is a complete computer input and output terminal that has a keyboard like a typewriter
and displays its data on a video screen. It fits into two compact packages; the keyboard is flexibly
attached to the main cabinet by a coiled cable. Refer to Figure 4-1-1.
The terminal consists of four basic components (not including enclosures) plus two important options.
The components are the terminal controller, the keyboard, the CRT monitor, and the power supply.
The options are the advanced video option and the current loop adapter option.

4-1

CRT
MONITOR

LED'S

TERMINAL CONTROLLER
• MICROPROCESSOR
• VIDEO PROCESSOR
RAM
ROM
NVR
UART
STP
CONNECTOR

•
•
•
•

KEYBOARD
•

ADVANCED
VIDEO
OPTION

CURRENT
LOOP
ADAPTOR
OPTION

TO HOST

EIA I/O

POWER
SUPPLY
MA-4656

Figure 4-1-1 VT I00 Components
The terminal controller is a single pc board module that manages all displays and all communication.
Everything else connects to it. The terminal controlller contains these functional components:
•

A microprocessor to manage terminal operations

•

A video processor for converting data to recognizable patterns

•

3072 bytes of random access memory (RAM) for screen data storage and microprocessor
scratch me.mory

•

8192 bytes of read only memory (ROM) containing the microprocessor's programming to
operate the terminal with its particular features

•

A nonvolatile RAM (NVR)to store the user-settable operating features without requiring
hardware switches

•

An asynchronous serial receiver-transmitter (UART) for data exchange with the computer

•

EIA level converters to adapt the on-board input and output (1/0) signals to the communications interface

•

A special connector, the standard terminal port (STP) that allows a plugged-in option to

))
'

intercept terminal communications inside the terminal enclosure.
The keyboard is the typewriter-style input device for the operator. It has a loudspeaker for user feedback keyclicks and bells, and indicators that show internal states of the terminal.

))
/

4-2

The CRT monitor is a video screen that displayg exchanges between the operator and the computer. It
can display data in two modes: 80 characters by 24 lines, or 132 characters by 14 lines (or, with the
AVO, 132 by 24). With the control circuitry on the terminal controller board, the CRT can perform
many special character display functions.
The power supply converts the ac power line to the four dc voltages required by the terminal. It has a
switching regulator for highest efficiency and coolest operation.
The advanced video option provides greater display capacity, plus it can carry extra firmware for the
expanded functionality of other products in the VT100 series.
The current loop adapter option converts the EIA output of the terminal controller to a more noise
immune standard when longer distances between the terminal and the computer are required.

)

4.1.5 Block Diagram Description
Figure 4- J-2 shows the VT 100 terminal as a set of functional blocks. The rest of this section describes
these blocks in greater detail.
4.1.5.1 Microprocessor - A microprocessor manages all terminal input and output operations. It also
provides the intelligence that enables the VT100 to respond to and generate a wide range of ANSI
control functions, and to emulate the characteristics of the VTS2. Several of the microprocessor's
program functions are effectively in series with data paths in the terminal. These functions have their
own blocks in the functional block diagram to clarify the processes involved. These blocks are shaded
to indicate that they are program functions or that the microprocessor controls data transfers between
the blocks. The microprocessor is a computer with its instructions in program ROM (read only memory) and working memory in the scratch RAM (random access memory). Terminal parameters for
start-up are stored in the nonvolatile RAM (NVR). The advanced video option (AVO) normally
contains extra RAM as described below but can also contain extra program ROM.
4.1.5.2 Program ROM - The VT100 program ROM is an 8K X 8 memory containing instructions
and data tables for the terminal's microprocessor. Memory comes in four 2K packages (later VT100s
may have a single 8K package). Checksum data stored in each ROM allows the terminal to confirm
the condition qf its programming at self-test.

)

4.1.5.3 Scratch RAM - The scratch RAM is the portion of RAM on the terminal controller that is
not used for the screen RAM. That is, the RAM is 3K bytes long, but only about 2.3K bytes are used
for screen display. The rest of the RAM contains the microprocessor stack, SET-UP data, various flag
bytes, the communication SILO, the keyboard buffer, and so on.
4.1.5.4 Nonvolatile RAM - The nonvolatile RAM (NVR) does not lose its data when its power is off.
It stores all user-settable features and parameters and the answerback message so that they are available each time the terminal is turned on. Even the screen intensity can be stored. There are no mechanical switches needed for configuration.
4.1.5.5 Advanced Video Option - The advanced video option (AVO) contains the I K X 8 of extra
screen RAM needed to expand the display from 14 lines of 132 characters to 24 lines, plus a 4K X 4
RAM to store an extended set of attributes for all characters. The AVO also contains an additional
segment of video processor to manage the four extra bits of data. Sockets for ROMs and jumper- or
switch-programmable decoders allow expansion or overlay of program memory.
The AVO is a replaceable subunit of the VT 100.

4-3

CRT
MONITOR

PROGRAM
ROM

SCRATCH
RAM

COMMUNICA nON
RECEIVER

EIA
DRIVERS

LEOS

J:..

STP
CONNECTOR

CURRENT
LOOP
ADAPTOR
OPTION

ro HOST
COMPUTER

t--'---.....;;.'-'-'~-=--4.otl COMMUNICATION
TRANSMITTER

KEYBOARD

POWER
SUPPLY
SHADED BOXES =MICROPROCESSOR PROGRAM FUNCTIONS
BOX=DEVICES INSIDE TERMINAL CHASSIS
II.....". .

Figure 4-1-2 Functional Block Diagram

',---"

-~'

~,/

)

4.1.5.6 Keyboard - The keyboard is the user's input device to the terminal. The keyboard's output is a
serial data signal that travels along the same wire as data coming from the terminal. The keyboard
contains a bidirectional interface circuit, a set of keyswitches arranged like a typewriter, circuits to
send the key information to the interface, LED indicators, and a small loudspeaker for keyclicks and
bells. The connection to the terminal is a 3-wire coil cord carrying signals, power, and ground.
The keyboard is a replaceable subunit of the VT100.
4.1.5.7 LEDs -The terminal can inform the user of some internal conditions such as on-line or local
and keyboard locked through the LEOs on the keyboard. The LEOs also may indicate the location of
a failure during self test and are user programmable during normal operation.
4.1.5.8 Keyboard Translator - The output for each key is a number that represents the key's location
(address) in the keyboard switch matrix. The microprocessor's keyboard translator translates the address to the industry-standard ASCII code.

)

4.1.5.9 Transmit Buffer.- The ASCII codes wait in a transmit buffer until they can be transmitted.
Some keys (such as the numeric keypad in application mode) produce control functions that are three
bytes long. The transmit buffer isnine bytes long and thus can store at least three keys of any kind.
4.1.5.10 Communication Transmitter - The communication transmitter is one-half of a programmable universal synchronous/asynchronous receiver transmitter (PUSART) and its associated circuitry and firmware routines. The PUSART is programmed to place data in a standard asynchronous
format by adding control and error detection bits to the original byte. The communication transmitter
thus takes the parallel ASCII keycodes out of the keycode buffer, converts them to serial form, and
delivers them to the EIA interface.

)

4.1.5.11
Communication Receiver - Data that the host sends to the terminal enters the communication receiver which is the other half of the PUSART. The PUSART accepts serial data from the
interface and converts it to parallel form. The PUSART also checks for errors and records them in a
status byte. The microprocessor reads the data and then the status byte to confirm the correctness of
the data. If an error is detected, a checkerboard is displayed instead of a character to symbolize the
error. If the microprocessor is busy managing the display, it devotes only enough time to the communication receiver to get the incoming code and check it for special codes requiring immediate action.
4.1.5.12 SILO - The rest of the incoming codes are stored in a part of RAM memory called a SILO.
This memory maintains the order of the data as it arrives; the first data to arrive leaves first. This
memory gives incoming data a place to wait when the microprocessor cannot transfer data from the
communication receiver to the screen RAM as fast as it arrives. The SILO control routine checks the
filling of the SILO and can issue XON and XOFF commands to the host to try to keep from overfilling. (XON and XOFF are explained in Section 4.3, Communication.)
In local mode, data from the keyboard bypasses the communication receiver and transmitter and the
SILO.
4.1.5.13 Control Function Parser - When the microprocessor has enough time, it takes data from the
SILO and puts it through a control function parser routine in the microprocessor. Each code is tested
to see if it is in the control range «20H or 7FH). If it is, the microprocessor acts on it immediately.
Line feed. for example, causes a one line scroll. If the code is IBH (escape), more codes are read from
the SILO until the characters that define a control function are seen. Then the microprocessor executes
the control function immediately or discards it if it is not a valid function. Noncontrol codes are
ignored by the parser and are written into the screen RAM.

)
4-5

4.1.5.14 Screen RAM - The screen RAM is a memory that stores data for display on the screen
(CRT monitor). The memory is organized according to the SET-UP line length specifications. Basic
terminal memory can hold 24 lines of 80 characters or 14 lines of 132 characters. With the extra
memory provided by the advanced video option, the screen size may increase to 24 lines of 132 characters and four additional bits are appended to each character location to allow expanded character
attributes. (A character attribute modifies the display of that character relative to the preset values for
the entire screen.)

')).

Most of the time, the screen RAM is readable and writable by the microprocessor. For about 10
percent of the time, during the tirst scan of each 10 scan line of characters, the video processor silences
the microprocessor and takes full control of the screen RAM, providing its own addresses to access the
memory. This complete control of memory (by a device other than the microprocessor, which is normally in control) is called a direct memory access (DMA). The DMA allows fast access of data in
memory because the microprocessor does not have to perform all the steps of addressing, reading, and
writing to a destination. The video processor needs the fast access because the data rate required to
display a line of characters on the CRT is greater than the microprocessor can handle.

4.1.5.15 Video Processor - The microprocessor puts displayable data in the screen RAM. The video
processor direct memory accesses (DMAs) data a line at a time from the RAM. It converts the ASCIIencoded data into streams of pulses which, when converted to light on a CRT screen, appear to form
characters on the screen. Custom ICs provide the complex timing and control signals required for this
conversion. The video processor can be programmed by the user to perform the conversion at different
rates(called refresh rates) to minimize flicker at different power line frequencies. A number of other
aspects of video processor operation can be programmed as well.
4.1.5.16 CRT Monitor - The cathode ray tube (CRT) monitor is a simplified monochrome television
set. It converts electrical pulse streams into dots of light by exciting a phosphor on the face of the tube
with a moving electron beam. A video input from the video processor enters a cathode drive circuit
that regulates the strength of the electron beam. The cathode driver is a linear amplifier that allows the
CR T to display different intensities (gray scale). The monitor uses timing pulses from the video processor to drive the horizontal and vertical electron beam deflection circuits. These circuits cause the beam
to travel down the screen slcwly while rapidly moving sideways to draw horizontal traces called scans.
This beam movement produces a pattern which is called a raster and the display system is called raster
scanning. Because the beam repeatedly passes by each location on the screen, the video processor, by
synchronizing its output with the motion of the beam, can make rows of dots of light align to form
characters.

')
' )

))

The screen does not hold its image after the electron beam has painted dots on it. For the eye to
perceive it as continuously illuminated (i.e., without flickering) the screen must be repainted (refreshed) repeatedly some 30 or more times per second. Also, stray magnetic fields and electrical noise
at the power line frequency can cause distortions in the display. If the refresh frequency is different
from the power line frequency, the distortions appear to move up or down the display. This is very
noticeable. Most distortions disappear by matching the refresh rate to the power line frequency. The
VT 100 can refresh at either ofthe two world power frequencies, 50 or 60 Hz, satisfying both flicker and
distortion requirements. (VT100 refresh is not locked to the power line but is close enough to conceal
most distortions.)

))

The CRT monitor, the monitor circuit board, and the flyback transformer are replaceable subunits of
the VT100.

j)
i

4-6

)

4.1.5.17 Power Supply - The VT100 power supply providesenough power to run the terminal and a
few options. It is a switching supply to allow highest efficiency and to minimize the heat load on the
rest of the terminal. It l~~iifies line voltage directly (without a transformer), and chops the resulting
high dc voltage with a transistor at about 30 kilohertz. This ultrasonic ac is stepped down in a relatively tiny transformer and then regulated at 'low voltage to give the various outputs.
The power supply is a replaceable subunit of the VT100.
4.1.5.18 Standard Terminal Port - The standard terminal port (STP) is a shorting connector in series
with the communication port, the modem control lines, and some power and timing lines. By connecting to the terminal through this port, options can exchange signals with the terminal controller or
intercept terminal-host communications from inside the terminal cabinet.

)

4.1.5.19 EIA Interface - The Electronic Industries Association (EIA) interface used on the VT I 00 is
the RS-232-C unbalanced bipolar voltage standard. The terminal controller has two types of ICs that
provide conversion between EIA levels and the TTL levels used on the controller board. One IC is a
line driver that outputs EIA levels for TTL inputs, and the other senses the EIA levels on the input line
and converts them to TTL.
4.1.5.20 Current Loop Interface Adapter - The current loop adapter option plugs into the terminal
controller board and mates with the EIA voltage interfaces to convert the terminal to 20 milliampere
current loop interfaces. The current loop interfaces can be individually selected to be active or passive.
The 20 rnA adapter is a replaceable subunit of the VT 100.

)

4.1.6 Firmware Introduction
Figure 4-1-3 portrays the VT100 as a system with four levels of operation.
At the deepest level, the background routines continually repeat to manage functions that do not
require precisely timed responses. The keyboard processor commands the keyboard to perform address scans, controls the bell and LEDs, and manages the conversion of key information from hardware-dependent codes to ASCII. The transmitter routine manages the transmission of ASCII data to
the host. The received character processor examines incoming data, manages the SILO, initiates special functions as specified in the data, and writes displayable data into the screen RAM. When the
terminal is in local mode, a logical shortcut bypasses the communication process and allows the keyboard data to enter the received character processor directly.
The background routines work on the internal buffers, registers, and flags which are logical devices
located in hardware and RAM. These are physical locations in hardware that contain information
placed there the hardware and by the firmware. The key buffers store up to three key addresses for the
keyboard processor to pass through the transmit buffer to the transmitter routine for transmission to
the host. The screen Ram, which stores the displayable data, is the largest segment of RAM. The
scrolling flags contain data for the scrolling process in the video processor. The SILO stores data
coming from the host in case the received character processor cannot transfer data to the screen RA M
as fast as it arrives. The Send XOFF flag, set by the SILO manager, signals the transmitter routine to
send the XOFF code to the host to halt transmission and prevent SILO overflow.

4-7

TBMT:

rRANSMIT BUFFER EMPTY

KBD ~
TBMT
~ LEOS

KEY
ADDRESS
(KBD RDI

KBD.DATA
AVAILABLE H
!INTERRUPTI

VIDEO CIRCUITS

KEYBOARD CONTROL CIRCUITS

~).

V SPEAKER
SCAN
STATUS
(KBD·UPI

12
(DMAI

4/
CONTROL
CHIP
COMMAND

KEY DOWN
INTERRUPT
ROUTINE

VERT
FREQ
INTER·
RUPT

PUSART

"HARDWARE"

/).

REC
FLAG
H
INTER·
RUPT

RECEIVED
DATA

Vi-'

V'
RECEIVER
INTERRUPT
ROUTINE

VERTICAL FREQUENCY
I!'lTERRUPT ROUTINE

TRA NSMITTED
OAT A

INTERRUPT
DRIVEN
ROUTINES

XMIT
FLAG
SCREEN
RAM
(BASIC <AVOI

KEY
BUFFERS

SCROLLING
FLAGS

SILO

INTERNAL
FIRMWARE
BUFFERS
REGISTERS
AND FLAGS

SEND
"XOFF"
FLAG

LOCAL

RECEIVED
CHARACTER
PROCESSOR

KEYBOARD
PROCESSOR

TRANSMIT

TRANSMITTER
ROUTINE

BACKGROUND
ROUTINES

BUFFER_J

RECEIVED
CHARACTER
PROCESSOR

TRANSMITTER
ROUTINE

MA"311

Figure 4-1-3 VT100 Firmware Block Diagram

''----"''.

-,--"

Interrupt routines are segments of the firmware that take precedence over other business in the microprocessor because of the short-lived nature of the data that they handle. The Key Down interrupt
routine is initiated by the appearance of data at the keyboard control circuits. It instructs the microprocessor to get the key address from the keyl;>oard UART and put it in the key buffer. The Vertical
Frequency interrupt routine occurs every 50th or 60th of a second and provides synchronization of
video routines with the actual display timing. The Receiver interrupt routine moves incoming data
from the communication transceiver to the SILO.
At the exterior level of the VT100 system, the hardware responds to the firmware by exchanging data
between the user and the host computer in forms that are understandable to each.
4.2 MICROPROCESSOR
The VT100 has an 8080 microprocessor at the heart of its intelligence. The 8080 performs all the usual
functions of a stored program computer, fetching instructions and data from ROM and RAM and
responding to service requests from various devices in the system. Because of technical limitations in
the implementation of the microprocessor hardware, some high speed counting and timing circuits and
some power circuits are located in peripheral ICs made with a different semiconductor technology.
These components are bipolar (8224, 8228) while the 8080 is NMOS.
4.2.1 8080 Microprocessor
The 8080 (Figure 4-2-1) contains a set of general and special purpose registers (the register array),
timing and control logic which responds to machine code instructions, and an accumulator and arithmetic logic unit that perform the computations associated with the microprocessor operation.

)

The stack pointer is an important register that points to the bottom of the stack. The stack is a lastin/first-out area in RAM that stores information about the current process when a subroutine or
interrupt branches away from the current instruction sequence. By storing this information at the
beginning of an interruption and restoring it at the end of the interruption, the 8080 can continue the
main program without any disturbance. This is the meaning of the interrupt process as it applies to the
interrupt-driven routines mentioned in the firmware block diagram in Figure 4-\-3.
Most of the pins on the 8080 are tristate data and address lines. Four pins are power supplies and
ground. The others, briefly, are:

)
F

HOLD - an input that lets another device get control of the buses when the 8080 finishes the current
machine cycle. Hold Request from the video processor is the input signal.
HLDA - Hold Acknowledge output indicates that the buses will be given up for the Hold state.
INT - Interrupt Request input accepts INTR H signal that causes read of the interrupt vector address
and branch to it.
READY - input for use with slow memory or I/O. If low, the 8080 enters the wait state (an indefinite
portion of a machine cycle). It is not used in the VT100.
WAIT - output acknowledging the wait state. Not used by VT100.
INTE - Interrupt Enable output indicates whether interrupts have been enabled or disabled with the
related instructions or by servicing an interrupt request. Not used by VT100.
0/ I, 0/2 - Clock phases I and 2. Inputs to drive the 8080.

)
4-9

D7 ~ DO
BIDIRECTIONAL
DATA BUS

~
I

I I I II

ACCUMULATOR
LATCH
(8)

POWER
SUPPLIES

(8 BIT!

INTERNAL DATA BUS

I-

F LlP·F LOPS

•

I-

u
w
-'
w

III

til

INSTRUCTION
DECODER
AND
MACHINE
CYCLE
ENCODING

I-

r.:J

TEMP. REG.
B
REG.
D
REG.
H
REG.

(8)1

TEMP. REG.
C
REG.

(8)1

(8)

E
REG.

(81

(8)

(8)1

(8)

STACK POINTER
PROGRAM COUNTER

G-

~ REGISTER
ARRAY

REG.
(16)1
(16)1

INCREMENTER/DECREMENTER (16)
ADDRESS LATCH

ADDRESS BUFFER

(16)

+12V
_+5V

_~5V

_GND
HOLD

A 15 ~ AO
ADDRESS BUS

READY

MA-4356

Figure 4-2-\ 8080 Block Diagram

'--'

'-~.

---...

"--./

)

SYNC - output that indicates the beginning of each machine cycle. Combines with clock in the 8224 to
produce Status Strobe (STSTB).
RESET - clears the program counter so that when it is released, program starts at location zero. Used
to initiate power-up sequence and self-test.
WR - Write output controls memory and I/O writes by indicating the stable period of the data bus
during a write instruction.
DBIN - Data Bus In output indicates that the data bus can accept data.
For more detail, see Intel's 8080A manual.

4.2.2

--)

Data Bus and System Controller

The microprocessor data bus passes directly to the 8228 bus driver and system controller. This device
provides TTL output buffering and level translation for the MOS 8080 bus. Because of the large
number of devices on the data bus, the drive capability of some devices would be exceeded at times if
all devices were on the same bus. (The keyboard UART is the weakest low-level current driver.) To
distribute the load, the bus is split into those devices that communicate bidirectionally or are only read
by the microprocessor, and those that are only written into. Read and writable devices are on the
bidirectional data bus (DB). The write-only devices are driven by a one-way bus buffer and are on the
data output tDO) bus. The terminal controller block diagram (Figure 4-2-2) shows which devices are
on each bus.
The ROMs do not have sufficient high-level current capacity to drive all of the other inputs on the bus.
Inactive devices have their outputs tristated but their inputs still draw enough current to accumulate a
significant load over the entire bus. Pull-up resistors are connected to the bus to provide the additional
current required. Their resistance is selected to supply enough current to meet high level needs on the
bus without exceeding the low level sink capacity of any driving device.
Pull-up resistors are used on the output-only DO bus because the baud rate generator, a MOS device.
has an input threshold higher than TTL driver E58 is guaranteed to deliver. Pull-up resistors ensure
that the data lines rise enough.

)

An additional function of the 8228 is a combinational logic decode of the 8080 status byte. This byte,
output by the 8080 at the beginning of each machine cycle, contains flag bits indicating the nature and
function of the cycle. The byte is latched into the. 8228 by the Status Strobe (STSTB) signal from the
clock generator. In combination with three control output bits from the 8080. the system controller
produces Interrupt Acknowledge (lNTA) and Memory and Input/Output Read and Write (M EM
WR,RD; I/O WR,RD). These signals reduce the external decoding requirements while saving pins on
the 8080 package.
When the video processor asserts Hold Request during a direct memory access (DMA) of the screen
RAM, the 8228 floats its outputs. Since these include the memory control signals, the video processor
must provide its own memory control. The DMA Enable signal, buffered into the MEM RD line by
gate E28 (pin 13), provides memory read enabling during the DMA. The other control signals are
pulled high by the pull-up resistors.

4-11

CONNECTORS

EIA

120MA lOOPI J5

~ E41, E61 f.--

SERIAL 12
DATA

GRAPHIC VIDEO

~ MODEM

E43

DIRECT DRIVE
GENERATOR

KEYBOARD INTERFACE
E2B, E48, E55

... ~BD
ClK

OPTION
PRESENT

1 B B

El0, E13
5

CHAR. f8
DATA

E14, E49

INTERRUPTS

~BUFFERI

BUFFEF

o ONE-WAY

DATA OUTPUT BUS

IlATCH &1
8UFFER
B lE15,E20
8

CHAR. ~
ADDR.
lATCH

18B

E16

INTER·
RUPT
lOGIC

1/0

E4O, E45, E52 E56

t" t----t
I

1/0 DECODE
E19, E27, E34

2
16

SCREEN &
SCRATCH RAM
3K X 8
E46, E47, ESO,
E51, E53, E54

PROGRAM
ROM
8K X B

1/0

_~AUD
ClK

8 1

ClK

1NT. REO.

_~VR

I NON,VOlATIlE RAM
E24, E29, E48, E61

INT. Ac

E32, E35, E36

NVR
DATA

,..

E18, E19
E23, E41

MICRO·
PROCESSOR

MEMORY DECOOE
E7, E23, E31

11
4 ADDRESS BUS

MEM. DISABLE

I I

1
I

ADVANCED
VIDEO OPTION
CONNECTOR
Jl

CHARACTER
GENERATOR
ROMS
2K X 8

E4, E9

ALT.
1
CHAR.

L I

"SIR
CONTROL
DATA 6
SCAN ADDR
.
4

VIDEO
CONTROL
AND
TIMING

TERMINATE

VIDEO CONTROL SIGNALS

ATTRI8UTES ';J

E3, E5, E6, E7,
E13, En

KBD ....
ClK

4
B

DMA ADDRESS COUNTERS
E21, E22, E25, E26, E30, E33

1
COM
SYN!

VIDEO

IIUNE BUFFERI
256 X 8 RAM
Ell, E17

f'6

NV~ .....

ClK

J
1

r
VIDEO CLOCKI
24.0734 MHz
E2

CONTROL BUS

---t--

81DIRECTIONAl DATA BUS

VIDEO OUT 1 + 2

I~HIFT REGISTER

INTEN·
110_ SITY
~ CONTROL
5
(D/A)

GRAPHIC
FLAG

SERIAL

~~NERATOR

BAjD
8
ClK

H & VBlANK

COMPo VIDEO
'3ENERATOR

rAUD RATE I \

T,o FLAGS

111 8

CNTRl

COMMUNICATION
TRANSCEIVER

E58

H& V
DRIVE

Iro

E37. E39, E44

1-"

IDIRECT DRIVE VIDEO OUTI J6 COMPOSITE VIDEO ."

f...rl~IA DRIVERS & RCVRS

MODEM
CONTROL

~ E42

PC BOARD FINGERS

GRAPHICS OPTlON1J2 ANALOG VIDEO IJB

I
r-------, Irr-

I STD. TERM. PORT

FLAG
BUFFER

1KEYBOARD IJ I

HOLD REOUEST
•

MICROPROCESSOR

VIDEO PROCESSOR.
MA .... 3'S2

Figure 4-2-2 Microprocessor Block Diagram

(
.~

'='

",-,l./

4.2.3
'\

Clock Generator

The 8224 provides the asymmetrical two-phase clock required by the 8080 logic. Its running frequency
is crystal-controlled. An LC circuit on the tank input improves the crystal's mode stability. The capacitor in series with the crystal compensates for internal phase shifts in the 8224 at the high operating
frequency. The 24.8832 MHz crystal frequency gets divided by 9 to produce a 361.69 ns clock period.
The buffered TTL phase 2 output clocks the PUSART and the baud rate generator. 361.69 ns is the
system clock period for the microprocessor.
The sync signal from the 8080 combines with an internal clock phase to produce the Status Strobe
(STSTB) during the last ninth of the first state of each machine cycle. STSTB latches the status byte
into the 8228 system controller.
The Ready input is not used in the VT100. It is an asynchronous input that gets synchronized to the
machine cycles in the 8224. The synchronized signal can cause the microprocessor to enter a wait state
during a memory access to wait for slow memory to respond. Memory used in the VT100 matches the
processor speed so Ready is not used.

--)

The Reset input is a Schmidt trigger. Its input is the + 5 volt supply delayed by an RC circuit with a
diode bypass for fast discharge. The Reset input is held low until well after all power supplies ha~'e
settled. When the capacitor voltage reaches the Schmidt threshold, reset is released and the microprocessor begins operation at memory location 0 (the start of the firmware ROM). The inverted and
noninverted reset signals clear other portions of the terminal controller.

4.2.4

)

)
/

Bus Timing

The bus timing diagram in Figure 4-2-3 illustrates the basic time relations between the address, data,
and control lines in the microprocessor system. The figure distinguishes between write and read cycles,
with the exception of the three top lines. These show the constants of the system: the two clock phases
from which all timing is derived, and the address bus, that always provides the current address for
either kind of cycle early in the cycle. Numbers on the diagram represent specified minimum/maximum nanoseconds between signals for normal operation.
Starting with the write cycle, note that the status strobe signal (STSTB L) shown in the read sectron
occurs at the same point in the write cycle. Therefore, the status strobe latches the status byte into the
8228 during the first clock cycle. The 8228 delivers the decoded Memory Write L or I/O Write L
(MEMW or lOW) signals in the third clock cycle when data to be written is stable. The 8080Data Bus,
DB Bus, and DO Bus graphs show the propagation delays between the three buses. The 8080 bus is the
output pins on the chip. The DB bus is the output of the buffered 8228 bidirectional bus, and the DO
bus is the unidirectional buffered output-only bus.
In the read cycle, the Memory Read L signal (along with I/O Read L and Interrupt Acknowledge L:
MEMR, lOR, INTA) starts early to enable the chip selects and allow the data buses to stabilize. Data
Bus In (OBIN) goes high before data settles but does not latch data into the 8080 until its falling edge,
when data is settled. The .two bus graphs show the assertion of stable data and also show the delay
through the bidirectional bus buffer to the 8080 data bus.

4.2.5

Microprocessor Memory

The basic terminal controller contains 8192 bytes (8K) of program ROM and 3072 bytes (3K) of static
RAM. (One byte = eight bits.) The original board utilizes four 2K X 8· ROM ICs; later versions may
use a single 8K X 8 ROM, The RAM is six 1K X 4 ICs arranged in pairs. The microprocessor can
address up to 64K memory locations. Some of these locations are reserved for future expansion. The
memory map of Figure 4-2-4 shows the portions of addressable space that are reserved and also those
areas that are available for new applications.

)
4-13

TIMES IN
NANOSECONDS
MIN/MAX

PHASE 1/11

PHASE 1/12

ADDRESS

WR
w

d MEMW.IOW
>-

~ 8080 DATA
~

;:
DB BUS

~
I

DO BUS

STSTB

~ DBIN

u MEMR. lOR

INTA

8080 BUS
DB BUS

MINIMUM SYSTEM REQUIREMENT
DATA CAN BE PRESENT MUCH LONGER
MA"'303

Figure 4-2-3 Microprocessor Bus Timing

<,"-......---",

-,--/

'---<-

-'-

'-.J-i

,,--,i

MA-4270

)

Figure 4-2-4 Memory Map

4-15

4.2.5.1 Memory Map - The basic terminal controller board contains 8K of program ROM and 3K
of screen and scratch RAM. These account for the locations from OOOOH to 2BFFH in memory. The
advanced video option adds another I kilobyte of RAM (2COOH to 2FFFH) to increase the number of
characters that can be displayed, The AVO also contains additional memory for character attributes.
This memory is only 4 bits wide, but to address each location in correspondence with each of the 4K
characters, it uses another 4K of addresses from 3000H to 3FFFH.
The next 16K locations (4000H to 7FFFH) are unassigned. However, 4000H to 4FFFH can be addressed by the video processor using the DMA address counters so 4000H to 4FFFH could be used for
additional screen RAM with 5000H to 5FFFH as associated attribute storage.
Above 7FFFH is an 8K area intended for additions or changes to the program ROM in 2K segments
and above that is an area intended for the same purpose but in one 8K segment. The top 16K are
unassigned.
4.2.5.2 Memory Devices - Each ROM has three ANDed chip select inputs that are mask-programmed to be either active-high or active-low, eliminating the need for external inverters. The programming provides I of 4 decoding of the two address lines A II and A 12. The third chip select line is
common to all four ROMs. In later VT100s jumpers W2 and W3 can be used to select either a high or .
low assertion for this single chip select to allow for 64K ROMs that do not have a programmable chip
select. Table 4-2-1 shows the addressing of the ROMs.
4.2.5.3 ROM Decoding - The program ROM is enabled when the top half of memory decoder E31 is
enabled and the correct address space is requested by the microprocessor. MEM RD or MEM WR
from the 8228, in the absence of MEM DISABLE, enables the decoder. The ROM is in addresses OH
to IFFFH, so A 13 (the 8K bit) is not asserted during ROM reads. Bit A 12 is low or high for 0-4K or
4-8K sections of ROM. The decoded outputs from both states are ORed to provide one of the select
signals for the ROM. Then bits A II and A 12 at two of the chip select lines enable the outputs in one of
the 2K segments. The low II bits address one of the 2K bytes in the enabled ROM. If the program is in
one 8K ROM, A 11 and A 12 are used as regular address lines; only one chip select line enables its
outputs.
4.2.5.4 RAM Decoding -If AI3 is asserted, the memory space between 2000H and 3FFFH is being
addressed. The ROM outputs are disabled and the decoded output from the top half of the decoder
enables the bottom half. Now bits AIO and A II are decoded to select I K segments of the screen RAM.
The I K X 4 RAMs are paralleled in pairs to make each location 8 bits wide. There is 3K on the basic
video board. The fourth I K is located on the A VO; its addressing is decoded separately.
4.2.5.5 Memory Disable - Memory Disable turns off all memory on the terminal controller board. It
is used by options that plug into the advanced video option connector:
1.

To replace (overlay) existing memory,

To disable the terminal controller memory when using memory above 4000H (since the
terminal controller does not decode A14, AI5).

The advanced video option can contain overlaying program memory. This is a set of address locations
that the AVO can be jumper-programmed to decode and provide data for. Since the main memory
may also be decoding the same location, the AVO must assert MEM DISABLE to disable the main
memory (ROM and RAM) so that only one data byte is asserted on the bus.

4-16

.
) )

)

4.2.6 I/O Decoding
The I/O address space is divided into two regions: one, containing the 8251 PUSART, has address bit
AOI always low; the other region contains all other I/O devices and has bit AOI always high. The list of
I/O addresses in Table 4-2-2 illustrates this by the presence of hex 2 in the low half of each nonPUSART address byte.
4.2.6.1 PUSART Read and Write - When bitAOI is low, the PUSART is enabled. The I/O RD and
WR signals from the 8228 control the read or write operation. Address bit AOO selects either the
command or data register for the I/O operation.
4.2.6.2 I/O Read and Write - When bit AOI is high, the PUSART is disabled. Bit AOI high is one of
two signals required to enable the I/O write decoder (E27). I/O WR is the other signal. Address bits
A05, A06, and A07 select one of the seven writable I/O devices. These are the baud rate generator, the
brightness control D/ A latch, the NVR latch, the keyboard UART transmit buffer, the DCOII and
DCO 12 video processor chips, and the graphics processor data port.

...

)

Table 4-2-1 ROM Chip Select Addressing
Address
Line 11

Address
Line 12

Chip
Select 2

Chip
Select 3

0
I
0
I

0
0
I
I

Active Low
Active High
Active Low
Active High

Active Low
Active Low
Active High
Active High

)
Table 4-2-2 List of Hex I/O Addresses
READ OR WRITE
OOH
PUSART data bus
01 H
PUSA~T command port

)

WRITE ONLY (Decoded with I/O WR L)
02H
Baud rate generator
42H
Brightness 0/ A latch
NVR latch
62H
82H
Keyboard UART data input
A2H
Video processor DC012
C2H
Video processor DCOII
E2H
Graphics port
READ ONL Y (Decoded with I/O RD L)
22H
Modem buffer
42H
Flags buffer
82H
Keyboard UART data output

4-17

ROM
Selected
I
2
3
4

··)1\

Only three I/O devices are readable so their addresses were chosen to allow read decoding directly
from the three address lines. When the 8228 asserts I/O RD, I/O WR deasserts and decoder E27 is
disabled. I/O RD enables the gates in E34 to allow reading of the keyboard UART receive buffer, the
flag buffer, or the modem control signal buffer, depending on which address bit is asserted.

4.2.7 Interrupt Vector
When any of the three interrupting devices (communication receiver, vertical frequency, or keyboard)
sets Interrupt Request (lNTR H) through gate E23, the current instruction is completed and then the
microprocessor sets its Interrupt Acknowledge (INTA) status bit and performs an instruction fetch.
The address requested in the fetch is the contents of the program counter. Its value is ignored in the
interrupt process, but it is not incremented, as it would be in a normal fetch cycle. The INT A bit is
decoded from the status word in the 8228 system controller; it enables the tristate interrupt vector
buffer (E41) which then presents a restart (RST) instruction to the data bus. Main memory does not
conflict with the vector buffer because the 8228, in decoding the interrupt status word, does not produce the memory read or write command bits needed for address decoding. (Refer to Table 4-2-3.)

Table 4-2-3

OOH
08H
10H
18H
20H
28H
30H
38H

Interrupt Addresses

Power-up (Not hardware driven)
Keyboard
Receiver
Receiver and keyboard
Vertical frequency
Vertical frequency and keyboard
Vertical frequency and receiver
Vertical frequency, receiver and keyboard

The RST instruction disables further interrupts, pushes the current program counter contents (the
location of the next instruction in the interrupted program) onto the stack and decrements the stacK
pointer. Then the program counter is loaded with the bits in the address field of the RST instruction.
This field is produced by the interrupt signals of the interrupting devices. The signals are passed onto
the data bus as bits in the address field (bits 3, 4, and 5). The rest of the instruction is hard-wired
through diodes that supply high level to the other data bits during INT A and that are reverse-biased to
isolate these data bus lines when the vector buffer is inactive.

-))

))

The RST address field, when mapped into the same bit locations in the program counter, defines a set
of eight 8-byte long memory spaces at the beginning of the memory. The first address, 0, is the starting
point for the terminal controller program and is not used for interrupts. The program is normally
started by a hardware reset signal that sets the program counter to O. It can also be started by an escape
sequence that the host can send to force the program to jump to zero, or in SET-UP mode the RESET
key can be pressed. The other seven memory spaces, starting at 8, 16, etc., contain jumps to places in an
interrupt handling routine that can mediate requests for service from any of the seven combinations of
interrupting devices.
At the end of the interrupt service routine, the stack is popped, interrupts are enabled, and the interrupted program continues.
Early VT100s can disable the receiver interrupt by programming D4 in the NVR latch. However, this
is never used by the VT100. Later VT100s instead, have the ability to add a Communications Transmit
Buffer Empty interrupt by adding W6. The 8080 would then have to distinguish transmit and receive
interrupts by testing the transmit flag on the flag buffer. This provision is not used on the VT I 00.
4-18

))

4.2.8

Power-Up and Self-Test

When power is first applied to the terminal controller board, the reset circuit in the 8224 holds the
microprocessor in a halt state. Within a second, after the voltages stabilize in the power supply, the RC
network at the reset input allows the input voltage to rise to the switching threshold of a Schmi tt
trigger. Then the reset is released with the 8080 program counter set to O. The low 64 bytes of program
are reserved for the eight interrupt service routines which can be addressed by the restart instruction
(see previous section). The low 8 bytes start the power-up routine by disabling the interrupts, setting up
the stack pointer, and then going immediately into the self-test routines.
Assuming there are no hard logic failures present on the board, the microprocessor attempts to perform a confidence check of the controller. Some failures are considered fatal and will stop the machine;
other failures limit its operation but will not prevent its use. Fatal failures are indicated by the LEDs
on the keyboard, while nonfatal errors
are indicated asa single character on the screen.
,.

)

)
/

The microprocessor first sends the number of the first ROM to the LEOs on the keyboard. Then it
calculates a checksum of the contents of the first 2K of program. (Since firmware is treated as fuur 2K
blocks of code, later VT100s with one 8K X 8 ROM chip operate the same way but any block failure
requires replacement of the one chip.) At the time of ROM preparation, a special byte was JOcludcci
within each block to make the checksum equal zero if there are no errors. If there is an error, the
microprocessor halts and the LEOs indicate the current ROM at the time of failure. OtherWise, the
LEOs are incremented to show the next ROM number and the process continues.
The next part of the test is writing and reading the RAM. Every bit in the RAM is written with a 0 and
a I and read each time. If the advanced video option is present (as indicated by the Option Present
flag), its RAM is tested immediately after the main RAM. In the main RAM a fai.1ure halts the
machine. Failure of a bit in the advanced video option RAM is indicated on the screen and the process
continues. In another terminal, like the VT52, one bad bit in the screen RAM means there is one
location that may not contain the right character. This can be annoying to the user but does not affect
the rest of the screen. If one bit is bad in a VT 100 line address, the entire screen below the affected line
can become garbled and unusable. A bad bit in the scratch area could disable communication with the
host. So this confidence check ensures that any RAM failure is detected immediately.
The next test checks the nonvolatile RAM by reading it. A checksum is calculated and compared with
the value stored the last time the NVR was written during a save. A bad NVR does notstop the VT100
because the SET-UP values can always be reestablished from the keyboard at power-up. The NVR test
is also the normal time when the terminal gets its auto SET-UP readings from the NVR. Time is saved
because reading the NVR is the most time-consuming part of both the self-test and the auto SET-UP.
If the NVR fails, the bell sounds several times to inform the operator, and then default settings stored
in the ROM allow the terminal to work. The operator must then manually reset any parameters that
differ from the default values.
To test the keyboard, the microprocessor commands the keyboard to scan once, lights all the LEOs,
for about a half second, and sounds the bell. It waits for the scan to finish and then looks for the last
key address 7FH at the keyboard UART. If the test fails, the terminal remains on-line, making it a
receive-only (RO) terminal.
This is the end of testing.
Once the NVR data is in the scratch area in RAM, the microprocessor uses that data to program the
hardware. All operating parameters that were last saved (see NVR) are recalled and the terminal is set
to match them. Finally the cursor appears at column I, line 1, and the microprocessor enters its
background routine, ready for operation.

4-19

Refer to the Communication chapter for a discussion of Data and EIA tests, and to the Service chapter
for a listing of the Self-Test Results tables.

4.3

))

COMMUNICATION TRANSCEIVER

The VT100 interfaces to its host system through a serial data port. An 8251 programmable universal
synchronous or asynchronous receiver-transmitter (PUSAR T), illustrated in Figure 4-3-1, drives the
port. This device translates between parallel and serial formats, adding or removing start and stop bits
as required. The data exchanged are ASCII characters; parity may be enabled or disabled; the selectably odd or even parity bit will take the most significant bit position.

4.3.1

PUSART Principles

Most data in computer systems is exchanged as groups of bits. The bit is the smallest unit of information, but to be useful, most information must be encoded into groups of bits in standardized patterns.
The VT100 operates with the standard patterns described by the American Standard Code for Information Interchange (ASCII). ASCII defines the use of 7 bits, specifying 128 different patterns that
correspond to almost all of the letters, numerals, and punctuation marks used in English and several
other languages. An eighth bit is reserved for an expanded standard.

--) )

The eight ASCII bits can be exchanged most' rapidly over eight separate wires (in parallel), but this is
very expensive to do anywhere outside the computer cabinet. Instead, the bits are rearranged to pass
over a single wire one after the other. This is called serial transmission. The circuitry for converting
parallel to serial and back again is complex but suited to large scale integration (LSI). The savings
allowed by the use of serial lines has encouraged the development of very sophisticated but inexpensive
conversion devices in LSI. At the same time, a variety of data exchange protocols has been developed.
The result is an LSI device that can operate with virtually any protocol depending on the programming
that it receives from its local processor. This is the PUSART. The VT100 uses one such device (Inters
8251A) only in asynchronous mode, plus two simpler, wire-programmed asynchronous-only UARTS
for the keyboard interface.
Synchronous and asynchronous describe the manner in which separate groups of bits (called bytes) are
exchanged. In order for a receiver to know which bit is arriving at any given time. it must know the
format of the byte and which bit of the byte is the first one. In synchronous mode, one or more special
bytes are transmitted which the interface recognizes as synchronizing characters. Then all data bytes
are transmitted together in rapid and precisely timed succession. Both the transmitter and receiver
must ha~e the same externally-supplied clock.
The VT100 only communicates asynchronously. Asynchronous transmission uses bits added to each
data byte to provide synchronization between the transmitter and receiver. Because any two data
exchanges can originate at random times with clock frequencies as much as I percent different, the
protocol assumes random arrival of any byte of data, and relies on the synchronization information in
the byte. This synchronization consists of extra bits appended to the beginning and end of the byte.
One bit at each end (one start bit and one stop bit) is the most common configuration. The start bit and
stop bit are defined to have specific states, and in particular, the start bit has a different state from the
idling condition on the line. The interface looks for the transition from the idling state (called mark)to
the start bit state (called space) and then clocks in the byte. The stop bit is the mark state, as is the
idling line, so an immediately following byte has the correct mark to space transition to provide
synchronization. The data bits, which occur between a start and a stop bit, are represented by a mark
for a one and a space for a zero: Figure 4-3-2 shows the asynchronous data format.

4-20

))

)
TRANSMIT
BUFFER
PARALLEL
TO
SERIAL

TxD

TxRDY
TRANSMIT
CONTROL

TxEMPTY
TxC

... j

RECEIVE
BUFFER

DSR
DTR

MODEM
CONTROL

CTS

SERIAL TO
PARALLEL

RxD

RTS

RxRDY
RECEIVE
CONTROL
INTERNAL
DATA BUS

PIN NAME

PIN FUNCTION

D, Do
C/O
WR
CS
CLK
RESET
TxC
TxD

DATA BUS (8 BITS)
CONTROL OR DATA IS TO BE WRITTEN OR READ
READ DATA COMMAND
WRITE DATA OR CONTROL COMMAND
CHIP SELECT
CLOCK PULSE (TTL)
RESET
TRANSMITTER CLOCK
TRANSMITTER DATA

RxC
RxD
RxRDY
TxRDY
DSR
DTR
SYNDETI
8RKDET
RTS
CTS
TxEMPTY

RECEIVER CLOCK
RECEIVER DATA
RECEIVER READY (HAS CHARACTER FOR CPU)
TRANSMITTER READY (READY FOR CHAR. FROM CPU)
DATA SET READY
DATA TERMINAL READY
SYNC DETECT/
BREAK DETECT
REQUEST TO SEND DATA
CLEAR TO SEND DATA
TRANSMITTER EMPTY

)

RxC
SYNDET/
BRKDET

MA-4302

)

Figure 4-3-1 8251A PUSART Block Diagram
4-21

TRANSMITTER OUTPUT

TxD

MARKING

START
BIT

BITS

DOES NOT APPEAR
ON THE DATA BUS

ST~

PARITY
BIT

DATA BITS

...

ST~

PARITY
BIT

DATA BITS

Do 0, ------Dx

RECEIVER INPUT

RxD

On 0,

))

GENERATED
BY 8251 A

BITS

PROGRAMMED
CHARACTER
LENGTH

))

TRANSMISSION FORMAT
CPU BYTE (7-8 BITS CHAR)

I C~;RACTER I
DATA

ASSEMBLED SERIAL DATA OUTPUT (TxD) ~

START
BIT

\\
DATA CHARACTER
)\

PARITY
BIT

\ \

BI,'
\

RECEIVE FORMAT
SERIAL DATA INPUT (RxD)

"
PARITY
DATA CHARACTER
{ {

START
BIT

\ 1

BIT

.?;-,

ST
BI.

I (

CPU BYTE (7-8 BITS CHAR)·

I <~RACTER I
DATA

·NOTE: IF CHARACTER LENGTH IS DEFINED AS 7
BITS THE UNUSED BITS ARE SET TO "ZERO"
MA-4284

Figure 4-3-2 Asynchronous Data Format

4-22

)

4.3.2

PUSART Operation

The complete functional definition of the PUSART is programmed by the system's software. A set of
contiOl words must be sent out by the microprocessor to initialize the PUSART to support tht: desired
communications format. Once programmed, the PUSART is ready to perform its communication
functions. The Transmitter Ready (TxRDY) output is raised high to signal the microprocessor that the
PUSART is ready to receive a data character from the microprocessor. TxRDY is reset automati(ally
when the microprocessor writes a character into the PUSART.
Upon receiving an entire character from the serial input, the Receiver Ready (RxRDY) 0utput is
raised high to signal the microprocessor that the PUSART has a complete character ready to be r.:ad.
RxRDY is reset automatically when the data!is read. The PUSART cannot begin transmis!!JOn until
the Transmitter Enable (Tx Enable) bit is set in the command instruction and it has received a Ciearl 0
Send (CTS) input. The Transmit Data (TxD) output will be held in the marking state when tht line is
idle.
For a more detailed description of the PUSART's operation, refer to Intel's 825lA specification.

4.3.3

PUSART Addressing

I/O read, write and enable addressing and commands are discussed in the microprocessor I; 0 Jc..:oding section.

4.3.4

)

PUSART Programming

The microprocessor can program the PUSART to opeate with several standards and parameters.
Many of these parameters are predetermined by the VT100 specifications. Character length, number of
stop bits, parity enabling and format, baud rate multiplication factor, and asynchronous operation are
all programmed in at power-up through the mode instruction (Figure 4-3-3) from the stored SET-U P
information. Address bit AOO selects either the command register in the PUSART for writing the byte
containing this information, or the transmit buffer for normal operation. A different programmable
device, discussed later, provides the selected baud rate from SET-UP data.
After the PUSART mode of operation is selected by writing a mode instruction, PUSART operation is
controlled by writing a command instruction (Figure 4-3-4). Once the mode instruction has been
written into the PUSART, all further "control writes" (C/D= I) load a command instruction. A reset
operation (internal or external) returns the PUSAR T to the mode instruction format.

)

The status of the PUSART can be read by the microprocessor by performing a read with C/D= I.
Some of the bits in the status word (Figure 4-3-5) have the same meaning as output pins on the
PUSART chip. The status word may be a maximum of 28 clock periods behind the event causing the
update. This clock, from the microprocessor's phase 2 TTL signal, is the clock for the internal operation of the PUSART, which is a dynamic device requiring internal refresh at regular intervals.

4.3.5

Data Transmission

When the microprocessor wants to send a character out, it checks the XMIT flag at the flag buffer. If
this flag is high the transmit buffer is empty and can accept data. If the flag is low, the microprocessor
continues through its background program and returns to check again later.
When the flag is high. the microprocessor loads a data byte into the transmit buffer. The PUSART is
double-buffered; this means that there is a second data buffer inside that is automatically loaded from
the first buffer. The second buffer's contents get start and stop bits and parity (if enabled) appended
·and are shifted out by the continuously running clock. Immediately after the second buffer is loaded,
the transmit flag goes high and the first (transmit) buffer can be loaded again.

)
4-23

EP PEN

BAUD RATE FACTOR
0

SYNC
MODE

(lX)

(16X)

(64X)

CHARACTER LENGTH
0

BITS

7
BITS

-) )

8
BITS

PARITY ENABLE
1 = ENABLE
=DISABLE

EVEN PARITY GENERATION/CHECK
1 = EVEN 0= ODD
. NUMBER OF STOP BITS
0

INVALID

1
BIT

1 %
BITS

2
BITS

(ONLY EFFECTS Tx; Rx NEVER
REQUIRES MORE THAN ONE
STOP BIT)

))

MA-4283

Figure 4-3-3 Mode Instruction

))
4-24

)
EH

RTS

SBRK

RxE

DTR

TxEN

TRANSMIT ENABLE
1 = ENABLE
0= DISABLE

DATA TERMINAL READY
"HIGH" WILL FORCE DTR
OUTPUT TO ZERO.

RECEIVE ENABLE
1 = ENABLE
0= DISABLE

--)

SEND BREAK
CHARACTER
1 = FORCES TxD "LOW"
0= NORMAL OPERATION

ERROR RESET
1 = RESET ERROR FLAGS
PE. OE. FE
REQUEST TO SEND
"HIGH" WILL FORCE RTS
OUTPUT TO ZERO

INTERNAL RESET: "HIGH"
RETURNS 8251ATO MODE
INSTRUCTION FORMAT
ENTER HUNT MODE·
1 = ENABLE SEARCH FOR
SYNC CHARACTERS
• (HAS NO EFFECT
IN ASYNC MODE)
NOTE: ERROR RESET MUST BE PERFORMED WHENEVER RxENABLE AND
ENTER HUNT ARE PROGRAMMED.
COMMAND INSTRUCTION FORMAT
MA-4276

Figure 4-3-4 Command Instruction

4-25

SYNDET

DSR

TxEMPTY

TxRDY

I J
PE

I
NOTE 1
SAME DEFINITIONS AS I/O PINS
PARITY ERROR
THE PE FLAG IS SET WHEN A PARITY
ERROR IS DETECTED. IT IS RESET BY
THE ER BIT OF THE COMMAND
INSTRUCTION. PE DOES NOT INHIBIT
OPERATION OF THE 8251 A.

OVERRUN ERROR
THE OE FLAG IS SET WHEN THE CPU
DOES NOT READ A CHARACTER BEFORE
THE NEXT ONE BECOMES AVAILABLE.
' - - - - - - + 4 IT IS RESET BY THE ER BIT OF THE
COMMAND INSTRUCTION. OE DOES
NOT INHIBIT OPERATION OF THE 8251A;
HOWEVER THE PREVIOUSLY OVERRUN
CHARACTER IS LOST

FRAMING ERROR (ASYNC ONLY)
THE FE FLAG IS SET WHEN A VALID
STOP BIT IS NOT DETECTED AT THE
END OF EVERY CHARACTER. IT IS RESET
BY THE ER BIT OF THE COMMAND
INSTRUCTION. FE DOES NOT INHIBIT
TH E OPERATION OF TH E 8251 A.

DATA SET READY INDICATES
THAT THE DSR IS AT A ZERO LEVEL.
STATUS BYTE FORMAT
NOTE 1:

TxRDY STATUS BIT HAS DIFFERENT MEANINGS FROM THE
TxRDY OUTPUT PIN. THE FORMER IS NOT CONDITIONED
BY CTS AND TxEN; THE LATTER IS CONDITIONED BY BOTH
CTS AND TxEN.
DB BUFFER EMPTY
i.e. TxRDY STATUS BIT
TxRDY PIN OUT
DB BUFFER EMPTY. (CTS-O) • (TxEN-1)

MA-4275

Figure 4-3-5 Status Byte Format

4-26

))

)

4.3.6 Data Reception
Any data that appears on the receive data line is shifted into the internal receive shifting buffer. When
a full character of bits has arrived, the start, stop, and parity bits are stripped. Parity is checked, and if
bad, the parity error flag in the status word is set. Data is transferred to the receive data buffer, and the
receive flag is set. This flag requests an interrupt from the microprocesor. The microprocessor then has
the amount of time it takes the next character to shift in to read the first character. After reading the
character, the microprocessor reads the status byte to check the integrity of the data. If the microprocessor does not read the receive data buffer in time, the second character writes over the first one
which is lost. Then, an overrun error is reported in the status word. The checkerboard character
appears for all errors.
4.3.7 Baud Rate Generator
PUSART clocks are derived from the microprocessor clock. The microprocessor clock crystal was
selected to provide a frequency within the limits of the 8080 which could be readily divided to provide
standard baud rates. The division occurs in programmable baud rate generator E60. This device contains two independent counters to allow different receive and transmit (split) baud rates. Each counter
has 4 input lines to select I of 16 rates. Th us, a I-byte load into the device can set up both send and
receive rates. The baud rate generator's input register is written into as a device in the I/O address
space.
E60 is factory mask-programmed with the division ratios required to get standard baud rates from the
crystal frequency. The 4-bit input is an address for a ROM location containing the SET-UP information for each rate. Table 4-3-1 lists the baud rate generator divisors.
E60 was originally designed to operate with a crystal as a self-contained crystal controlled oscillator
and divider. The oscillator is located elsewhere, so the oscillator inputs EXTI and EXT2 receive the
microprocessor clock driven out of phase by two inverters in E38.

)
Table 4-3-1 Baud Rate Generator Divisors
(Input frequency = 2.76480 MHz)
Freq
(Hz)
Error

XJ6

)

Baud Rate

Divisor

Output

50
75
110
134.5
150
200
300
600
1200
1800
2000
2400
3600
4800
9600

3456
2304
1571
1285
1152
864
576
288
144
96
86
72
48
36
18
9

800
1200
1760*
2152*
2400
3200
4000
9600
19200
28800
32000 *
38400
57600
76800
153600

19~OO

)

-0.006%
-0.019%

+0.465%

307?00

*Output frequency shown is nominal value. Include percentage error to get actual frequency.
4-27

4.3.8 Serial Interface
The serial transmit and receive interfaces are ICs that convert between TTL signals and EIA RS-232-C
unbalanced bipolar signals. The electrical specifications and connector pinouts are described in Chapter 3, Installation.

4.3.9 Modem Control
Certain pins on the PUSART are labeled with standard modem control designations. These pins are
readable [Data Set Ready (DSR)] or writable [Data Terminal Ready (DTR)] and [Request to Send
(RTS)] as buffered bits in the PUSART's status and control bytes. Other signals from the modem pass
through EIA level translators with Schmi tt trigger inputs to a tristate buffer which, when enabled by
the MODEM RD command, presents them to the data bus. Another signal [Speed Select (SPDS)] is
written from the data bus into the NVR latch which is a convenient extra latch position. None of these
signals are used to support modem control in the basic VTl 00. They are always programmed at powerup to allow normal full duplex operation with some modems when a standard EIA cable is installed
between the modem and the VT100. See Chapter 3 for more interface information.

4.3.10

Data Types

Three kinds of data can be exchanged between the VTl 00 and the host: control characters, control
functions, and displayable c h a r a c t e r s . - ) ,)
Control characters are any ASCII characters in the range 0 - 1FH. TheyincIude carriage return and
line feed.
Control functions start with a control character (escape) and contain additional characters which extend the range of special actions that the terminal can perform. Cursor home is such an action. Some
control functions can contain numeric parameters to modify the special actions. Direct cursor addressing is a typical example. Appendix A of this manual describes the programming and use of the control
characters and functions in detail.

))
j

Displayable characters are those ASCII codes that are stored in the screen RAM, causing a character
to be displayed on the screen.

4.3.11

SILO

The microprocessor checks each character as it comes in from the PUSART. Only four characters
cause any special action at this time. Control codes XON and XOFF (see below) are immediately processed while NULL and DELETE are discarded. Everything else gets put in a 64 character (128 characters in later model terminals) first-injfirst-out space in the scratch area of RAM called the SILO.
The SILO processor routine maintains this area by updating two locations called SILIN and SILOUT.
These point to the current entry and exit points in a loop of memory locations. The farther apart they
are, the more characters are in the SILO. The addresses of the two points are subtracted from each
other to determine the filling of the SILO. The subtraction is performed in modulo 64 (or 128) arithmetic to accommodate the rotation of the locations.

\ )

The SILO is necessary because when the microprocessor reads a character, it acts on that character
completely before taking another one. For example, a line feed character causes the processing needed
to accomplish a scroll. The processing time often exceeds the time between characters at the port. To
save the characters that might arrive and get lost during a special action, the microprocessor responds
to the received data interrupt by quickly moving the data to the SILO. Only the examination described
above gets performed. After the current action is finished, the microprocessor gets the longest waiting
character out of the SILO and range checks it. If it is less than 20H, it is a control character and the
microprocessor processes it. If it is 20H or above, the microprocessor puts it in the next character position in the screen RAM.

;

4-28

4.3.12 XON jXOFF
XON and XOFF are two control codes that the terminal and host may send to each other to control the
pace of data transmission. The host usually has a buffer space similar in function to the VT100's SILO.
If either device has a large processing load at the same time that a lot of data is being received, the
buffer or SI LO can fill up. The receiving device monitors its buffer and sends Transmit Off (XOFF)
when the buffer contains a predetermined number of characters. On the VT100 this value is 32 characters. (A second XOFF is sent at 112 characters in late model VTl OOs.) The sending device should stop
transmitting until it receives Transmit On (XON). When the receiving device empties its buffer to another predetermined number of characters (16 characters on the VT100), it sends XON.
The NO SCROLL key on the keyboard enables and disables SILO fetching. If data continues to arrive,
the SI LO management routine sends XOFF or XON as required. The microprocessor keeps track of
the current state and sends the opposite command the next time the key is pressed. CTRL-S and
CTRL-Q send XOFF and XON directly. Coordination of various causes of XON and XOFF is discussed in Appendix A.
Because of the serial interface, several character times may elapse before the transmitter acts on XOFF
and its last character passes through the receiver. This partly determines how full the buffer can be
before XOFF must be sent. The other determinant is the worst case condition in split speed operation.
If the terminal transmits at a low speed and receives at a high speed, the interface delay can allow
several characters to arrive before XOFF stops their transmission. A detailed discussion of this problem
is in Appendix A.

)

4.3.13 Control Function Parser
A parser is a routine that examines a sequence of characters. It then starts processes and extracts parameters based on the contents of the sequ'ence. The term comes from the grammarian'S practice of
parsing (separating) a sentence into its component parts: SUbject, verb, object. The meanings of many
sequences are being standardized throughout the data processing industry through American Standards
Institute (ANSI) and International Organization of Standardization (ISO) committees. The VT100
employs a subset of several of these standards, plus it has several private sequences to allow it to perform certain DEC-specific functions like behaving as a VT52.
A character in the range below 20H is always the starting flag for the parsing process. Some functions
have only the first character for their sequence; line feed (OAH) is an example. After the function is
performed, the next character is taken from the SILO. If it is not in the control range, it is put on' the
screen. Escape (1 BH) is the flag for a longer sequence. After escape, characters are taken from the
SI LO and range-checked as intermediate (20H to 2FH) or final (30H to 7EH) until a final character
appears. The sequence is interpreted and the appropriate function performed. Then the next character
is taken from the SILO and displayed if not in the control range. If a sequence is not supported by the
VT 100, it is parsed and then ignored.
Detailed descriptions of the functions are in the VT100 User Guide and in Appendix A of this manual.
4.3.14 Local
In local mode, keyboard output bypasses the communication transmitter and receiver and SILO and is
acted on directly. The Data Terminal Ready signal (DTR) at the EIA interface is unasserted in local.
4.3.15 Standard Terminal Port
The standard terminal port (STP) is a printed circuit board edge connector on the basic video board
that contains twenty pairs of contacts. When no board is plugged into the connector, the pairs of contacts meet. All EIA data and modem control signals plus the two baud clocks pass through this connector. Future options may utilize the signals present at this connector. For a complete discussion of the
details of the STP, see the STP Option Interface Guide, Chapter 7.

)
/

4-29

4.3.16 Communication Self-Test
The VT100 can test its communication circuits, but because the operator must temporarily plug in a
specially wired loop back connector, the test is not performed automatically. Refer to the Service
chapter for instructions. There are two tests. One is a data test for either 20 rnA current loop or EIA
interfaces. This test transmits a pattern of data out the transmit line and examines the data as it returns
through the receive line. Each bit of the data bus is tested with the eight test characters: 01H, 02H,
O4H, 08H, IOH, 20H, 40H, 80H. This group of eight passes through the circuit repeatedly while the
microprocessor changes the baud rate after each pass.
The second test, for EIA only, tests the modem control lines to ensure that they can be controlled.
Some terminal controller modules have an etch revision at E39 that does not permit EIA testing. These
modules can be recognized by the presence of extra wires attached at E39.
Table 4-3-2 lists the test connector wiring.
Table 4-3-2 Loopback Test Connectors
From

EIA:

Pin 2
Pin 4
Pin 20
Pin 19

Pins 3 and 15
Pins 5 and 8
Pins 6 and 22
Pins 12 and 17

20 rnA:

Pin 1
Pin 2
Pin 5

Pin 3
Pin 7
Pin 8

4.4 KEYBOARD
The VT100 keyboard is a typewriter-like array of momentary, normally-open switches. The array is
mounted in a small case with a speaker for audio feedback and electronics for interfacing the array to
the terminal. The VT100 keyboard connects to the terminal controller board through a three conductor cable that plugs into the back of the terminal cabinet. The cable carries power, ground, and a
complex bidirectional data and clock signal. The terminal sends the clock signal and a status word that
controls the LED indictors, the bell, and the keyboard scan process. The keyboard sends the row and
column address of each key that is pressed. The terminal's microprocessor then translates that address
into an ASCII character and transmits it serially to the host through a UART or performs internal
actions such as SET-UP and no-scroll. Refer to Section 4.3 for a discussion of UART principles.
This section describes the operation of the keyboard and its interface hardware and software in the
.
terminal.
4.4.1 Keyboard Block Diagram (Figure 4-4-1)
The bidirectional interface separates incoming from outgoing data on the single signal line. Incoming
data from the terminal contains a clock that passes directly to the UART and the address counter
circuit. Then the data and clock are integrated. The duty cycle encoded data becomes serial data. The
UART deserializes the terminal data and produces an 8-bit parallel output. These bits control the bell
and LEDs, and start address scanning, depending on which bits were set by the terminal in the keyboard status byte.

4-30

))

,..----+ CLOCK

SERIAL
LINE
TO
TERMINAL

BIOIRECTIONAL

INTEGRATOR

BELL
CIRCUIT

INTERFACE
'--~ _ _...J

CLOCK

SERIAL
DATA IN

UART

CLOCK
SERIAL
DATA OUT

ADDRESS
COUNTERS

DATA
STROBE

MA-4661

)

Figure 4-4-1 Keyboard Block Diagram

The address counters start on command and send row and column addresses to the keyboard switch
array. When an address matches a key that is down, the Key Down signal strobes that address into the
UART. The UART serializes the address and the bidirectional interface sends it to the terminal.
4.4.2 Keyboard UARTs
Information is exchanged between the terminal and the keyboard in serial form. One start bit, eight
data bits, and one stop bit are transmitted using a clock derived from the horizontal timing circuits in
the terminal. Besides timing the serialization of data in the terminal, the clock is also transmitted to the
keyboard to time its circuits. The terminal data is a status byte that controls the keyboard by commanding key scans and other functions.
The keyboard transmits its data in the same format and with the same clock. Its data are the addresses
of whatever keys are down during a scan. The last key address sent has the highest address possible and
is always sent to indicate to the microprocessor that the scan is complete.
4.4.3 Keyboard Status Byte
The terminal controls the keyboard through a status byte that it sends through the interface along with
the keyboard clock. The first six bits of the 8-bit byte (Figure 4-4-2) control the On-Line/Local,
Keyboard Locked, and four user programmable LEDs on the keyboard. Every time the status byte is
sent, it refreshes the LEDs even if no new action is being taken. The seventh bit is sent only once in a
vertical interval and initiates the scanning process in the keyboard. The eighth bit, if sent only once,
causes the keyboard speaker to click. (Keyclick is defeat able at SET-UP.) If the bit is sent approximately two hundred times in a row, for about a quarter second, it sounds a bell.

4-31

SP~R.

CLICK

~INE/

ON
LOCAL
START
SCAN

LJO

L!O

KEYBO.
LOCKED

LED

4
MA04293

Figure 4-4-2 Keyboard Status Byte
The operating clock for the keyboard interface comes from an address line in the video processor
(LBA4). This signal has an average period of 7.945 microseconds. Each data byte is transmitted with
one start bit and one stop bit, and each bit lasts 16 clock periods. The total time for each data byte is
160 times 7.945 or 1.27 milliseconds. Each time the Transmit Buffer Empty flag on the terminal's
UART gets set (when the current byte is being transmitted), the microprocessor loads another byte
into the transmit buffer. In this way, the stream of status bytes to the keyboard is continuous.

--7)

4.4.4 Key Address Counter
Keyboard addresses are outputs from a counter that correspond to locations in a keyswitch matrix.
The counter can address all locations in the matrix. If a keyswitch is closed, the counter output addressing that location is transmitted. The address counter is a pair of 4-bit binary counters arranged so
that only the top seven bits go to the switch matrix. These outputs also connect directly to the U ART
transmit data inputs. The output of the first bit in the counter is a squarewave with a period equal to
the amount of time that each key is sampled during a scan. Key Down L appears at the input to flipflop E6 when the counter reaches the address of a key that is pressed. Half a sample period later, the
first counter bit clocks the Key Down signal through flip-flop E6. The half period delay allows any
glitches in the address counter to settle before Key Down asserts Data Strobe. Data Strobe loads the
UART transmit buffer with the address count present at the data input at that moment. That address
count represents the key that was down when Key Down was asserted.

))

The UART, which is double-buffered, deasserts Transmit Buffer Empty while the transmit (outer)
buffer is full. This stops the counter by blocking the clock at gate E4. As soon as the data moves into
the inner shifting register, the transmit buffer empties, TBMT is asserted, and the count con tin ues. In
this way, any number of key addresses can be sent to the terminal. The time the scan takes tofinish
varies with the number of keys down. On the first address load, when transfer from the transmit buffer
into the shifting register is immediate, the UART reasserts TBMT almost immediately. For loads later
in the scan, the UART deasserts TBMT for a longer period because an address that loads into the
transmit buffer must wait with the counter stopped until the previous transmission is done. Because
the transmission time is 160 counts and the complete address scan takes only 128 counts, there is a
minimum wait of 32 counts or about 20 percent of a character transmission time between the scan of
address 7FH and the transmission of address 7FH if the first key in the scan was down. (Refer to
Figure 4-4-3.)
Because it is hard-wired, the highest address (7FH) always asserts Key Down and gets transmitted,
indicating end of scan to the terminal. The highest bit also clocks flip-flop E3 which clears the counters
and stops the count. This is the only way that the scan is terminated. The scan begins again when the
terminal sends the Start Scan bit in the status byte to the keyboard. When the data arrives, the U ART
asserts its Data Available flag. On the next clock transition, flip-flop E6 passes a short, c1ock-synchro-

4-32

nized Data Available pulse to gate E5, resetting the flag in the process. E5 combines the Data A vailable pulse with the Start Scan bit if present to clear E3, allowing the count to begin.
After the terminal starts the scan, it waits for the 7FH address and then for a vertical interrupt (a
synchronizing signal explained in VIDEO) before it will start the scan again. If there are a lot of keys
down, the scan may take more than one vertical interval to finish. The keyboard ignores further
requests to scan until the current scan is complete.

BEST CASE
SCAN: ADDR 1.......... 127 COUNTS ........ ADDR 7FH
TRANS: ADDR 1................. 160 COUNTS ............ ADDR 7FH

·1

I·

DELAY BETWEEN SCAN & TRANS
OF ADDR 7FH

--1
WORST CASE

SCAN:
ADDR 1. ADDR 2 ... NO COUNTS ..... START COUNTS ..... 126 COUNTS ..... ADDR 7FH
TRANS: ADDR 1............... 160COUNTS......... ADDR 2................... 160COUNTS ............. ADDR 7FH

)

DELAY BETWEEN SCAN &
TRANS OF ADDR 2

DELAY 1 COUNT MORE
THAN BEST CASE
MA·4278

Figure 4-4-3 Key Address Transmission Delays

)

Here is a formula that estimates the delay between the time that the terminal asserts the start scan bit at
its UART and the time that the terminal UART asserts data available upon receiving the final address
7FH. The clock driving the address counter and UART has a period of 7.945 microseconds, there is
the number of addresses to the first closed key to be counted, and the serial transmission of an address
takes 160 clock periods (16 clocks per bit, 8 data bits. 1 start and 1 stop bit).
160
start scan command word to keyboard
+
m
counts to first key
+ n X 160 for n keys down
+
160
transmission of 7FH
Total X 7.945 = microseconds
For example, suppose that one key, #24, is down.

)

+
+
+

160
start scan command word to keyboard
24
counts
160
transmission of key #24 address
160
transmission of 7FH
504 X 7.945 - 4004 microseconds (4.0 #lS)
4-33

4.4.S Key Scanning and Address F o r m a t i o n ' ) )
The keyboard is an array of contact pairs arranged in 16 rows and 8 columns (Figure 4-4-4). One side
.
of each pair is connected to all the others in its column and each column connects to +5 volts through
a resistor. Each of the eight columns also connects to an input on the eight-to-one multiplexer E14.
The other side of each contact pair connects to all the others in its row and to an output on one of the
one-to-eight demultiplexers Ell and E13. The seven outputs ofthe keyboard scan counter control the
mUltiplexer and demultiplexers. The rows are selected by the lower four bits and the columns by the
upper three.
A low input to the selected column line on the multiplexer causes a Key Down L signal. If a key is
pressed when the row and column address is not on its position, the un selected demultiplexer line is
high and there is no change in level across the contacts. If the demultiplexer line is selected, it is low,
and the closed contact pulls the column low through the resistor. Each column is scanned top to
bottom as the low four address lines count; then the next column is selected and scanned. The complete
keyboard scan takes about 1 millisecond when no keys are down.

The last row and column position (address 7FH) is wired to always indicate key down. This value
indicates to the terminal that the scan is ended.
4.4.6 Bidirectional Interface Operation
The terminal sends data and clock to the keyboard; the keyboard sends data only. Transmission is
asynchronous, full duplex, serial, 8-bit data with one start bit and one stop bit over a single signal line.
Four states can exist on the line, representing the two signal states from each end of the line.
Both signals may coexist on the same wire, originate at opposite ends, and simultaneously communicate provided that sensing resistors are put at each end. The interface works by observing the voltage
variations on its input (across the sensing resistor) while biasing the input in an opposite direction with
its own output signals so that only input variations can cause enough change to exceed the threshold of
detection.
4.4.6.1 Interface Line - Refer to the keyboard interface schematic, Figure 4-4-5a. If one side of the
line is at + 12 volts and the other is at ground, then by Ohm's Law, the center of the two equal resistors
will be at V /2 = 6 volts. If both sides are at 0 or 12, the center will be identically at 0 or 12. Thus the
signal line can either have no current flow but with the junction of the two resistors at either 0 or 12
volts, or the junction can be at 6 volts with current flow to the left or to the right, thereby representing
the four required states.
4.4.6.2 Receiving Side - The receiving side of the keyboard interface separates the incoming signal
from its own output by delivering a sample of its output to the inverting input of a comparator. Refer
back to Figure 4-4-5. This provides an additional input for the comparator which compensates for the
variation that occurs on the noninverting input at the same time that the output changes. R 12 and R 13
(both approximately 20K) and R11 (10K) are a divider providing bias to the comparator. If E8 (an
open collector driver) is off (high), then the end of R 11 connected to E8 is essentially at + 12 volts
through RI5 (470). The Rll, R12, R13 divider outputs 9 volts as shown in Figure 4-4-5b. If E8 is on
(low), the end of RII is at ground, and the divider outputs 3 volts (Figure 4-4-5c).
Meanwhile, the signal from the other end of the signal line is admitted to the noninverting input of the
comparator through a resistor (R 7) whose value is small compared to the comparator's high input
impedance. The positive feedback resistor R8 provides a small amount of hysteresis to improve the
circuit's noise immunity.

4-34

--j)

COLUMNS

)
7
LSD

1(\
1\,"/

I~'/

\...."/

0 1(\0 0 t:F~
0 115\8 8
8
I\" /' 0 G
1(" _\ If '\1(,\
1"'- / I'" .) 8

~\
PF'

\.../

~.J

SPACE

LINE

--1

BACK

RETURN

FEED

)

ALWAYS
OPEN

!
BREAK

/"\./
\

DELETE

}

1
{

RETURN

'"
L

&
7

%
5

NO
SCROLL

SETUP

CTRL

SHIFT

CAPS
LOCK

SPACE
BAR

)

MSD

(

[

#£
3

TAB

ESC

ALWAYS

F CLOSED

MA-4361

Figure 4-4-4 Keyboard Switch Array

4-35

R7
>-""",-.COMPARATOR
OUTPUT
12

r- ---- -------,
I
R12
I
I

20K
I
BIAS
I
:
NETWORK:
I
R11
R13
I
IL _ _
10K
20K
_ _ . _______
.JI
I

\-SIGNAL LlNE---l

SENDER
2K

I RECEIVER I
I JUNCTION
:11

OUTPUT
DATA

'1'" I
~
'\J

----J)

CABLE
CAPACITANCE
MA-4659

Figure 4-4-SA Keyboard Interface Circuit

20K

10K

...--_---.L___ 9 VO LTS
20K

Figure 4-4-SB Bias Network - E8 High

12
20K
...---__~--+3VOLTS
20K

10K
MA-4660

Figure 4-4-SC Bias Network - E8 Low

4-36

))

For the case where the junction is at 12 volts, there is no ambiguity about what signal the sender is
sending. The comparator must produce a high output because the sender is outputting a high level.
(The receiver is also sending a high level but no distinction between the two needs to be made.) The
bias network presents a 9 volt signal to the inverting input, so the comparator sees a + 3 volt difference
and goes high.
For the case where the junction between the sender and the receiver is at 0 volts, there is again no
ambiguity. The comparator must produce a low output because both sender and receiver are outputting low levels. The bias network presents a 3 volt signal to the inverting input, so the comparator
sees a -3 volt difference and goes low.

'.
/

F or the case where the junction is at 6 volts, either end could be the high or low sender. The decision is
made at each end by each circuit examining its own output to decide whether it or the other end is
sending a particular level. If the sending end is at 0, the non inverting input sees 6 volts. The receiving
end is outputting 12 volts, and the network outputs 9 volts to the inverting input. The comparator sees
a difference of -3 volts and outputs low in recognition of the low level at the sending end. If the sending
end is at + 12, the noninverting input sees 6 volts. The receiving end is outputting 0 volts, and the
network outputs 3 volts to the inverting input. The comparator sees a difference of + 3 volts and
outputs high in recognition of the high level at the sending end.
Table 4-4-1 summarizes the effects of the various signals.

Table 4-4-1 Keyboard Receiver Signals

Send.
End

Rec.
End

NonInvert.
Input
(Junction)

0
0
12
12

0
12
0
12

0
6
6
12

)

Invert.
Input

3
9

Difference

Output

-3
-3
+3
+3

0
0
5
5

4.4.6.2.1 Keyboard Cable Compensation - C7 compensates the circuit for the capacitance of the cable.
If C7 were not present, the output of the comparator would glitch when the output driver on the same
end of the line changed state. This would be caused by the output signal propagating immediately to
the inverting input of the comparator but being delayed (by the RC time constant of the line) to the
noninverting input. C7 is chosen so that the time constant of the line is the same at both inputs.
Because the resistance seen at the middle of tn ... line is approximately 1000 ohms (two 2K resistors in
parallel) and the resistance seen at the inverting input is approximately 5000 ohms (one 10K and two
20K resistors in parallel), C7 is about one-fifth of the cable capacitance. The value is not critical
because the ratio of the two time constants can be as large as two and still provide acceptable noise
immunity.
4.4.6.3 Terminal Data Encoding - The keyboard requires a clock for its operation and is provided
with one by the terminal controller side of the interface. To transmit a clock independently of data on
the same wire, the terminal side of the interface generates a clock signal within which data is encoded
as a pulse width modulation. The terminal circuit produces a 75 percent high pulse width output for
the mark state. Data transmission causes the clock output to switch between 75 and 25 percent pulse
width (duty cycle).
4-37

Figure 4-4-6 is a timing diagram that illustrates the formation of the pulse width modulation. Three
nand gates, I, II, and III, combine three signals, Data, LBA 3, and inverted (not) LBA 4. The three
gate outputs are wire-ANDed so that all three must have high outputs to produce a high to the out
inverter, which drives the terminal's end of the bidirectional interface. The timing is not precisely
correct in this drawing because the LBA signals, addresses to the line buffer in the video processor, are
not pure squarewaves and have variations in their periods (see the DCOlt section). These variations
give the keyboard signal the appearance of clustering in groups of four, but they do not affect the
operation of the circuit.
The negative transition of each output pulse occurs at the clock interval regardless Of the presence of
data. This transition is therefore the reference point for the keyboard clock at the receiving end.
4.4.6.4 Combined Interface Signal - Figure 4-4-7 illustrates the four possible conditions on the interface line when the effect ofthe clock is included. Figure 4-4-8 shows all four states and the transitions
between each of them. Two series sensing resistors in the interface circuit divide the signal in half at the
wire. Therefore, although the drivers swing t2 volts at each end, the figures show only 6 volt variations.
4.4.6.5 Decoding of Data from Terminal- The keyboard recovers the modulated clock signal sent by
the terminal but must also separate the data from the clock. The signal is sent directly to the UART
and address counter circuits as a clock. The negative edge of the clock occurs at a fixed interval while
the positive edge varies according to the duty cycle modulation. The keyboard circuits use the negative
edge and ignore the variable pulse width.
Data is extracted from the combined clock-data by a simple resistor-capacitor filter on one input to a
comparator. The other comparator input is referred to one-half the power supply. Because the duty
cycle of incoming data is either 1/4 or 3/4, the capacitor charges to that proportion of the supply
voltage over the 16 clock periods of each bit. The comparator switches when the capacitor voltage rises
or falls past the reference value. The short duty cycle of zeros averages to a low voltage that holds the
comparator output low. The long duty cycle of ones averages to a high voltage and switches the
comparator to a high state. The comparator output goes to the serial data input of the UART where it
is deserialized. When all ten bilS of a transmitted character are loaded into the UART, it asserts Data
A vailable. This signal enables the bell and scan start if the appropriate bits were set in the byte. The
LED bits remain latched at the parallel output ofthe UART until the next command byte arrives.
4.4.6.6 Keyboard Output - The keyboard UART serial output goes directly to an open collector
driver that swings its end of the bidirectional signal line between 0 and + 12 volts. This is shown as E8
and R 15 in Figure 4-4-5a. The circuit is identical at both ends of the interface.
4.4.7 Bell
The keyclick/bell circuit provides audio feedback and attention signals for the user. A bit in the
keyboard status word controls the bell. Refer to Figure 4-4-9 for the circuit. Capacitor C8 charges to
+ 5 volts through resistor R 16. The speaker connects between the capacitor and the collectors of the
transistors in E I. When a single status word contains the bell bit, flip-flop E3 toggles and turns on E I.
C8 discharges through the speaker and EI, generating a click. When the voltage on C8 falls low
enough, it clears E3. E I turns off and C8 charges up for the next click. The value of C8 is selected to
determine the volume of the click. D8 protects the transistors from inductive spikes from the speaker.
If the bell bit is set for many words in succession, the UART latch holds the data output constant. A
one clock period pulse from the Data Available flip-flop E6 gates the bell bit through E5 to form a
clock to E3. As C8 discharges through the speaker and EI, E3 clears itself. turning.EI off. Then C8
charges up again until the next Data Available pulse clocks E3 on again to repeat the discharge cycle.
C8 discharges fast enough so each Data Available pulse (which arrives every 1.28 ms) triggers a cycle,

4-38

I
'-"---/

, ~""-."..........

'---.../

-, r-

LBA3

L_J

L_Jr--l

LBA 4

L-_--I

DATA

' - -_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _---J

r-------- - - -------------,

III

--~J

OUT

--l.J

.j::.

U
U

U
1
rL __ J-'
n
'-----v---'!CLOCKING
"--v----J
16 OF THESE
16 OF THESE

1"-.-/

OUT

AVERAGE HIGH
.NOTE:

EDGE

U
Lfl
nJ

rL
U--'

AVERAGE LOW

LBA3 & 4 APPEAR DIFFERENT
IN WAYS NOT AFFECTING THIS
CIRCUIT.

OPEN COLLECTOR WIRE-AND:-.J
ALL MUST BE HIGH TO
GET HIGH OUT

1= DATA AND LBA4
I' = DATA AND LBA3
III = LBA3AND LBA4
MA-4369

Figure 4-4-6 Encoding of Terminal Data and Clock

allowing the circuit to produce an 800 hertz tone. Bell is generated by setting the bell bit for 0.25
seconds (about 200 status words). Each cycle of the tone is at a ·reduced amplitude compared with the
single keyclick because R 16 is selected to limit es's charging rate. The overall effect of the tone burst
on the ear is that of a beep.

INTERFACE
STATE

KEYBOARD TERMINAL

---------- 12
6

...).)

111111 i

--------- 12

II
I

I
I

IIIII ! 0
12
11111 ! 6

---------- 0
12

! IIIII i 6

---------- 0
MA-4667

))

Figure 4-4-7 Four Keyboard Interface States

-, r--II

II
II

II
II
II
II

---,r..

......
....
II
II

II
II
II

TERM XMITS 1

TERM XMITSO

KEYBOARD XMITS 0

TERM XMITS 1

KEYBOARD XMITS 1
MA-4666

Figure 4-4-8 Keyboard Interface Signal

4-40

))

)
+5

R16

FLIP
FLOP
E3

BELL
BIT

CLEAR

MA-4658

Figure 4-4-9 Bell Circuit
4.4.8 Keyboaril Interrupt Routine
When the microprocessor responds to an interrupt from the keyboard UART and reads a key address
from the UART data output, it immediately range tests the address. The function key addresses are all
above the regular key addresses, with SET-UP the lowest at 7BH and the always down Last Key at
7FH. An incoming function key address causes a bit to be set in a flag byte called Keys (Figure 4-4-10).
When a key address below the function range arrives, the microprocessor checks the low three bits in
Keys for the key count. If the count is less than three, the key address is stored in a three place New
Key Address Buffer and the counter is incremented. If the count is already three, the counter is in'cremented but the key address is discarded.

KEY: COU7TER

)

CAPS
LOCK
LAST
KEY

CONTROL
SHIFT

OVER
FLOW
BIT

SETUP
MA-4291

Figure 4-4-10 Keys Flag Byte
4.4.9 Logical Keyboard Processor
The logical keyboard processor is that portion of the operating firmware that manages the interpretation and transmission of keyboard data. It consists of several processes.

4-41

4.4.9.1 Key Recognition - For a key to be recognized as a new key, it must not have been down in the
scan before entry is accepted. The microprocessor checks each key's history at each scan. If a key was
down during the last scan, it is old and is not entered. Only new keys, those not previously seen, are
entered. This system allows a key to be held down without being continuously entered at each scan.
(This process and auto repeat are discussed later.)

'\
))

If a key is detected as down for the first time in a while, the microprocessor assumes that it has been
pressed. During the scan when the key is first down, the contacts may bounce for several milliseconds.
The time window when the contact is scanned is very short. If the contact happens to be bouncing open
during that interval, it is not detected in that scan but the time between scans is long so it is finished
bouncing by the next scan and is detected normally. If the contact happens to be closed for the first
scan while still bouncing, it is detected. It is also detected on the next scan after it has stopping
bouncing but now it is an old key and is not reentered.
When the microprocessor is free to perform its background routines, it processes the results of the last
keyboard scan. First it checks the high bit of the Key's byte to see if the scan is finished. If it is not, the
microprocessor continues with other work (exits). If it is , the microprocessor checks the key counter
overflow bit representing four or more keys sent in the scan. If the bit is set, the microprocessor clears
the New Key Address Buffer and exits.
If the overflow bit is not set, the first key is taken from the new key address buffer and tested. If it is
either the SET-UP or NO SCROLL key, the microprocessor acts on it immediately and branches to
the appropriate routine. It then returns to the keyboard process. (Refer to the microprocessor section
for a discussion of SET-UP and to the Communication section for no scroll.)
Another three place buffer contains the keys from the last scan. The microprocessor compares the
three new keys with those in the old key buffer. If an old key is not in the new key address buffer, that
key is cleared from the old key buffer. New keys then go into the old key buffer. Each key address is
only seven bits long, so the high bit in each key entry is used to indicate how long that entry has been in
the buffer. If a key is new, the bit is set to O. This means that the microprocessor has only seen the key
once. If the key appears in the new key address buffer on the next scan, the microprocessor sets the
high bit to I, converts the key address to an ASCII code, and sends it to the keycode buffer. Only one
key is converted and sent in each cycle of the logical keyboard processor routine. Since the set bit
indicates that the key has been sent, in the next cycle that key will be ignored and the next key will be
processed.
4.4.9.2 Key Rollover - Key rollover means that more than one key can be down at the same time and
be accepted. Normally the VT100 accepts 3-key rollover. If the keys were pressed during different
keyboard scans, they will be accepted in the order in which they were pressed. If they were all pressed
during the same keyboard scan, they will be accepted in the order of their addresses.
Certain conditions will limit the rollover to only two keys. The 2.5 key rollover specification reflects
the presence of these conditions. Because there are no isolating diodes in the switch matrix, certain
patterns of contact closure can produce false key down indications. Specifically, three switches closed
in three corners of a rectangular pattern, as in Figure 4-4-11, will cause a fourth apparent key address
to be sent to the terminal. (Refer back to Figure 4-4-3 to see how the pattern fits into the row-column
configuration.) For this reason, although any number of key addresses may be sent by the keyboard
during a scan, no more than three may be sent if any are to be accepted. If four or more appear, the
terminal ignores all of them and waits for a scan with only three. The special function keys (SHIFT,
CAPS LOCK, CONTROL, SET~UP) are in a part of the matrix where there is no pattern sensitivity
and no ambiguity. They all are accepted in the same scan with the regular keys and are not counted in
the 3-key maximum.

4-42

) )

})

/V"""

---------,

I
I
I
I
I
I
I

/ H

VG
/'

SCANNING ORDER
COLUMN 5-ROW 7
C5-RB
C4-R7 (C4 SEES C5-R7
THROUGH G, H)
C4-R8

OUTPUT

J
H
K (FALSE 4TH KEY)

G
MA-4668

Figure 4-4-11 Sneak Path

)

The sneak path problem is also the reason that the keyboard processor looks for the appearance of a
key in two successive scans. In Figure 4-4-11, the keys are scanned in the order J, H, K, G. Normally.
the keyboard processor counts the number of keys in a scan and ignores all of them if the count is over
three, as it is in this case. But it is possible for a user to briefly press keys J, H, and G and then release
G before the scan is completed. Then the keyboard processor might see J, H, the false key K, and.
because G was lifted just before it was scanned, register only three keys. One key would be false, but
with a count of only three, all would be accepted. To prevent this, the keyboard processor looks for the
same keys two scans in a row. Because the G key is released, the K key does not appear in the second
scan, and only J and H appear and are accepted by the keyboard processor.
4.4.9.3 Generation of Codes - Because the keys are essentially randomly ordered with respect to the
ASCII standard, the program ROM includes several look-up tables that assist in the generation of
ASCII codes. Most keys convert directly from an address to an ASCII code. The codes for alphabetic
keys with SHIFT or CAPS LOCK down are formed from the tables with bit 5 forced to 0 to produce
uppercase alphabetic codes. Holding down the CONTROL key when another key is pressed causes
another table look-up. If the key is in the table of valid control codes, that keycode is changed by
forcing bits 5 and 6 to O. For example, the ASCII code for the letter "g" is 67H. Holding down the
CONTROL key and typing G transmits the ASCII code 07H.

Normal code
Control code
Shift code

x
x
x

o
1

o
o
o

o
o
4-43

o
o
o

Nonalphabetic keys have a second look-up table that provides the shifted code given the unshifted'),
code. CAPS LOCK performs the shift function only on alphabetic codes, leaving the nonalphabetics
J
to be performed with an extra shifting action. If a key that does not make a standard control code is
pressed with CONTROL, it is not sent.
.
4.4.9.4 Keyboard Transmit Buffer - Some keys generate three byte ASCII codes. The auxiliary keypad, for example, when in alternate mode, transmits escape sequences that a system can be programmed to interpret in a special way. Cursor control keys also send escape sequences which the operator
can use to control the cursor position. To store the codes that might be generated by three simultaneous keys, a 9-byte buffer is provided for the communication port. If the buffer fills up, the keyboard
is locked and the appropriate LED lights. When a key is about to be converted to ASCII and stored in
the buffer, the microprocessor checks the buffer to see if there is room. Since any key might produce
three bytes of code, the microprocessor must be sure that there are at least three places available in the
buffer. If six I-byte keys have been sent, the next I-byte key leaves only two places. Therefore, the
.
keyboard locks with only seven places filled in the buffer.
4.4.9.5 Auto Repeat - If only one key is in the old key buffer (in the logical processor), and if it
appears continuously, it may be a candidate for auto-repeating. A key with control cannot repeat, and
a nonrepeat table contains a few keys that do not repeat. These are SET-UP, NO SCROLL, ESCAPE.
RETURN, BREAK, and ENTER.
A count-down timer gets loaded with a value each time the buffer changes. When there is no change
for a while, the timer decrements to zero. This takes about one-half a second. Then the kev is sent to
the keyboard buffer a second time and the timer is loaded with a smaller v·alue. This tim~ the count
lasts about one thirtieth of a second. The key is sent and the timer reset the same way until the key is
lifted or another key is pressed while the first is still down. If the second key is lifted, the count
continues at the fast rate. The timer is decremented every time a status byte is sent to the keyboard
(every 1.27 ms).

'---j,

)

4.5 NONVOLATILE RAM
The nonvolatile RAM (NVR) used in the VT100 can retain its data for about 10 years and 1 billion
reads; This type of device is also called electrically alterable read-only memory (EAROM). It contains
the programmable configuration information that would otherwise have to be reentered every time power was turned on or else stored in mechanical switches.
While the microprocessor sets up initial conditions using the specifications in the NVR, a user can
change those settings at any time from the keyboard. However, the new settings are only stored in the
scratchpad memory until a special Save sequence is initiated from the keyboard to store the settings in
the NVR.
This section contains an overview of NVR principles, the circuitry used with this particular device, and
the microprocessor's procedure for using the NVR. Information about the SET-UP data contents may
be found in Section 4.7.11.
4.5.1 Principles
An NVR memory cell consists of an MNOS (metal nitride oxide semiconductor) field effect transistor
whose gate is insulated with a material (silicon nitride) that can accept charge movement through itself
and yet hold a charge as a superior insulator. Under high voltage conditions, electrons can migrate between the insulating layer and the substrate, leaving a net charge buried in the insulator. This is the
process of erasing and writing.
An FET transistor normally has a physically determined voltage required betwc;c;u its gate and channel
for it to conduct. A charge in the insulator between the gate and channel adds algebraically with the
voltage on the gate and shifts the value of threshold voltage accordingly.

4-44

}

A reference voltage is applied to the gate of a memory transistor in the read process. If the threshold is
below the reference, the transistor will conduct. If the threshold is above the reference, there is no
conduction. Decay of the stored charge causes the difference between high and low thresholds to
become smaller. At normal operating temperatures this decay takes 10 years to affect recovery of data.
Erasure consists of writing all the memory cells in a word to the same low threshold. Writing pushes
the selected cells to a high threshold. The time required to write or erase is determined by the thickness
of the gate insulation, the voltage used, and the temperature of the device. The thickness of insulation
is a trade-off between writing time and data longevity.
The writing process damages the insulating properties of the nitride layer. With the ER 1400's layer
characteristics, its data retention time is at least 10 years provided that it is written less than 1000 times.
The data retention time is at least 1 year if it is written between 1000 and 10,000 times.
4.5.2 NVR Device
The ERI400 (E24) is a 1400-bit memory arranged as 100 14-bit words. Data and addresses enter or
leave the device in serial form through a single bidirectional line. Information is shifted with an external clock; identification of the meaning of the bit stream is made by setting three control lines that
specify the operation in progress (Figure 4-5-1).

DATA REGISTER

14 BITS

.......

10 BITS

TENS
ADDRESS

MEMORY
ARRAY

.......

READ
100 x 14
WRITE
ERASE

I/O
BUFFER

DATA
I/O

t
MODE

DECODE
LOGIC

ADDRESS

DECODE
CLOCK
GEN

CL OCK

UNITS ADDRESS

10 BITS
MA·4308

Figure 4-5-1 ERl400 NVR Block Diagram

4-45

4.5.3 NVR Control
Seven states of the three control lines give commands to the NVR device:

))

Data Bus
State

Standby. The device output floats.

Accept Address. Two 1 of 10 addresses are shifted in at the clock rate.
The first group is the 1O's decade and the second is the 1's decade of the
1 of 100 word address.

Erase. The word stored at the addressed location is erased to all zeros.

Accept Data. The data register accepts serial data shifted from the I/O
pin. The address register remains unchanged.

Write. The word contained in the data register is written into the
location designated by the address register.

Read. The addressed word is read from memory into the data register.

Shift Data Out. The output driver is enabled and the contents of the data
register' are shifted out.

Not Used.

-j)

))

4.5.4 NVR Support Circuits
Several factors in the ERI400's design require special treatment by its support circuitry. It requires 35
volts for the write and erase processes. Its signal levels are specified to be Vss (the positive power
supply) for the high level and 12 volts lower than that for low. The logic of these signals is negative: I is
asserted as a low level, and 0 is asserted high. When driving data, the NVR·s output actually switches
between + 12 and -23 volts with a high impedance in series with the negative sour~e. This requires that
an external clamping diode (D4) be present to pull the low level up to ground through the high
impedance. 12 volts and ground are thus the I/O logic levels. The terminal controller is made with
TTL, so open collector buffers with pull-up resistors to + 12 volts are the interface.
The Standby command of the ERI400 is not used in the VT100. Instead, Accept Data serves as the
command to the NVR during idle periods. The code for Accept Data is all inputs low. This arrangement protects the contents of the NVR from spontaneous writes that might occur due to the powerdown behavior of E29, the 7417 open-collector buffers. E29, in powering down, tends to drop its
outputs from the high to the low state in an unpredictable order. This could present a write command
to the NVR before the power supply voltages drop low enough, if the NVR had been maintained in the
Standby mode by keeping its inputs high. By keeping all outputs low with the Accept Data command,
no new command can occur accidently during power-down.

4-46

))
.

)

The pull-up resistor used to drive the data input is too Iowan impedance for the ERI400 output to
drive. Line C2 is high during state 5 when the output driver is enabled, and low when data is being
input to the NVR, so transistor Q1's base is connected there. When the line goes high, Q1 turns off,
raising the impedance at that point to the leakage values of Q 1 and E29, and the input impedance of
comparator E48. D4 clamps the output to -0.6 volts to protect the comparator input from leakage to
-23 volts in the ER 1400.
Later VT100s may have power down circuitry in series with the Vgg pin on the ERI400 to ensure that
the -23 volt supply is cut off when the + 5 volt supply is low. This would prevent writing during powerdown.
The comparator is a high impedance load for the ERI400 output driver and is biased to switch at +8
volts. This threshold ensures that a wide range of output levels will still pull the low state down enough
to switch the comparator. The comparator output, pulled to +5 volts by a resistor, provides the NVR
DATA bit to the flag buffer. The microprocessor reads the flag buffer after it sees the NVR clock flag
change, and samples the NVR DATA bit to get the serial data.

---j

)

The NVR clock is the lowest frequency line buffer address signal and occurs at the horizontal line rate,
15734 kHz. This is the bit shifting rate during address and data transfers. A read operation requires
only a single clock cycle to transfer data into the data register but an erase or write of a word each
takes 20 milliseconds.
4.5.5. Microprocessor Management
During an NVR operation, the microprocessor uses 21 bytes of scratch pad memory to set up a group of
NVR address or data bits. Each group consists of 20 bits of address or 8 bits of data with another 6 bits
of fill for the 14 bit NVR word. These groups must enter the NVR serially with precise timing. The
microprocessor cannot calculate the groups fast enough to keep up with the shifting process while also
managing the shifting process, so it saves time by precalculating and storing each group of bits. Then it
reads the stored groups, and delivers them to the NVR at the NVR clock rate through the NVR latch.

The microprocessor serializes its data by rotation in a register. It combines the data with command bits
to control the NVR device. Output from the microprocessor reaches the NVR through the NVR latch.
The latch is updated at each clock during an NVR operation. One unrelated signal from the communication port is routed through an extra bit in the NVR latch device.

)

The microprocessor reads the NVR during the power-up process. The bits are deserialized by rotation
as they are read in. Although 14 bits from each NVR word are written or read, only 8 bits are used by
the microprocessor. The data goes into location in the scratch portion of RAM for use during a later
portion of the power-up.
4.5.6 NVR Timing
Use of the ER 1400 in the VT100 takes place during normal operation of the video circuits. The 8080
must be free to operate on every scan line in order to load data and address and shift data out (the
ER 1400 is clocked at the horizontal rate and this signal is provided as a clock to the flag word for the
8080). Because of the above two conditions, it is necessary to ensure that all DMAs performed by the
video processor are short.

The processor works fast enough to permit the display of the short message "WAIT" at the left edge of
the screen. This causes the DMA to be longer but informs the operator during long save and recall
·sequences. The timing for all of these conditions is shown in Figure 4-5-2.

)
4-47

-Vss

CLOCK SIGNAL ON --,
PIN 5 OF NVR

CLOCK FLAG

HOLD REQUEST

BO COLUMN MODE
HOLD REQUEST

132 COLUMN MODE

' - , - - - - - - -....

--.J

~----TTL H (1)

--lI Ir-2~5
J

-TTL L (0)
us

-;!-1.5 "'

1
-,

' - - - - - - V s s - 12

I-- 10 us + 623 ns FOR EACH CHARACTER DISPLAYED

I
1--11.3 us + 373 ns FOR EACH CHARACTER DISPLAYED
'MA-4299

Figure 4-5-2 NVR Signals

-))

All data and command changes to the ER 1400 occur immediately following the falling edge of the
clock flag (the rising edge of the signal on pin 5 of the ERI400); the data is strobed into the NVR on
the opposite edge. The two one-of-ten codes for address have a single zero and nine ones (at DO of the
8080 bus).
When the "shift data out" mode is entered as per the timing diagrams in Figure 4-5-3, the first data bit
may appear when the mode is entered but will become stable 25 p's later; subsequent data bits will be
shifted out on the falling edge of the clock flag (rising edge of the signal on pin 5 of the chip) and will
become valid 25 p.s later. This means that, following a change to "shift data out" mode or a falling
edge of clock flag during this mode, a delay of 25 p.s must occur before reading of the NVR data flag is
attempted. The first bit shifted in when writing is the first bit shifted out when reading.

II
J

When using the ER 1400, it is necessary to operate with interrupts off.

4.6

VIDEO PROCESSOR

The video processor is the heart of the VT 100 display. It is composed of the devices shown on the right
side of the terminal controller block diagram (Figure 4-6-1). This section discusses the general mechanism that converts data into a visible display, followed by a discussion of the two central devices (the
timing and control chips) followed by a general discussion of all the other blocks on the video processor side of the block diagram. The interactions between the data paths on the left of the diagram and
the processor on the right 'are covered in Section 4.7, Microprocessor - Video Processor Interface.

4.6.1

Introduction

The video processor converts data into an electrical signal that a CRT monitor can turn into visible
letters, numbers, and symbols. The video processor works with the characteristics of the CRT monitor
to do this. A brief description of these characteristics precedes an introduction to the video processor's
operation.

4-48

))

-.........

ClK

LJlSLf1JLIl.~:

,---------/

CLKrI n
I L....{

C31

I ~

n
L....J

Lf,..

n
L....J

n
L...J

rH
L...J
l

I
I

-..,

I
I

• OK TO CHANGE MODE

1+-1 CLOCK CYCLE REQUIRED

20 CLOCK PERIODS

n
L....J

Cl~/

L.....{

READ

ACCEPT ADDRESS"

ClK

LJUlSLflSl.,~~

ClK~JlJlSLJL.J:

C11

~
I

C3 - - ,

;:;

DATA
(OUT)

",.

I H

H
l

ERASE

14 CLOCK PERIODS

iI'

=---=---=~""~'~~ ~

-I

SHIFT DATA OUT"

ClK

lSULJLI1Il.,JLIl.JLJlJ~ HL

Cll
C21

//
//

C3 - - - ,

DATA

I•

I: 25~

S MAX.

~JlJlSLJL.J~

Cl - - ,

C2!

",.

H
l

H
L

H
l

71 ~

WRITE

25~SMAX.==:j

14 CLOCK PERIODS-----.l
ACCEPT DATA"

ClK

"OUTPUT DATA CHANGES ON THE POSITIVE-GOING
CLOCK EDGE. DATA AND ADDRESS INPUTS ARE
SHIFTED ON THE NEGATIVE-GOING CLOCK EDGE.
MA·4929

Figure 4-5-3 NVR Timing Diagrams

PC BOARO FINGERS

120 MA lOOP J5

EIA

IGRAPHICS OPTION In

ANALOG VIDEO b8

H& V
DRIVE

I
MODEM
CONTROL

E37, E39, E44
SERIAL
DATA

~I/O
~ E42

GRAPHIC VIDEO

2 ? MODEM
. CNTRl

COMMUNICATION
TRANSCEIVER

E43

1!1

KFYOOARDINTERFACE

I-

E60

SERIAL

OPTION
PRESENT

BAJD
I!O
8
ClK

8;10

GRAPHIC
FL.AG
1

t..DATA OUTPUT BUS

-'lATCH &l
BUFFER
8

-1~15,E20

NonVOLATilE RAM
E24, E29, E4B, E61

1/0

E40, E45, E52 E56
1/0

r---"'----.

MICRO·
PROCESSOR
E32, E35, E36

t, , t----,

f-+8AUD
ClK

SCREEN &
SCRATCH RAM
3K X 8
E46, E47, ESO,
E51, E53, E54

PROGRAM
ROM
8K X B

INT. REO.

I-

8 1

CHAR
ADDR

1/0 DECODE
E19, E27, E34
2

,LINE BUFFER
256 X B RAM
Ell, E17

+ ,I

ADVANCED
VIDEO OPTION
CONNECTOR
Jl

MEM. DISAB lE

L 1

VIDEO

SIR

E4, E9
ALT.
1
CHAR.

+
VIDEO
CONTROL
AND
TIMING

ATTRIBUTES .~
TERMINATE

E3, E5, E6, El,
E13, E22

KBD __

ClK
5
NV~

1
COM P
SYN

CONTROL

CHARACTER DATA 6
GENERATOR SCAN ADDR.
ROMS
2K X 8
4

VIDEO CONTROL SIGNALS

DMA ADDRESS COUNTERS
E21, E22, E25, E26, E30, E33

{16

ClK

I
1

r
VIDEO ClOCKI
24.0734 MHz
E2

CONTROL BUS

4 ADDRESS BUS

--t-t"
8

MEMORY DECODE I.
El, E23, E31
I

CHAR.
DATA

lATCH

E16

elK

E18, E19
E23, E41

t-- NVR

8
1

INTERRUPTS

NVR
DATA

flO, E13

E 14, E49

FLAGS

VIDEO OUT 1 + 2

I~HIFT REGISTER

INTEN
1/0_ SITY
CONTROL
5
m/A)

elK

DR. F48, E55

INT.AC~

COMPo VIDEO
'iENERATOR

H & V BLANK

.~HD

BIDIRECTIONAL DATA BUS

INTER
RUPT
++- lOG'IC

IBAUD RAT\I
GENERATOR

BUFFER I'DONEWAY

-t18UFFER
8
E58
I

DIRECT DRIVE
GENERATOR

~EIA DRIVERS& RCVRSI

~ E41, E61 f----

FLAG
BUFFER

(DIRECT DRIVE VIDEO OUTI J6 COMPOSITE VIDEoL!I

I:i:r-

r-------,

STD. TERM. POAT

KEYBOARD JI

HOLD REQUEST
•

MICROPROCESSOR

VIDEO PROCESSOR

~
MA-43S2

Figure 4-6-\ Video Processor Block Diagram

.~~

'--.-/

)

4.6.1.1 The Raster - The raster is that area on the CRT screen that is scanned (passed over) by an
electron beam moving in a regular pattern. Deflection coils cause the beam to quickly scan a series of
horizontal lines while moving relatively slowly down the screen. This scanning repeats quickly and
constantly, and persistence of vision makes the entire screen look continuously scanned. The beam
may be on or off, lighting the fluorescent phosphor on the face of the CRT or leaving it dark, but the
beam's continuous motion traces and defines the raster.
4.6.1.2 Character Formation - As the beam moves in its horizontal scans, it can be turned on and off·
very fast. This means that a spot of light (called a dot) can be produced anywhere along each horizontal scan line. Each scan contains the same number of potential dots. If only one dot is lit in a scan but
dots in the same position on several successive scans are also lit, the screen appears to have a vertical
line on it. This ability to line up dots on different scans is the key operating feature of the video
processor.
Limiting our discussion to 80 column lines for this description, each character that can be displayed on
the VT 100 is made up of a matrix of dots, ten wide and ten high (Figure 4-6-2). There are 800 dots in a
scan and the raster is made of 240 scans. So there are 80 groups of ten dots in each scan horizontally,
and 24 groups of 10 scans vertically. Each 10 dot X 10 scan group is a character cell, where a character
can be displayed, and there are 80 X 24 character cells on the screen. As the electron beam scans the
raster, the video processor turns the beam on and off, assembling the characters scan by scan.

INTERCHARACTER
SPACE
CHARACTER ~
CHARACTER
,...-___
1.,___-.,,,____----1.,-------.,

..•..
•••••

)

......
••••• .. •••••••
..•••••••••••
•••••••
........
••
••
• ••••••••
•

10 SCANS

)

DOTS

·1· •••••• ·1·
•••••••
.

. . . . .

••
••
. ••

. ~ .. :~~.

...•••

· ..•..
. . . . • ••
....•..

.....
••••••

•
••
•• ••
••••••
•
•

I II

I I

DOT INTERVALS

•

CRT BEAM MOVES
THIS WAY

CHARACTERS ARE SHOWN
BEFORE DOT STRETCHING

SCAN 1
SCAN 2
SCAN 3
SCAN 4
SCAN 5
SCAN 6
SCAN 7
SCAN 8
SCAN 9
SCAN 10

CHARACTER INTERVALS
MA-4655

)

Figure 4-6-2 Dots, Scans, and Characters

4-51

4.6.1.3 Video Processor Data - When we say the characters are assembled scan by scan, we mean
that only one scan of information about a character is displayed and then the same scan for the next
character is displayed. This continues to the end of a line of characters. Then the next scan of the first
character is displayed, followed by the same scan of the next character, and so on. Therefore. each
character must enter the video processor ten times, once for each scan, and remain only until the next
character in the line is displayed.
Each character in a line is stored in one of a group of adjoining locations in the screen RA M. (See
Figure 4-6-3 for the video processor functional diagram.) The process of moving a line of character
data from the screen RAM to the video processor takes the same amount of time as a scan. During
data movement, the microprocessor cannot use the RAM. So, to give the microprocessor as much
working time with the RAM as possible, the video processor accepts the line of character data for
display during the first scan and stores it in its line buffer at the same time for use during the other nine
scans. Movement of data from the screen RAM to the line buffer by the video processor is called direct
memory access (DMA). During the DMA, the video processor provides addresses to the screen RAM
with the address counters. The data is stored in the line buffer which gets its addresses from the line
buffer address (LBA) outputs from the DCOll timing chip. Both kinds of addresses change values
when they receive a clock (called character clock) that occurs after each 10 dots. This clock makes the
character data in the video processor change at the right times to provide proper alignment of the
display.
The character latches hold the data coming from the screen RAM or line buffer and ensure that the
data remains stable for long enough periods to be written into the line buffer or the video shift register.
The buffer between the latches is tristatable and, during non-DMA time, prevents the normal data
flow on the microprocessor data bus from interfering with the video processor's reading of data from
the line buffer.
4.6.1.4 Video Processor Character Generation - Data, coming either from the screen RAM for scan 1
or the line buffer for scans 2 through 10, becomes part of an address to a character generator RO M.
(See Figure 4-6-4, Character Generator Example.) The rest of the address comes from a scan counter
in the DCOl2 control chip. The scan counter addresses the ROM according to which of the ten scans is
to be displayed. The 4-bit scan counter skips over the other 6 possible addresses to the ROM. so the
ROM contains data in only 10 out of 16 locations. The output of the ROM is eight bits that represent
the pattern of sequential dots to be displayed for that character on that scan. The eight bits enter the
video shift register, a serializer that converts the eight parallel bits into a one-bit-wide stream. An extra
flip-flop stores the last bit so it can be output to the stream two or three extra times (depending on line
length) to fill the intercharacter space. The stream enters the DC012 control chip, where the final
adjustments for video display are made, and then the video signal goes to the CRT monitor.
4.6.1.5 Attributes - Three attributes apply to the VT100 display: character, line. and screen. Character attributes provide a special appearance to characters as they appear on the CRT screen. In the basic
VT100 (without an advanced video option) only one bit of memory (called the base-attribute bit) is
available to each character on the screen. The original character data from the screen RAM is eight
bits wide but only seven bits define the character itself. The eighth bit defines the base attribute. The
attribute bit bypasses the character generator ROM and video shift register and enters the DCO 12.
There it controls the presence or absence of the attribute as that character is displayed. The base
attribute is displayed as either reverse video or underline, depending on the selection of the cursor at
SET-UP, and is invoked by the base attribute bit. Reverse video appears as all 10 scans of a character
cell reversed (black changes to white and vice versa). Thus, if two vertically adjacent characters are in
reverse video, no black space appears between them. Underline forces the ninth scan on (off with
reverse video screen attribute.) Once a character attribute or combination of attributes is set, all displayable characters sent to the terminal have that attribute regardless of where they are placed on the
screen. This continues until the attribute selection is changed.

4-52

)

1
-)
.

""-'"'

LINE BUFFER ADDRESSES
FROM DCOll

",J

'---.-/

-.~--

ATTRIBUTES
LINE BUFFER
AVO

AVO
AVO

EXTENDS WIDTH TO 12 BITS

AVO ATTRIBUTES

BASE ATTRIBUTE

SCREEN RAM
AVO
3K
EXTENDS
RAM
(INCLUDES
T04K
SCRATCH RAM)

~
I

VI
W

ALT CHAR SET

1ST
CHAR
LATCH
AND
BUFFER

2ND
CHAR
LATCH-

CHARACTER
GENERATOR
ROM

SCAN
COUNTER
IN DC012

LINE
BUFFER

CHARACTER
CLOCK

LINE
..I BUFFER

VIDEO
OUT

DOT CLOCK

8
12

ADDRESS
COUNTERS

SERIALIZER
'VIDEO
I
_I ATTRIBUTES
SHIFT
r - - l I N DC012
REGISTER

ADDRESSES~ ....

HORIZONTAL RATE CLOCK

TO AVO
LINE BUFFER

IN DCOll
MA·.a&67

Figure 4-6-3 Video Process~)f Functional Diagram

FLIPFLOP STORES DO AND
, . - - - - - - . SHIFTS IT INTO VIDEO SHIFT REGISTER
1 BIT
OUT

DO
FLIP
FLOP
L

CHARACTER
GENERATOR
ROM

ASCII "B"
7 BITS

--...I

7 BITS OUT

OUTLINED DOTS
IN FIGURE 4-6-2

SHIFT
OUT

------) )

L
+ - - LAST DOT OF PREVIOUS
CHARACTER DEFINES
FIRST DOT OF NEW
TO DC012
CHARACTER.

SCAN COUNT
"SCAN 2 I I
4 BITS

MA-4569

Figure 4-6-4 Character Generator Example
Line attributes are double height, double width, and scroll. The VT 100 displays single-width, doublewidth or double-height, double-width characters on a line by line basis. All characters on one line
appear in the same mode. Double-width lines are generated by displaying each dot of a character twice
in the horizontal direction. Double-height, double-width lines are generated by displaying each dot of
a character four times (twice horizontally and twice vertically). The top and bottom halves of a doubleheight, double-width line must be entered as two separate lines of characters. The scroll attribute
indicates that a line is part of the scrolling region.
Screen attributes affect the entire screen's characteristics at once. They include the base attribute
selection (reverse or underline as mentioned under character attributes), reverse video over the entire
screen, 80 or 132 character line length, 50 or 60 Hz refresh rate (chosen according to the local power
supply), interlaced or noninterlaced operation, and jump or smooth scrolling of data over the screen.
4.6.1.6 Advanced Video Option (AVO) - The AVO extends the length of the screen RAM so more
characters can be displayed in 132 column mode. In addition, it allows each character to have four
more attributes, for a total of five. Three of the AVO attribute bi"ts enter the DCOl2 to control displayable features of each character. The fourth AVO attribute bit controls the selection of an extra
character set by switching in an optional alternate character generator ROM that can provide nonASCII characters or other special displays.

4-54

.))

)

To provide the extra attribute bits, the AVO widens the entire screen RAM by 4 bits to make each
character location 12 bits wide. It also contains a 4..bit wide extension of the character latches and a 4bit wide attribute line buffer, addressed by the sarne LBA signals as the re'gular line buffer. This extension treats the attribute data with the same timing as the character part of the circuit, and matches
each character with its attributes.
With the A YO present, reverse video, underline, bold, and blinking are all available singly or in combination. Reverse and underline appear as described above with the addition that if both rev.erse and
underline are asserted, the underscore is forced dark instead of light. Bold increases the intensity of the
display. The blink rate is about half that of the cursor or about 0.5 Hz. Cursor selection is independent
of character attributes when the AVO is installed.
4.6.2 Timing Chip Description
The DCOII is a custom designed bipolar integrated circuit that provides most of the timing signals
required by the video processor. Internal counters divide the output of a 24.0734 MHz oscillator (located elsewhere on the terminal controller module) into the lower frequencies that define dot. character. scan, and frame timing. The counters are programmable through various input pins to control the
number of characters per line, the frequency at which the screen is refreshed, and whether the display
is interlaced or noninterlaced. These parameters can be controlled through SET-UP mode or by the
host. In the following discussion, refer to the block diagram in Figure 4-6-5.

)

4.6.2.1 Input Decoder - The input decoder responds to commands on the 00 Hand 01 H pins (connected to 04 and 05 of the 8080 bus respectively) whenever the VIOEO WR I L pin is low. The outputs of the decoder select 80/132 column, 60/50 hertz refresh, and interlaced/noninterlaced modes of
operation. Table 4-6-1 shows that when 01 H is low the number of columns is programmed according to
the state of 00 H, and when 01 H is high the refresh rate is programmed. Interlaced mode is always
selected when the column mode is set, and non interlaced mode is selected when the refresh rate is set.
The interlace mode in use depends on whether "number of columns" or "refresh rate" was selected last.

Table 4-6-1 Video Mode Selection
(Write Address = C2H)

)

Inputs
D5
D4
Pin 21
Pin 20

o
o

o
1

Configuration
80 column mode
132 column mode

Sets interlaced mode

60 hertz mode
50 hertz mode

Sets noninterlaced mode

In addition to strobing data into the input decoder, VIO WR 1 L also acts as a reset signal for the
OCOII. Whenever VIO WR 1 L is low, the counters in the OCOlI are held in a cleared state. Resetting
the counters serves no purpose in the VT100 because the remainder of the VT100 synchronizes itself to
the DCO 11. but a reset is useful for testing both individual chips and complete modules. Because writing into the DCOII would cause the counters to reset and disturb the display, this is never done unless
the mode is being changed.

4-55

~_~,L-~

DOUBLE WIDTH
MUX
__________________
______________
__
A~

DOT rOCK

VSR LD

ADDR CNT

I
L-

HOLD
REOUEST
H

OW
H

WRITE

CHAR

)
D,1H

I-- t----

J-

I--

.--11
MASTE
CLOCK
24.073
MHz

~2 (132:,!
: 1.5 IBO)

BO
MUX

ADDR
ON H

1/2 DOT
i-CLOCK (BO)

-LE

';'10IBO)
-791132)
DOT COUNTER

HORIZ
BLANK
H

BOH

BaH

1
H
DIVIDER

RESET

4> l~lr

HORIZ
TIMING

DOT
DIVIDER

RESET
H

OUTPUTS

CaMP.
SYNC
GEN

BOH
INTERLACE

60 Hz
CaMP
SYNC L

VERT

f-+ DRIVE H
VERT
TIMING

VERT

f-+ BLANK L

f--+ VERT

10 VERT DIVIDER
10

INPUT
DECODER

.----

L-:Jr-,.,

HORIZ
DRIVE
L

f--

~~UI~~

lOT CLK 1132)~

VID WR
1L

HOLD
REO H

DWH
L...--

DO:

RESET H

I}{~
~17

';'3IBO)

Y-7511321

9 INTO B STATE MAPPING

VF - VERT
RESET L

HF
2 HF

VERTICAL COUNTER
-7 524/60 Hz NON-INTERLACED
-7 525/60 Hz INTERLACED
-7 630/50 Hz NON-INTERLACED
-7 629/50 Hz INTERLACED

LBAO THRU LBA7
MA-4269

Figure 4-6-5 DeOl1 Block Diagram

'---/

~-,_/

)

4.6.2.2 80/132 Column Selection - The column mode is changed by modifying the divisors of three of
the counters in the DCOll. The first of these counters divides the input clock (MASTER CLK) by 1.5
to produce the dot rate clock for 80 column mode. The DOT CLK output provides the signal that
controls the shifting of dots out from the video shift register. A multiplexer determines what rate DOT
CLK will have for the entire screen by selecting either the output of the divide-by-l.5 in 80 column
mode, or by selecting the 24 MHz MASTER CLK directly in 132 column mode. The other two
counters affected by 80/132 selection are the dot counter and the horizontal counter.
4.6.2.3 Dot Counter - The dot counter uses four flip-flops to divide the DOT CLK that was selected
by the multiplexer by 10, in 80 column mode, or by 9, in 132 column mode. The output of the dot
counter is the character rate clock, which is used to move character codes in the latches that are outside
the DCOIl. Character clock is further divided by the horizontal counter. The timing of CHAR CLK is
shown in Figures 4-6-6 and 4-6-7 for each of the two column modes. CHAR CLK is unaffected by
double-width mode. The output of the next-to-Iast flip-flop is used for the write enable signal for the
line buffer RAMs (WRITE LB L). WRITE LB l is also shown in Figures 4-6-6 and 4-6-7. This signal
allows the data and address changes, caused by the rising edge of CHAR CLK, to become stable
before writing ·is enabled and then disables writing before CHAR ClK rises again. WRITE LB L is
gated directly with HOLD REQ H so that it is active only during DMAs. Intermediate signals from
the four flip-flops are used by various other functions in the DCOll such as the double-width multiplexer and the composite sync generator.
80 COL - SINGLE WIDTH

--I

1--62.3094 ns

DOT CLOCK

)

CHAR CLOCK , ' - -_ _---'
VSR LD

__...............~,r - I'L..........~......................................................................~,r - I'L-..........__

WRTLBL~

80 COL - DOUBLE WIDTH

)

--l
DOT CLOCK

CHAR CLOCK
VSR LD

~
,---,
__.....~,
'~.....................................................................................~I
'L....................___

WRTLB L ~
MA,4297

Figure 4-6-6 Video latch Timing - 80 Column

4-57

})

132 COL - SINGLE WIDTH

1- 41.5396 ns
DOT CLOCK

CHAR CLOCK

IL.-___

..J

VSR LD - - - - - '

WRTLBL~
132 COL - DOUBLE WIDTH

-l

1-83.0792 ns
--

))

DOT CLOCK

CHAR CLOCK

I . . _~

VSR LD

WRT LB L

~
MA-47AO

Figure 4-6-7 Video latch Timing - 132 Column
4.6.2.4 Double-Width MUltiplexer - The double-width mUltiplexer (MUX) produces the three signals
whose timing must be changed when a line of characters is switched between single- and double-width
modes. The frequency of DOT ClK must be divided in half on a double-width line so that the video
shift register will shift half as often, making each dot (and therefore each character) twice as wide as it
would be in single-width mode. In order for the video shift register to work properly with the half-rate
DOT ClK in double-width mode, the load signal for the shift register (VSR lD H) must still come
every 10 dots (80 column mode) or 9 dots (132 column mode). Therefore loads must occur at every
other CHAR ClK. Similarly, incrementing the DMA address counters occurs on every other CHAR
ClK to ensure that characters that are stored sequentially in the screen RAM are presented to the shift
register at the correct time for each VSR lO H pulse. The different modes of DOT ClK and VSR lO
H are shown in Figures 4-6-6 and 4-6-7.

I)
/

In single-width mode, the double-width MUX directs the output of the 80/132 MUX to the DOT
ClK pin, providing either a 16 MHz or 24 MHz output. To get the half-rate OOT ClK for 80 column
mode, the double-width MUX selects the output of the first flip-flop in the dot counter, that acts as a
divide-by-2 because the dot counter is dividing by 10 (10 is an even number). In 132 column mode the
same selection cannot be made because the dot counter is dividing by 9. But the divide-by-I.5 is not
needed in 132 column mode, so this divider is converted to a divide-by-2 and the double-width M UX
selects its output when a double-width line is displayed in 132 column mode.
..~

4-58

)

The load input of the video shift register used in the VT100 is a synchronous input. This means that
when the load input is high, the rising edge of DOT CLK causes a parallel load to be performed instead
of a shift. To get one load and many shifts for each character, VSR LD H can only last for that one
cyCle of DOT CLK that is adjacent to CHAR CLK. Furthermore, transitions of VSR LD H must satisfy SET-UP and hold times with respect to the rising edge of DOT CLK. In single-width modes, VSR
lD H is one dot time wide, generated from the outputs of the dot counter, and its SET-UP and hold
times are guaranteed by internal propagation delays. This relationship is shown in Figures 4-6-6 and 46-7 by a slight shift in the transitions of VSR LD H with respect to DOT CLK. In double-width mode,
VSR lD H is created by selecting every other CHAR CLK (Figure 4-6-5 shows a flip-flop that divides
CHAR ClK by two for this purpose) and then delaying this signal by one single-width dot time.

80 COLUMN OPERATION
~-----12.46us----~~
HOLD REO H

--,
.I

-+-_..r-- 9 .97 us Min·--.....,I 1111111111111111
I-- 2.4924 us
I-I 2.243 us Min.
1111111111111111

ADDR CNT ON H - f . - - - - - - - - - - - '

I
I
SINGLE WIDTH:

.ADDR CNT H

I
I

DOUBLE WIDTHI
I

)

ADDR CNT H

I-- 623n5
.------11.465U Si-----L.___________

HORIZ BLANK H

-1.495us

)

132 COLUMN OPERATION

HOLD REO. H -4.-J----11 .215us

---;-......,'~1l!!.1l!.!.11!!11!!11!.!1.!.!11.!.!1l..!..!11~11~11~11~ln!.!..

~----11.34us------I

11----1.37jJs Min
I

ADDR CNT ON H - ; . . - - - - - - - - - - - - '

111111'1111111111111111111

SINGLE WIDTH I

ADDR CNT H

111111111111111111.111111111

DOUBLE WIDTH:
ADDR CNT H

.......- - - - 12.088jJs

----1..___________

HORIZ BLANK H
MA·4279

Figure 4-6-8 Address Count Timing

4-59

The signal that increments the DMA address counter (ADDR CNT) is shown in Figure 4-6-8. ADDR
CNT has the same timing as CHAR CLK;the difference is that it does not run continuously. Figure 46-8 shows that ADDR CNT can only be generated if HOLD REQUEST H is high and that it is further
controlled by a signal from the horizontal timing section (ADDR CNT ON H) that allows ADDR CNT
to provide exactly three pulses (in single-width mode) before HORIZ BLANK H goes low. The three
pulse delay primes the external character latches so that the dots for the first character on a line are
being loaded into the video shift register at the same moment that HORIZ BLANK H enables the
video at the beginning of a scan. The only change made to ADDRCNT for double-width operation is
that every other pulse is deleted, beginning with the first pulse (Figure 4-6-8).
4.6.2.5 Horizontal Counter - The block diagram in Figure 4-6-5 shows the horizontal divider broken
into three stages. The first divider is programmable according to the number of columns selected and is
driven by CHAR CLK from the dot counter. For 80 column mode the divisor is three; for 132 column
mode the divisor is five. The total division from MASTER CLK to the output of this first divider is 45,
independent of mode (for 80 columns: 1.5 X lOX 3 = 45, for 132 columns: I X 9 X 5 = 45). Therefore, the operation of all of the remaining dividers in the DCOII, which are driven from the first horizontal counter, are also independent of the column mode. The second stage of the horizontal divider has
a divisor of 17, which is chosen to give the required number of displayable columns plus about 28 percent more to allow time for the monitor to execute a horizontal retrace. The last stage is a simple divide-by-2 that provides the horizontal frequency. Designing the last stage to be a divide-by-2 guarantees
that the signal at its input will have a frequency twice that of the horizontal frequency as required by
the vertical dividers to create interlaced operation. The total division from CHAR CLK provided by
the horizontal divider is 102 in 80 column mode and 170 in 132 column mode. In either mode the frequency at the output of the horizontal counter is 15.734 kHz.

---1\
/

4.6.2.6 Horizontal Drive and Horizontal Blank - Two timing signals are generated from the horizontal
counter to control those system functions occurring at the scan rate. These signals begin at the end of a
scan and last until the horizontal counter is incremented past a specific state that is decoded to turn the
signals off. The monitor requires a pulse at the end of every scan to tell it when to initiate a retrace and
begin the next scan; the duration of this pulse must be between approximately one-quarter and one-half
of one scan. Figure 4-6-9 shows HORIZ DRIVE L as produced by the DCOll; the slight difference in
timing between 80 and 132 column modes is the result of design convenience and is not significant to
the operation of the VT100. HORIZ BLANK H is designed to allow 83 characters during the forward
scan in 80 column mode and 137 characters in 132 column mode. The extra characters are included for
possible future use such as a field of indicators along the right margin of the screen or as extra symbols
inserted to mark text. The rising edge of HORIZ BLANK H is synchronized to CHAR ClK to eliminate the accumulated delay of the horizontal counter. The falling edge of HORIZ BLANK H occurs
between two CHAR ClKs (Figure 4-6-8) to meet some requirements of the DCOl2, but inside the
DCOl2 HORIZ BLANK H is delayed to the following edge of CHAR ClK so that the beginning of
each displayed scan will coincide with a character boundary. The actual video blanking occurs inside
the DCOl2. Therefore there is no signal outside the DCOl2 that has the exact length of horizontal blank
time.
4.6.2.7 Line Buffer Addressing - The line buffer memory stores one line of characters during a scan on
which a DMA occurs and then recalls these characters on each successive scan until the next DMA.
Because the line buffer is a random access memory, it has address inputs that must be provided with a
sequence of addresses that change at each CHAR CLK such that each character is stored in a unique
location. The horizontal counter can provide such addresses because it is incremented through a series
of unique states that repeat in the same sequence on every scan. Because of the three stages that comprise the horizontal counter, there are nine flip-flops whose outputs must be converted into the eight
line buffer address (lBAs) outputs. The conversion is possible because the 9 flip-flops represent a maximum of 170 states in 132 column mode (8 bits can represent 256 states). The DCOII contains gates
that combine the output of the ninth flip-flop in the hcrizontal counter with the outputs of the other

))
4-60

80 COLUMN OPERATION

--1-

63.55 us

1--623 ns
HORIZ
~
11.465us
BLANK H
1--12.088us

--l

HORIZ
BLANK H

\--12.462us
--11--;373.85 ns
~
12.088us

132 COLUMN OPERATION

I
80 COLUMN OPERATION

-------'\

)

HORIZ
DRIVE L

""'l-===28.288us

·1-

35.262us

I
132 COLUMN OPERATION
HORIZ
DRIVE L

)

VERT
BLANK

f.:=28.164us

35.386us

·1

1----1.869., FOR 80TH HIGH + LOW TRANSITIONS

:]--- - - --

~
VERT
RESET

-I-

____________________________

r-5.608us

JII~'

NOTE: HORIZONTAL BLANK H IS SHORTER THAN ACTUAL HORIZONTAL BLANK TIME.
MA-4272

Figure 4-6-9 Horizontal Timing
eight to generate some addresses that are not otherwise represented by the eight. The resulting LBAs
do not follow a normal binary counting sequence but the sequence of unique addresses repeats exactly
on each scan. Figures 4-6-1 o and 4-6-11 show the LBA sequence for one-half of a scan; the other half is
identical except that LBA 7 is low.
Several of the LBAs are used as general purpose clocks in the VT100. LBA 3 and LBA 4 are used to
generate timing for the keyboard. These signals satisfy the keyboard's requirement of two squarewaves, one twice the frequency of the other, even though every 16th transition is delayed (the second
stage of the horizontal counter divides by 17, not 16). LBA 7 is used by the nonvolatile RAM. The
31.468 kHz signal on LBA 6 could be used for power supply synchronization, although this is not done
in the VT100.

4-61

CHAR
CLOCK

--I

!-1.246188/lS

LBA 0

LBA 1

LBA2n·

LBA 3

1--3.738561/lS

!--5.60684/ls---I
LBA 4

J
I-

~
I

LBA 5

-II

9.34641/ls

1...Jl
I- .

LBA

,--.-,

7.47713/ls---j

LJ1.
-I

16.82353/ls

r-I- - - - - - - - - - - : . ._ _

-1

31.77778/lS

J
IH DRIVE L 1

ul,-__

LBA 7

28.288/ls

"'1- - - - -

SECOND HALF OF SCAN
IS IDENTICAL EXCEPT
LBA 7 IS LOW

-.l
1
MA-4300

Figure 4-6-10 Line Buffer Address Outputs - 80 Column

'~JI

"-./'

,"--'"'

,,-1

,.~

CHAR
CLOCK

--l

'~..

-~'

(

1--747.7n5

LBA 0

LBA 1

LBA 2

LBA 3

LBA 4
~
I

0Vol

LBA 5

J
L.Jl
I-

1"--_-----'

LJl

16.82353 VS

14.95425 \.IS

LBA6l

ILBA 7 J
H DRIVE L l-

,-I_ _

31.7778 VS

' 1....__
-I

28.164 \.IS

SECOND HALF OF SCAN
IS IDENTICAL EXCEPT
LBA 7 IS LOW

MA-4274

Figure 4-6-11 Line Buffer Address Outputs - 132 Column

4.6.2.8 Vertical Operation - To paint a complete picture on the screen, the monitor moves the
electron beam slowly from the top of the screen to the bottom, while it is also moving the beam quickly
from left to right to paint each scan. The vertical sweeps of the beam must be repeated continuously so
t,hat the picture is refreshed often enough to prevent the appearance of flicker. In television terminology, a single pass of the beam from the top of the screen to the bottom (and the data displayed during
that time) are referred to as one field. A complete picture, which may contain one or more fields
depending on the type of interlacing in use, is called one frame. When the VT100 is used in noninterlaced mode, each successive field is identical and therefore only one field is contained in each frame.
During interlaced operation in the VT100, there are two types of fields that alternate with each other
so that each frame consists of two fields. Even fields start at the top of the screen and display 240 scan
lines before reaching the bottom. Odd fields place their first scan line between the first and second
scans of the preceding even field and then place each additional scan between succeeding scans of the
even field. Interlacing the even and odd fields gives a whole frame of 480 scans, instead of 240 scans, to
provide increased vertical resolution. In noninterlaced operation, commands to the monitor to begin a
new field are always coincident with commands to begin a new scan. This causes the beam to always be
in the same vertical position when the first displayed scan is begun. But, in interlaced mode, odd fields
begin with a command for a new frame that occurs halfway through a scan line. This causes the beam
to have moved down the screen from where it would have been during an even field (by the distarice
that it moves in one-half of a scan) when the first displayed scan is begun. Even and odd fields are
made to alternate by including an odd number of half-scans in every field. This is in contrast to
non interlaced operation, where each field contains only complete scans. The VT100 always displays
the same video information on both even and odd fields. Interlaced mode is provided for future use by
options that desire increased vertical resolution.
4.6.2.9 Vertical Counter - The lO-bit vertical counter, shown in Figure 4-6-5, determines the frequency at which the screen is refreshed by counting the number of horizontal scans to be included in
each field. The vertical counter uses the 31.468 kHz output of the horizontal counter so that it can
count the half-scans required for interlaced operation. Figure 4-6-5 lists the four available divisors that
select the interlace mode and keep the refresh frequency as close to the local power line frequency as
possible (to minimize interference with the screen from nearby equipment). The vertical frequencies
produced by these divisors are approximately 1/20 Hz above or below the nominal power line frequency.
.
4.6.2.10 Vertical Outputs - Three outputs are derived from the flip-flops in the vertical counter to
control the vertical refresh operations in the VT 100. These signals are shown in Figures 4-6-12 and 4-613 for all four modes. VERT DRIVE H is issued at the bottom of the screen to initiate a vertical
retrace followed by a new vertical scan. This operation is analogous to the effect of HORIZ DRIVE L
on horizontal scans. The time between any two VERT DRIVE Hs is a constant, equal to an even
number of half-scans in noninterlaced mode and equal to an odd number of half-scans in interlaced
mode. VERT BLANK L always enables exactly 240 scans during any field and blanks any remaining
scans. Furthermore, VERT BLANK L is always turned off exactly 20 scans after VERT RESET H in
60 Hz mode and 50 scans after VERT RESET H in 50 Hz mode. VERT BLANK L is always adjusted
to display complete scans even during odd fields in interlaced mode. VERT RESET H initiates the
DMA process at the start of every field. When VERT RESET H goes high, the DMA address counters
are reset to point to address 2oo0H in the screen RAM, all line attributes are cleared, and the scroll
counter in the DCOl2 is preset to the value stored in the scroll latch. (See the DCOl2 description for
more explanation.) During noninterlaced operation and on even fields, VERT RESET H occurs at the
. same time as VERT DRIVE H; but, on odd fields VERT RESET H is delayed one-half of a scan to
match the start of a horizontal scan. The relationship of VERT RESET Hand transitions of VERT
BLANK L to HORIZ BLANK Hand HORIZ DRIVE L is depicted in Figure 4-6-9. Notice that
VERT BLANK L always turns on or off when the video is already blanked by HORIZ BLANK H.

4-64

A" ••

.,-,.~

I
,--',

"-----'

60 HERTZ NON-INTERLACED

VERT DRIVE

•

, -I -12511S (2 SCANS)

16.65 m,

-I 1.40 ms

VERT BLANK l~

"j::=

--I

16.65 m'

::fl-

5 .6 I1S

12511S (2 SCANS)

-i 1.40 ms

I
VERT RESET

F =t 1.02ms

~ 1.02ms

60 HERTZ INTERLACED
~
I

0'\

VERT DRIVE

..:::I

~ 1.02 ms

16.68 ms
"
I
-I -15711S (2.5 SCANS)

VERT BLANK

l
11

..:::rc

5.611S

4 -

-.j 1--12511S (2 SCANS)
I

EVEN FI ELD

_
-I 1.40 ms

_1 4
16.65m'

16.68 ms

I-- 31.811S (.5 SCAN)

VERT RESET

,I
..

1.46 ms

Ir- .

1.02 ms

ODD FI ELD

16.72 ms

IL
nL___________
MA-4301

Figure 4-6-12 Vertical Signals - 60 Hz

... ~...

50 HERTZ NON-INTERLACED
VERT DRIVE

-=:IC::
I-

VERT BLANK

-1

1 .02 ms
20.02 ms

4 . 77mS

::1

-I4 . 77

--I

!---1.59 ms (25 SCANS)

"'"

VERT BLANK

19.99 ms

4.77 ms

EVEN FIELD

19.99 ms

20.02 ms

4.70 ms

-1

- 1 i t ' · 5 5 m. (24.5 SCANS)

ODD FIELD

I- .

-I

ms--j

50 HERTZ INTERLACED

0-0--

20.02 ms

1--1.59 ms (25 SCANS)

VERT RESET--=:jt= 5.6 ~s

VERT DRIVE -=:j(:-1.02 ms

t1.52 m. (24 SCANSI

\:=31.8 ~s (.5 SCAN)
VERT RESET

5.6

1
19.96 ms

•

"--

MA-4301

Figure 4-6-13 Vertical Signals - 50 Hz

"-.-/

..~ .

...

'-. /

4.6.2.11 Composite Sync - The CaMP SYNC L signal supplied by the DCOll is combined with
video information by the terminal controller board to produce the composite video signal that appears
on J9 at the back of the VT100. An external monitor can use the composite video signal to reproduce
the image displayed on the VT100 screen. This is accomplished by using the video information to control beam intensity and the composite sync waveform to synchronize the raster to the video information.
The composite sync generator in the DCOll uses outputs from the dot, horizontal, and vertical counters
to generate the complex timing of CaMP SYNC L. COMP SYNC L consists of one of the vertical
intervals depicted in Figure 4-6-14 followed by 240 horizontal sync pulses, another vertical interval, etc.
The vertical synchronizing interval consists of a transition from horizontal sync pulses to six equalizing
pulses, six vertical sync pulses, six more equalizing pulses, and then a return to horizontal sync pulses.
Two vertical intervals are shown in Figure 4-6-14. The vertical interval that begins an odd field is similar to that which begins an even field except that the equalization and vertical sync pulses are shifted
by one-half scan with respect to the horizontal sync pulses. In noninterlaced mode all fields are even
fields; but, in interlaced mode every other field is an odd field. Figure 4-6-14 also shows the relationship
of CaMP SYNC L to both the horizontal blank time (HORIZ BLANK H modified by the DCOI2)
and VERT BLANK L. CaMP SYNC L meets the requirements of EIA RS-170 and the NTSC standards for sync pulse generators.

)

4.6.2.12 Hold Request, Address Load, and Double-Width - The logic associated with HOLD REO H,
ADDR LD L, and DW H is shown in Figure 4-6-5. The falling edge of HOLD REQ H sets AD DR LD
l to the low state; ADDR lD L is subsequently cleared by the falling edge of CHAR ClK, thus creating a short low pulse on ADDR LD L at the end of each DMA. ADDR LD L stores, in their respective
registers, all line attributes and the memory address of the next line to be accessed by a DMA. The
rising edge of HOLD REQ H causes the value of DW H that was stored in the holding flip-flop by the
previous ADDR LD L to be transferred to a second flip-flop whose output controls the double-width
MUX. This means that the value of DW H stored at the end of one DMA by ADDR LD L does not
actually become effective until the beginning of the next DMA. The holding flip-flop for DW H is
cleared by VERT RESET H at the start of every field. HOLD REQ H is also used to enable ADDR
CNT and WRITE LB l only during DMAs. Interactions of HOLD REQ H with other signals during a
DMA are further defined in Figure 4-6-19.
.
4.6.3 Control Chip Description
The control chip (DCO 12), like the timing chip, is a custom bipolar device. It accepts attribute specifications and timing signals and delivers addresses for the character generator ROM and attributes for
the video output to the monitor. It also generates the HOLD REQUEST signal that halts the microprocessor and initiates DMAs to get lines of characters. Refer to the block diagram, Figure 4-6-15.

The DCOl2 performs three main functions.
I.

Scan count generation. This involves two counters, a multiplexer to switch between the
counters, double-height logic, scroll and line attribute latches, and various logic controlling
switching between the two counters. This is the biggest part of the chip. It includes all scrolling, double-height logic, and feeds into the underline and hold request circuits.

Generation of HOLD REQUEST. This uses information from the scan counters and the
scrolling logic to decide when to generate HOLD REQUEST.

Video modifications: dot stretching, blanking, addition of attributes to video outputs, and
multiple intensity levels.

4-67

EVEN FIELD

60 Hz NON-INT = 22 SCANS}
= 22 SCANS
{ 60 HzlNT
50Hz NON-INT =-75 SCANS
50 Hz INT
= 74 SCANS

VERTICAL BLANKING INTFRVAI

.. I

60 Hz NON·INT = 2 SCANS
= 2 SCANS
{ 60 Hz INT
50 Hz NON-INT = 25 SCANS
50 Hz INT
= 24 SCANS

LAST DISPLAYED SCAN OF

FIRST OF 240 DISPLAYED
SCANS IN EVEN FIELD

~---

COMPOSITE
SYNC
I

:\;\

TIME _ _

I
:
I

ODD FIELD

I -1

START OF
EVEN FIELD

H PE RIOD = 63.56 lIS

I
160 Hz !fliT = 23 SCANS
50 Hz INT = 75 SCANS

: VERTICAL BLANKING INTERVAL

I-

.. ,

60 Hz INT
{
50 Hz INT

LAST DISPLAYED SCAN OF
EVEN FIELD

2.5 SCANS
= 24.5 SCANS
=

FIRST OF 240 DISPLAYED
SCANS IN ODD FIELD

I
I
I

~---

I
I

f>'
0'1

:\n

TIME_

-1 ~

START OF

ODD FIELD

---,I Ir

H PERIOD

= 63.56 lIS

H BLANK {1 1.84 IlS / 80 COLUMN
12.34Il s /132COLUMN

HORIZONTAL

8~NK

----LJ

L-J

EO = 2 JJ iJS

~~~OSITE
FRONT PORCH = 1.54 115
H SYNC = 4.7 1 IlS

j+-

U
..j I-

I
-.J

LIL

I-V SYNC = 27.28 115

H PE RIOD = 63.56 IlS

NOTES:
•

LJr---------U

IN NON INTERLACED OPERATION THE
EVEN FIELD IS REPEATED CaNTIN
UOUSL Y. AND THE ODD FIE LD IS NOT
USED.

•

IN INTERLACED OPERATION THE
EVEN FIELD ALTERNATES WITH THE ODD
FIELD.

•

ALL FIELDS CONTAIN 240 DISPLAYED
SCANS.

MA·4J04

Figure 4-6-14 Composite Sync Output

~-..<_/

,--../

"-~

DATA <0-3>

SCROll
lATCH

VIDW2lL - -

LO~

DECODER

...

BLINK
F.F.

i
. -.1-.

,-----,/

~-...---

OFFSET
COUNTER

CNT

-.--~./

SCAN COUNT MATH

SCROLL
MUX

NORM: ADD 15GET 15,0,1,2 ... 7,B
o HI TOP: +2 & ADD 15 FOR 15, 15,0,

0 ... 3,3
o HI BOT: +2 & ADD 4 FOR 4,4,5,5 ... B,B

BOUNDRY DET.
4
DHIGH

VERT FREQ
INTRPT

VERT
RESET

VER,
FREQ
INT
F.F.

SCAN
~ COUNTER

TOP HALF

?PSCAN

BASIC ATR
LATCH

tCLR

REV
FIELD
F.F.

SCAN
COUNT
<0:3>

r.NT

rJ3

REV VID

DHIGH
DWIDE

ADDR lD

....

CLR

lATCH
1

SCROll
F.F.

I NEW

SCROll
ZONE

'-CLK

LATCH
2

t ClK

4
CHAR ClK

1
I

rl.

ATTRIBUTE
LOGIC

,-~

VIDEO OUT 1
VIDEO OUT 2

jHHORIZ
BLANK
F.F.

rERM
F.F.

CL~ ICLR·

'---

fCLK

~
HORIZ CLK
GENERATOR
3 CHAR DELAY

HOLD
~Q

;i5
~
2 CHAR
DELAY

HORIZ BLANK

-<r}

CLR

fLO
~

HOLD RQ

ClK
SCROLL NG
I

H CLK
TERM

UNDER
LINE

BOLD

DOT
}STRETCH
DELAY

BLINK

VID IN DOTfCLK
MA-43~

Figure 4-6-15 DCOl2 Block Diagram

4.6.3.1
Input Decoder Functions - The input decoder accepts a 4-bit command from the micro-')I'
processor when VID WR 2 L is asserted. Table 4-6-2 lists the commands.
'J
The low eight values are used to load the scroll latch with the offset for smooth scroll. The scroll latch
is loaded in two passes, first writing the two leas.t, and then the two most significant bits. Because the
offset is a decimal value controlling 10 display scans, the combination setting the most significant bits
to lIB is not used in the VT100 (lIB covers the range 12-16).
The input decoder also toggles the blink flip-flop by complementing the state of the flip-flop whenever
lO00B is written. The blink flip-flop invokes blink only where the blink attribute is set.
To save external hardware the vertical frequency interrupt flip-flop is located in the DC012 because a
spare pin was available. It is set by the falling edge of Vertical Reset. It is cleared by writing 1001B
into the input decoder.
Set and clear of reverse field are not toggled because the absolute state is important and there is no
feedback for the system to detect the current state. Therefore, the two states are explicitly set to their
desired values.
1100B means set base attribute to underline and 1101B means set base attribute to reverse video.
1110B and 1111 B are for future specification.
Any time the input decoder is loaded with 11 XXB, the blink flip-flop gets cleared. This is the only way
to initialize blink in the chip testing process. The firmware does not currently use the ability to clear the
blink flip-flop but if hardcopy output was being implemented, it could be used to set the blink to a
known state during a freeze.
Table 4-6-2 Control Chip Commands
(Write Address A2H)

D3 D2 Dl DO

Function

0
0
0
0

0
1
0
1

Load low order scroll latch = 00
Load low order scroll latch = 01
Load low order scroll latch = 10
Load low order scroll latch = 11

0
0
1
1

0
1
0
1

Load high order scroll latch = 00
Load high order scroll latch = 01
Load high order scroll latch = 10
Load high order scroll latch = 11 (not used)

0
0

0
1

Toggle blink flip-flop
Clear vertical frequency interrupt

0
1

Set reverse field on
Set reverse field off

0
I
0
1

Set basic attribute to underline·
Set basic attribute to reverse video·
Reserved for future specification·
Reserved for future specification·

0
0
0
0

0
0
0
0
0
0

0
0
I

0
0
0
0
1
1

·These functions also clear blink flip-flop.
4-70

)

4.6.3.2 Attribute Latches - The line attributes are managed by two latches that store the scrolling,'
double height (DH), and double width (DW) bits. The first latch stores the incoming data when
Address Load (ADDR LD) goes low at the end of a DMA. Scrolling means that the next line will be
part of the scrolling region, DH means the next line will be double height, and DW specifies top or
bottom half for double-height lines. There is no double height, single width combination, so in double
height, double width is assumed. The DW pin also tells the DC012 to extend HOLD REQUEST
during double width. The second set of latches for double width and double height are clocked by the
rising edge of HOLD REQUEST (similar to the second latch for double width in the DCOII) to invoke
the attributes at the beginning of the new line. The first latch stores those attributes from the end of
one DMA to the beginning of the next. It is the outputs of the second latch that invoke attributes in the
chip. The scroll bit is invoked by a different signal that will be discussed later. The pin defining
scrolling is the same pin as reverse video; it means scrolling when loaded in by ADDR LD L and it
means reverse video at all other times. The reverse video signal passes from the input pin around the
first latch directly to the attribute logic.
4.6.3.3 Scroll Counter - The scroll counter consists of two 4-bit, divide-by-IO counters called the
Scan counter and the Offset counter. Both are clocked at the horizontal rate to count scans. The Scan
counter is cleared by VERTICAL RESET so that it starts at 0 and counts by lOs down to the end of
the screen.
The Offset counter is loaded with the contents of the scroll latch by VERTICAL RESET. The Scroll
latch is loaded by the microprocessor and defines the offset between the Scan and Offset counters for
an entire frame because the Offset counter is only loaded at vertical reset time. The microprocessor will
load the latch with the offset for the next frame during the current frame.

)

At the beginning of a frame, the two counters divide by 10 but start at different numbers. If not
currently scrolling, the offset is 0 and there is no functional difference between the counters. If in the
middle of a smooth scroll, the offset will be some other value from I to 9. The scan address outputs
from the chip to the character generator ROM are either the output of the Offset counter or of the
Scan counter, depending on whether the current line is in or out of the scrolling region, respectively.
One of the two counters is selected by a 4-bit wide multiplexer (MUX) whose output is the Counter In
Use. The MUX is controlled by the scroll flip-flop which is the second latch for the scroll attribute bit.
Most line attributes (double height, double width) take effect when HOLD REQUEST goes high for
the line in which they are effective. They always take effect with the actual data being displayed.
However, the scroll flip-flop can only change state when crossing one of the fixed 10 scan boundaries
that ate defined by the Scan counter reaching O.
Each scan when the Scan counter reaches 0 is a scan on the screen where a change can occur from a
nonscrolling to a scrolling region, or from a scrolling to a nonscrolling region since this is normally
where one line of data changes to the next if the line were not scrolling. This is the only place changes
can occur because the bottom of a nonscrolling line of characters is the place for moving into a
scrolling region and the 'top of a nonscrolling line is the place for getting out of a scrolling region. To
get into or out of a scrolling region the Scan counter must be at zero. A 4-bit Boundary Detect decoder
gate is connected to the outputs of the Scan counter and clocks the scroll flip-flop when all its inputs
are zero. If the input to the scroll flip-flop (from the scrolling attribute latch) has been low and goes
high at the end of the previous line's DMA, then on 0 (boundary detect), the output of the scroll flipflop, which is the control line of the MUX, causes the MUX to switch from the Scan to the Offset
counter. At the bottom of the scrolling region, the last DMA in the region gets a line from the screen
RAM with its scroll attribute not set so when the next 0 boundary is reached, as defined by the Scan
counter. the scroll zone is exited. This is because the scroll flip-flop will get a zero input again which
will switch the MUX back to the Scan counter right in the middle of the line that was partway through
scrolling.

4-71

4.6.3.4 Scan Count Math - The scan count output of the scroll MUX goes through combinatorial
logic that looks at the double height bit and the top and bottom half bit and decides whether those scan
counts need to be modified for double height before going out of the chip. If double height is not
asserted, the top and bottom half bit is ignored and the scan is passed through with I subtracted in
modulo 16 arithmetic. (Thus, 0 becomes 15, 3 becomes 2, 9 becomes 8.) If double height is asserted and
top half is asserted, then the operation is to divide the scan count by 2, and continue to subtract 1 after
dividing by 2 so the first scan = 15, second scan = 15, third scan = O. If bottom half is asserted, the
operation is to divide by 2 and add 4. This particular arithmetic arrangement was designed for an
external component that is no longer needed. Otherwise, dividing by 2 (and adding 5 for bottom half)
would be sufficient. Table 4-6-3 shows the scan count sequences for the various modes. The scan count
changes on the rising edge of HORIZ BLANK H.
Table 4-6-3 Scan Count Sequence
Normal and Double Width
SC3
SC2
SCI
SCO

0
0
0
0

1
0
0
0
0

0
0
0
0
1

0
0

Double Height Top
SC3 SC2 SCI

SCO

1
0

0
0
0

0
0
0
0
0

1
1
1
1

1
1

1
1
1
1

0
0

1
1

1
I

Double Height Bottom
SC3 SC2 SCI SCO

0
0
0
0
0

1
1
1
1
1

0
0
0
0

0
0

0
0
0

I
I
1

1
1
1

0
0
1
1

I
1

0
0

1
1

0
0
1

I
1

1
1

NOTE: Top line of table is first scan of character line (where HOLD REQ H occurs).

4.6.3.5 Generation of HOLD REQUEST - HOLD REQUEST is the signal to the microprocessor
that makes it give up control of the data bus. Then the video processor can DMA a line of data out of
the screen RAM and place the data in the line buffer. Two principle conditions can generate HOLD
REQUEST. The most common is when the output of the Counter in Use = 0, meaning on the first
scan of a new line of characters. (The Counter in Use, which is the output of the scroll MUX, is either
the output of the offset or scan counter, depending on whether the current line is in or out of a scrolling
region.) Whenever a new line of characters starts, a HOLD REQUEST is needed to get the line's data.
Therefore, a detector at the output of the scroll MUX detects scan 0 of the Counter in Use.
The other condition for generating HOLD REQUEST is at the top of a new scrolling region. This is
necessary because as the CRT beam moves from a nonscrolling region into a scrolling region, it
switches from the last scan of a normally registered line to the first displayed scan of a line that is
scrolling. Assuming the scrolling.region is in midscroll, the first scan is not the Oth scan of the scrolling
line, so the Counter in Use is not O. The new scroll zone flip-flop (whose clock input comes from the
scroll flip-flop that controls the scroll MUX) is triggered by entrance into a scrolling region. When the
new scroll zone flip-flop is set, it forces a HOLD REQUEST even if the Counter in Use is not also o. If
the Counter in Use is 0 (implying an offset of 0 between scrolling and nonscrclling lines) there are two
simultaneous causes for HOLD REQUEST.

4-72

)
.)

The new scroll zone flip-flop is cleared by ADDR lD that occurs at the end of the Hold Request
generated by the new scroll zone flip-flop. That is, the new scroll zone flip-flop generates HOLD
REQUEST. HOLD REQUEST is cleared either by Terminate or Horizontal Blank, whichever comes
first (the way the VT100 is programmed, Terminate is always first) and termination of that HOLD
REQUEST feeds back through ADDR lD and clears the new scroll zone flip-flop. More about
HOLD REQUEST follows a discussion of Horizontal Blank and Terminate.
4.6.3.6 Horizontal Blank and Terminate - Horizontal Blank, in addition to blanking the video output,
clears the terminate flip-flop and also generates an internal timing signal (horizontal time reference)
for clocking counters.
A short signal, that occurs on every horizontal scan and lasts only a few character times, is needed in
the chip to clock flip-flops and to disable the decoder gates that detect boundaries and generate HOLD
REQUESTS while the counters settle. Horizontal Blank cannot be used for this purpose because
HOLD REQUEST and Boundary Detect (and other signals) need to be settled well before Horizontal
Blank ends. The required short signal comes from a small counter triggered by Horizontal Blank and
further clocked by character clock. The counter has two outputs: a two-character clock wide signal
that enables and disables the boundary detect flip-flop (that drives the scroll flip-flop) and a threecharacter clock wide signal (H ClK) that enables the Hold Request gate. Boundary detect is enabled
earlier than the HOLD REQUEST gate so that the existence of scrolling can set up the new scroll zone
flip-flop before HOLD REQUEST is enabled. A gate combines the output of the new scroll zone flipflop and the output of the boundary detector (Counter in Use = 0). The output of that gate is combined with H ClK (three character clocks long following Horizontal Blank) to generate the rising edge
of HOLD REQUEST three character times after the rising edge of Horizontal Blank, and is also
combined with the signal from the terminate flip-flop to end HOLD REQUEST when a terminator is
detected.

)

If Terminate did not cut off HOLD REQUEST, then HOLD REQUEST would be disabled by the
next falling edge of the H ClK signal, but the VT100 is programmed to always end HOLD REQUEST
with Terminate. If the VT100 was not working right, HOLD REQUEST might be ended by Horizontal Blank.
Terminate causes a number of functions inside the DC012. The Terminate input is not direct; it is
sampled on each rising edge of character clock and latched into a flip-flop. When detected, it ends any
HOLD REQUEST in progress. In normal screen mode, it blanks the video output, but in reverse
screen mode it forces the video output to the dim intensity level. The terminate flip-flop output feeds
back 1'0 its own input, so that as soon as the flip-flop is clocked with Terminate asserted, the flip-flop
latches itself up.

Once the terminate flip-flop is latched up by feedback from its output, there is only one wl;ly to clear it:
through the asynchronous clear input. The flip-flop must not be cleared until Horizontal Blank has
taken over blanking the video output; therefore the clear is delayed slightly (by one character time after
the onset of Horizontal Blank). The clear is maintained until just before the character clock which
corresponds to the first character on the screen because data on the video data bus may be undefined
and might contain extraneous Terminates. These must not be detected during the horizontal blanking
interval because they would latch up the terminate flip-flop for an entire scan. However, the internal
signal that blanks the video outputs of the DC012 during the horizontal blanking interval cannot be
used to clear the flip-flop directly because it must release the video output at the exact beginning of the
first character on the screen, and the terminate flip-flop must be capable of detecting the terminator in
the first character position. If the internal blanking signal was used to clear the terminate flip-flop, the
release time of the flip-flop would not be satisfied, and it might miss a terminate in the first character
position (as would be found in SET-UP and the top and bottom fill lines). Therefore the Horizontal
Blank output of the DCOII is made to end approximately one-half character time before video unblanking to release the clear on the terminate flip-flop. Inside the DCOI2, the falling edge of Horizontal Blank H is delayed to the following character clock to provide the correct video blanking.
4-73

4.6.3.7 Double Width and Hold Request - The first occurrence of Terminate or Horizontal Blank at
the end of a DMA normally gates Hold Request off. But in double-width mode, Hold Request is
extended by two character times. This is required because each character appears twice in the external
character latch pipeline. To get the first and second byte of the address of the next line correctly placed
with respect to Address Load, the end of Hold Request must be delayed by two character times after
Terminate or Horizontal Blank. This delay occurs either when double width is asserted alone or any
time double height is asserted (because double height implies double width) by combining Hold
Request with a two character time delayed version of Hold Request to give a Hold Request that starts
at the normal time but ends two character times later.

'
./)

4.6.3.8 Attributes - The attribute section of the DCOl2 basically implements Truth Table 4-6-4 which
decides how to interpret various combinations of attribute inputs. A number of different inputs determine attributes applied to each character. There are the four attribute pins: Reverse Video H, Underline L, Bold L, Blink L. But there are internal signals (mostly from the input decoding section) that
affect attributes also: the blink flip-flop (toggled by the input decoder to provide blink timing), the
reverse/normal field flip-flop (set by input decoder), the base attribute flip-flop (set, by the input
decoder to select whether the reverse video pin is interpreted as reverse or underline) and the scan
counts (from the output of the double height section, that enable the underline on the correct scan).-----j)
These inputs are applied to a combination of gates that feed into a 4-bit latch before controlling the
' '
video outputs. This latch is clocked by character clock to change the attributes during the intercharacter space.
Table 4-6-4 defines the following appearance for characters with attributes.
Normal characters appear uninverted at normal intensity by asserting VID OUT 2 whenever VID IN
H is asserted. Bold characters are the same as normal characters except that VID OUT I and 2 are
enabled. Reverse characters (exclusive-OR of reverse video and reverse screen) normally have dim
backgrounds with black characters so that the large white spaces have the same impact on the viewer's
eye as the smaller brighter white areas of normal characters. Bold and reverse asserted together give a
background of normal intensity. Blink applied to nonreverse characters causes them to alternate between their usual intensity and the next lower intensity. (Normal characters vary between normal anp
dim intensity. Bold characters vary between bright and normal intensity.) Blink applied to a reverse
character causes that character to alternate between normal and reverse video representations of that
character. Underline causes the ninth scan of a character to be forced to white of the same intensity as
the character for nonreversed characters, and to black for reverse characters.
4.6.3.9 Dot Stretcher - The dot stretcher reduces the video bandwidth required in the monitor (especially in 132 mode) by making the minimum dot width 80 nanoseconds. Wider dots give the CRT time
to reach full intensity before turning off again. This makes vertical lines appear to have the same
intensity as horizontal lines, rather than looking dimmer because of the brightness loss as illustrated in
Figure 4-6-16. The dot stretcher works by delaying the VIDEO IN H signal by one dot time (using a
flip-flop clocked by Dot Clock) and then ORing the undelayed and delayed signals. Figure 4-6-17
shows an example of the dot stretcher's operation.
4.6.4 Address Counter and Data Structure in RAM
Refer to the print set and to Figure 4-6-20. The address counter (E21, E22, E25, E30) is three presettable 4-bit binary counters cascaded to form 12 bits with an additional flip-flop that provides a 13th
bit of address to the screen RAM. Vertical reset clears the counter to an initial hardwired value of
2000H so that the video processor always begins to process from that location after a vertical reset.
The counter is loaded with a new address at the end of each DMA when the address load signal
appears. The counter counts forward from this address at the character clock rate, using the address
count (ADDR CNT H) signal. ADDR CNT H only occurs during Hold Request; thus, the counter
only counts during the DMA portion of each line. The address is loaded at the end of the DMA scan

4-74

)

and held until the next DMA begins. The 13th bit is programmed by 04 in the high byte of a DMA
address. The VT100 firmware programs this bit high to access address 2000H plus the contents of the
12-bit counter. But if D4 were programmed low DMAs would access address 4000H plus the contents
of the 12-bit counter.
Table 4-6-4 Character Attribute Combinations
Attributes

----1

)

Reverset

Underline L

Bold
L

Blink
L

L
L
L
L

H
H
H
H

H
H
L
L

H
L
H
L

L
L
L
L

H
H
L
L

H
L
H
L

H
H
H
H

H
H
L
L

H
L
H
L

H
H
H
H

L
L
L
L

H
H
L
L

H
L
H
L

Off
Dim
Normal
Bright
Not Applicable

D
N
B
X

NjD

B
BjN

a
a

x
X
x

N
D

DjO
N

NjO

L
H
H
L

x
x
x

B
BjN

NjO

L
H
L
H

Underline
Video:\:

DjO

L
L
H
H

NjD

Reverse

Character
Video (VID
IN H 1)
N

a
0
a

Rev Vid H

Beam
Intensity

a
a
a

Rev Field H

Code in
Table

Effect*
Background
Video (VID
IN H 0)

VID2H

VID IH

L
L
H
H

L
H
L
H

OjN
OjB

a
a

OjN

OjB

'" For blinking, outputs are shown as OFF ION where OFF and ON are the blink flip-flop states.
t Reverse = (reverse field H) XOR (reverse video H)
:j: Intensity of beam on underline scan

4-75

BRIGHTNESS LOSS

INPUT
MONITOR
PULSE
RESPONSE
THE MONITOR RISETIME IS TOO SLOW TO REACH MAXIMUM
BRIGHTNESS BEFORE THE SHORT PULSE FALLS
80 ns

"1--_
BY STRETCHING ALL PULSES TO A MINIMUM
OF TWO DOT TIMES, FULL BRIGHTNESS IS ATTAINED

SIMPLE LOGIC ADDS ONE PULSE TO THE
TRAILING EDGE OF ANY INCOMING PULSE
INPUT

OUTPUT

S:L
)
MA-4665

Figure 4-6-16 Dot Stretching

CHARACTER AS STORED IN
SCAN
CHARACTER GENERATOR ROM
NUMBER
1 • . • • • • • . • • • . • . . . . . •.

2· . . . • . . . . . . . . . . . . . . .
3· . . • . • . . . . . . . . . . . . . .
4· . • . . . • . . . . • . ••••. ..
5·· • . . . . . • . . .•• . . . . • • .
6· •••••••. . .••. . . . • . .

ACTUAL CHARACTER DISPLAYED
AFTER DOT STRETCHING

....
.....
•••••••• ••••••••

· ..

............ .
........... .

· ..

•

••

•

8·.· ....•...•......

••• ••••••••• ••••
••••••••• •••• ••••
•••
•••• ••••••••
.
..
..... .

9· . . . . . . . . . . • . . . . . .
10· . . . . . . . . . . • . . . . . .

7·.· ....•...•. ••••.

•
•

••

•

... ........
......... ....
•

••

. ......

20 DOTS FOR 2 CHARACTERS
IN 80 COLUMN MODE

•

..... .
..... .

MA-4663

Figure 4-6-17 Dot Stretcher Example

1
/

4-76

)

In double-width mode, the address count pulses occur half as often as in normal width. The line buffer
receives the normal number of WRITE LB pulses, however, so each character gets copied into two
adjacent locations in the line buffer.

4.6.5 Address Latch Buffer
Tristatable latches (E26, E33) store the address counter outputs for a character period to increase the
speed of RAM accesses that would otherwise be slowed by the long propagation delay of the counters.
They provide sufficient output power to drive the address bus of the basic terminal controller board
and the A VO if present. The outputs are disabled during the non-DMA period to prevent conflict with
the microprocessor address bus.
4.6.6 Line Buffer
Data arrives at the screen RAM latch (E20) during a DMA cycle and is latched in by the character
clock. During the DMA the tristate buffer (EIS) is enabled. Data passes through it to the line buffer
(Ell, EI7) for storage and to the character generator latch input (EI6). The line buffer is a 2S6 X 8
RAM that can hold one full line of data including the three end bytes. The line buffer is written when
WRITE LB L is asserted in the middle of each character clock period. Its outputs are disabled during
the DMA by the Hold Request signal. During non-DMA operation, the screen RAM latch samples
the 8080 data bus at the character rate but the tristate buffer is off so the 8080 data has no effect.
However, the outputs of the line buffer are enabled and the same data is presented to the character
generator latch as was presented during the previous DMA scan. The address signals for the line buffer
are described in Section 4.6.2.7.

)

4.6.7 Character Generator
The character generator is a ROM that is addressed by the coded representations of the desired characters stored in the screen RAM. Each code is used as the high 7 bits of the address to a 2K X 8 ROM
(that provides enough storage for 128 characters). (The eighth character bit is the base attribute input
to the DCO 12.) The low four address bits are provided by the scan counter in the DCO 12. The seven
character bits com bine with the 4-bit scan count from the DCO 12 to give an II-bit address to the
ROM. The data stored at each (scan + character) address are 8 bits representing the presence or
absence of dots of light at sequential horizontal positions within that scan. Figure 4-6-18 shows the
patterns supplied in the standard VT100 character generator ROM.
If the alternate character generator ROM and the advanced video option are present, the AVO may
assert AL T CHAR SET L to disable ROM E4 and enable ROM E9 when the alternate character set
attribute is set.

)

If the AVO is installed without the alternate character ROM, any character cell in which the alternate
set is selected will appear white (black if reverse video). See Appendix A (SCS) for selection of alternate
characters.
If it is necessary to use a different main character set than the one provided in the VT100, but an
alternate character set is not required, the following arrangement may be used.
I.

Cut jumper WI to disable the main ROM.

Plug a new main ROM into the alternate character ROM socket.

The new main ROM must be programmed exactly as the alternate ROM described in Section 6.S.1 except that the chip select on pin 18 must be programmed for high assertion.

To use a UV erasable PROM in the socket for E9 (which must have Intel 2716 pinouts), cut jumper W4
and insert jumper WS to put +S volts on pin 21. Only later VT100s have these jumpers. The access
time of the ROM must be less than 300 ns to guarantee operation in 132 column mode.

4-77

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MA-4927

Figure 4-6-18 Character Generator ROM Patterns (Sheet 1 of 2)

4-78

---))

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t-------~t

MA-492B

Figure 4-6-18 Character Generator ROM Patterns (Sheet 2 of 2)

4-79

4.6.8 Video Shift Register
When VSR LD H is asserted, seven of the eight output bits from the character generator ROM are
latched into the shift register and one is latched into a flip-flop (Figure 4-6-4). At the same time, the
last bit shifted out by the video shift register (VSR) during the previous character time is latched into
the first bit position in the VSR. The VSR is continuously clocked by the dot clock. The first bit shifted
out in the new character time is the same value as the last bit of the previous character, providing
horizontal continuity of characters from one character cell to the next. The seven new bits are then
shifted out.
Meanwhile, the flip-flop that stored the eighth bit has delivered that bit to the serial input of the shift
register. This bit was shifted into each successive register position as the first eight bits were shifted out.
Now the shifting continues, for one more bit in 132 mode, and for two more bits in 80 mode, causing
the last bits shifted out to be the value that was stored in the flip-flop.
VSR LD H then latches the next character into the VSR and flip-flop, with that last, multiply-shifted
bit in the first position. In this way, one bit from the character ROM defines the two or three dots
between characters, while seven bits define the character itself.
At the end of the scan, horizontal blanking forces the flip-flop output low. Since blanking lasts more
than one character time, the low level will be shifted to the first position before the start of the next
line. This ensures that the first dot on the next scan will be at the screen background level.

4.6.9 Terminator
The 7 bits of each character address in a line go to an 8-input terminator detector gate as they are
passed to the character generator ROM. Only seven bits are examined because the eighth bit is an
attribute and does not contribute to the uniqueness of a character. The last bytes in each line are a
terminator character and'two address bytes. Only the terminator character activates the gate. During
the character time when the terminator reaches the detector gate, the first address byte is at the input to
the character generator latch and the second address byte is at the input to the screen RAM latch. On
the next character clock, the terminator causes the DCOl2 to blank the display and end the Hold
Request. The latch outputs now contain the two address bytes. The low order byte is at the inputs to
counters E30 and E25, while the low four bits of the high byte are at E21. The fifth bit is at the input to
flip-flop E22. This bit is normally high; it allows future development of screen RAM size. The high
three bits go to the line attribute inputs to the DCOI2.
The address load (ADDR LD L) pulse, timed to arrive before the next character clock after the
terminator, loads the address counters, flip-flop, and DCOl2 inputs with the two final bytes. The
counters and flip-flop designate the address of the first character of the next line to be displayed.

4.6.10 DMA Cycle Timing Diagram
Figure 4-6-19 shows the complex timing in the video processor during the first scan of each new line of
characters. The entire scan is shown, with the repeating portion compressed into the area represented
by dashed lines. The diagram is for 80 columns, single width display as the VT100 is currently programmed. Diagrams in the DCOll timing chip description (Section 4.6.2) show the differences in
signals in other modes.
The first line shows the Character Clock. This continuous signal is the time reference for all DMA
events.
The terminator occurs at the end of the previous scan. TERM L goes low for one character clock ,md
then goes to an unspecified state which depends on the random characters that appear at the terminator detector gate. Character Clock continues to shift (lata through the character latches but invalid
data are present in the character latches and stored in the line buffer until just after DMA ENA is

4-80

.)
.)

..-'-.

.,---.• ,'.

·,--Y

,--",.

'-,.."./

._/

'VRESET

CHAR CLK

--tkj
H~f

TERM L

HORBLANKH

•

:
!---3CHARS

:
L___

1"---1

13ps
UNDEFINED REGION

-:11.

:
I

~3 CHARS-----i

HOLD REOUEST H

LI U u Lf-------UlJ
i 51 ps

-I,
---Ls:
I

:
I '
'SYNCHED TO END OF
iCHARACTERINDC012

I
I
:

---------~

~3CHARS____l
DMAENAH
E22-9

~
I

•

---------l-L

-----------------------~I

CHARACTER LATCH
DATA BUS
•

E15, LINE BUFFER, AND ADDRESS LATCH
OUTPUTS TRISTATED DURING THIS INTERVAL

.______________________________________________________

__________________________________+___ I

I
I
I

VALID DATA AVAILABLE
AT E150UTPUT

:
":
:

I
I

ADDRCOUNT

~------lrT

ADDR LD

-------LJ

WRITE LINE BUFFER

------~

I
I

MA-43S3

'VERTICAL RESET DURING
lSTDMAOF FIELD ONLY

Figure 4-6-19 DMA Cycle Timing Diagram

asserted. The terminator forces the video output to black or white (depending on normal or reverse
screen) effectively blanking the end of the line (Section 4.6.11). It also ends any DMA process in
progress. The terminator's blanking effect is taken over by Horizontal Blank, and the terminate flipflop in the DCOl2 is held cleared until just before the first character in the next scan. This prevents
undefined data from triggering the flip-flop and blanking the whole scan.
Horizontal Blank occurs 3 characters after Terminate in 80 column mode and 5 characters after in 132
column mode. It forces all video to black (regardless of normal or reverse screen) for the horizontal
retrace interval. The Horizontal Blank signal, as an output from the DCO 11, ends one-half character
before the video actually needs to unblank. This early transition releases the clear for the terminate
flip-flop in the DCOl2. The blank signal is resynchronized to Character Clock in the DCOI2.
Hold Request silences the microprocessor so that the video processor can DMA data out of the screen
RAM. It is initiated when the scan counter in use is equal to 0, or at the setting of the new scroll zone
flip-flop. The terminate flip-flop must be cleared for Hold Request to occur. The start of Hold Request
is delayed by three character times from Horizontal Blank to allow the counter in use to settle before
the O-boundary is detected. Hold Request is ended by the detection of terminate at the end of a DMA
scan.
DMA Enable is generated from Hold Request and the microprocessor's hold acknowledge (HLDA)
by discrete logic on the board (E22). It is enabled by the first rising edge of LBA4 if HLDA from the
8080 is available. This ensures that the 8080 has given up the bus before DMA ENA H enables the
DMA address counters onto the bus and drives MEM R low. DMA ENA H is cleared by the end of
HOLD REQUEST H. While the use of LBA4 ensures that enough time has elapsed from the start of
HOLD REQUEST H to guarantee that the 8080 is in its hold state, the use ofHLDA H on the D input
of the DMA Enable flip-flop is required to prevent HOLD REQ from preventing a power-up cycle in
the 8080.
The character latch data bus is shown to be tristated from the start of Hold Request until the start of
DMA Enable. Then it starts moving data through the video circuits.
Address Count begins three character times before the end of Horizontal Blank (Section 4.6.2.4).
Address Load is triggered by the falling edge of HOLD REQUEST H. It stores the line attributes for
the next line and loads the address counters with the pointer address bytes at the end of the D MA line
of characters. This address is the location of the first character of the next line to be DMA'd.
Write Line Buffer L is described in Section 4.6.2.3.
Figure 4-6-20 is an expansion of the DMA timing showing the contents of the various storage devices
at different times at the beginning of the active display of a scan.

4.6.11 Video Blanking
The display is blanked by Horizontal Blank during the horizontal retrace interval, and by Vertical
Blank during the vertical retrace interval. These hardware signals ensure that the CRT beam is turned
off while it moves backward through the active screen area. Horizontal Blank controls the video signal
inside the DCO 12. Vertical Blank controls the video signal at the video output circuit. Figure 4-6-21
shows a frame of video divided into segments of one horizontal and vertical movement of the electron
beam over the CRT face with respect to time. Figure 4-6-21 plots the screen and its invisible regions by
relating the screen area to the state of blanking at any given time. The top-left corner represents the
first visible dot position. The terminator position is the place where no data is displayed but H orizontal Blank has not yet taken effect. Horizontal Blank then takes effect all through the horizontal retrace
period and unblanks just in time for the first dot of the next scan. After 240 scans, Vertical Blank turns
off the beam while the beam returns to the top of the screen.
4-82

.)
)

r--"I Ir---'Lrlf
I

CHARACTER CLOCK

------~

Ti:RfYlINATE L

HO RIZONTAL BLANK

HORIZONTAL BLANK
I
;-3CHARSl? SYNCHED TO
I
END OF CHARACTER ~~_DC012 :

---

I
I
I
I

HOLD REQUEST H

--- --------L
I

DMA ENABLE H

I·

I
. I
I

I
ADDRESS COUNT

I
I
I

ADDRESS LOAD L

--- --------~
I

VALID DATA AVAILA BLE
AT E150UTPUT

-u---u---u--u

11.n.rU--.,....r---1J

I I

----L-j

I
I

WRITE LINE B U F F E R L n J l . . f l j " I l f r - t . J 1 S l l S l S

--,I

l.SnJTLf~.J--U

CONTENTS OF ADDRESS COUNTER jADDR1 ADDR1 ADDR2 ADDR3 ADDR4 ADDR, ADDR6 ADDR7
CONTENTS OF ADDRESS LATCH ADDR1 ADDR1 ADDR1 ADDR2 ADDR3 ADDR~ ADDR5 ADDR6
DATA AT E20 IN CHAR1 CHAR1 CHAR1 IrHAR2 CHAR3 CHAR4 CHAR5 CHAR6
DATA AT E150UT
DATA AT E160UT
DATA LOADED INTO VSR

· .
·
· . .

CHAR1 CHAR1 CHAR1 CHAR2 CHAR3 CHAR4 CHAR5

• = UNDE FINED

CHAR1 CHAR1 CHAR1 CHAR2 CHAR3 CHAR4
CHAR1 CHAR1 CHAR1 CHAR2 CHAR3

HORIZONTAL BLANK "LANK BLANK BLANK BLANK BLANK
DISPLAYED DATA

CHAR1 CHAR2 CHAR3
MA·4669

)
Figure 4-6-20 Character Latch Timing

The horizontal blanking signal is timed to provide 83 columns of unblanking in 80 column mode, and
137 columns of unblanking in 132 column mode. The VT100 is currently programmed so that the three
or five extra columns are blanked by Terminate. In normal screen mode, this termination forces the
beam to black, but in reverse screen, termination forces the beam to the screen background intensity.
This means that in reverse screen mode, the last character that can fit on the screen is three or five
characters in from the right edge of the illuminated screen. Because hardware blanking allows 83 or
137 columns on the screen, future program developments may allow the microprocessor to format the
screen RAM for this ability. The main adjustment is to the position of the terminator and address
bytes relative to the starting location of the line. Terminate, as the first character in the line, creates
short lines during the vertical blanking interval to conserve memory while maintaining synchronization of the line address system.

4.6.12 Video Input and Output
Refer to the print set for the circuits discussed in the next three sections.
4.6.12.1 Direct Drive Video - The two video outputs from the DC012, Video Out 1 and Video Out 2,
are combined with Vertical Blank in two open collector nand gates (E7). The outputs of the open
collector gates are wire-ANDed with Graphic 1 and 2 IN (E29) from any option board that may be
present. The combined outputs of E7 and E29 pass through and either float the base of Q4 to + 12 V
through R38, R3 Q , ~nri R ,6 or R37, or pull Q4's base down to some value determined by one, the
other, or both resistors R36 and R37 in parallel. These four conditions represent the four levels of
intf't"lsity visihle 0:1 thE' screen~ black. dim. normal. and bright.

4-83

))

3 CHAR TIMES (80 COL)
5 CHAR TIMES (132 COL)
TERMINATOR

- - HORIZONTAL --+
TIME

HORIZONTAL
RETRACE
TIME

VERTICAL
TIME

DISPLAY TIME

)

+
BLANKED BY
VERTICAL
BLANK

~BLANKED~

SHADED AREA: BLANKING BY
TERMINATOR

f
))

BY
HORIZONTAL
BLANK
MA-4285

Figure 4-6-21 Video Blanking
4.6.12.2 Composite Video Out - The combined outputs of E7 and E29 control the voltage at Q2's base
through R31, R32, and R33 in the same manner as described above. At the same time, composite sync
controls Q2's base through R33 and R30. The result is a 75 ohm output from the terminal controller
consisting of four intensity levels (including black) and composite sync. This signal can directly drive a
standard video monitor or sync can be extracted to synchronize an external device for input to the
terminal. The output is dc coupled. Although the use of dc coupling is not in strict agreement with EIA
specification RS-170, this presents no problem with most monitors because they are usually ac
coupled. See Figure 3-4 for an illustration of the composite video output.

))

4.6.12.3 Video In - The video input stage terminates a standard video signal in 75 ohms. R43 and 07
bias Q5 into linear operation, while C 16 passes video around the bias network. R44 suppresses oscillation in the stage and D6 protects the transistor from a reversed polarity input. D5 biases the base
of Q3 and R41 is the load resistor for the amplifier. R42 stabilizes the amplifier by emitter feedback.
The video input affects only the picture on the internal monitor as described below; it does not appear
at the Composite Video output.
4.6.13 Intensity Control
The video input and output circuits can produce a range of voltages (as compared with circuits that
can produce only two values, white or black levels for example). To do this they are biased into linear

4-84

'j )

operating conditions. 'High current biasing provides the necessary high speed operation. Because of the
high power dissipation that results, the video circuits use discrete transistors.
Direct Drive Video is the output from the terminal controller to the video monitor's cathode driver
transistor. It is a combination of Video Out 1 and 2 signals from the video processor, and by means of
a parallel transistor Q3, the signal from the video input (labelled Graphics Video In on schematic sheet
BV4). The monitor already receives horizontal and vertical deflection signals directly from the terminal controller, so the terminal's video input only requires picture information. If the blanking level on
the video input is greater than zero volts, the screen background intensity cannot be black. Note that
composite sync on the input is also ignored by the terminal controller, because the terminal's timing is
produced entirely by a crystal and cannot be synchronized to external signals. As explained in Chapter
3, external sources may be synchronized with the composite sync signal that appears at the output jack.

1
(

)

Q3 and Q4, in parallel, are one amplifier with common emitter and collector resistors. The emitter
feedback resistor is R40, connected to + 12 volts. The collector load is the input impedance of the
monitor to ground in parallel with R54 and anyone of32 parallel combinations ofR49, 50, 51, 52, and
53. This arrangement allows either transistor to set the minimum current through the collector and
emitter resistors with the voltage that appears on its base. The other transistor then cannot reduce the
current by being turned off. It can increase the current, however, if it receives a larger base voltage.
Thus the output is always proportional to the greater of the two inputs. Two identical input signals do
not add up because of the common emitter resistor. If the voltage on Q3's base is more positive than
the voltage on Q4's base, a voltage at the base of Q4 causes a 0.6 volt different voltage at R40 (both
emitters), and current to maintain that value will flow through Q4 and R40 (as well as the collector
load). If the same voltage as at Q4's base is applied to Q3's base, the voltage at the emitters will not
change due to the identical voltage drops across the two base-emitter junctions. There is no current
change and so the output remains constant. 08 and R48 provide an extra 0.6 volts of bias for the
cathode drive transistor in the monitor. C 17 bypasses the diode for video signals.
The 32 combinations of resistors in the collector load of the output stage are produced by connecting
any of the selected resistors to ground through an open-collector buffer. The buffer inputs come from
the 0/ A (digital to analog) latch. This latch is written into as an I/O device by the microprocessor. In
SET-UP mode, the microprocessor uses the up and down cursor keys as inputs to a 5-bit software updown counter. At each vertical reset, the microprocessor writes the current contents of the counter into
the latch. Thus there are 32 possible intensity levels available, controlled from the keyboard. and
frequently updated to minimize the effects of soft errors. Note that the variable intensity only applies
to the internal monitor.
4.7 MICROPROCESSOR - VIDEO PROCESSOR INTERFACE
The microprocessor communicates with the video processor in the following ways.
I.

During setup, the microprocessor reads the setup specifications and writes them into the
DC011 and DCOl2 to establish screen attributes.

The contents of the screen RAM directly control the display of the lines and characters. This
region of memory contains the displayable characters, their attributes, the line attributes,
and the addresses that link one line to the next. The microprocessor modifies and updates
this information in the intervals between DMAs. During each DMA, the video processor
copies one line of characters from the screen RAM for display on the screen.

During smooth scrolling, the microprocessor updates the scroll latch in the DCOl2.

This section describes the processes of control through the screen RAM and scrolling. The meaning
and mechanism of the line and character attribute bits and the DMA process are discussed in the video
processor section. The setup process is discussed in the 8080 section.
4-85

4.7.1 Screen Memory Organization
Three bytes of control data are located at the end of each line of characters (Figures 4-7-1 and 4-7-2).
The first byte, called the terminator, is 7FH and is a unique character that the video processor recognizes as the end of the line. (The high bit is not tested by the terminator gate but is set to zero to avoid
complications in the attribute circuits.) Five bits of the next byte and all of the last byte are an address

))

DATA
80 CHARACTER SINGLE WIDTH
.
STARTING LOCATION
135

DATA
132 CHARACTER SINGLE WIDTH
41

83
BLANK

DATA
40 CHARACTER
DOUBLE WIDTH

~
6J..-.L:

COPIED FROM END OF LINE
135

DATA
66 CHARACTER DOUBLE WIDTH

Figure 4-7-1 Line Organization

HIGH ADDRESS
BYTE

TERMINATOR

~
10

(7FH,177Q)
I

SCROLLING REGI01\!

07
1

PART OF SCROLLING REGION
NOT PART OF REGION

05
1
0

o
o

1
0

1 ID7 0605 A

1=200H+
Q=4000H+
DOUBLE HEIGHT
DOUBLE WIDTH

LOW ADDRESS
BYTE

AlA

------

AI
I

\ \

ADDRESS (INPUT TO ADDRESS
COUNTER AT END OF DMA)

THREE HARDWIRED BITS ARE ADDED TO THE THIRTEEN
VARIABLE BITS. WHEN THE ADDRESS COUNTER CLEARS
AT VERTICAL RESET, IT POINTS TO THE START OF RAM.

NORMAL LINE 80/132
DOUBLE WIDTH 40/66
TOP HALF, DOUBLE HEIGHT
BOTTOM HALF, DOUBLE HEIGHT

ALL LINE ATTRIBUTES APPLY TO THE LINE
STARTING AT THE GIVEN ADDRESS.

MA·4664

Figure 4-7-2 Terminator and Address Bytes

4-86

) I

pointing to the first character of the next line to be displayed. The three remaining bits in the first byte
define the line attributes of the line pointed to by the address. The high bits of the address are hardwired at the address latch so that when vertical reset clears the address counter, it will point to the
ROM-RAM boundary at 2000H.
During power-up or reset the microprocessor writes terminators and addresses into the screen RAM
according to the specified line length arid refresh rate. The 50/60 Hz refresh choice causes the microprocessor to arrange fill lines to place the beginning of the display in the right time slot relative to the
vertical reset and vertical blanking signals. The line length determines the locations of the control
bytes. For 80 column lines, the memory space is arranged in 83 byte intervals; for 132 columns, the
interval is 135.

)

The memory organization for 80 column, 60 Hz mode is shown in Figure 4-7-3. Location 2000H is the
start of the RAM space. When Vertical Reset resets the DMA address counter in the video processor
to zero, the counter latch points to this location. The first 18 bytes are fill lines. Byte 2 is the only one
written differently when 50 Hz refresh is selected. The change causes a longer fill time during the longer
vertical blanking interval.

About 700 bytes of RAM are reserved for the microprocessor stack, scratch pad, and setup areas. The
rest of the 3K RAM is devoted to screen information. At program start, the microprocessor reads the
contents of the NVR into the setup area (Section 4.7.11). Then the microprocessor reads the setup
parameters, erases the screen area and writes in terminator and address/attribute bytes at the selected
line length intervals. The address at the end of each line points to the first location of the next line. At
start-up, this is the next physical location. The end of the last line points to a fill line which points to
itself. The fill line repeats until vertical reset.
4.7.2 Fill Line Operation
The video processor clock is constant so it always takes the same amount of time to refresh the screen.
At any refresh rate there are some fill lines needed at the, beginning of the frame so the data can be
displayed starting a few lines down from the top edge of the picture tube. At the slower refresh rate, the
video processor must idle for awhile between frames. Figure 4-7-4 illustrates the vertical position of the
electron beam in the CRT as a function of time for the two refresh rates. The beam sweeps down the
display area at the same rate for both refresh rates but because 50 Hz has a longer interval between
sweeps, the beam travels farther off the ends of the display. While the beam is off screen, fill lines
maintain synchronization in the video processor address system. More fill lines are needed at the top
and bottom of a 50 Hz screen than a 60 Hz screen. Figures 4-7-5 and 4-7-6 show the top-of-screen fill
line operation for both refresh rates. Starting at location 2000H, the terminator-address triplets point
successively to one another and then to the first displayable line of data in the screen RAM. The
change of one byte in the group changes the fill line delay from two lines for the 60 Hz refresh rate to
five lines for the 50 Hz refresh rate. Figure 4-7-.7 shows how the single bottom fill line repeats itself as
many times as needed until vertical reset stops it and clears the address counter to 2000H.

4-87

0123456789 ABCDEF
300
01
02

4- K X 4

2C
2D

2E
2F
30
31
32

ATTRIBUTE
RAM

FF~

__________________J
MA-4268

Figure 4-7-3 Screen RAM Organization - 80 Column, 60 Hz
(Sheet 1 of 2)

4.7.3 Line Organization
In 132 column mode 25 lines are set up in the RAM for a 24 line screen, and 15 lines are set up for a 14
line screen. With the advanced video option, 25 lines are set up for either 80 or 132 column lines.
Except during a smooth scroll, only 24 lines are seen. But the microprocessor keeps the 25th line erased
in reserve, and when a scroll takes place, the 25th line becomes visible as the new top or bottom line
(depending on scroll direction). All new characters after the command that caused the scroll go into
this line. In preparation for a full screen scroll up, the microprocessor writes the address of the repeating fill line into the 25th line pointer as well as the 24th line pointer. This saves processing time during
the address rearranging which is a part of each scroll.
The line organization changes when the first scroll occurs. Pointer addresses are revised any time a line
is added to or removed from the screen. The extra line is used whenever the screen needs to be scrolled
up or down, Then the extra line is displayed and the old top or bottom line is scrolled off screen,
erased, and made available as the new 25th line.
To understand the pattern of address changes, consider the display to consist of two parallel entities: a
physical screen and a logical screen.

4-88

for 50 Hz

)

0123456789ABCDEF
200 7F 70:O~ 7F F2 DO 7F 70 10617F 70 06 7F 70 OF 7F
01 70 03 ~ ~
02

/'~ ~ [,:Wi'l,,; ~ ;;;-.

t:m ~ ~ ~ ~j;
"'~

03 ~ %

3K X 8

~:~ ~1

c'/'/-

STACK

/.')"////

'7';"

W/h
~;;

04 l'X~ ~~ if~ ~~ r//0
05

-%!. 0;

~~~~~~~-L~~~~~~~~~~

SCRATCH
PAD

06
SETUP AREA

)

~g D A

2F
30
31

FOR

~r-+-+-4-~~-+-+-4~--~+-+-+-4-~

NOTE: ADDRESSES ARE
TYPICAL BUT MAY VARY

~~+-+-~~~-+-+-4~~~+-+-+-~~
~r-+-+-4-~~-+-+-4~~~+-+-+-4-~

32 7 F F3123 D

N D

FOR

SEC

N E

)

SCREEN
RAM

END
BF N

)

FIR

LAS

E 7F 70 06

CO
ADVANCED VIDEO
SCREEN RAM

FF~

1K X 8

________________________________

MA-4273

Figure 4-7-3 Screen RAM Organization - 80 Column, 60 Hz
(Sheet 2 of 2)

- --

4-89

~BEAM

POSITION
ON SCREEN

2 SCANS FILL

~--;---60 HERTZ FRAME TIME ---~

I r2~SCANs-BOTTOM

-_--.l_ . (2.5L1NES) FILL

ACTIVE SCREEN_T_IM_E_----.l
___
r - - - - - - - - 5 0 HERTZ FRAME TIME

VERT
RESET
MA·4281

Figure 4-7-4 Need for Fill Lines

))

STARTING
ADDRESS AFTER
VERT RESET

HARDWIRED

•

ADDRESS POINTS
HERE

! 2

+
3

~ o0 O~~~
__;_--~~~:-~--~~~~--~--~. .~--~--~~~~--~--~~~
1
I

!----"-----'

I
I

TWO FILL LINES AT TOP OF SCREEN

I
2

CROSSHATCHED BYTE = TERMINATOR
60 HERTZ FILL LINE OPERATION

Figure 4-7-5 Fill Line Operation - 60 Hz
4-90

MA-4360

STARTING
ADDRESS AFTER
VERT RESET
CHANGE FROM 3 TO 9
FOR 50 HERTZ

)

I
I

I
I
I

FIVE FILL LlNESATTOPOF SCREEN

2
CROSSHATCHED BYTE = TERMINATOR
MA-4357

}

Figure 4-7-6 Fill Line Operation - 50 Hz

STARTING ADDRESS
AFTER VERT RESET
HARDWIRED

)

dJo

+
0

0
I
I

I
I

I
I
I

2 D

FILL LINE REPEATS UNTIL VERTICAL RESET

I
I

[~---~

)

CROSSHATCHED BYTE =TERMINATOR

Figure 4-7-7

End of Screen Fill Line Operation

4-91

4.7.3.1 Physical Screen - The physical screen is the memory organization already described. The key
feature of the physical screen is the pointer address, contained in each line, that causes the hardware to
chain the lines into a sequence for display. After a series of split screen scrolls, the physical screen will
contain some arbitrary sequence of line addresses.

\
))

For example, if the original order of lines was 1,2,3, ... 23,24, the new order might be 16,13,24, ... 1,8.
With only this organization, a command to insert a line at the fifth position on the screen would
require the microprocessor to follow the various pointers around the RAM until it reached the fourth
such pointer in order to learn where the physical address of the current fifth line was in the RAM.
Instead, a list of locations is maintained in the logical screen.

4.7.3.2 Logical Screen - The logical screen is a 25-entry table that points the microprocessor to the
proper locations for shuffling (rearranging) line addresses and placing the cursor in the screen RAM.
The table is set up in a contiguous area of memory starting at the location named LATOFS (Line
Address Offset Table). Figure 4-7-8 shows LA TOFS after at least one split screen scroll and in the
process of another split screen scroll. Each entry in LA TOFS is the number of a line in the physical
screen. The position of an entry in LA TOFS refers to the position of a line on the screen. The microprocessor updates LA TOFS just before each scroll so that the microprocessor can rewrite line addresses during the vertical interval. The microprocessor reads the table to learn which line in RAM is
available for writing as the new 25th line or which line is being used at a given position on the screen.
For example, to insert a character in the fifth line, if the fifth entry in LATOFS is "20," the microprocessor calculates the starting address of the 20th physical line in RAM. The character address can
be calculated from that starting address and the cursor position within the line.
4.7.4 Address Shuffiing
Consider the case of a full screen upward jump scroll. The line in LA TOFS #25 is the extra line, not
seen except during smooth scrolls. When a line is jump-scrolled off the screen, it becomes the extra
line, and the RAM area that was line 25 becomes the new last line. The scrolled-off line is erased and
its first location is noted as the starting point for new data entry into the RAM. The physical screen
addresses that chain in which the displayed lines must be revised. And since the last fill line must point
to the new first line, which formerJy was the second line, the revisions must be done before that last fill
line is DMA'd into the video processor.

))
.

\))

When a line feed is received, four numbers are stored in memory. These are Shuffle Address 1 and 2
(SHUFAD) and Shuffle Data I and 2 (SHUFDT). SHUFAD contains the location of the address that
must be changed, and SHUFDT contains the new address that will be inserted there. The two sets are
for the pointer above the line that will be scrolled off, and for the pointer on the end of the line that w i l l ) )
be the new next-to-Iast line (Figure 4-7-9).
The process of changing the addresses is called shuffling because most of the computation is done in
advance. The change is made quickly in a simple point-swap-point-swap sequence. One other item
prepared in advance is the pointer on the 25th (to become the new last) line. The pointer on the present
last line is copied onto the new line. The shuffling process takes less than 550 microseconds (the time
between DMAs). The screen's apparent jump up or down is entirely the result of the revision of pointer
addresses. If, for example, the shuffle occurs during the 15th line, no effect will be visible until the end
of the frame, when the extra line is pointed to but is outside the viewable area. After vertical reset,
however, the shuffled address at the top of the screen will point to the old second line. The first line is
gone, and now there is room on the screen for the extra line and it appears.

4-92

LATOFS

)

seR OLUNG

REG ION

~
)

t
22
25

#10

#11

#12

#13

#14

#15

#16

#17

,#18

#19

#20

#21

#22

#23

#24

(EXTRA) #25

25
20

t
MA-4305

Figure 4-7-8 Line Address Offset Table

4-93

SHUFAD 1

I FILL 2 PNTR I
SHUFDT 1
LINE 2
BEFORE
SHUFFLE

PNTR = LOCATION
OF POINTER ADDRESS

AFTER
SHUFFLE

SHUFAD 2

I LINE 24 PNTR I
-'

SHUFDT 2
EXTRA
AFTER SHUFFLE

----EXTRA LINE
MA·4294

Figure 4-7-9

))

Full Screen Address Shuffle

4.7.5 Shuffle Timing
The shuffles for smooth and split screen scrolls must be synchronous with vertical reset to avoid
disrupting the appearance of the display. Therefore, only one line feed can be executed during a frame.
However, in the case of full screen jump-scrolling, the address shuffle may occur at any time. This
increases the rate at which line feeds can be executed and improves the terminal's throughput.

While full screen jump scrolling may occur at any time relative to vertical reset, the two other kinds of
scroll are sensitive to the time when shuffling occurs. If a split screen scroll region (say 10 lines in the
middle of the screen) is revised while the display is scanning that region, only the change at the bottom
of the region would take effect during that frame. The extra line would appear at the bottom of the
region and the following lines on the screen would be pushed down one line from their locations in the
previous and succeeding frames. The appearance would be that of a flash of mixed-up data below the
scrolling region. Therefore, all changes are made during the vertical interval, ensuring that the proper
number of lines is consistently displayed because the video procesor cannot ever attempt to execute
only part of a shuffle sequence.
LogicaIly the screen still flashes in full screen scrolling, but because the extra last line is displayed
beyond the end of the screen, Vertical Blank blanks the display. The shuffle is a two part process, with
the two address bytes transferred in separate operations. If the shuffle is not complete before the DMA
that accesses the shuffled address starts, the address that gets read may not be valid, and garbage may
be displayed thereafter. The random start of the shuffle process relative to the video processor's DMA
timing causes this error.

4-94

}

)

4.7.6 Scrolling Region
A scrolling region may be established on the screen, on a horizontal line basis, within which data may
be inserted or lines scrolled without affecting the positions of data outside the region. Only one region
may be defined at a time, but it may be repeatedly redefined. Full screen scrolling is a special case in
which the region margins and the screen margins are the same. A control sequence defines the region
by specifying the line numbers of the top and bottom of the region. When the control sequence arrives,
the microprocessor stops taking characters from the silo and waits for the current scroll to end. It then
checks the parameters for legality (top less than bottom, bottom less than 14 or 24). If they are bad, the
sequence is ignored and the next character is taken from the silo. If they are good, they are stored in
locations labelled Top and Bottom. Then starting at the pointer address on the last fill line, the microprocessor sets the fill line's scroll attribute bit to scroll or no scroll (depending on the specification for
top margin) and jumps to the end of the addressed line, setting its scroll attribute bit. The microprocessor continues down through the screen RAM until it reaches the point to line #Top,the line that
will be at the top of the scrolling region .. It sets the attribute bit there to scroll. Jumping down through
the region, the microprocessor continues setting line attributes to scroll until it reaches line #Bottom,
the bottom line in the scrolling region. From there to the end of the screen it sets the bit to no scroll.

)

)
j

4.7.7 Split Screen Jump Scrolling
When a line feed is received, during the remainder of the current frame the microprocessor rotates
LA TOFS and prepares for the shuffle. The line number at LATOFS #25 (25 in Figure 4-7-8) moves up
to the bottom position in the scrolling region (LATOFS #8). The line at LATOFS #5 (20) moves to
LA TOFS #25,.and all the other lines in the scrolling region of LATOFS are moved up one position. The
shuffle buffers are set up. (See Figure 4-7-10.) SHUFAD 1 contains the location of the pointer address
at the end of the last line before the scrolling region and SHUFDT 1 contains the pointer address for
the new first scrolling region line. SHUFAD 2 contains the location of the pointer address at the end of
the new last scrolling region line and SHUFDT 2 contains the pointer address for the first line after the
scrolling region.
While the LATOFS and shuffle preparations are underway, data may be coming into the terminal.
Data received after the line feed stay in the silo until the LATOFS rotation is completed. Just before
rotation, LATOFS 25 points to the number of the next RAM line that data should go to. The microprocessor records that location and then performs the rotation and shuffle preparation. Then it resum~s
writing the data into the line previously noted. When vertical interrupt occurs, the shuffle is performed
and the new line is displayed. If no new line feed is received, data will continue to be written into the
same line.

4.7.8 Smooth Scroll
Become familiar with the operation of the scan counter as described in the video processor section
(4.6.3.3) before studying this description.
In smooth scroll operation, the VTl 00 changes the positions of the lines on the screen slowly, so the eye
can follow and read the data as it goes by. Instead of moving one character height or 10 scan lines in a
single frame, the data moves up or down one scan line in each frame. The smooth scroll rate is thus 6
lines per second at 60 Hz frame rate. Operation is the same for full or split screen smooth scroll. For
any given line feed, the effect on the screen is delayed by two frames. Line feeds may queue up so that
scrolling is continuous.
Smooth scroll is enabled by a bit in the setup memory that causes the microprocessor to perform all
scrolls as smooth scrolls. Whenever a line's scroll attribute bit is set, it receives its scan count from the
offset counter. At vertical reset, the scan counter, used for nonscrolling lines, always resets to zero, but
the offset counter is loaded with the contents of the scroll latch. The microprocessor loads the scroll
!~t~h with an offset value. If a jump sc!'c!! i~ involved, the offset is always zero. If a smooth scroll is
iilvolved, the microprocessor loads an offset value according to the number of frames that have passed
si!:'~'~ th.' line hf:'e~)" mrtvine

4-95

SHUFAD 1

BEFORE
SHUFFLE

SHUFDT1
LINE 6
PNTR = LOCATION
OF POINTER ADDRESS

AFTER
SHUFFLE

--- ---

SHUFAD 2

"'-

r---------------------------------~

BEFORE
SHUFFLE

I
I

LINE 8 PNTR

SHUFDT2
EXTRA

AFTER
SHUFFLE
SCROLLING REGION IS LINES 9·15. AT SHUFFLE, .LINE 9
DISAPPEARS, REGION MOVES UP, EXTRA LINE APPEARS
TO FILL SPACE BETWEEN LINES 15 AND 16.
MA·4662

Figure 4-7-10 Split Screen Address Shuffle
When a line feed is received the microprocessor waits for the current scroll to end. It sets an internal
scroll-pending flag. Then, at the next vertical interrupt, during the frame before scrolling begins. the
shuffling process makes its preparations but only to add the 25th line to the display. During smooth
scrolling, all the lines will be visible together. The microprocessor, which keeps count of the number of
frames in the scroll, loads the scroll latch in the DC012 with an offset value of one. Then, when the
second vertical interrupt occurs, the microprocessor performs the address shuffle, the offset counter
receives the 1 from the scroll latch, and the microprocessor loads the scroll latch with 2. As the frame
begins, the character generator ROM, driven by the offset counter, displays its second scan row on the
top line of the scrolling region. The rest of the scrolling region is moved up by the same amount. so the
last scan in the scrolling region is the top scan of the 25th line.
Succeeding frames contain the data moved up scan by scan as the microprocessor loads the scroll latch
with larger offset values each time. During the ninth such frame, en!y one scan from the top line and
nine scans of the bottom line are visible. The microprocessor loads 0 into the scroll latch. If the

4-96

))

microprocessor made no other change, then on the next frame, the data which had been moving slowly
up would jump back down to its original position. Therefore, another address shuffle is performed at
vertical reset. The top line of the scroll region is discarded and the 0 offset applies to the former second
line. The old top line is erased and becomes the new 25th line that will appear in the next scroll.

4.7.9 Split Screen Smooth Scrolling
In a split screen smooth scroll, the scan count to the character generator ROM must switch between
the normal scan counter and the offset counter. (Please remember that full screen scroll is a special case
scrolling region, and that jump scrolling is a special case offset count of 0.) The scrolling attribute bits,
which were set when the control sequence that defined the region arrived, operate the counter multiplexer (MUX).
Refer to Figure 4-7-11 and suppose that the smooth scrolling region extends from line 5 to line 8. The
scroll is shown at its midpoint. The scrolling attribute bits at the ends of lines "Fill" through three
define lines one through four to be outside the scrolling region. Each line's DMA has occurred when
the scan counter, which always starts together with the top scan on the screen, is equal to zero. A
DMA always occurs:

)

I.
2.

when the counter in use (MUX output) is zero, or
when the MUX first selects the offset counter.

The offset counter is loaded with its offset at vertical reset. The switch between scan counter and offset
counter may only occur when the scan counter (NOT the MUX output) is zero.

)

SCAN
COUNTER

OFFSET
COUNTER

COUNTER
IN USE

0--

(FILL)

0-5--

0--

)

SCROLLING ATTRIBUTE
READ AT END OF DMA

5-0-5--

0-0

5--

0-0

0--

B
D E F
G H I
J K L
A

LINE 1
LINE 2
AT NEXT SCAN COUNTER'
LINE 3

TO OFFSET COUNTER
LINE 4

'" R
P

LINE 5

LINE 6

S T U
V W X

LINE 7
LINE B

LINE 25

2 3 4
5 6 7

LINE 9

IVI

o BOUNDARY. WILL SWITCH

LINE 10

COUNTER STARTS AT 5.
SWITCH TO OFFSET
COUNTER CAUSES DMA.
NEXT DMA AT O.

SWITCH OUT OF OFFSET
COUNTER AT NEXT
SCAN COUNTER
o BOUNDARY. COUNTER
IN USE = O. THEREFORE
DMA.

MA~4282

Figure 4-7-11 Split Screen Smooth Scroll at Midpoint

---'I;
4-97

The scrolling attribute bit on line 4 enters the DCOl2 during the DMA. However, it does not cause the
MUX to switch until scan 0 of the next line. Because of the switch to the offset counter, line 5 begins
with a DMA but it starts displaying with scan 5 because of the offset counter. Its attribute bit enters
the DC012 at the endof the DMA. Five scans later the counter in use is 0, so another DMA occurs.
Because the scan counter did not reach 0 before the DMA, the scrolling attribute bit from line 5 is
ignored. Line 6 DMAs and its attribute bit enters the DCOl2. Line 6 is present for all JO scans and so
as the scan counter passes through zero, the DC012 accepts control from the line 6 attribute bit.

) )

This special case (pointer to the second line in the scrolling region while the smooth scroll is in progress) is the only occasion when an attribute bit is ignored. The double height and width bits in the same
byte are accepted at the moment the DMA for the corresponding line begins.
As line 5 begins to roll off the screen, the extra line in memory, line 25, becomes visible. It becomes the
new line 8 when line 5 is gone. Line 25 carries a non-scroll attribute bit because the next line (which the
address on line 25 points to) is not in the scrolling region. Line 25 was DMA'd in when the counter in
use was 0 so it starts with its top scan. But the attribute bit is accepted when the scan counter reaches 0
five scans later. Now the counter in use is 0 again because of the shift back to the scan counter so
another DMA occurs, bringing in line 9, which also carries a non-scroll attribute bit. The rest of the
screen thus appears in non-scroll fashion.

~} )

4.7.10 Cursor
The microprocessor keeps a running account of the cursor position. It starts at the reset position, line I
- column 1 (top left corner) and responds to any change in position that might result from receiving a
new character, a cursor position command, a line feed, etc. The microprocessor calculates the appropriate screen RAM address from the com bination of previous line and column plus change in position,
or from the specified line and column in a direct cursor address (DCA) control sequence. The microprocessor then records the current setting of the base attribute bit of the character at that address and
then periodically inverts the attribute bit, showing the operator that the cursor is located at the spot on
the screen where the attribute appears and disappears.

) )

When a line scrolls on the screen, its location in RAM remains the same. To keep the cursor at the
same screen location as before, representing the cursor moving down through the lines, the microprocessor calculates a new address for the cursor at.the same column number but in the RAM location
corresponding to the next line. Then, after resetting the attribute at the old location to its original
value, it records and then toggles the attribute bit at the new location.
The attribute bit that the microprocessor toggles and the appearance on the screen depend on the setup
selection of the cursor attribute and on the absence or presence of the AVO. If the AVO is not installed, the microprocessor toggles the eighth bit in each character word. A setup selection of reverse
video causes the microprocessor to set the DCOl2 to interpret its base attribute input (REV VIO H) as
reverse video. If setup specifies that underline is to be used for the cursor, the DCO 12 is set to interpret
the base attribute as underline instead of reverse. Notice that without the AVO the cursor selection
determines the appearance for all characters on the screen that have the attribute asserted. Furthermore, the microprocessor will accept the commands for either reverse or underline to assert the attribute.
With the A VO present, the microprocessor tells the DCD 12 to interpret the base attribute as reverse
video. The cursor selection in setup determines whether the microprocessor toggles the base attribute
bit (eighth character bit) to get reverse video or one of the bits in the AVO RAM to get underline. With
the AVO present, cursor selection is independent of attributes and all attributes are available at the
same time.
-

4-98

)

4.7.11 SET-UP
The SET-UP area is a portion of the scratch RAM. It contains 8-bit bytes that are passed between the
RAM and the NVR. Here is a list of the SET-UP area contents. This list is subject to change.

)

22 bytes
17 bytes
1 byte
1 byte
1 byte
1 byte
1 byte
I byte
1 byte
1 byte
1 byte
1 byte
1 byte
1 byte
1 byte

Answerback message (20 characters and 2 delimiters)
TabsencodedinbiU
80/132 column mode
Intensity
Mode byte for PUSART
On-Line/Local
Switchpack 1 (scroll, autorepeat, screen, cursor)
Switch pack 2 (bell, keyclick, ANSI/VT52, XON/XOFF)
Switchpack 3 (US/UK #, wrap, new line, interlace)
Switchpack 4 (parity sense, parity, bits/char, power)
Switch pack 5 (STP - visible only when option installed)
Transmit baud rate
Receive baud rate
Parity
Checksum for NVR

When the SET-UP key is pressed, the SILO locks and any scroll in progress is allowed to finish. Now
the 25th line (Extra) is available for one of the lines at the bottom of the screen. Another 135 bytes are
available as line Extra2 for use as the other of the two bottom lines, or for the NVR buffer area and the
"Wait" message displayed during NVR operations. Another 125 bytes are the SET-UP screen RAM.
They store the words "SET-UP A" twice (for double height) and the words "To exit press SET-UP"
with the three lines' terminator and address bytes, plus 19 more terminators and addresses for the 19
blank lines in the middle of the SET-UP screen.
For SET-UP A, the microprocessor performs a routine to fill the line at the bottom of the screen with a
ruler. The other free line displays the cursor and letters "T" to indicate tab positions. Then the microprocessor counts from the beginning of the tab bytes in the SET-UP area to find the bit corresponding
to the column number where the cursor is. If the bit is set, the microprocessor writes the letter "T" in
the current cursor position. Keys (Set/Clear Tab, Clear All Tabs) on the keyboard can toggle the
setting of the bit in the SET-UP area, and the microprocessor writes or erases the "T" to agree.
For SET-UP B, the microprocessor displays the data contained in the switchpack and baud rate area
by writing corresponding information into the bottom line. Switch pack 5 is only displayed if the
Option Present flag is asserted at the STP. The other free line is not written into but is used to display
the cursor position and to display the answerback message when the message is being entered. The
switch pack data is changed by a key on the keyboard (Toggle 1/0).
The non-switchpack data is changed by separate keys. These include parameters that display themselves - line/local (displayed in the LEDs) and 80/132 column, seen on the screen. Transmit and
receive are displayed numerically as is parity in later models. Switching between SET-UP fields and
starting Reset can also be done with separate keys on the keyboard.

4.8 MONITOR
Two monitors have been used in VT 100 prodection. Early VT 100s used DEC part n urn ber 30-14590 .
. Later terminals used DEC part number 30-16080. Most of the circuitry in these monitors is standard
television technique, and this description simply highlights the signal path. An overview of the general
principles of the horizontal section is included because this circuit is not intuitively understandable
from an examination of the schematic. It is also the likely candidate for failure because of high stresses
in the components.

4-99

4.8.1

Monitor Description: 30-16080 (Elston)

4.8.1.1 Video Driver - The cathode driver stage gets its operating supply of 38 volts from a winding of
the flyback transformer. R 108 is the collector load resistor. C 10 I bypasses the emitter resistor to
improve high frequency response. The stage is intended to be biased by the driving circuit (direct video
out in the terminal controller). R I OS couples the video signal to the cathode limiting current flow if the
CRT arcs.
4.8.1.2 Brightness - The brightness control circuit gets its -ISO volt operating supply from the horizontaloutput circuit. A charge pump (C104, ClOS, CRI03, CR104) produces a large negative voltage
by inverting the large positive swings in the stage. The brightness voltage returns to the + 38 volt video
output power supply, rather than ground, to allow the brightness control output to vary from -42 to
+ 17 volts. The brightness control potentiometer R 109 varies the bias on the first grid of the CRT.
C107 bypasses currents resulting from internal CRT arcs from the high voltage anode and also bypasses video signals from the grid to ground.
4.8.1.3 Vertical Oscillator - The vertical oscillator receives its synchronizing drive through inverterbuffer Q298. R331 and C314 are a low pass filter on the input for noise immunity. The input is accoupled so that if the drive stops in either a high or low state, the circuit can self-oscillate to keep the
beam moving on the CRT.

,
))

Basically,the oscillator is an RC timer with R333 and R334 through CR304 charging a capacitor made
of C318 and C319 in series. As the voltage across the capacitor rises, emitter follower Q309 drives the
output amplifier Q31 0 so that its collector current rises at the same rate. CR304 is two diode drops to
compensate for the drops in Darlington Q309 so the voltage at the base of Q31 0 is the same as the
voltage at the top of C318. When Q308 receives a vertical drive pulse. it discharges the capacitor.
R341 and R342 are positive feedback tothejunction of C318 and C319. As Q310's emitter voltage rises
with increasing current, the two resistors couple that voltage back to the timing capacitor. This makes
the voltage across the capacitor rise faster than it would with only the charging current from R333 and
R334. But because the rise in capacitor voltage causes Q310 to turn on more, the feedback voltage'
increases as well. The exponential rise in Q31O's output current that results closely matches the variations in L30 I 's current. This is explained more under Linearization (Paragraph 4.8.1.6).

4.8.1.4 Self-Oscillation - If the vertical circuit does not receive a drive signal, it oscillates by itself to
keep the electron beam moving to prevent a phosphor burn. The self-oscillation period is longer than
the period between vertical drive pulses so the drive pUlses, when present, always control the vertical
scanning rate.
Without a drive signal, Q308 is off. The regular RC circuit produces a rising voltage at the base of
Q310. While Q31O's output moves the electron beam, its base voltage is coupled to Q308 through R33S
and CR302. When the voltage rises above the diode drops of CR302 and Q308 base-emitter junction,
Q308 turns on, discharging the timer and cutting off Q3l O. The retrace voltage that forms at L30 I is
coupled by R344, C316, and R339 back to Q308 to keep it on long enough to complete a retrace even
though the voltage from Q31O's base (that started the cycle) has started to decrease. After the voltage
at L301 falls (due to current flow to the yoke), the drive to Q308 is gone, Q308 turns off, and the selfoscillation cycle begins again.

4.8.1.5 Vertical Output - Consider L301, which is a large inductor, to be a constant current source.
Assume that the yoke starts with a current that deflects the beam to the top of the screen, and Q310 is
off. The current flows out of L301 through C321 and into the yoke. Q310 now turns on with a gradually rising voltage on its base. Q310 thus draws an increasing amount of current from the junction of
L301 and the yoke. Because L301's current is relatively constant, this means that less current is avail-

4-100

-) )

able for the yoke, so as Q31 O'S current increases, the yoke's current decreases. Eventually Q31 0 passes
all of L301's current and the yoke current is 0 with the beam in the center of the screen. As the current
in the collector of Q310 increases further, the current in the yoke reverses and adds to the current
coming from L301. This oppositecurrent deflects the beam toward the bottom of the screen. Now the
vertical drive pulse turns Q310 off, and L301 produces a large positive voltage to try to maintain its
constant current. This voltage rapidly reverses the current in the yoke and makes the beam return to
the top of the screen very fast. Q310 turns back on and the cycle repeats.
C320, R338, and R340 limit the voltage across L301 during vertical retrace.
4.8.1.6 Linearization - Because L301 is not an infinitely large inductor, it is not a perfect current
source. Current through it decreases gradually until the beam reaches the center of the screen and then
increases again toward the end of the cycle. This non-ramp component of the current would cause
character height to vary from top to bottom if allowed to pass to the yoke. The exponential drive to
Q310 as a result offeedback causes Q3l 0 to accept the varying current from L30 I at the same time that
a ramp current through the yoke is maintained.

)

4.8.1.7 Horizontal Driver - The horizontal driver receives a TTL level drive pulse from the terminal
controller. R468 limits the base current to Q413. C435 provides noise immunity for the drive input.
When the input is high, Q413 is on. With pin 6 of transformer T403 pulled low by Q413, and with
about half the supply voltage at pin 4, there is approximately 6 volts across the primary winding 4-6.
The secondary winding 3-2 sees one quarter of this voltage across itself due to the 4: I turns ratio, but
no current flows because Q414's base-emitter junction is reverse biased. Meanwhile, current increases
through winding 4-6 while Q413 is on, storing energy in T403's core. When the drive signal falls,
turning Q413 off, winding 3-2 reverses polarity. Current now flows from T403 winding 3-2 through
Q414's base-emitter junction, Q414 saturates, and the horizontal output functions as discussed below'.
When the drive signal goes high again at the beginning of horizontal retrace, Q414 needs to turn off
very quickly to minimize dissipation. The purpose of opposite polarities in T403 is to force Q414 off hy
turning Q413 on. When Q413 turns on, winding 3-2 reverses polarity again and this voltage forces
Q414's base-emitter junction into a reverse biased state, rapidly discharging Q414's stored base charge
and cutting off Q414's collector current.
R470 limits the peak current through Q413. R470, and R471 (if present), limit the on time of Q414.
C443 (if present) speeds up the turn-off of Q414. C436 limits the peak voltage that develops across
Q413 caused by leakage inductance in the primary of T403 that prevents complete coupling of the
primary energy into the secondary. C437 filters Q413's power supply.
4.8.1.8 Horizontal Deflection Operation - The horizontal output circuit consists basically of Q414,
CR406, C438, C441,.and the horizontal deflection yoke. Assume that the beam is at the center of the
screen during a scan. Refer to the waveform diagram (Figure 4-8-5) for reference points TO through T4.

4-101

Figure 4-8-1 Horizontal Deflection Current - TO

YOKE

+15

C441

TO
MA-4670

TO:

Right Half of Scan (Figure 4-8-1)

Initial Condition:

Current through the yoke is zero. C441 is charged to + 15 volts. Q414 is on.

Action:

Current now flows out of the yoke through Q414, pushed by the voltage across
C44l. The voltage across C441 is nearly constant so the current through the
yoke's inductance increases linearly. As the current increases, the beam moves
to the right of the screen. The magnetic field building in the yoke stores energy.

))

Figure 4-8-2 Horizontal Deflection Current - Tl
C438

YOKE

T1
MA·4671

))

Tl:

Start of Retrace (Figure 4-8-2)

Initial Condition:

Current out of the yoke is maximum. Q414 is switched off. C438, which had
been grounded by Q414, has 0 volts across it.

Action:

Current continues to flow out of the yoke by inductive inertia as the stored magnetic field collapses. C438, which is Ii small valued capacitor, quickly charges
to a high voltage. This voltage opposes current flow and causes a rapid reduction in current. The beam quickly returns to the center of the screen.

Figure 4-8-3 Horizontal Deflection Current - T2
YOKE

T2
MA·4672

T2:

Middle of Retract (Figure 4-8-3)

Initial Condition:

Current through yoke is zero. C438 is charged to + 175 volts.

Action:

The high voltage across C438 causes a rapid rise in current into the yoke. The
beam moves to the left of the screen.

4-102

( )

Figure 4-8-4 Horizontal Deflection Current - T3
+15

C441

T3
MA-4292

T3:

Start of Scan (Left HalO (Figure 4.,8-4)

Initial Condition:

Current into the yoke is maximum. C438 is discharged.

Action:

Inductive inertia makes the top of the yoke slightly negative, forward biasing
CR406 to provide a return path for current out of the bottom of C441. The yoke
current gradually decreases as the magnetic field collapses and discharges into
C441 against the voltage across C441. The beam moves to the center of the
screen.

T4:

Return to TO

)

4.8.1.9 Horizontal Output Circuit - (Refer to Figure 4-8-5.) L403 adds a variable inductive reactance
that controls current in the circuit to vary the scan width. L402 is a magnetically biased inductor whose
reactance varies nonlinearly with changes in yoke current. This, with R481 and C442, provides correction for nonlinearity caused by the decaying exponential rate of current increase in the yoke caused by
the yoke's resistance. It does this by allowing more voltage across the yoke at the end of the scan. This
is needed because as current increases, the voltage drop increases across the resistance of the yoke,
tending to reduce the rate of current increase.
II

)

~r-

T1 T3
T2

CURRENT
THROUGH
YOKE
DAMPS

DVOLTS
•
- -L
VOLTAGE

ACROSS
YOKE

T1 T3
T2

MA-4930

Figure 4-M-5 Horizontal Detlection Waveforms

4-103

Power enters the circuit through the primary of the flyback transformer. CR408 and C439 work with
an autotransformer winding on the primary to boost the stage's operating voltage to + 15 volts. The
output circuit shuttles yokvbe current back and forth between C438 and C441, losing only a small
amount of power in resistive losses. Therefore, the average current through the flyback primary is small
(approximately 800 rnA). C438 is relatively small so that the voltage drop across its reactance can provide "S" correction for the horizontal scan. This compensates for the difference between the arc of the
tube face and the arc of the beam's deflection. Each time Q414 turns off, a 175 volt pulse, lasting the
interval of horizontal retrace, appears at the dotted end of the flyback primary. This puise passes into
the flyback secondary to provide high voltages for the CRT.

) )

4.8.1.10 High Voltage and Focus - When the 175 volt pulse appears on the flyback primary at retrace,
the fly back secondary produces 12 kV which is rectified by CR407 and filtered by the capacitor made
by the aquadag (graphite) coatings on the inside and outside of the CRT's glass envelope. Screen and
focus grid voltages of about 370 volts come from a lower voltage winding on the flyback and are rectified by CR409. For the cathode driver, 38 volts comes from another winding and is rectified by CR102.

4.8.2

))

Monitor Description: 30-14590 (Ball)

4.8.2.1 Video Amplifier - The video amplifier consists of Q 101 and its associated circuitry. The incoming video signal is applied to the monitor through J 10 1-8 and RIO I to the base of Q 10 I.
Transistor Q 10 I has a nominal gain of 15, and operates as a class B amplifier. Q 101 remains cut off
until a dc-coupled, positive-going signal arrives at its base and turns it on. R 103 provides series feedback that makes the terminal to terminal voltage gain relatively independent of transistor parameters
and temperature variations. R 102 and C 101 provide emitter peaking to extend the bandwidth to 12
MHz.

"") )

The negative-going signal at the collector of QIOI is direct coupled to the CRT cathode. The class B
biasing of QIOI allows a large video output signal to modulate the CRT's cathode and results in a
maximum available contrast ratio.
Overall brightness of the CRT screen is also determined by the negative potential at its grid which is
varied by the brightness control.
4.8.2.2 Vertical Deflection - QI02 is a thyristor used as a programmable unijunction transistor and
with its external circuitry forms a relaxation oscillator operating at a vertical rate. The sawtooth
forming network consists of A 101, C 103, and C 104. These capacitors charge exponentially until the
voltage at the anode of Q 102 exceeds its gate voltage at which time Q 102 becomes essentially a closed
switch, allowing a rapid discharge through LIO!. The rate of charge or frequency is adjustable by
A 101. The oscillator is synchronized by a negative pulse coupled to its gate from the vertical drive
pulse applied externally at J 10 1-9.
A divider network internal to AIOI sets the free running frequency by establishing a reference voltage
at the gate. This programs the firing of Q 102 and amounts to resistive selection of the intrinsic standoff
ratio. Frequency is controlled by passive components only. CR 101 provides temperature compensation for Q I 02 while controlling the gate impedance to allow easy turn on and off of Q I 02. L 101
forms a tuned circuit with C 103 and C 104 during conduction of Q 102 which provides a stable control
on the dropout time of Q! 02 to assist in maintaining interbce. Q 103 collector to base forward diode
clamping action prevents the voltage from swinging too far negative during this flywheel action.

4-104

))
"

The sawtooth at the anode of Q I02 is direct coupled to the base of Q I 03. This stage functions as a
Darlington pair emitter follower driver for the output stage Q 104. It presents an extremely high impedance in shunt with A 101 and prevents the Beta dependent input impedance of Q 104 from affecting the
frequency of the sawtooth forming network.
Linearity control of the sawtooth is accomplished by coupling the output at Q 103 emitter resistively
back into the junction of CI03 and C104. This provides integration of the sawtooth and inserts a
parabolic component. The slope change of the sawtooth at Q I 03 output is controlled by the setting of
A 102. The output at Q 103 is coupled into a resistive divider.
Height control R 110 varies the amplitude of the sawtooth voltage applied to the base of Q 104 and
controls the vertical raster size on the CRT. CI05 limits the amplitude of the flyback pulse at Q104
collector.

)

The vertical output stage Q 104 uses an NPN power transistor operating as a class AB amplifier. The
output is capacitively coupled to the yoke. LI provides a dc connection to B+ for QI04. It has a high
impedance compared to the yoke inductance that causes most of the sawtooth current of Q 104 to
appear in the yoke. R 114 prevents oscillations by providing damping across the vertical yoke coils.

4.8.2.3

Horizontal Deflection

Low Level Stages - The purpose of QI05 and QI06 is basically to process the incoming
horizontal drive signal into a form suitable to drive the output stage Q lOS. The duty cycle of QIOS
becomes essentially independent of the amplitude and pulse width of the drive pulse. This is a necessary condition to assure stability and reliability in the output stage. In addition, these stages provide a
horizontal video centering adjustment by delaying retrace with respect to the horizontal drive pulse.

4.8.2.3.1

)

The drive pulse is presented to Q 105 via] 101-6. The base circuit of Q 105 includes a clamp and a
differentiator that makes QI05 output insensitive to drive pulse amplitude and width changes. The
only requirement is that pulse amplitude be of 2.5 volts minimum and pulse width should be 10 to 40
microseconds. Q 105 with Q 106 functions as a monostable multivibrator with Q 107 being a slave that
provides a positive feedback. Specifically, when QI05 is turned on by the drive pulse, it discharges
CI12 at a rate determined by the setting of A 103. When CI12 is discharged to 1.75 volts, QI06 turns
off. This change of state turns QI07 on and the base drive to QI06 from RI2S is shunted through
Q107. Q106/Q107 remains in this state for nominally 25 microseconds until Cll2 recharges through
A 103 to S.25 volts. At this time, Q 106 is biased on again by the current through A 103. The multivibrator is now in a state in which QI06 is on and Q105/Q107 is off. It remains in this state until the
next drive pulse occurs or power is turned off. C 112 is the only timing capacitor in the circuit and has
two time constants associated with it. Primarily, the charge path between pin I and pin 3 of A I 03
determines the on time of Q 107. The discharge path through the video centering control and Q I 05
determines the delay between application of the drive pulse and the start of retrace (turn on of Q 107).

4.8.2.3.2

High Level Stages - These stages consist of Q 107 driving the output stage, Q lOS, and its
associated circuitry through TIOI. QI07 is an inverting slave of QI06 and is driven alternately into
saturation and cutoff as are all stages in the horizontal circuit. Q I 07 output is transformer coupled to
the output stage with phasing of TIOI chosen such that QIOS turns off when QI07 turns on. This
allows QIOS to turn off quickly, thus minimizing dissipation. A careful review shows that QIOS turns
off at a variable delay time after receipt of the drive pulse. This action causes retrace to begin.

------i
.I

4-105

During conduction of the driver transistor, energy is stored in the coupling transformer. The polarity
at the secondary is then phased to keep QI08 cutoff. As soon as the primary current of TIOI is
interrupted due to the base signal driving Q 107 into cutoff, the secondary voltage changes polarity.
Q 108 now saturates due to the forward base current flow. This gradually decreases at a rate determined by the transformer inductance and circuit resistance. However, the base current is sufficient to
keep Q 108 in saturation until the next polarity change of T 101.

))

The horizontal output stage has two main functions:
I.

to supply the deflection coil with the correct horizontal scanning currents and

to develop high voltage for the CRT anode and de voltage for the CRT bias, focus, and
accelerating grids, as well as the dc voltage for the video output stage.

QI08 acts' as a: switch that is turned on or off by the rectangular waveform on the base. When it is
turned on, the supply voltage plus the charge on C 123 causes deflection current to increase in a linear
manner and moves the beam from near the center of the screen to the right side. At this time, the
transistor is turned off by a polarity change ofTIOI that causes the output circuit to oscillate. A high
reactive voltage in the form of a half cycle negative voltage pulse is developed by the deflection coil
inductance and the primary of T2. The peak magnetic energy that was stored in the deflection coil
during scan time is now transferred to CI22 and the deflection coil distributed capacity. During this
cycle, the beam is returned to the center of the screen.
The charged capacitances now discharge into the deflection coil and induce a current in a direction
opposite to the current of the previous part of the cycle. The magnetic field thus created around the
coil moves the scanning beam to the left of the screen.
A fter slightly less than half a cycle, the decreasing voltage across C 122 biases the damper diode CR III
into conduction and prevents the flyback pulse from further oscillation. Magnetic energy that was
stored in the deflection coil from the discharge of the distributed capacity is now released to provide
sweep for the left half of scan ar.d to charge CI23 through the rectifying action of the damper diode.
The beam is now at the center of the screen. The cycle repeats as soon as the base of Q I08 becomes
positive with respect to its emitter.
C 123 serves to block dc current from the deflection coil and to provide "S" shaping of the current
waveform. "S" shaping compensates for stretching at the left and right sides of the picture tube because the curvature of the CRT face and the deflected beam do not follow the same arc.
L I 03 is an adjustable width control placed in series with the horizontal deflection coils. The variable
inductance allows a greater or lesser amount of deflection current to flow through the horizontal yoke
and varies the width of the. horizontal
scan.
.
Linearity control is provided by modifying the deflection coil voltage. During retrace, an auxiliary
winding on the fly back transformer supplies a pulse that charges C 119 through rectifier diode CR 112
and L I 02. This voltage is then applied in series with the deflection coil when the damper diode turns on
at the start of trace. The voltage is sawtooth shaped and has the effect of decreasing the deflection coil
current as a function of the sawtooth shape. This compensates for the stretch normally found on the
left side of the screen due to the deflection coil and system RL time constant. Linearity is optimized by
adjustment of L 102 that acts as an impedance to the pulse from T2.
The negative flyback pulse developed during horizontal retrace time is rectified by CR 110 and filtered
by C 117. This produces approximately -130 V dc which is coupled through the brightness control R 117
to G 1 of the CRT.

4-106

)

This same pulse is transformer-coupled to the secondary of T2 where it is rectified by CR2. CR 113.
and CR 114 to produce rectified voltages of approximately 12 k V, 400 V, and 32 V respectively. 12 k V
is the anode voltage for the CRT, 32 V is used for the video output stage, and the 400 V source is used
for 02 and 04 voltages for the CRT.

4.9

POWER SUPPLY

The VT100 series power supply is a switching supply that has 95 watts capacity with four separate
voltage outputs. Figure 4-9-1 is a block diagram of the power supply.

4.9.1 Power Input
The input to the supply consists of an electromagnetic interference (EMI) filter. an on/off switch. a
fuse, and a 120/230 volt select switch. Either on 120 Vac or 230 Vac input. thermistors Rl and R50
reduce the start-up inrush currents to safe levels. For 120 Vac operation a voltage doubler rectifieris
used; for 230 Vac operation a full wave bridge rectifier is used. The storage voltage across both C9 and
C 14 varies with the input line voltage from 200 to 360 Vdc. R 19 and R20 are bleeder resistors.

)

4.9.2 Start-Up Circuit
Transformer T2 is a start-up transformer that supplies the power to start the control circuit. The ac
voltage on the primary of T2 is stepped down. rectified. and filtered by C33. The dc voltage is applied
to the input of + 12 volt regulator Z2. The output passes through two diodes D24 and 025 to the
control circuitry. When the unit is turned on. the only voltage supplying power to the control circuit is
from the regulator Z2. After the outputs come up. the + 12 output is fed back through diode D34 to the
control circuitry. This voltage. being higher than that delivered by the start-up circuit. will back bias
D24 and 025. isolating the Z2 regulator. This scheme of returning the output through 034 enables the
power supply to also meet its ride-through specification (ability to supply power during brief outages).

INPUT
SWITCHING
RECTIFIER·
r+ FILTER
f-+ TRANSISTOR ~
Q7
400 VOLT

AC
IN

-23 VOLT
RECTIFIER·FILTER

-12 VOLT RECTIFIER
AND REGULATOR

MULTI· f-+
TAP
30 KHZ
TRANS·
FORMER f-+

)
4

START·UP
SUPPLY
12 VOLT

5 VOLT RECTIFIER·FILTER
AND CURRENT LIMIT SENSE AMP

I--

12 VOLT RECTIFIER·SWITCHING-REGULATOR-

f-+ FILTER &CURRENT LIMIT SENSE AMP

t
PULSE WIDTH SYNCHRONIZATION
SLOW
TURN-ON ~ MODULATOR
E4
CIRCUIT

PULSE WIDTH
MODULATOR
E2 & E3

MA-4295

Figure 4-9-1 VT100 Power Supply Block Diagram

4-107

4.9.3 Control Circuit
The control circuit in the VT100 power supply is designed around the 3524 pulse width modulator
(PWM) Ie. It houses a voltage reference, error amplifier, oscillator, pulse width modulator, pulse
steering flip-flop, dual alternating output switcher, current limit circuit, and a shutdown circuit. Refer
to the regulator Ie block diagram in Figure 4-9-2 and the power supply timing diagram in Figure 4-9-

))

REFERENCE 1 - - - - - " - -..... +5V TO ALL
REGULATOR
INTERNAL CIRCUITRY

OSCILLATOR

+5V

+5V
FLIP
FLOP
OSCILLATOR I--_ _..L.-_ _~

CT
(RAMP)

COMPARATOR

-= +5V

+5V
INV. INPUT

N.I. INPUT
GROUND
(SUBSTRATE)

1K
10 t--'Vll'If-----,.-+-I

COMPENSATION

SHUT DOWN
10K

MA-5265

Figure 4-9-2 3524 Regulator Block Diagram

--))

4-108

t
RAMP -1

jL ~
DC lEVEL
OSC. OUTPUT

FLIP FLOP OUT
TO NOR 13,14
OUTPUT OF
COMPARATOR
OUTPUT OF
NORGATE 13,14

)

013,14 Call

02 BASE

L--1

.--'""'1

l..-'1

.--1

L 1.0- I---

L I--"

L ~~

r
I

I- ~

l-I-I-

02 Call
Q7

)

V CE

FLIP FLOP OUT
TO NOR 11,12

PIN 6,7

)

PIN 3 555
COll01

08V CE
Q7 Call CURRENT

"- ~

I
I

5 .IV

I
II

---

....

PIN 2 555
PIN 5

-----~

-----

r--

- lJ
-n

I-

- V--

I--

I-

MA-4873

Figure 4-9-3 Power Supply Timing Diagram

4-109

The oscillator frequency is determined by resistor R I 8 and C7. The ramp generated by C7 is used as a
reference for the comparator. The discharge time of C7 determines the pulse width of the oscillator
output pulse. This pulse is used as a blanking pulse to ensure that the outputs of the 3524 cannot turn
on simultaneously. Capacitor C5 ensures that the blanking pulse is wide enough.

The error amplifier in the 3524 is a transconductance amplifier with an output (pin 9) impedance of
approximately 5 megohms. Pin 9 is a convenient point to place the compensation network, R30, C 17
and R 13. It is also an ideal point at which to turn off the switching regulator and also to put the slowturn-on circuit. The slow-tum-on circuit consists of 039, R51, 040, and C34. When the power supply
is initially turned on, pin 9 is pulled to ground by C34 through 039. As C34 charges slowly and
exponentially, pin 9 follows, turning the power supply on slowly and exponentially. The current that
charges C34 comes from pin 9 and the + 12 volt start-up. Pin 9 operates at a de level that is determined
by the input error voltage into the amplifier. C34 eventually charges to + 12 volts, thus back biasing
039. When the supply is turned off, C34 discharges through 040, allowing the supply to be ready for
another soft start.
The inverting side (pin I) of the input to the error amplifier is tied to the + 5 volt output through a
))
dividing network, R32 and R48. The noninverting side (pin 2) is referenced to a 5.1 volt zener, D38.
through a divider network, R41 and R49. An error appearing at the input causes pin 9 (output of the
error amplifier) to shift. This dc level shift is tied into an input of a comparator. As mentioned earlier,
the ramp generated by C7 is tied into the other input of the comparator. This is the means by which the
regulator is pulse width modulated. The actual frequency of the oscillator is 60 kHz. The switching
frequency of the power supply is one-half that of the oscillator, 30 kHz, because only one of the singleended outputs is used. This output, pin 13, a pulse width modulated wave, turns transistor Q4 on and
off. When Q4 is off, Q3 supplies base current to Q2, turning it on. D7 and D8 are antisaturating
diodes. R 14 limits the base drive, and R 15 and C4 are a speed-up circuit to turn Q2 on and off f a s t . ) )
When Q2 is on, Q7 (high voltage switch) is off. Current flows from + 12 through R II, D6. and winding
/
5,6 of pulse transformer T3 through transistor Q2 to ground. The circuit remains in this state until the
base drive to Q2 is removed by the control circuit. When Q2 turns off, the voltage across T3 winding
5,6 reverses, causing all the dotted ends of windings in T3 to become negative. Current starts to flow
into the base of Q7. This causes collector current to flow. This current flows through winding 1,2 of
TI, through the transistor and winding 2, I of T3. The current in winding 2, I of T3 produces current
flow in windings 3,4 and 7,8 of T3. The action that takes place is regenerative and causes Q7 to snap
on. When Q7 conducts, energy is transferred from the primary of TI to the secondary. The positive
voltage at terminal 5 of TI charges capacitor C6 through DI4 and R21. The voltage at terminal S
depends on the conduction time of the switch. When the control circuit switches Q2 on, C6 discharges
))
through D9, winding 5,6 of T3 and transistor Q2. The current through winding 5,6 of T3 makes all
dotted ends positive on T3. The negative polarity on the base of Q7 turns it off. D II is across T3 and
Q2 for noise immunity. ~iodes D41, D12, and D3 are antisaturation diodes. C19, R27, and D27 form
the snubbing network.
Winding 1,3 of T I is a reset winding. On turn-off, this winding clamps the collector of Q7 to two times
the storage voltage. It is also used for resetting the high frequency transformer core.

4.9.4

Outputs

There are four secondary outputs. The -23 volt output is rectified by D28, filtered by C 18, and passed
on through a 300 ohm resistor. The output goes to two zeners mounted on the video board. The 300
ohm resistor limits power dissipation in the zeners and also dissipates all power under short circuit
conditions.

4-110

)

The -12 volt output consists of a rectifier diode (029), filter capacitor (C20), and a regulator Z I.
The voltage at winding 9,10 ofTI is rectified by 036 and goes into an averaging network L2 and C31.
The catch diode, part of 036, completes the conduction path when Q7 is off. When Q7 is on, there is a
voltage on the secondary with pin 9 being positive. 036 is forward biased and current flows through
L2. This causes the inductor current to increase at this time. With the switch open, stored energy in the
inductor forces the current to continue to flow to the load and return through the catch diode (D39).
The voltage across L2 is now reversed and is approximately equal to the output voltage. During this
time the current in L2 decreases. The average current through L2 equals the load current. Since C31
keeps the output voltage constant, the load current will also be constant.
The + 12 volt output circuitry works the same way as above except the voltage at winding 4,5 is turned
on by Q8 and turned off by the voltage at terminal 5 going negative.

)

The current limit circuits for the + 5 and + 12 outputs have identical configurations; only some resistance values differ. The voltage at pin 5 of the comparator referenced to the junction of R 16 and R37 is
compared to the voltage drop across R37. When the voltage across R37 increases to a point where pin
6 is more positive than pin 5, El switches and pin 7 goes low, pulling pin 9 of E4 to ground and thus
turning off the switching regulator.
The + 12 volt control circuitry is synchronized to the 3524 pulse width modulator at a 30 kHz rate.
When pin 12 of E4 is high, capacitor C 15 charges up from the + 12 start-up through R29, R26, and
D 13. The polarity on the capacitor is such that when pin 12 of E4 goes low, the emitter of Q6 is pulled
negative with respect to the base which is at ground. Q6 saturates, pulling pin 2 of E3 down, thereby
allowing the ramp generated at pins 6 and 7 by C 13 to ramp up. Pin 6 is the threshold pin. The input to
this pin is compared to the voltage at pin 5. When the voltage at pin 6 is equal to that of pin 5, pin 7
discharges capacitor C 13 and pin 3 of E3 goes low. Whenever the voltage atpins 6 and 7 is ramping up,
the output of E3, pin 3, is high.
C13 is the timing capacitor for the +12 volt pulse width modulator. C13 is charged from a voltagecontrolled current source. The voltage at pin 5 of E3 is connected to the 5 volt reference. Thus, when
the voltage across C 13 reaches 5 volts (pin 6 of E3), the output of E3 pin 3 drops low, turning Q 1 off
and thereby turning Q8 on. Q7 and Q8 are turned off simultaneously when Q2 is turned on. The pulse
width that determines the length of time Q8 is on is determined by the rate of voltage rise on capacitor
C 13. The faster C 13 charges, the longer Q8 conducts. The error voltage that determines the rate of rise
on C 13 is generated by error amplifier E2. The noninverting input of the error amplifier is referenced
to D38, a 5.1 volt zener. The inverting input is divided up by R2 and R3 and senses the + 12 volt
output. Any difference creates an error signal on pin 6 of E2. This increase or decrease in voltage at pin
6 divided by R33 generates a proportional change in current through Q5, thereby changing the rate of
voltage charge on capacitor C 13. As described above, the change in rate of charge affects the pulse
width.
R4, C I, and R5 are the compensating components of error amplifier E2. D 1 and D2 are antisaturating
diodes. R6 limits the base drive and R 7 and C3 are a speed-up circuit to turn Q I on and off.

4-111

4.9.5

Power Supply Specifications

4.9.5.1

Input Specifications

Voltage
Single phase, 2 wire

90-128 V rms (switch-selected)
180-256 V rms

. Frequency

47-63 Hz

Current

2.2 A rms max. @ 115 V rms; 1.1 Arms
max. @ 230 V rms.

Input power

2S0 VA apparent ISO W max.

Power factor

Ratio of input power to apparent power is
greater than 0.6 at full load and minimum
input voltage. The VT100 appears
capacitive to the line.

Leakage current

When installed in VT100 terminal, current
to ground is 0.5 rnA max. Each line to
ground at 250 V rms sine, 50 Hz.

Current limiting

When installed in video terminal, 3.0 A
normal blow fuse.

Electrical Magnetic Interference Susceptibility
Conducted transients
Single voltage transient without causing
system degradation: 600 V @ 2.5 W /sec
max.
Single voltage
transient, survival

1000 V @ 2.5 W Isec max.

Average transient power,
survival

O.SWmax.

Conducted cw noise

Unit operates without error at conducted
noise levels up to 10 kHz - 30 MHz: 3 V
rms.

RF Field Susceptibility: 10 kHz - 30 MHz:
2 V1M; 30 MHz - 1 GHz: 5 V1M with power
supply installed in video terminal.
Power line disturbance

.--}
/

4-112

Ride through capability

The VT100 power supply provides a minimum
of 20 ms ride through during a power
outage condition. Ride through capability
is at low line and full load. During this
time, all power supply outputs must be
within their specified limits.

Overvoltages

The VT100 power supply withstands for one
second, an overvoltage of 110% of the
maximum input voltage(s) specified in
Section 4.9.5.l.1 without causing system
degradation or damage.

U ndervol tages,
disturbances, and
outages

The VT100 power supply is capable of
withstanding undervoltage disturbance and
power interruptions without physical
damage.

Hi-Pot

Input to frame for 1 min. - 2.15 kV dc;
input to output for 1 min. - 2.5 kV rms

Output Specifications

General

DC outputs are provided by the dc output
distri bution.
DC output specifications are defined at
the dc distribution buses.

)

Distribution systems must be designed per
this document since no output voltage
adjustments are provided.
Output power

+ 5 V @ l.5 A min to 1l.0 A max.
+ 12 V @ 1.0 A min to 2.75 A max.
- 12 V @ 0.0 A min to 0.5 A max.
-23 V@ 10 rnA

+ 5 V Specifications
Total regulation
Static line regulation
Static load regulation
Long term stability
Thermal drift
Ripple
Dynamic load regulation
Noise

±5%
±0.75%
±2.0%
0.1% - 1000 hrs
± 0.025% - deg C
150 mV p-p for f < 66 kHz
± 2.2% (see notes)
1% peak at f greater than or equal to 100
kHz (noise is superimposed on ripple)

4-113

+ 12 V Specifications
Total regulation
Static line regulation
Static load regulation
Long term stability
Thermal drift
Ripple
Dynamic load regulation
Noise

±5%
±0.5%
±1.0%
0.1 % - 1000 hrs
± 0.025% - deg C
240 mV p-p for f < 66 kHz
± 1.1 % (see notes)
1% peak @ f greater than or equal to 100
kHz (noise is superimposed on ripple)

- 12 V Specifications

Total regulation
Static line regulation
Static load regulation
Long term stability
Thermal drift
Ripple
Dynamic load regulation
Noise

±5%
±0.5%
±1.0%
0.1 % - 1000 hrs
± 0.025% .:. deg C
240 mV p-p for f < 66 kHz
± 1.1 % (see notes)
1% peak @ f greater than or equal to 100
kHz (noise is superimposed on ripple)

- 23 V Specifications
Total regulation
Static line regulation
Ripple

±10%
±2%
500 mV p-p

NOTES
1. For all outputs, the long term stability and thermal drift specifications apply after a 5 minute
minimum warmup, measured at the dc distribution buses with an averaging meter.
2.

Dynamic load regulation is measured in + 25% load steps from a starting point of 75% of full load.
Measurements are made at the power supply terminals.

Zener diodes for -23 V supply are located at the load and not on the power supply.

Regulation limits

Regulation limits are measured at the dc
distribution buses. The root sum squared
of errors due to the following
conditions:
1.

2.
3.
4.
5.

Initial tolerance
Static and dynamic input voltage
Gradual load changes over the load
range minimum to full load
Operation over the temperature
environment specified for the VT100
Long term stability per 1000 hours
Ripple and noise must remain within
the total regulation limits stated
above.
4-114

))

Ripple and noise must be measured with a
wide band oscilloscope in the
differential mode between ground and the
output under test. The oscilloscope must
be grounded so as to minimize spurious
responses. The specification applies only
to repetitive voltage variations that
occur while operating with a constant
input voltage and fixed load.

Ripple and noise

Overload protection
+5 V output

Current limit with fold back. Limit point
is 18 A max; fold back is 6.0 A max.

+ 12 V output

Current limit with foldback. Limit point
is 6 A max; foldback is 3 A max.

-12 V

Internal current limit of the 3-terminal
regulator is per DEC specifications
PS-19-12048-02.

-23 V

Current limited to less than 150 rnA.

Voltage adjustments

None. 1.68 kg (3.7Ibs) max.

)

4.9.5.2 Cooling - Cooling is by natural convection. Adequate space must be provided around the supply to allow a free flow of air. The power supply was designed with capability greater than the basic
VT100 requirements so that options could be easily added in the future. Since the form factor and
power dissipation of these options is not known, additional air flow and thermal profile t.esting should be
conducted as part of the option design to determine if convection cooling is sufficient.
4.9.5.3 Base Product Power Requirements - The basic VT100 terminal controller, monitor, and keyboard have the following combined maximum power requirements:
+5V
+12 V
-12 V
-23 V

2.5 A
1.8 A
0.03 A
12 rnA (no other device should use this Voltage).

The advanced video option uses +5 V, 1.1 A maximum power.

4-115

CHAPTER 5
VT100 SERIES SERVICE

5.1 INTRODUCTION
This chapter contains troubleshooting and repair information for VTI 00 series terminals. All terminals
in the series are based on the VT100. Therefore, the VT100 can always be isolated as a separate unit.
This allows you to determine if the trouble lies within the VT100 hardware or the hardware added to
the terminal to make it a variation.

)

5.2 TROUBLESHOOTING
This section contains the troubleshooting information for the VT100 series of terminals.
5.2.1 Troubleshooting the Basic VT100
The VT100 has a series of internal self-tests that help isolate failures to a field replaceable unit (FRU).
Paragraph 5.2.7 describes the tests and how to run them. Tables 5-1 and 5-2 show the error codes, the
detected failure, and the FRU you should replace.

)

Table 5-1

Keyboard LED Error Codes

Keyboard LEDs
Ll
L2

Error

Replace FRU

Off

ROM I

Terminal controller

Off

ROM 2

Terminal controller

Off

ROM 3

Terminal controller

Off

ROM 4

Terminal controller

Off

Main Data
RAM

Terminal controller

5-1

Table 5-2

VT100 Display Error Codes

Error Detected
Error
Char.

AVO

2
3

NVR

8
9

X
X

X
X
X

X
X

EIA

X
X

Data
Loopback

X
X

KBD

X
X
X

B
C

F
G
H
I

X
X

Replace
Advanced video
Terminal controller
Advanced video and
terminal controller
Keyboard
Advanced video and
keyboard
Terminal controller
and keyboard
Advanced video,
terminal and keyboard
Terminal controller
Advanced video and
terminal controller
Terminal controller
Terminal controller
and advanced video
Keyboard and terminal
controller
Advan.ced video,
keyboard and terminal
controller
Terminal controller
and keyboard
Advanced video,
keyboard and
terminal controller
Terminal controller
Advanced video and
terminal controller
Terminal controller
Advanced video and
terminal controller
Keyboard and terminal
controller
Advanced video,
keyboard and terminal
controller
Keyboard and terminal
controller
Advanced video,
keyboard and terminal
controller
Terminal controller
Advanced video and
terminal controller

})

)

))

-, )

._--\
/

5-2

Table 5-2

)

Error Detected
Error
Char.
K

Data
Loopback

EIA

Replace

X
X

Terminal controller
Advanced video and
terminal controller
Keyboard and terminal
controller
Advanced video,
keyboard and terminal
controller
Keyboard and terminal
controller
Advanced video,
keyboard and terminal
controller

AVO

NVR

X
X

J
L

M
N

)

VT100 Display Error Codes (Cont)

KBD

5.2.2 Troubleshooting Basic VT100 Variations Without Self-Test
If the terminal appears to be faulty, pei-form the following procedure. If the problem is not solved by
this procedure. refer to Table 5-3 for a list of typical problems.

)

Turn the power switch to the off position and check the following items.
a.

Power cord. Make sure the cord is connected securely at both the terminal and the wall
outlet. Check the wall outlet with another device, such as a lamp. to make sure it is
providing ac power.

Voltage selection switch and fuse. Make sure the switch is in the correct position and
the fuse is good.

Keyboard coiled cord. Check that the cord is securely plugged into the keyboard connector at the back of the terminal.

Turn the power switch to the on position. The terminal performs the power-up test. Refer to
the power-up test description (Paragraph 5.2.7.1) for more information about the power-up
test. If the terminal does not power up correctly, refer to Table 5-3.

If needed, perform the Computer Port Data Loopback Test. Refer to the data and EIA loopback test descriptions (Paragraphs 5.2.7.2 and 5.2.7.3) for more information about the data
loopback test.

Table 5-3 lists the most common VT100 failures and the associated symptoms. To use this table select
the symptom that matches the terminal failure.

5-3

Table 5-3

Basic VT100 Troubleshooting Procedure
1.

NOTES
This procedure assumes that only one field
replaceable unit (FRU) has failed. Symptoms
displayed may be representative of a multiple
failure; as a result, symptoms may change as
FRUs are replaced. Always troubleshoot to
the current symptoms.

Spare parts do fail. Do not ignore the possibility of a failure just because the module has
been replaced once.

You must turn off the power before disconnecting or replacing any FRU.

Symptoms

Probable Cause

Corrective Action

No response when
power switch is set
to ON. The CRT
filament and LEOs
are not on.

Not plugged in; no
power at wall socket.

Plug in VT100; use different
wall socket if possible.

Main power fuse

Replace fuse. (If fuse blows
again there is a possible
shorting problem. Use
appropriate troubleshooting
methods).

No response when
power switch set
toON and CRT
filament is on.

No audible tones
when terminal is
turned on. LEOs
are on. (Most
usual hard failure
symptoms.)

Power supply

Replace power supply.

AC line cord

Check for open or short
and replace line cord.

Power distribution
harness

Replace harness.

Power supply

Replace power supply

Terminal controller
board

Replace board.

Power distribution

Replace harness.

Controller

Replace.

Keyboard

Replace.

Keyboard cable

Replace.

Speaker

Replace.

5-4

)

____ )

Table 5-3

)

Basic VT100 Troubleshooting Procedure (Cont)

Symptoms

Probable Cause

Corrective Action

No audible alarms
and indicators
when terminal
turned on and none
of the LEDs are
on.

Keyboard disconnected.

Connect.

Keyboard cable

Replace.

Keyboard

Replace.

Connectors

Check and reconnect.

Controller

Replace.

Screen brightness
too low.

Adjust monitor
brightness.

Controller

Replace.

Monitor board

Replace.

Flyback transformer

Replace.

DC power harness

Replace.

CRT and yoke assembly

Replace.

Monitor fuse open.

Replace monitor board.

Monitor board

Replace.

DC power harness

Replace.

CRT and yoke assembly

Replace.

Random characters
appear on screen.

Controller

Replace.

Horizontal or
vertical line
appears on
screen.

Monitor connectors

Check and reconnect.

Monitor board

Replace.

CRT and assembly

Replace.

Cursor does not
appear on screen
after terminal
powered up.
CRT filament is
on.

Cursor does not
appear on screen
after terminal
is powered up.
CRT filament is not
on. Keyboard
functional.

5-5

Table 5-3

Basic VT100 Troubleshooting Procedur~ (Cont)

Symptoms

Probable Cause

Corrective Action

Screen display
distorted.
Characters narrow
on left or right
side of screen.

Monitor is
misadjusted.

Adjust monitor.

Monitor board

Replace.

Flyback transformer

Replace.

CRT and yoke assembly

Replace.

Controller

Replace.

Ball monitor
misadjusted

Adjust.

DC power harness

Check.

Controller

Replace.

Monitor

Replace.

Display
presentation bows
in or out

Yoke pincushion
incorrectly
misadjusted.

Replace CRT and
yoke assembly.

Display
presentation
jumpy.

Interlace feature
on.

Turn feature off.

Power feature set
incorrectly.

Set feature to
correct line
frequency.

Controller

Replace.

Power supply

Replace.

Monitor board

Replace.

Flyback transformer

Replace.

Graphics or alternate
character set, or
alternate keypad
mode or cursor key
mode is selected.

Clear condition with
power up or reset.

Keyboard

Replace.

Controller

Replace.

Screen rolling.

Wrong character
appears on
screen when typed
in LOCAL

))

5-6

))

. --~j

TableS-3

)

Basic VT100 Troubleshooting Procedure (Cont)

Symptoms

Probable Cause

Corrective Action

Wrong character
appears on
screen when typed
in ON-LINE with
loopback installed.
Terminal functional
in LOCAL mode.

Graphics or alternate
character or alternate
keypad mode or cursor
key mode is selected.

Clear condition with
power up or reset.

Transmit and receive
speed not the same.

Change speed setting.

Controller

Replace.

Wrong characters
Receive and/or
appear on screen
transmit speed set
when typed ON-LINE incorrectly.
and connected to
computer. Terminal
Bits per character
functional with
feature set
incorrectly.
loopback connector.

Set speeds to match
computer.

Parity feature set
incorrectly.

Set parity and/or
parity sense feature
to match computer.

Communications line

Check communications
facilities.

Double characters

. Local echo set on
full-duplex system
(VTIXX-AC. VT132 only)

Set correct feature.

Messages received
are incomplete.

XON /XOFF feature set
incorrectly.

Set feature.

Computer doesnot
recognize XON /XOFF
sequence.

See Chapter 3 of
VTl 00 User Guide.

Controller

Replace.

Parity feature set
incorrectly.

Set parity and/or
parity /sense feature
match computer.

Controller

Replace.

Communications
facility problem

Check communications
facility.

Set feature to match
computer.

)

Checkerboard
character appears
on screen instead
of character typed
(ON-LINE with
computer).

5-7

Table 5-3

Basic VT100 Troubleshooting Procedure (Cont)

Symptoms

Probable Cause

Corrective· Action·

All characters
displayed as a
white area (black
with reverse
screen).

Alternate character
set selected and
not available.

Clear condition with
power-up or reset.

Only top or bottom
half of characters
appear on the
screen.

Incorrect use of
double-height escape
sequence

See Chapter 3 of
VTl 00 User Guide.

SET-UP features
do not work correctly
(multiple alarms
may sound on power
up or recall). Error
2 displayed on
screen.

Save operation not
performed.

Perform save operation.

Controller

Replace.

Terminal does
not respond to
Escape sequences ..

ANSIjVT52 feature
set incorrectly.

Set ANSIjVT52 feature
to correct compatibility.

)l
5.2.3 Troubleshooting the VTI05
The troubleshooting procedure for the VTI05 consists of the following two steps.
1.

Isolate to the base VT100 configuration and troubleshoot the terminal.

Perform the internal VTI05 waveform generator tests described in Chapter 6.

To isolate to the base VTI 00, disconnect the graphics interface cable from 12 on the VT100 controller
board. Now perform the troubleshooting procedures outlined for the basic VT100.
To troubleshoot the VTI05 graphics module, reconnect the graphics interface cable to 12 on the VT100
controller board. Perform the testing procedure outlined in Chapter 6. When performing the test sequence remember to type the sequence exactly as written. If an error appears retype the sequence to
verify that there is an error and then replace the VTI05 graphics board.

5.2.4 Troubleshooting the VT132 The troubleshooting procedure for the VT132 is the same as the basic VT100. The only difference between the two machines is the advanced video option (AVO) board. On the VTI 32 the AVO board is
standard and contains four ROMs. In addition the ROMs on the video controller board are different.
To replace the terminal controller board, remove the ROMs from the new VT100 controller board and
substitute the VT132 ROMS. To replace the advanced video option board, configure the new AVO
board jumpers or switches for VT132 application, and plug in the VT132 ROMs. Paragraph 5.4 contains information on board configuration.

5-8

)
)

5.2.5 Troubleshooting the VT125
The VTI25 is a VT100 terminal that has an intelligent graphics processor installed between the terminal and the communications connectors (Figure 5-1). A problem in the graphics components can also
prevent the terminal from working as a text terminal. You can isolate any problem with the VTI25 to
the VT 100 components or the graphics components.

If any of the following conditions exist, remove the graphics components from the terminal by performing the appropriate procedures in Paragraph 5.4.
•

The VTI25 cannot complete its power-up test

•

Any of the keyboard indicators L 1 through L4 are on at the end of the power-up test

•

The complete screen flashes

After you remove the graphics components, test and troubleshoot the terminal as a VT100. When the
terminal passes its VT 100 power-up and data looback tests, reinstall the graphics components and perform the VTl25 tests (Paragraph 5.2.8).

)

INTERNAL
MONOCHROME
MONITOR

)

GRAPHICS
AND
TEXT

EXTERNAL
COLOR
MONITOR

VIDEO SYNC

MONO VIDEO
VT100
TEXT
TERMINAL

~ ONLiNE!
I

L ___

GRAPHICS
PROCESSOR

.--

r--

CURRENT
LOOP
ADAPTER
OPTION

4--

--+

COMPUTER

.....II

r---,

Q
I
L

LOCAL I
I

_ _ _ _ .J

EXTERNAL
GRAPHICS
PRINTER

MA-9436

Figure 5-1

VT 125 General Block Diagram

5-9

5.2.5.1 VT125 Self-Test Error Codes - If a character appears under the blinking cursor at the top left
corner of the screen,or a message appears in the center of the screen, check Table 5-4 for the meaning
of the error code character or message. Note that a character displayed can indicate more than one
error. To clear the error indication after the error is corrected, reset or power up the terminal.

)
/)

The self-test code characters indicate the following three types of errors.
Advanced video option
(A VO) if installed

If the advanced video option fails,
the terminal operates with the basic
VT100 text capabilities.

User permanent SET-UP
feature memory failure
(Memory)

If the user permanent SET-UP feature
fails, the terminal operates using
default SET-UP feature selections
for each feature. (Refer to Chapter
6 for more information on the SET-UP
feature memories.)

Keyboard missing or
malfunction
(Keyboard)

If only the keyboard fails, the
terminal ends the test ON-LINE
so it may operate as a receive-only
terminal. The SET-UP feature
selections cannot be changed.

There are several other problem indicators. The solutions are listed in the suggested order of FRU replacement.
Indication

Problem

Solution

VTI25
Off-Line

VT I 00 text terminal
is LOCAL. LOCAL was
saved in SET-UP, or
there is a user
permanent memory
problem. Terminal
can only process
received graphics.

Put terminal ON-LINE
and save the feature in
SET-UP.

VTI25
BM Error

Video bit map memory
has one or more bad
pixels.

Replace graphics board.

VTI25
EC Error

Computer port failed
data loopback test.

Replace graphics board,
paddle board, 24-pin
cable.

Terminal can only
operate as text
terminal in LOCAL.

5-10

))

)

Indication

Problem

Solution

VT125
IC Error

Internal communications failure VT125 cannot communicate with VT100.
Terminal can only
process received
graphics.

Replace graphics board,
paddle board, 24-pin
cable.

VT125
SC Error

Auxiliary port failed
data loopback test.

Replace graphics board,
paddle board, 24-pin
cable.

Terminal cannot send
data to printer.

)

VT125
VG Error

Vector generator
could not draw
sample shape.
Graphics probably
does not work,
but terminal may
communicate normally.

Replace graphics board.

A box is drawn around the margin of the graphics screen area. * If any part of the box is missing or
distorted, replace the graphics board.

)

If no graphics data appears, but the VT100 section of the terminal operates correctly, the 16-pin cable
may be bad.

I f an external color monitor has trouble displaying graphics information, but the internal monitor displays correctly, the ground wire from the chassis to the graphics processor board video connector
bracket may be loose.
Table 5-4 shows the possible error characters displayed on the screen and the failure indicated by each
character. Note that a character displayed can indicate more than one error.

)

*Not on all units

5-11

Table 5-4

VT125 Displayed Error Codes

Character
Displayed

Faults Detected
AVO
Memory

I
2
3
4
5

X*
X

X
X

Keyboard

Message
Displayed

Faults Detected

VTl25 OK

None (normal power-up test results)

VTl25 Offline

VT 100 text terminal is LOCAL - LOCAL
was saved in SET-UP, or there is a User
Permanent Memory problem.

VT125 BM Error

Video bit map memory one or more bad
.
pixels

VT 125 EC Error

Computer port

VT125 IC Error

Internal communications

VT 125 SC Error

Auxiliary port

VT125 VG Error

Vector generator

))

*Also, bell tones are generated - perform a save and recall in SET-UP.
tCheck that keyboard is properly connected.
\)
}

5-12

5.2.6 Troubleshooting the Options
Troubleshoot the options after the basic terminal is checked and found to be good. Then perform the
option checkout procedure for the suspected faulty option as outlined in Chapter 6. If the option does
not check out correctly, replace the option module.
5.2.7 VT100 Internal Self-Tests
The VT100 contains five self-test programs in the basic ROM memory.
Power-up test
Data loop back test
EIA test
Video adjust pattern keyboard test
SET-UP display test
Each test isolates a failure to the faulty module, so you can repair the terminal in a minimum amount of
time. If the VTI XX-AC Printer Port Option is installed, use the test procedures for that option.
NOTE
In command sequences, control keys are labelled in
angle brackets <KEY>. Control keys are The special keys that do not display on the screen. Press
that key and type the rest of the sequence as shown.

)

5.2.7.1 Error Codes - If executing any self-test produces an error, the test stops automatically. The
screen may display a random pattern of characters. Error indications are error codes displayed on the
keyboard LEDs (L 1 through L4, listed in Table 5-5), or an error code character displayed in the upper
left corner of the screen, under the blinking cursor. If one or more errors occur while the test is repeating continuously, the entire screen flashes from white to black to white about once a second as an alarm.
This continues until the terminal is reset.
Table 5-2 shows the error characters that may appear on the screen, and the FRU that you should
check or replace.
5.2.7.2

Power-Up Test - The terminal automatically performs the following tests during power up.

Writes a 1 and a 0 in each bit location of RAM on the basic terminal controller board to
verify that the RAM can store each bit.

Writes a 1 and a 0 in each bit location of RAM on the advanced video option (AVO) board to
verify that the option RAM can store each bit. If the AVO board is not present, The terminal
automatically skips this part of the test.

Reads the contents of the nonvolatile RAM (NVR), calculates a checksum, and then compares the checksum to the checksum stored in the NVR.

Reads the contents of each ROM chip, calculates a checksum, then compares the checksum
to the checksum stored in each ROM chip.

Turns on all keyboard LEDs on, rings the keyboard bell for one-quarter second, and looks for
the end-of-scan character from the keyboard to determine if the keyboard is functioning
properly.

)

5-13

Use one of the following procedures to start the power-up test.
1.

Turn the terminal power on.
or

Type the following sequence to perform the test once. The terminal must be in ANSI-compatible mode (in SET-UP B, group 2 switch 3 = 1).

<ESC>[2;ly
or
3.

Type the following sequence to perform the test continuously. The terminal must be In
ANSI-compatible mode (in SET-UP B, group 2 switch 3 = 1).

<ESC>[2;9y
or
4.

Enter SET-UP and press the 0 key (reset).
NOTE
The continuously running test ends only if it finds an
error OT you turn power off.

Any error found by the power-up self-test appears on the terminal screen and/or LEDs LI through L4
on the keyboard. Tables 5-5 and 5-6 define the error c o d e s . ) )

-~)
.I

5-14

Table 5-5
Error

ROM 1 checksum error
ROM 2 checksum error
ROM 3 checksum error
ROM 4 checksum error
Main data RAM error

)

Fault Detected
AVO NVR KBD Data

X
X

)

5
6
7
8

X
X

Off

Off
Off
Off
On.
On

Off
On
On
Off
Off

On
Off
On
Off
On

EIA

Displayed Error Codes
Display
Char

Fault Detected
AVO NVR KBD Data

®
A

B
X
X
X
X

X
X

Off
Off
Off

Display
Char

.off

Table 5-6

2
3

LED Error Codes

X
X
X
X

X
X
X
X
X
X
X
X

C
D
E
F
G
H
I
J
K
L
M
N
0

X
X

X
X
X
X

X
X

X
X
X
X

NOTE
See Table 5-3 to determine the correct module to replace.

5-15

X
X
X
X
X
X
X
X

EIA

X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X

5.2.7.3 Data Loopback Test - In the data loopback test the VT100 transmit and receive lines connect
to each other via a special external connector. The test transmits a predefined set of characters. The
terminal receives the characters and compares them to the characters transmitted. If the characters do
not match, an error is then flagged.
Use the following procedure to perform the data loopback test.
I.

I nstall the appropriate data loop back connector. Connector PN 12-15336 is for EIA communications (Figure 5-2); connector PN 70-15503-00 is for 20 rnA current loop communications
(Figure 5-3).

Make sure the transmit and receive speeds are the same.

Place the terminal in ANSI-compatible mode (in SET-UP B, group 2 switch 3 = I).

Place the terminal ON-LINE.

Type the following sequence to perform the test.

<ESC>[2;2y
or
6.

Type the following sequence to perform the test continuously.

<ESC>[2;lOy
NOTE
The continuously running test ends only if it finds an
error or you turn power off.
Any error displays an error code on the screen. Table 5-6 defines the error codes.

5-16

))

)
TOP CONNECTOR
USED ON VT125
AND VTIXX-AC

VT100} BOTTOM
VT125 CONNECTOR
VT132

)

CONNECTORS

FROM PIN

TOPIN

TO PIN

*DATA LEADS ONLY
NOTE: ACCESS COVER
SHOWN IS VT125
MA-7868A

Figure 5-2

EIA Loopback Connector

5-17

FROM PIN

TO PIN

1
2

3
7

))

NOTE: ACCESS COVER
SHOWN IS VT125
MA-7867A

Figure 5-3

20 rnA Loopback Connector

5-18

5.2.7.4

)

EIA Test - The EIA test checks that the following signals can be set to a I or a O.

Data Terminal Ready
Data Set Ready
Carrier Detect
Speed Select

Request to Send
Clear To Send
Ring Indicator
Speed Indicator

Use the following procedure to perform the EIA test.

)

Install the EIA data loopback connector, PN 12-15336.

Make sure the transmit and receive speeds are the same.

Place the terminal in ANSI-compatible mode (in SET-UP B, group 2 switch 3 = I).

Place the terminal ON-LINE.

Type the following sequence to perform the test once.
<ESC>[2;4y
or

Type the following sequence to perform the test continuously.
<ESC>[2;12y

)

NOTE
Tl1e continuously running test ends only if it finds an
error or you turn power off.
Any error displays an error code on the screen. Table 5-6 defines the error codes.

)

5.2.7.5 Keyboard Tests - The power-up test verifies the presence or absence of a keyboard. You can
test individual keys by placing the terminal OFF-LINE and enabling keyclick. All keys typed should
produce a click except NO SCROLL, CAPS LOCK, SHIFT, BREAK, and CTRL. Typing the keys
should cause the corresponding character to appear on the screen. Typing CTRL-G should cause the
bell to sound. This checks both the bell circuitry and the CTRL key. Keys typed with CAPS LOCK and
SHIFT verify the operation of those keys. After you enable XONjXOFF and place the terminal ONLINE, the BREAK and NO SCROLL keys should produce a click when typed.
5.2.7.6 SET-UP Screen Test - In SET-UP the terminal exercises all its display functions. The functions available in the base VT100 include double-height, double-width characters (SET-UP A). doublewidth, single-height characters (TO EXIT PRESS "SET-UP"), and reverse or underline base attribute
(selected by the cursor setting and displayed by the cursor and the tab ruler).
The VT I 00 with advanced video option includes these basic functions, plus other functions such as 24
lines with 132 columns (test with the video adjust pattern, Paragraph 5.2.7.7), and more character attributes.

.'\
,/

5-19

You can check the following features using the keyboard controls.
Intensity (up and down arrow keys)
80/132 column
Transmit and receive speeds
Reset
Smooth/jump scroll
Auto repeat
Reverse screen
Margin bell
Keyclick
US/UK character set
Wraparound
ON-L1NE/OFF-LINE control
Check smooth scrolling by filling the screen with E's (Paragraph 5.2.7.7) and pressing line feed while
OFF-LINE.
To perform the test, enter the two SET-UP displays and check that the details agree with those in the"
sample displays in Chapter 6. To test the advanced video option, check that the words SET-UP A blink
in bold, the words TO EXIT PRESS "SET-UP" are underlined, and the tab ruler has alternating normal and reverse video sections, even if you selected the underline cursor.

) I

5.2.7.7 Video Adjust Test - The video adjust test provides a screen full of E's for display height,
width, and linearity adjustments. The test pattern is internal to the terminal and is not sent to the host
computer.
Use the following procedure to display the test pattern.
I.

Place the terminal in ANSI-compatible mode (in SET-UP B group 2 switch 3 = I).

Place the terminal LOCAL.

Type the following sequence.
<ESC>#8

5.2.8 VT125 Tests
The VTI25 terminal has several self-tests available for checking terminal operation. Some of these are
the self-tests of the VT100 terminal that the VTI25 Graphics Processor resides in. The other tests are
for the graphics processor itself. To perform the VT100 tests as described in Paragraph 5.2.7, disconnect the graphics components from the t~rminal controller.
The power-up test is performed each time the terminal is powered up. The other tests may be performed after the power-up test is completed. To perform the other tests, the terminal must be disconnected from the communication line, have an external loopback connector installed, and be ONLINE with the ANSI/VT52 SET-UP B feature selected to ANSI (SET-UP B switch 2-3 = I). When
the tests are completed, turn the power switch off, remove the loopback connector, and conned the
communication cable.
NOTE
A continuously running test ends only if it finds an
error or you turn power off.

~I
/

5-20

)

S.2.S.1 Loopback Connector Installation - Use the following procedure if the test needs a loopback
connector installed on the terminal.
1.

With the power switch off. disconnect the communication cable.

Install the loopback connector on the EIA connector. Refer to Figure 5-2. The EIA loopback
connector is PN 12-15336.

If the 20 rnA Current Loop Adapter Option is installed, perform one of the following steps.
a.

Use the loopback connector (PN 70-15503-00) included with the option.
or

Disconnect the current loop option cable from the terminal controller board (inside the
access cover) and use the EIA loop back connector.
NOTE
Do not use the EIA loopback connector at the same
time as the current loop connector.

)

S.2.S.2 VT125 Power-Up Test - The VT125 has a built-in power-up test to check terminal opcra,tion.
The test checks general operation of the VTl 00, including the advanced video option (if installed). user
permanent SET-UP feature memory, and keyboard. The test also checks general operation of the
graphics processor, including the bit map memory.

)

To perform the power-up test, either turn the terminal on or (if power is already on) enter SET-UP and
press the 0 (RESET) key. Install a loopback connector and use the following procedure if the test must
run continuously.

Turn the power switch on. The terminal performs the power-up test.

Type one of the following sequences to perform the test.

)

<ESC>[4;ly

Performs test once.

<ESC>[4;1;9y

Performs test continuously.

NOTE
You can also perform this test with the VT100 power-up test sequence (ESC[2;ly). The continuously
running test ends only if it finds an error or you turn
power off.
The test gives the following indications.
•

Keyboard and screen flash on and off.

•

All keyboard indicators turn on and off, and either the ON LINE or LOCAL indicator turns
on.

•

The wait message appears on the screen and then is erased.

•

A bell tone sounds.

5-21

•

A band of light appears at the top of the screen and then is erased.

•

A second bell tone sounds.

•

A message on the screen gives the result of the VTI25 power-up test, and a box is drawn*
around the margins of the graphics screen area.

))

NOTE
Messages do not appear on the screen until the terminal warms up.
•

The. text cursor appears in the upper-left corner of the screen.

The power-up test provides three possible error indications. (Refer to Paragraph 5.2.7.1 for more information about error indications.)
•
•
•

Displays error on screen as a character or a message.
Displays error on keyboard indicators L 1 through L4.
Sounds several bell tones.

))

5.2.8.3 VT125 Computer Data Port Loopback Test - The computer port data loopback test checks
that the VT 125 can transmit and receive characters over the computer dat port. To perform this test,
you must connect the transmit and receive lines to each other with an externalloopback connector. The
terminal transmit and receive speeds must be the same (300 baud or faster). Use the following procedure to perform the test.
I.

Turn the power switch on. The terminal performs the power-up test.

Type one of the following sequences to perform the test.

))
)

<ESC>[4;1;2y

Performs power-up test and computer data port loopback
test.

<ESC>[4;1;2;9y

Performs power-up test and computer data loopback test
continuously until failure.

The test gives the following indications. (Refer to Paragraph 5.2.8.2 for power-up test indications.)
•

Either the ON-LINE or LOCAL indicator turns on.

•

The wait message appears on the screen and then is erased.

•

The cursor appears in the upper-left corner of the screen.

•

If the computer data loopback test fails, the message "VT125 EC Error" appears on the internal monitor.

*Not on all units

5-22

5.2.8.4 VT125 Auxiliary Port Loopback Test - The auxiliary port data loopback test checks that the
VTI25 can transmit and receive characters over the auxiliary data port. To perform the test, you must
connect the transmit and receive lines to each other with an externalloopback connector. The auxiliary
port speed must be 300 baud or faster. Use the following procedure to perform the test.
I.

Turn the power switch on. The terminal performs the power-up test.

Type one of the following sequences to perform the test.
<ESC>[4;1;3y

Performs power-up test and auxiliary data port loopback
test.

<ESC>[4;1;3;9y

Performs power-up test and auxiliary data port loopback
test continuously until failure.

The test gives the following indications. (Refer to Paragraph 5.2.8.2 for power-up test indications.)

)

•
•
•
•

Either the ON LINE or LOCAL indicator turns on.
The wait message appears on the screen and then is erased.
The cursor appears in the upper-left corner of the screen.
If the test fails, the message "VT125 SC Error" appears.

5.2.8.5 VT125 Display Test - This test requires you to check the internal monitor screen and the color
monitor screen (if present) for correct operation. The screen cycles through the four intensity levels of
each of the three primaries and white to test the output memory. The computer data port must have the
loopback connector installed. Use the following proc~dure to perform the test.
I.

Turn the power switch on. The terminal performs the power-up test.

Type one of the following sequences to perform the test.
<ESC>[4;1;4y

Performs power-up test and display test.

<ESC>[4;1;4;9y

Performs power-up test and display test continuously until failure.

)

The test gives the following indications. (Refer to Paragraph 5.2.8.2 for power-up test indications.
Monochrome
White
Step I

Light Grey

Dim Grey

Black

Step 2

Black

White

Light Grey

Dim Grey

Step 3

Dim Grey

Black

White

Light Grey

Step 4

Light Grey

Dim Grey

Black

White

Step 5

White

Light Grey

Dim Grey

Black

5-23

Color
Step I

Light Green

Light Red

Light Blue

Black

Step 2

Black

Step 3

Black

Light Red

Black

Step 4

Dim Blue

Dim Red

Dim Green

Dim Grey

Step 5

Light Blue

Light Red

Light Green

Light Grey

5.2.8.6 VT125 Video Bit Map Memory Test - This test checks that every bit in both video bit map
planes can be written to both I and o. The computer data port must have the loopback connector installed. Use the following procedure to perform the test.
I.

Turn the power switch on. The terminal performs the power-up test.

Type one of the following sequences to perform the test.

<ESC>[4;1;5y

Performs power-up test and video bit map memory test.

<ESC>[4;1;5;9y

Performs the power-up test and video bit map memory
test continuously until failure.

The test gives the following indications. (Refer to Paragraph 5.2.8.2 for power-up test indications.)
•

The screen fills with levels of intensity (or color) moving from top to bottom.

•

If this test fails, the message "VT125 BM Error" appears.

5.3 VIDEO ALIGNMENT
This section describes the alignment of both the Ball Brothers and Elston video monitors. Always check
each adjustment, because many adjustments affect each other. However, if a check shows the correct
indication, skip that adjustment and go to the next procedure.
Make all adjustments under the following conditions.
•
•
•
•
•
•
•
•
•

Normal video (white characters on a dark background; SET-UP B, switch 1-3 = 0)
80 characters per line
ANSI mode (SET-UP B, switch 2-3 = 1)
LOCAL mode
Out of SET-UP
CRT alignment template attached to the screen
Top cover removed
Screen filled with E's (after brightness adjustment)
Type ESC#8

5-24

))

5.3.1 Alignment Template
Figure 5-1 shows the alignment dimensions used in this procedure. If the mylar adjustment templates
(PN 94-03220-03 and PN 94-03270-03) are not available, use the dimensions in Figure 5-4 to make a
paper template. Paragraph 5.3.1.1 and Figure 5-5 describe the procedure. You can perform the adjustments with the paper template or mylar template.
5.3.1.1 Making a Paper Template - You need a pencil, a ruler, a triangle (or anything that can help
you draw a right angle), and a sharp knife.

)

Mark a dot in the center of a piece of stiff paper.

Measure the dimensions to the top and bottom, and mark with dots.

Draw the top and bottom lines (using the triangle if needed).

Mark off the left and right sides and draw those lines at a right angle to the top and bottom
lines.

Measure the width of the "red shaded area" in Figure 5-4 and mark a dot at each of the four
sides.

Draw a line through each dot to form a box around the box already drawn.

Mark an area in the middle of each side to leave for support, and cut out the rest of the band
of paper.

Measure from the center through each outer corner and cut the paper away so you can center
the paper template on the CRT between the marks at the bezel corners.

)

5.3.1.2 Attaching the Template - The CRT template (PN 94-03220-03) stays attached to the screen
during all adjustment procedures. (If you are using the paper template, you may have to remove it for
brightness and linearity.) Use the following procedure to attach the template to the CRT.

)

Using the bezel around the screen as a guide, mark the four edges of the screen with a watersoluble marker (felt-tip pen).

Remove the terminal top cover (Paragraph 5.4.11).

Tape the CRT alignment template to the screen, with the four corners of the template all the
same distance from the marks you made in step 1.

5.3.2 Monitor Adjustments (Ball Brothers)
The following paragraphs describe the alignment of the Ball Brothers video monitor. You can easily
identify the Ball monitor by the location of the f1yback transformer (on the horizontal brace above the
CRT's neck). This monitor also has a small connector board on the CRT yoke assembly.
Make all adjustments under the conditions listed in Paragraph 5.3.

5-25

CHARACTER·
WIDTH
1.9 «
MIN----tJ:° 75 ) ~

~III

CO')

2.2

7.0
..11114- (0.275)

)Z871~
VI
I

211 (8.30)

129/
/f5.,1

60
(2.36)

134
(5.28)

RED
SHADED
AREAS

120
(4.72)

\.1114..------ (3.90)

0'1

•

(C- _. - . ---- -------MIN 3.0

MAX 3.7
-

VT 100 ALIGNMENT TEMPLATE
9403220·3
REV.C
7·1·79
7 1 79 VT·l 00 9403246·38

RED SHADED AREA
NOTE: MEASUREMENTS ARE IN MILLIMETERS
EXCEPT VALUES IN PARENTHESIS ARE IN INCHES.
MA·2069A

Figure 5-4

"-..--l,..-r'

'--'

Video Alignment Templates

'-....-/'

'~...--/

-.~//'

)

CUT OUT

'\.
MA-9340

Figure 5-5

Paper Alignment Template

5-27

5.3.2.1
board.

Brightness - Use the following procedure to adjust the brightness control on the monitor
\

))

Let the terminal warm up for at least five minutes.

Increase the screen brightness to the maximum level by pressing the T key in SET-UP.

Adjust R 117 (Figure 5-6) until you see the display raster. Then turn R 117 in the opposite
direction until the raster disappears.

Return the brightness to a medium level by pressing the 1 key in SET-UP.

J101

VERT. HOLD

BRIGHTNESS

R117

c::J

FOCUS

))

BVIDEO

R122
c::::J

A103

CENTERING

VERT
LINEARITY

B
R'8

VERT.
HEIGHT

))
HaRZ
LINEARITY

J104
1

HaRZ
WIDTH

® ®

0
MA·2078

Figure 5-6

))

Ball Video Monitor Board Adjustments

- 'I)
!

5-28

5.3.2.2

)

Yoke Rotation - Use the following procedure to check and adjust the yoke.

Check that all four sides of the screen display are parallel to the red shaded area of the alignment template.

Loosen the yoke collar clamp screw (Figure 5-7) and turn the yoke until the four sides of the
screen display are parallel to the edges of the red shaded areas of the alignment template.

Check that the yoke is pushed all the way forward toward the face of the CRT. Hold the
yoke in place and tighten the yoke collar clamp screw.

)

VERTICAL
CENTERING
RINGS

)
YOKE COLLAR
CLAMP SCREW

Figure 5-7

Ball Monitor CRT Adjustments

5-29

MA-2081

5.3.2.3 Vertical Height - Use the following procedure to check and adjust the height of the screen
display.
I.

Check that the top and bottom display lines are covered by the red shaded area of the adjustment template.

Adjust the vertical height control RllO.

))

5.3.2.4 Horizontal Width - Use the following procedure to check and adjust the width.
I.

Check that the first and last columns of the display are covered by the red shaded area or the
adjustment template.

Adjust the horizontal width coil L I 03 with a monitor alignment tool (PN 29-23190-00).

5.3.2.5

Centering - Use the following procedure to check and adjust the display centering.

Check that the display is centered in the red shaded area on the alignment template.

Turn Al 03 fully counterclockwise.

Center the presentation by rotating the centering tabs on the yoke (Figure 5-7).

NOTE
You can use Al 03 to move the display presentation a
small distance horizontally. If AI03 is incorrectly
adjusted, the left side of the screen may be distorted.
5.3.2.6 Vertical Linearity - Use the following procedure to check and adjust the character height over
the complete screen.
I.

Use the character height template (PN 94-03270-03) to check that the heights of the charac-·
ters located near the four corners and the center of the display are between the minimum and
maximum sections of the template.

Adjust the vertical linearity control Al 02.

Check and adjust the vertical height and vertical hold if needed.

5.3.2.7 Horizontal Linearity - Use the following procedure to check and adjust the character width
over the complete screen.
I.

Use the character width template (PN 94-03270-03) to check that the widths of the characters located near the four corners and the center of the display are between the minimum and
maximum sections of the template.

Adjust the horizontal linearity coil L 102 with a monitor alignment tool (PN 29-231290-00).

Check and adjust the horizontal width if needed.

5-30

))

5.3.2.8

Vertical Hold - Use the following procedure to check and adjust the vertical hold setting.

Turn vertical hold control AlOl fully counterclockwise. If the display is not stable (tearing or
rolling), turn AlOI clockwise untii the display is stable. Note the position of the control and
continue turning clockwise until the display is not stable. Then turn the control counterclockwise to the position halfway between the positions where the display is not stable.

Check and adjust the vertical height and vertical linearity if needed.

5.3.2.9

Focus - Use the following procedure to check and adjust the focus of the screen display.

t key in SET-UP. Decrease the

Increase the brightness to the maximum level with the
brightness eight steps by pressing the key eight times.

Look at the characters at the four corners and in the center of the screen. All the dots in the
vertical segment of the E should be visible in each character.

NOTE
Some operators may want the focus incorrectly adjusted to suit personal preference.

)

If the focus is adjusted correctly, go to step 4.
3.

Adjust R 122 (Figure 5-6) for the best overall character display.

Remove ,the CRT alignment template, clean the marks from the screen, and replace the terminal top cover if no more adjustments are needed.

5.3.3 Monitor Adjustments (Elston and DIGITAL)
The following paragraphs describe the alignment of the Elston video monitor. You can easily identify
the Elston monitor by the location of the flyback transformer (on the monitor sideplate opposite the
monitor board). Also, the E.\.ston CRT yoke assembly does not have the connector card.
Make all adjustments under the conditions listed in Paragraph 5.3.

)

5.3.3.1
board.

Brightness - Use the following procedure to adjust the brightness control on the monitor

t key in SET-UP.

Increase the brightness to the·maximum level with the

Adjust R I 09 (Figure 5-8) until you can see the display raster. Then turn R I 09 in the opposite
direction until the raster disappears.

Return the brightness to normal intensity by pressing the

)
5-31

l key in SET-UP.

L403
WIDTH

R477

FOCUS
R334
HEIGHT
R342
V ERTI CA L--~~;;;ov
LINEARITY

)
MA-4572

Figure 5-8
5.3.3.2

Elston Video Monitor Board Adjustments

Yoke Rotation - Use the following procedure to check and adjust the yoke_

Check that all four sides of the display are parallel to the red shaded area of the alignment
template.

Loosen the yoke collar clamp screw (Figure 5-9) and turn the yokeuntil the four sides of the
display are parallel to the red shaded area of the alignment template.

Check that the yoke is pushed all the way forward toward the face of the CRT Hold the
yoke in place and tighten the yoke collar clamp screen_

5.3.3.3 - Vertical Height - Use the following procedure to check and adjust the overall display heighL
1.

Check that the top and bottom lines of the display are covered by the red shaded area of the
adjustment template.

Adjust the vertical height control R334_

5.3.3.4

Horizontal Width - Use the following procedure to check and adjust the overall display width_

Check that the first and last columns of the display are covered by the red shaded area of the
adjustment template.

Adjust the horizontal width coil L403 with a monitor alignment tool (PN 29-23190-00)_

5-32

)

CRT ANODE CAP

)

Figure 5-9

)

5.3.3.5

Elston Monitor CRT Adjustments

Centering - Use the following procedure to check and adjust the display centering.

Check that the display is centered in the red shaded area on the alignment template.

Center the presentation by rotating the frontmost centering rings on the neck of the CRT
(Figure 5-6).

5.3.3.6

Vertical Linearity - Use the following procedure to check and adjust the character height over
the complete screen.

)

Use the character height template (PN 94-03270-03) to check that the height of the characters located near the four corners and the center of the display are between the minimum and
maximum sections of the template.

Adjust the vertical linearity using R342.

Check and adjust the vertical height if needed.

5-33

5.3.3.7

Focus - Use the following procedure to check and adjust the focus.

t key in SET-UP. Decrease the brightness

Increase the brightness to maximum level with the
eight steps by pressing the ~ key eight times.

Look at the characters at the four corners and in the center of the screen. You should see the
individual dots in the vertical segment of the E in each character.

))

NOTE
Some operators may prefer the focus incorrectly adjusted.
If the focus is adjusted correctly, go to step 5.

5.4

Adjust R477 (Figure 5-8) for the best overall character presentation.

Adjust the focus rings furthest from the yoke for the best focus at all four corners (Figure 59). Readjust R477 if needed.

Remove the CRT alignment template, clean the marks from the screen, and replace the terminal top cover if no more adjustments are needed.

))

MODULE REMOVAL AND REPLACEMENT

This section contains information for removal and replacement of mechanical subassemblies in VT100
series terminals. Unless otherwise noted, each procedure applies to all terminals in the series.
Figure 5-10 lists all of the removal procedures in this chapter and the sequence to perform them in. For
example, Figure 5-10 shows that to remove the dc power harness, you must first remove the top cover,
bottom cover, and power supply removal procedures.

5.4.1

Access Cover

Use the following procedure to remove the terminal access cover.

Remove power from the terminal by disconnecting the ac plug from the ac outlet.

Unplug the keyboard.

Unplug any connectors from the composite video input/output jacks.

Disconnect the communications cable.

Disonnect the printer interface cable if installed.

Use a flat-blade screwdriver to loosen the four captive screws holding the access cover (Figure 5-1 1).

If the 20 rnA current loop option is installed, gently pull the access cover away from the
terminal about two inches. Then reach in and disconnect J5 from the terminal controller
board.

To install the access cover, perform steps 1 through 7 in reverse.

5-34

))

ACCESS
COVER
(PARA 5.2)

•

20Ma CURRENT
LOOP BOARD
(PARA 5.9)

•

ROM
(PARA 5.5)

)

•

TERMINAL
CONTROLLER
BOARD
(PARA 5.3)

VT125
GRAPHICS
BOARDS
(PARA 5.8)

VTl05
WAVE FORM
GENERATOR
BOARD
(PARA 5.7)

•

TOP
COVER
(PARA5.15)

ADVANCED
VIDEO
BOARD
(PARA 5.4)

PRINTER
PORTNTl25
STP BOARD
(PARA 5.6)

1
+

BOTTOM
COVER
(PARA 5.20)

•

POWER
SUPPLY
(PARA 5.21)

VIDEO MONITOR
BOARD
BALL (PARA 5.16)
ELSTON (PARA 5.18)

CRT DISCHARGE
AND ANODE CAP
(PARA 5.26)

FLYBACK
TRANSFORMER
BALL (PARA 5.17)
ELSTON (PARA 5.19)

CRT-YOKE
ASSEMBLY
(PARA 5.25)

RF SHIELD
(PARA 5.22)

KEYBOARD
TOP COVER
(PARA 5.10)

DC.
POWER HARNESS
(PARA 5.23)

t
)

VT105/VT125
EXPANSION
BACKPLANE
(PARA 5.24)

KEYBOARD
(pARA 5.11)

t
KEYBOARD
CABLE
(PARA 5.12)

SPEAKER
(PARA 5.13)

KEYCAPS
(PARA 5.14)
MA-9341

Figure 5-10

Removal Procedures Sequence

5-35

TOP COVER

))

Figure 5-11

VT100 Terminal (Rear View)

5-36

)

5.4.2 Terminal Controller Board
Use the following procedure to remove the terminal controller board.
I.

Remove the terminal access cover (Paragraph 5.4.1).

Pull the terminal controller board partially out of the card cage. Disconnect the ground wire
(if present) from the metal bracket at the EIA connector.

)

If the .terminal is a VTI05, disconnect the graphic interface cable from the terminal
controller board (Paragraph 5.4.6).

If the terminal is a VT125, refer to Paragraph 5.4.7. After separating the boards, remove the terminal controller board from the card cage. Remove the screw holding the
STP board to the terminal controller. Then remove the board.

Remove the advanced video board and/or the printer port/STP board if they are installed on
the terminal controller boara (Paragraphs 5.4.3 and 5.4.5).
.

To install the terminal controller board, perform steps I through 3 in reverse. Make sure to reconnect
any ground wires.

NOTE
Always replace an FCC-certified terminal controller
board with another certified board. There is a key in
the connector to prevent swapping. Do not use an
FCC-certified board in a noncertified terminal. The
VT125 must use a certified board. The noncertified
board is PN 54-13009-00. The certified board is PN
54-13009-03.

)

Check the terminal controller board for the correct
ROM configuration. On -W series and printer ports,
you must move the old ROMs to the new board. Refer to Paragraph 5.4.4 for ROM procedures. Refer
to Paragraph 5.5 for ROM location information.
Remember to reconfigure the terminal to the customer's original SET-UP selections (which should
be listed on the keyboard SET -UP label).
The VTI05 and VT125 cables are 16-pin cables
connecting to an IS-pin connector. The VTI05 fits
into the top 16 connector sockets and VT125 fits
into the bottom 16 connector sockets. Do not misalign the cable in the connector. Refer to Paragraphs 5.4.6 and 5.4.7.

5-37

5.4.3 Advanced Video Option Board
Use the following procedure to remove the advanced video (AVO) board.
I.

Remove the access cover (Paragraph 5.4.1).

Remove the controller board (Paragraph 5.4.2), and, if present, the graphics boards (VTl05,
Paragraph 5.4.6; VT125, Paragraph 5.4.7)

Grasp the advanced video board by its edges and gently but firmly lift the board straight up
and off the terminal controller board (Figure 5-12).

To install the advanced video board, perform steps 1 through 4 in reverse.

NOTE
Check the AVO board for the correct ROMs, jumpers, and switch settings (Paragraph 5.5).

))
/

COMPONENT SIDE
OF ADVANCED
VIDEO BOARD
COMPONENT
SIDE OF
TERMINAL
CONTROLLER
..

J1 ADVANCED
VIDEO
CONNECTOR

J2 GRAPHICS
CONNECTOR
TERMINAL
CONTROLLER
BOARD

J4 EIA
COMMUNICATIONS
. CONNECTOR

J6
J9 VIDEO OUT
-CONNECTOR
~-------

"'----.,....,..4-- J7
KEYBOARD
CONNECTOR

______________~~

MA·1995B

Figure 5-12

Terminal Controller with Advanced Video Board

5-38

))

)

5.4.4 ROMs
If a ROM is defective, or if the terminal is a variation with special ROMs, this procedure describes
how to replace them. Refer to Paragraph 5.5 for ROM positions and part numbers.
I.

Remove the old ROM.

Unpack the new ROM from its container, but do not remove the ROM from the conductive
foam. Gently press the foam against the surface of the terminal controller board to remove
static charges. Then remove the ROM from the conductiv~ foam.

Refer to Figure 5-13 and align the ROM so the notch on the end is on the same side as the
notch on the board socket. Check that all ROM pins are evenly spaced and straight before
trying to insert the ROM into the board socket.

Using even pressure, press the top of the ROM until it is completely seated into the socket. If
you feel a lot of friction, remove the ROM and check for proper pin placement.

After the ROM is inserted, make sure all ROM pins are properly seated in the socket~

5.4.5 Printer Port/VT125 STP Board
Use the following procedure to remove the printer portjVT125 STP board.
I.

Remove the access cover (Paragraph 5.4.1).

Remove the terminal controller board (Paragraph 5.4.2), and, if present, the graphics boards
(VTI05, Paragraph 5.4.6; VT125, Paragraph 5.4.7).

Remove the screw and washer holding the STP connector on the terminal controller board
(Figure 5-14).

Disconnect the STP board from the STP connector on the terminal controller board.

On a VT125, remove the 24-pin flat cable.

To install the printer portjSTP board, perform steps 1 through 5 in reverse. For a VT125, refer lo
Figure 5-15 to install the 24-pin cable.

)

NOTE
Check the printer port/option board to make sure
the correct jumpers are installed or switches set. Refer to Paragraph 5.5 for configuration information.
If you remove the printer port/STP board from the

terminal, check that pairs of contacts of STP connector J3 short together to make electrical contact.
Adjust separated contacts with a scribe.

5-39

NOTCH

TERMINAL
CONTROLLER
BOARD
SOCKET

)i
IC BODY

SOCKET

BOARD

ROM IC PINS

mvwtAM1W
\

PARALLEL AND
EVEN SPACING
PROPER ROW
ALIGNMENT

)

NOTE
CHECK ALIGNMENT BEFORE INSTALLING

Figure 5-13

MA-55318

Terminal Controller Board ROM Installation

5-40

VT125 GRAPHICS STP BOARD

o o00 0 I:
n'l:nOil
i
!:.J i
I
DO
'I

~ i:~

ilo ~ 00
I; , I

;

i"l ~
I~I.
i i 0 DOD
I! I
~

!i I ~
jL-J°n

nO inOllrlLJ -

I 0--

ni i'UInil nn ucoD

,u,

I_I

~~_~:LJ

)

J2 .J

GRAPHICS

Lrf

MOUNTING

~J""""'-CONN.

~Jl:

ADVANCED

~~~RS~~;

)

20MA OPTION

VIDEO
. ?J3
OPTION·'
CONN'

CONN.
.
:

EIA
CONN.

STP
CONN.

PRINTER STP OPTION BOARD

DODD

)
/

MA·9350

Figure 5-14

Installing Printer Port on VT125 STP Board

5-41

))

D
MA·9~2

Figure 5-15

VTl25 24-Pin Flat Cable on STP Board

5-42

S.4.6

{

)

VTI0S Waveform Generator Board

Use the following procedure to remove the VTI05 waveform generator board.
1.

Remove the access cover (Paragraph 5.4.1).

Disconnect the graphic interconnecting cable from XE90 on the waveform generator board
(Figure 5-16).
CAUTION

The graphic interconnecting cable connectors are
easily damaged. To prevent damage, use a small
screwdriver to alternately lift each end of the connector until it releases from the module.
Refer to Figure S-16 for correct orientation when
reconnecting this cable.
3.

)

Remove the waveform generator board by gently but firmly pulling the board straight out of
J2 on the backplane.

To install the waverform generator, perform steps 1 through 3 in reverse.

5.4.7

VT125 Graphics Board

Use the following procedure to remove the VT125 graphics board.

)

Remove the access cover (Paragraph 5.4.1).

Pull the terminal controller board and the graphics board partially out of the card cage. Disconnect the ground wire from the BNe connector bracket on the graphics board.

Remove the screw holding the STP board to the terminal controller and remove the board
(Figure 5-14). Remove the 24-pin cable from the STP board and the graphics board. Remove
the 16-pin cable from the terminal controller connector.

Pull the graphics board out of the cage.

To install the graphics board, perform steps 1 through 4in reverse (Figures 5-17 and 5-18).

)

NOTE

The 16-pin cable connects to the bottom 16 connector sockets. Do notmisalign the cable in the connector (Figure S-17).

)
5-43

USE SCREWDRIVER
TO GENTLY PRY

CONNECTOR OUT
OF SOCKET
.
XE90..a

))

M7071
WAVEFORM
GENERATOR

XE90

RED LINE ON CABLE
MUST BE IN POSITION
AS SHOWN

).)
J2

PINS 9 AND 10
ARE NOT USED
MA-4577

Figure 5-16

VTI05 Interconnections

5-44

)

MA·9343

Figure 5-17

VT125 16-Pin Flat Cable on Terminal Controller

5-45

))

MA-9344

Figure 5-18

VT125 Graphic Cable Connections

5-46

,( )

5.4.8 20 mA Current Loop Board
Use the following procedure to remove the 20 rnA current loop board.
1.

Remove the access cover (Paragraph 5.4.1).

Use a Phillips screwdriver to remove the two screws holding the Mate-N-Lok connector to
the bottom of the access cover.

Use a Phillips screwdriver to remove the three screws holding the 20 rnA current loop board
to the access cover. Then remove the board.

To install the 20 rnA current loop board, perform steps 1 through 3 in reverse.
5.4.9 Keyboard Top Cover
Use the following procedure to remove the keyboard top cover.

)

Remove power from the terminal by disconnecting the ac plug.

Unplug the keyboard from the monitor.

Use a flat-blade screwdriver to loosen the captive screws holding the keyboard together (Figure 5-19).

Remove the top cover by lifting it straight up.

To install the keyboard top cover, perform steps 1 through 4 in reverse.

)

5.4.10 Keyboard
Use the following procedure to remove the keyboard.
1.

Remove the keyboard top cover (Paragraph 5.4.9).

Disconnect keyboard cable J2 from the keyboard.

Remove the keyboard.

To install the keyboard, perform steps 1 through 3 in reverse.
For a terminal with a printer port, move the PRINT jENTER key from the defective keyboard to the
new keyboard. For a VT132 keyboard, move the special keycaps to the new board.
5.4.11 Keyboard Cable
Use the following procedure to remove the keyboard cable.
1.

Remove the keyboard top cover (Paragraph 5.4.9).

Remove the keyboard (Paragraph 5.4.10).

Disconnect the keyboard cable from the speaker.

Remove the keyboard cable.

To install the keyboard cable, perform steps 1 through 4 in reverse.

1
...

5-47

/,VER

KEYBOARD TOP

CAPTIVE SCREW S (2)
MA-2071

Figure 5-19

Keyboard DOIsassembly

5-48

5.4.12 Keyboard Speaker
Use the following procedure to remove the keyboard speaker.
1.

Remove the keyboard top cover (Paragraph 5.4.9).

Remove the keyboard (Paragraph 5.4.10).

Remove the keyboard cable (Paragraph 5.4.11).

Remove the speaker by sliding it toward the front edge of the keyboard cover.

To install the keyboard speaker, perform steps 1 through 4 in reverse.

5.4.13 Keycap
Normally the only tool you need to install keycaps is a keycap pUller. In some cases you may need a pair
of long nose pliers. Use the following procedure to remove and replace keycaps.

)

Remove power from the terminal by turning it off.

Starting at the upper-left corner of the keyboard, insert the keycap puller between the first
cap to be replaced and the adjacent keycap (Figure 5-20).

Squeeze the puller legs around the cap and slowly pull straight up to remove keycap (Figure
5-20).

If the adapter does not come off with the keycap (Figure 5-20), carefully remove adapter
with long nose pliers. Reinstall adapter in the keycap, making sure the pointed end faces the
front of cap.

)

NOTE
In some keyboards the adapter and the keycap may
be one molded part.

In a very few cases the plunger may also come off. If so, take care to prevent the spring from
flying out. Refer to Figure 5-20 and replace the plunger and spring as follows.
CAUTION
You must follow this procedure carefully to prevent
damage to key contacts.

)
a.

Slide spring into plunger. Place spring and plunger in slot, and over key contacts. Make
sure cross piece of plunger is vertical, not horizontal.

Slowly push plunger down until cross piece. touches contacts. Release plunger and check
that contacts are separated.

Locate proper replacement keycap. Gently press key cap straight down on plunger until it is
fully seated.

Repeat steps 1 through 6 for each keycap you replace.

Store all unused keycaps in shipping bag for later use.

5-49

))
KEYCAP
YPULLER

KEYCAP

Figure 5-20

Keycap Removal

5-50

)

}

Use the following procedure to test the new keycaps.
1.

Place terminal off-line.

Press each new keycap (both lowercase and uppercase) and make sure the proper characteres) appear on the screen.

5.4.14 Top Cover
Use the following procedure to remove the top cover.
1.

Remove power from the terminal by disconnecting the ac plug.

Unplug the keyboard.

Use a small flat-blade screwdriver (or a scribe on newer terminals) to release the two front
pop fasteners located under the front lip (Figure 5-.21).

Use a small flat-blade screwdriver to release the two rear pop fasteners located on the lower
rear edge of the bottom cover (Figure 5-21).

Remove the top cover by lifting it straight up.

)

To install the top cover, perform steps 1 through 5 in reverse.

5.4.15 Video Monitor Board (Ball Monitor)
Use the following procedure to remove the video monitor board on a Ball monitor.

)

Remove the top cover (Paragraph 5.4.14).

Remove the circular connector from the base of the CRT (Figure 5-22). Bend the wire harness up and out of the way.

Disconnect the four wires from the yoke connection card.
Tab I - White with Yellow
Tab 2 - Red
Tab 3 - Blue
Tab 4 - Brown

)
4.

Disconnect 4-wire connector J 102 at the bottom of the video monitor board.

Disconnect 7-wire connector J104 at the bottom-center of the video board. Move the harness
up and out of the way.

Disconnect the green ground wire.

Disconnect video input cable JIOI from the top edge of the video monitor board.

Release the four standoffs and remove the video monitor board.

To install the video monitor board, perform steps 1 through 8 in reverse .

.... i
l

5-51

Figure 5-21

Top Cover Removal

))

5-52

STANDOFFS (4)
FLYBACK
TRANSFORMER

J101-,,--_-t:I:-_)

)

J102

J104
GROUND
WIRE

)

Figure 5-22

MA-2082

Ball Video Monitor Board Removal

5.4.16 Flyback Transformer (Ball Monitor)
Use the following procedure to remove the flyback transformer from the Ball monitor.

WARNING
The CRT anode may contain a stored high voltage.
Refer to Paragraph 5.4.25 for the anode discharge
procedure.

)
1.

Remove the top cover (Paragraph 5.4.14).

Disconnect 7-wire connector J 104 at the bottom-center of the video monitor board. Move the
harness up and out of the way (Figure 5-22).

Discharge the high voltage at the CRT anode. Refer to Paragraph 5.4.25 for the procedure.

Disconnect the CRT anode wire from the CRT. Refer to Paragraph 5.4.25 for the procedure.

Use a 1/ 4-inch nutdriver to remove the two nuts mounting the flyback transformer to the
horizontal crossbrace, and remove the transformer.

To install the fly back transformer, perform steps 1 through 5 in reverse.

5-53

5.4.17 Video Monitor Board (Elston or DIGITAL)
Use the following procedure to remove the video monitor board from an Elston or DIGITAL monitor.
1.

Remove the top cover (Paragraph 5.4.14).

Remove the circular connector from the base of the CRT (Figure 5-23).

Disconnect the white wire connecting the video monitor board to the yoke ground lug.

Disconnect the 8-pin connector from the video monitor board.

Disconnect the 4-pin connector from the video monitor board.

Disconnect the video input connector from the top edge of the video monitor board.

Release the four standoffs and remove the video monitor board.

To install the video monitor board, perform steps 1 through 7 in reverse.
CAUTION
Do not misalign the 8-pin connector when reconnecting the cable. You may damage the monitor
board if the connector is not correctly installed.

GROUND WIRE

YOKE COLLAR
CLAMP SCREWS
MA-4574

Figure 5-23

Elston/DIGITAL Video Monitor Board Removal

5-54

. ))

5.4.18 Flyback Transformer (Elston or DIGITAL)
Use the following procedure to remove the fly back transformer from an Elstonor DIGITAL monitor.
WARNING
The CRT anode may contain a stored high voltage.
Refer to Paragraph 5.4.25 for the anode discharge
procedure.

)

Remove the top cover (Paragraph 5.4.14).

Disconnect the 8-pin connector from the video monitor board.

Remove the screw and washer securing the vertical choke to the monitor chassis. Remove the
vertical choke.

Discharge the high voltage at the CRT anode. Refer to Paragraph 5.4.25 for the procedure.

Disconnect the CRT anode wire from the CRT. Refer to Paragraph 5.4.25 for the procedure.

Use a 1/4-inch nutdriver to remove the two nuts mounting the flyback transformer to the
monitor chassis.

To install the flyback transformer, perform steps 1 through 7 in reverse.
CAUTION
Do not misalign the 8-pin connector when reconnecting the cable. You may damage the monitor
board if the connector is not correctly installed.

)

5.4.19 Bottom Cover
Use the following procedure to remove the bottom cover.

)

Remove the top cover (Paragraph 5.4.14).

Disconnect the power cord from the terminal.

Turn the terminal over on its side and remove the four shipping screws (if present) securing
the bottom cover to the chassis. Discard the screws and turn the terminal up.

Locate the four pop fasteners holding the frame to the base and release the fasteners by pull.
ing the plungers up.

Grasp the metal frame and lift the frame up and out of the terminal bottom cover.

To install the bottom cover, perform steps 1 through 6 in reverse.

\}
5-55

5.4.20 Power Supply
Use the following procedure to remove the power supply.

Remove the top cover (Paragraph 5.4.14).

Remove the bottom cover (Paragraph 5.4.19).

Locate the three pop fasteners on the side of the chassis. Release the pop fasteners by pulling
the plungers out (Figure 5-24).
WARNING
Capacitors C9, C14, and the surrounding circuits
contain a 300 Vdc charge. To discharge the capacitors, leave the power supply plugged into the terminal for a minimum of four minutes after the power
cord is removed.
To assure complete discharge of the capacitors after
the power supply is out of the terminal, short the capacitors with an insulated wire as shown in Figure 525.

Grasp the power supply by the end plate and remove by pulling it straight out.

To install the power supply, perform steps 1 through 4 in reverse. Note that there is a grounding tab on
the chassis at the back that fits into a small slot in the end of the power supply's switch plate.

)1
POP
FASTENERS

CHASSIS

MA·2077

Figure 5-24

Power Supply Removal

5-56

1)
/

)
CAPACITORS
C9 & C14

)

SHORT THESE 2 POINTS

)

MA-2072

Figure 5-25

Power Supply Capacitator Discharging

5-57

5.4.21 RF Shield
Use the following procedure to remove the RF shield for the VTl25.
1.

Remove the top cover (Paragraph 5.4.14).

Remove the bottom cover (Paragraph 5.4.19).

Remove the power supply (Paragraph 5.4.20).

Remove the terminal controller board (Paragraph 5.4.2) and, if present, the graphics boards
(VTI05, Paragraph 5.4.6; VT125, Paragraph 5.4.7).

Loosen, but do not remove, two Phillips screws at the top of the shield (Figure 5-26).

Set the chassis on end with the cage up.

)

NOTE
The terminal rests on several parts that stick out on
the bottom. Set the terminal on a protected surface
.
to prevent damage.

))

The bumpers attached to both sides of the chassis
protect the terminal during shipping. You can remove them now if necessary.
7.

Loosen, but do not remove, two Phillips screws at the bottom of the shield.

Slide the shield off the cage. Push the ribbon cable (front of the terminal) and the ground
wires (at access opening) as needed to avoid catching the wires on the sheet metal.

Tighten the four screws temporarily and set the terminal on its bottom.

))

To install the shield, perform steps 1 through 9 in reverse. At step 8, compress the metal contact fingers
on the shield (Figure 5-26) by pressing on the shield until the indicated parts touch.
5.4.22 DC Power Harness
Use the following procedure to remove the dc power harness.

\)
j

Remove the top cover (Paragraph 5.4.14).

Remove the bottom cover (Paragraph 5.4.19).

Remove the power supply (Paragraph 5.4.21).

Remove the terminal controller board (Paragraph 5.4.2) and, if present, the graphics boards
(VTI05, Paragraph 5.4.6; VT125, Paragraph 5.4.7).

Remove the RF shield if present (Paragraph 5.4.21).

Disconnect 10-pin edge connector J 101 from the video monitor board.

Remove the card cage by releasing the two pop fasteners (remove two Phillips screws on newer terminals) that hold the top of the card cage to the chassis. Tilt the card cage out slightly,
then lift the card cage out of the bottom holding clips (chassis slots on newer terminals).

5-58

\,)
J

SCREW +
4 PLACES
SCREW + LOCKWASHER

LOCKWASHER
OR POP
FASTENERS

SHOCK

GROUND
WIRE FOR
GRAPHICS

)

GROUNDWIRE
FOR TERMINAL
CONTROLLER

)
HOLE FOR
CONNECTOR·

-------. SHIELD
WIRE FRAME +
SPRING CLIPS

tJll"I Il""l

MAKE PARTS
TOUCH, THEN

TIGHTEN SCREW\

~[J~t

.,,,~~t
~~[JE

~~
MA·9345

Figure 5-26

FCC Shield on VT100 Cage

5-59

Disconnect the ground wire from the 10-pin connector to the monitor chassis.

For a VT100: Remove the two E-ring fasteners (spread the spring clip~ on late model terminals) holding the 22-pin edge connector to the card cage, then remove the connector (Figure
5-27).
For a VT105 or VTJ 25: Disconnect the 22-pin edge connector from the expansion backplane
(Figure 5-28).

10.

Remove the two E-ring fasteners (spread the spring clips on newer terminals) holding the 18pin edge connector to the chassis, then remove the connector.

11.

Remove the dc power harness.

To install the dc power harness, perform steps 1 through 11 in reverse.

POP FASTENERS (2)

CARD
CAGE

18 PIN

~----~CONNECTOR

MA-2D73A

Figure 5-27

VT100 DC Power Harness Removal

)I
5-60

)

EXPANSION BACKPLANE

SPACER

TERMINAL
CONTROLLER
EDGE
CONNECTOR

DC POWER CABLE
MA-4580A

Figure 5-28

Expansion Backplane

5-61

5.4.23 VTI05jVT125 Expansion Backplane
Use the following procedure to remove the VT105jVTI25 expansion backplane.
I.

Remove the access cover (Paragraph 5.4.2).

Remove the terminal controller and graphics boards (Paragraphs 5.4.2, 5.4.6, 5.4.7).

Remove the top cover (Paragraph 5.4.14).

Remove the bottom cover (Paragraph 5.4.19).

Remove the power supply (Paragraph 5.4.20).

Remove the RF shield if present (Paragraph 5.4.21).

Disconnect the ground wire from the lO-pin connector to the monitor chassis.

Disconnect the 22-pin edge connector from the expansion backplane (Figure 5-28).

Remove the four screws and nuts, and two spacers securing the expansion backplane to the
card cage.

To install the expansion backplane, perform steps 1 through 6 in reverse.
5.4.24 CRT and Yoke Assembly
You can repair Elston terminals with new CRT and yoke assemblies without replacing the complete
chassis assembly (Field Service Monitor Assembly). If you replace the CRT and yoke on Ball monitors,
replace the f1yback, choke, and monitor board with Elston parts at the same time.
WARNING
High voltage is present at the CRT anode. Refer to
Paragr~ph 5.4.25 for the anode discharge procedure.
I.

Disconnect the terminal from the power outlet.

Remove the top cover (Paragraph 5.4.14).

Discharge the CRT anode (Paragraph 5.4.25).

Disconnect the CRT high voltage anode wire (Paragraph 5.4.25).

Disconnect the circular connector from the CRT base (Figure 5-23) .

. 6.

Disconnect the white wire connecting the video monitor board to the yoke ground connector.

Disconnect the 4-pin yoke connector from the monitor board.

5-62

Remove the top two, then bottom two screws from the CRT frame while supporting the assembly from the bottom of the CRT.

Remove the CRT yoke assembly.

)

WARNING
Handle the tube by the sides next to the face, never
by the neck. Do not rest the tube on its neck.
To replace the CRT yoke assembly, perform steps 1 through 8 in reverse.
NOTE
Refer to Paragraph 5.4.26 for the proper CRT storage and disposal procedures.
5.4.25 CRT Discharge and Anode Cap
Use the following procedures to discharge the CRT anode, and remove and install the anode cap.

)

CRT Anode Discharge
Before you service the CRT or its associated circuits, you must discharge the anode. You can safely use
either of the following methods (Figure 5-29).
•

Connect the plug end of a YOM lead to chassis ground, and discharge the anode by touching
the probe to the CRT anode.

•

Gently slip the end of an insulated-handle screwdriver under the plastic anode cap on top of
the CRT while shorting the other end of the screwdriver to an unpainted area of the CRT
frame.

)

Anode Cap
Disconnect the heavy CRT anode wire from the CRT as follows.
1.

Note the position of the anode wire.

The connector holds itself in place with spring barbs in the depression of the tube (the anode
socket). You must push against one barb, allowing the other side to clear the lip of the anode
socket (Figure 5-30).

Push at a right angle in the direction that the wire enters the rubber cap.

While pushing across, push up on the cap to free one barb.

Now push up in the opposite direction to free the other barb.

5-63

})
INSULATED HANDLE
SCREWDRIVER
(ALTERNATE METHOD)
CRT

METAL

~FRAME

CRT
MA·2070A

Figure 5-29

CRT Anode Discharging

)I
5-64

A. PUSH IN THE DIRECTION
OF THE ARROW

)

B. WHEN ONE BARB IS FREE
PUSH IN THE OPPOSITE
DIRECTION (SEE ARROWS) TO
REMOVE ANODE CAP

)

MA·6735A

Figure 5-30

Removing Anode Cap

)

5-65

S.4.26 Field Handling of CRTs
You must use the following procedures when handling CRTs in the field.

))

CRT Replacement
1.

Replace CRTs only in areas where risks and exposure are limited to trained Field Service
personnel.

Only DIG ITAL service personnel should be in the area during replacement.

Any serviceperson relacing a CRT must wear, at least, safety glasses with side guards, or
goggles and gloves. The part numbers are as follows.
Goggles
Gloves

PN 29-16141
PN 29-16146

Before servicing the CRT or its associated circuits, you must discharge the anode (Paragraph
5.4.25).

CRT Handling
Take the following precautions when handling any CRT.
•

Handle the CRT by the sides next to the tube face. Never handle the CRT by the neck.

•

Do not allow the neck to strike anything.

•

Never rest a CRT on its neck.

•

Never scratch the glass of the CRT. Be particularly careful of this when working around the
CRT with any tool.

•

Never touch the glas'i of a CRT with a hot soldering iron.

CRT Storage
All CRTs must be stored in a closed shipping container or correctly mounted in the product. Never
store a CRT without a protective enclosure.
CRT Disposal
If a regional disposal procedure does not exist in your area, use the following steps to safely dispose of a
CRT. Destroy any CRT that is to be scrapped and not returned for repair.
WARNING
Use extreme caution when performing this procedure.
I.

Place the bad CRT in the container that the new CRT was received in.

Close and seal the container, leaving only the end of the CRT neck exposed (Figure 5-31).

Slowly crush the CRT evacuation point with a pair of pliers or wire cutters. The evacuation
point is located in the center of the CRT neck end. If the procedure is done correctly you will
hear a quick rush of air. This means that the CRT no longer has a vacuum and can no longer
implode.

5-66

))

)
Figure 5-31

CRT Disposal

5-67

5.5 BOARD CONFIGURATIONS
This section contains the information required to configure all boards in the following terminals.
VTl 00 and all optional configurations
VTlO5
VTl25
VTI32
5.5.1 Terminal Controller Board
Use Table 5-7 with Figure 5-32.

5.5.2 Advanced Video Option Board with Jumpers
Use Tables 5-8 and 5-9 with Figure 5-33 .
5.5.3 Advanced Video Option Board with Switches
Use Tables 5-9 and 5-10 with Figure 5-34.
5.5.4 VTIXX-AC Printer Port Option
The factory settings are switches I and 5 on; switches 2, 3, and 4 off. Do not change these settings. See
Figure 5-35.

5.5.5 VTI05 Waveform Generator Board
See Figure 5-36.
5.5.6 VT125 Graphics Terminal STP Board
See Figure 5-37.

))
Table 5-7

Terminal Controller Board ROMs
ROMs Used

Used In

ROM 0

ROM 1

ROM 2

ROM 3

VT100

23-031E2
or
23-061E2

23-032E2

23-033E2

23-034E2

VT100-WC-WK

23-095E2

23-096E2

23-139E2

23-140E2

VT100jVTIXX-AC

23-095E2

23-096E2

23-139E2

23-140E2

VTI32

23-095E2
or
23-180E2

23-096E2

23-097E2

23-098E2

23-181E2

23-182E2

23-183E2

23-061E2

23-032E2

23-033E2

23-034E2

VTI25 and
VTI05

Character
Generator

))
23-094E2

y
5-68

ROM3~

ROM 2 - - r - : : H

GRAPHICD
OPTION
CONN.

ROM'----iR

20 mA J5
OPTION
CONN .. -

I I

ADVANCED
VIDEO
OPTION
CONN.

ROM 0

~b~NECTOR J3

EIA
I
CONN. J4,
I
I

CHARACTER
GENERATOR

KEYBOARD
CONN.

)

MA·4582

Figure 5-32
Table 5-8

Terminal Controller Board

Advanced Video Option (AVO) Jumpers

)

Jumpers Installed
Used In

)

VT100
VT100-W A-WB
VT100-WC-WK
VT100jVTIXX-AC
VTI32 (ROMs 99-100)
(with basic video
ROMs 95-98)
VTI32 ROMs 236-239
or PROMs 224-227
(with basic video
ROMs 180-183)
VT125

- X
X -

10 11 12

X -

1,
/

5-69

X -

Table 5-9

Advanced Video Option (AVO) Board ROMs

)

ROMs Used
Board
Used In
VT100
VT100-W A-WB
VT100-WC-WK
VT100jVTIXX-AC
VTl05
VTl32

ROM A

ROMB

23-069E2
23-152E2
23-284E2
or
23-186E2

23-185E2
or
23-187E2

23-099E2
or
23-236E2
or
23-224E2

ROMe

ROMD

23-237E2

23-238E2

23-239E2

23-225E2

23-226E2

23-227E2

23-100E2

VTl25

rc::::J1
c:::::J 2

JUMP::RS-< c::::J10

B"12
'-

B13
14

ROM

MA-43B9C

Figure 5-33

Advanced Video Option Board with Jumpers

5-70

))

Table 5-10 .Advanced Video Option Switches
Switches Closed
Board
Used In

)

VT100
VT100-W A-WB
VT100-WC-WK
VT100jVTIXX-AC
VT132 ROMs 99-100
(with basic video
ROMs 95-98)
VTl32 ROMs 236-239
or PROMs 224-227
(with basic video
ROMs 180-183)
VTI25

8 7 6 5 4: 3 2 1

x -

S2
5 4

x -

X - X

-----x-x

)

--x--X--

,..,
w

"'"
....

,..,
w

)

"'"

t1I

ell

....

ROM
ROM
ROM
ABC

ROM
D

NOTE:
DOT ON SWITCH ILLUSTRATION
INDICATES THE SIDE DEPRESSED
MA-4584A

Figure 5-34 Advanced Video Option Board with Switches
5-71

DODD
D~
DO 0 DO
DOT ON SWITCH
SHOWS THE SIDE
NOT DEPRESSED.
MA·4583C

Figure 5-35

VTIXX-AC Printer Port Option Board

XE905

U TBl

c:::=s

E7l
W2_ c::::IW3

c:::::3 E64

)
c::=J E27

....-

_Wl
c=JE20

'--

JUMPER IN

c:::J

JUMPER OUT
MA-45B6

Figure 5-36

VTI05 Waveform Generator Board

5-72

'-"

DOD

o'------' D~ 00

ODDO

)

MA-9348

Figure 5-37

VT125 STP Board

5.6 COMPONENT LEVEL TROUBLESHOOTING
This section helps you to troubleshoot the terminal controller, microprocessor, power supply, and CRT
monitor to the component leveL
')

5.6.1 Troubleshooting the Terminal Controller
Refer to Paragraph 5_2 to .isolate problems to a field replaceable unit (FRU) such as the terminal controller board. This section explains diagnosis of problems within the board.
Check the power supply for proper voltages and ripple. Make sure the voltage switch is not set to 230
volt on a, 115 volt system. Check that all board edge contacts are clean and well seated.
The following four self-test results may appear on the screen or keyboard.
No valid LED code
LEDs indicating fatal error
Nonfatal errorS with screen display
No indicated error
This section explains the major areas in the terminal to check for each of the self-test results.
5.6.1.1 Microprocessor - This section explains the four possible self-test results and how to troubleshoot them.

5-73

5.6.1.1.1 No Valid LED Code - If the LEOs show some rahdom value not found in Table 5-1, several)
areas must be checked. ROM 1 containing the self-test routines 'may be at fault; the 8080 may be haltJ)
ed or in an inappropriate routine; there may be a hard failure on the data or address bus. (See Paragraph 5.6.1.2.)
With ROMs, first try to reseat them in their sockets to ensure good contact. Check the chip select lines
to be sure the ROM is being selected.
Check the clock signals to the 8080 from the 8224 (E32-6, -10, -1 I). Check that the Ready and Reset
signals are both high (E32-4 and -I). Check for the Sync signal (E32-5) every 3 to 5 clock periods. If
there is no Sync, check Hold Request L (E35-I3) which should go high for approximately 60 microseconds every 635 microseconds. Check for the Status Strobe (STSTB L) (E32-7). Hold Acknowledge
(HLOA, E35-2I) should toggle in step with Hold Request. If the clocks are present and Sync is entering E32 but Status Strobe does not appear, E32 is probably bad. If all these signals are okay, the problem is most likely not in the 8080 circuits.
If Hold Request is always high, the video processor is bad even though it may be able to drive the
monitor. Grounding pin I of the Test MUX (E 1) disconnects Hold Request, and while the self-test
reports keyboard and NVR failures, the microprocessor is able to work. E 1 also removes the master
clock from the video processor, so the monitor should be disconnected (to protect it). Connect EI pin 2
to 3, and EI-6 to 5 and II to run the microprocessor.

'))
I

Check the data and address buses for toggling during the power-up test. Almost every address gets
looked at during the test,and every data bit changes state, so that is the best time to check for stuck bus
lines. Restart the test for each line by shortingC 10 momentarily with a clip ,lead.
Connections made at E I can serve to force either the microprocessor or the video processor off the
buses. That is, asserting Hold Request L to the 8080 (with a clip lead from ground to pin 11) keeps the
video processor off the buses, and asserting Hold Request H (with a clip lead from EI-I6 to El-6 or10) keeps the microprocessor off the buses.

))

Check that the interrupt input is not stuck high (E35-14) if Interrupt Enable goes high. If the video
processor works there should be a vertical frequency interrupt every 16.7 ms. If the processor is stuck in
some kind of loop because some device is always calling it, the possibilities can be narrowed by looking
at each of the devices' interrupts and flags.
.
5.6.1.1.2 LEDs Indicating Fatal Error - If the power-up self-test never manages to finish. the LEOs
may display a number indicating the area of failure. Table 5-1 relates LEO indications to areas of
trouble. LEO indications mean the error is a fatal one, which means the terminal cannot function at all.
(If the self-test finishes, the LEOs are cleared.) Only the ROMs and the main RAM cause such a
failure. The table indicates the ROM to check if a ROM number is displayed. Try reseating the ROMs
in their sockets to ensure good contact. Check the chip select lines to be sure the ROM is being selected. There is no simple way to determine which of the six RAM chips is bad if a RAM failure is
indicated. One-at-a-time replacement of all six chips is the only way to find and fix a bad RAM chip.
But check the memory decoders' ability to address the RAMs before the RAMs themselves are
blamed.
5.6.1.1.3 Nonfatal Errors with Screen Display - Nonfatal errors do not disable the terminal completely, although they may limit its usefulness. A keyboard failure, for example, still lets the terminal
operate in receive-only mode. Nonfatal errors have codes assigned to them for display on the screen at
the end of self-test. (See Table 5-2.) The data loopback and EIA tests are considered nonfatal because
the microprocessor must work properly before the tests can run at all; but if they fail, the terminal
cannot communicate. To troubleshoot nonfatal problems, s,?e the following paragraphs.

5-74

))

5.6.1.1.4 No Indicated Error - Many video processor and monitor problems (see below) do not affect
the microprocessor, so most self-tests cannot detect the problems. But if the screen fills with garbage or
goes blank after the keyboard beeps with all LEOs on at the same time (indicating completion of selftest), the video processor or monitor may be at fault. (If the NVR fails its test, a number appears on the
screen and the keyboard beeps several times. If garbage or nothing appears when the beeps occur, the
problem is probably in the video processor rather than in the NVR circuit.) Because you know the microprocessor is good after completion of self-test, the screen RAM probably contains proper control
data (terminators and line addresses). Therefore, you should be able to troubleshoot the video processor
(garbage: Paragraph 5.6.1.2) or the monitor (blank: Paragraph 5.6.4) as the first step. Run the self-tests
again after fixing the video processor; other nonfatal errors may appear then.
Areas not directly tested by self-test (because they have no effect on the microprocessor) are as follows.

)

NVR Write (tested by Saving)
Graphics port
Video processor (check with SET-UP screen test)
Monitor
Correctness of baud rates (only confirms match between send and receive)
Keyboard switches
Some video processor functions must work for the self-test to pass. For the microprocessor to work,
these video processor signals must be close to frequency.
LBA 3 and 4
LBA 7 (NVR clock)

)

In addition, Hold Request cannot be stuck asserted and the NVR flag buffer must work.
5.6.1.2 Video Processor - There are two classes of video processor failure: 1) those that prevent proper movement of data from the screen RAM and through the character latches, terminator detection,
line address counters, (OMA problems in general), and 2) all other problems. Those include the line
buffer, character generator ROMs, video shift register, smooth scroll, attribute, and scan count control
in the OCOI2, the analog video circuits, and monitor drive signals.
The areas in class 1 are difficult to troubleshoot because failures at any point in the loop from screen
RAM data to screen RAM line addressing can cause the same kind of symptoms. The following section
contains a suggested basic approach to the problem. The areas in class 2 have more straightforward
troubleshooting, because the character data can be considered valid and failures with a particular
symptom are probably caused by a single kind of problem. No further detail is provided here. In either
case, make sure you understand the technical descriptions in Chapter 4 before spending any time trying
to troubleshoot these circuits.
A dual-trace, triggered oscilloscope with delayed sweep is essential for servicing the video processor.
Trigger the scope on Vertical Reset and examine each line for signals according to the OMA timing
diagram (Figure 4-6-19) in Chapter 4. Check the continuous signals from the OCO 11. These include
everything except Address Load, Address Count, and Write Line Buffer. All others should be checked
against their timing diagrams for the respective screen modes. Any output affected by double
width/ double height may be in either possible state.
Now check signals in this order.
1.

Vertical Reset

Address counter outputs E21, E22, E25, E30 (not the latches). All should be zero except
E22-5 at Vertical Reset.
5-75

Hold Request starting after time specified in Figure 4-6-19.

DMA Enable

Address Count and Write Line Buffer

While triggered on Vertical Reset, delay sweep to the region shown in Figure 4-6-20. Watch
for the data defined by Figure 4-7-5, Fill Line Operation at positions within the character
latches as shown in the chart in Figure 4-6-20.

Check for Terminate L during the time that Horizontal Blank is going low.

On the following character clock, Hold Request should go low causing Address Load to go
low and Address Count to stay high.

Check the address in address counter outputs (E2l, E22, E25, E30) according to Figures 4-7.
2 and 4-7-5.

10.

Check for unasserted line attributes at E6-23 and E5-1, -26, -27.

11.

Adjust sweep delay to observe the second DMA period.

12.

Check the length of Hold Request. If twice as long as normal, this means that double width
mode has been invoked by some incorrect signal at Address Load time. Because the screen
RAM does not contain properly formatted data for double width, the video processor cannot
display a normal screen.

Any other failures of address loading cause the second DMA to read from the wrong area of the screen
RAM.

))

Later DMAs may fail when early ones are okay if, for example, a high bit on the address counter is
stuck low. Then only higher address lines (lines later in the screen) are affected. This assumes that no
scrolling has been attempted since power-up, because that would eliminate the correspondence between
screen position and address order.

5.6.1.3 Communications - Most aspects of the communications channel operation are under program
control, so the microprocessor must be fully functional (power-up self-test passed) for the interface to
work at all. The communications channel is tested with the data loopback connector (either EIA or 20
rnA) installed. Then you can start self-tests to check the channel operation. If there is a failure, leave
the loopback connector installed. Typing on the keyboard in on-line mode provides data movement to
the transmitter (and receiver if the transmitter works).
Check the STP connector contacts. All interface signals pass through these normally shorted contacts.
Check the baud rate clocks. The baud rate generator (E60) or its two-phase input drivers (E38) may be
bad. Check that the transmit and receive frequencies are as selected (16 times baud rate). E60 may
accept changes on one side but not the other. At the PUSART, check the EIA line driver and receiver
chips (E39 and E44) and check for pulses on the address (E43-11, -12) and I/O control lines (E43-10,
-13) and on the interrupt flag lines (E43-14, -15).
Modem control problems are probably either the STP contacts or the EIA driver or receiver chips.
Check the enable line to the modem flag buffer (E-41). (Also check the modem control signal at the
NVR latch E-61.)

5-76

)1
/

5.6.1.4 Nonvolatile RAM - The NVR is the only device that uses -23 volts (E24-2). The video. processor must be providing LBA 7 as a clock with 12 volt swing at E24-6. Q1 must present a very high
impedance when it is off or read data will be lost. Check the NVR latch E-61 and the flag buffer E-42.
5.6.2 Troubleshooting the Keyboard
During the self-test, check to see that the LEDs flash and that the bell sounds. Failure of these does not
cause the keyboard to fail but this is the best time to make sure that they work.

If the keyboard fails self-test, the terminal automatically becomes a receive-only terminal. The number
4 is then visible on the screen under the blinking cursor. Keyboard failure means the microprocessor
attempted to start a scan but did not get any results back. (The always-down "last key" should always
be returned.) The problem could be at either end of the keyboard interface but is more likely at the
cable or connectors, or in the keyboard circuitry (because of its greater exposure to the environment).
Either the on-line or local LED should be on if the keyboard has 12 volt power and the 5 volt regulator
works. If power is good, check the interface line for the presence of signals from the terminal. Refer to
Paragraph 4.4 in Chapter 4 for an illustration of normal signal appearance.
On the interface, if the clock from the terminal is on the interface line, it should also be at E9-8. Serial
data input (I bit for every 16 clocks) should be at the output of the integrator E2-1. Serial data output
should be at E9-12 (TTL level) and E8-8 (12 volt swing).

)

A clock signal always comes from the terminal, and command bytes come continuously, starting every
1.28 milliseconds. Every 50th or 60th of a second, one command byte includes a bit that triggers the
START SCAN L signal (E5-11) which should start the address cuunters. KEY DOWN L (E6-12)
should appear once each scan to represent the last address (for simplicity, make sure that no keys, including CAPS LOCK are down) and should drive the DATA STROBE L input to the UART (EI2-23).
TBMT H (Transmit Buffer Empty) (E 12-22) should go low when DATA STROBE is asserted. TBMT
H goes low for only a few clock periods when only one key address is being sent. More keys held down
causes TBMT H to stay low for more time for each additional key. When TBMT H is low, the clock
cannot pass through E4 to drive the counter clock input (E7-14); it can at all other times. At the end of
a scan, E3-1 goes high and holds the counter CLEAR inputs. The START SCAN L signal (E5-11)
resets E3 to allow the count to continue.
If no signals are coming from the terminal (and this assumes that the phone jack on the board is good)
check for clock signals LBA3 and LBA4. These come from the video processor, so if the display works
th~y should be present. If they are, check E 18-1 and E 18-2 (TTL and 12 volt respectively). Also, if the
terminal works otherwise, the microprocessor should be regularly loading the keyboard status byte into
the UART for transmission. Check for Keyboard Write (KBD WR L) at E55-23 approximately every
1 ms in response to KEYBOARD TBMT (E55-22) which should go high every 1.28 ms. Make sure that
serial data comes out from pin 25.
If data is coming from the keyboard but is not getting read by the microprocessor, check that data
reaches the serial input pin 20, and that each received word sets KEYBOARD DATA AVAILABLE
H. Check that the KEYBOARD DATA AVAILABLE H interrupt signal reaches the Interrupt Vector
Latch E41-2 and the microprocessor through E23-8. Finally, in response to the interrupt, the microprocessor should strobe KBD RD L at E55-4 and -18. KBD DATA AVAILABLE should go low.
5.6.3 Troubleshooting the Power Supply
The power supply has two basically different sections: one is low power control circuitry and the other is
. highly stressed power components. The current limiting circuits are supposed to protect the supply well
enough to prevent fuse blowing in spite of short circuits, so if the fuse blows repeatedly there is probably a major problem.

5-77

The power switching transistor Q7 and its companion 019 are the usual failure, because they are subjected to the most stress in the circuit. When they fail, other components may also fail. Check 041,
R22, the input rectifier diodes, and the two thermistors. Heavy currents could cause the thermistors to
crack. Sources of stress include connecting the supply to 230 volts when set to 115 volts, and shorting of
any transformer secondary. If the fuse blows but Q7 is okay, check C9, C14, the input rectifiers (the
two connected to the thermistors are the ones used in 115 volt operation), R19, and R20.

)

If Q7 repeatedly blows, the control circuit may be driving it incorrectly. Lift one side of R22 to shut off
the power circuit and examine the control circuit. (The absence of voltage at the output causes the
control circuits to produce maximum pulse widths, and the 12 volt startup supply's regulator Z2 gets
hot from having to provide continuous output. But it is safe.) Look for a strong pulse on Q2's colle,ctor,
a ramp on Q5's collector, a squarewave on Ql 's collector, a negative going pulse on pin 2 of E3 and a
12-13 microseconds squarewave on pin 3. (Refer to Figure 4-9-3, Power Supply Timing Diagram, in
Chapter 4.)

The - 12 and - 23 volt supplies are individually current-limited; the - 12 by its three terminal regulator's self-protection, and the - 23 by a 10 watt resistor in series. The + 5 and + 12 volt supplies shut
down the entire supply if either supply's current limiter is triggered. Check for shorts by checking the
voltages across R37, R44, and R45. If there are no shorts and the supply does not turn on, and startup
voltage (10.8 volts at the banded end of 025) is present, lift one side of 010. This diode carries the
SHUTDOWN L signal from the current limiters to pin 9 of the 3524 control chip. This isolates the
problem to the comparators (E 1). Check the comparators by measuring their inputs: for the + 12 circuit, measure the input with respect to the inboard side of series resistor R37; for the + 5 circuit, measure with respect to the inboard side of R44 and R45. Pin 5 should be positive with respect to pin 6, and
pin 3 should be positive with respect to pin 2. '
The slow turn-on circuit might also keep the supply off. Lift 039 to disconnect it and use an autotransformer to bring the supply up gradually.
I f there are oscillations or instability in the + 5 volt control loop, check C31 and L2, C 17, R31, and
R30. If there are oscillations in the + 12 volt control loop, check C26, Ll, R4, C 1, R5. Excess sensitivity in the current limit circuits can also cause apparent output instability. There is some interaction
in the operation of the supply because the + 5 volt supply must deliver at least its minimumm current
for the + 12 volt supply to be able to meet its regulation specification. Therefore the supply should not
be operated without some load.

5.6.4 Troubleshooting the CRT Monitor
Most monitor problems are due to blown horizontal output transistors and diodes. These components
are the most stressed in the circuit. Listen to the monitor when checking it: the only normal sound is a
slight 15 kHz whistle.
Other problems include a dark screen (with normal sounds), a horizontal line on the screen, and sync
problems (rolling or tearing horizontally or vertically). If the screen is dark, turn up the brightness control on the monitor board, press SET-UP, and bring the intepsity to maximum with the cursor up key,
If there is a faint display or no display, check the horizontal drive operation, check the CRT voltages
and filament, and check the video output from the terminal controller. The high voltage is probably
okay if the horizontal frequency sound from the flyback is normal. The voltages to check then are the
control, focus, and screen grid biases.
A horizontal line on the screen indicates a failure in the vertical oscillator.
For rolling or tearing, check the horizontal and vertical drive signals coming from the terminal controller and be sure that these signals reach the monitor board. (On the Ball monitor board the vertical
hold control should be properly adjusted to ensure good lockup to the drive signal.)

5.6.5 Troubleshooting the Options
See Chapter 6.
5-78

))

CHAPTER 6
OPTIONS

6.1 SET-UP PROCEDURES
This section chapter summarizes the SET-UP information for the following terminals.

)

VT100 - all variations, including VTI XX-AC
VTI05

VTI25 - including VTIXX-CB, -CL
VTl32

This section does not provide detailed information on any SET-UP features, or explain how to change
any of the features. For detailed information, see the user guide shipped with the terminal.

6.1.1 SET-UP A
Figure 6-1 shows a typical SET-UP A display. The same display appears in all VT100 series terminals.

SET-UP
A
m EXIT PRESS "SET-UP"
\

MA-2737

Figure 6-1

SET-UP A with Advanced Video Option (AVO)

6-1

6.1.2 SET-UP 8
Figure 6-2 shows the SET-UP 8 display for the basic VT100 terminal, and Figure 6-3 the SET-UP B
display for the VT100 terminal with VTIXX-AC printer port. Note the fifth switch group in Figure 6-3.
This group appears if an option is installed in the STP connector on the terminal controller board.

))

Figure 6-4 shows the SET-UP B summary for the VT100-AA, -AB, -WA, and -WB terminals. Figure 6S shows the SET-UP B summary for the VT100-WC and -WK word processing terminals, the VT132,
and the VT100 with VTIXX-AC printer port. Figure 6-6 shows the SET-UP B summary for the VTI0S.
Figure 6-7 shows the SET-UP B summary for the VTI2S.

6.1.3. SET-UP C
Figures 6-8, 6-9, and 6-10 show the three different types of SET-UP C displays. Only some word processing VT100s (VTI00-WC through VT100-WK), the VT100s with printer port (VTIXX-AC), and
the VT132s contain a SET-UP C. Figures 6-11 and 6-12 show the VTIXX-AC and VT132 SET-UP C
features, and the possible settings.

))

SET-UP
B
Tn FXIT PRESS "SET-UP"
))

T SPEED

~€:JO

R SPEED

9600
MA2733

Figure 6-2

VT100 SET-UP A

6-2

SET-UP B
TO EXI i

~ =~~~

==C~T
=-l;.-

-Ur:
r ==

VT1XX-ACONLY

~
~ ~igg~

)

:.:..=..=..:

r=!~

MA·3575A

Figure 6-3

VT100-We, -WK, VT132, VTlXX-AC SET-UP B

6-3

~~~-~-~------------------------------

SCROLL

I~£ I

(SHIFTED/O = #
=£

= JUMP
t1 = SMOOTH

AUTOREPEAT

\.1
WRAP AROUND fo = OFF
l1 = ON

fo
l1 == OFF
ON

SCREEN {O = DARK BACKGRND
1 = LIGHT BACKGRND

NEW LINE {O = OFF
1 = ON

CURSOR{O = UNDERLINE
1 BLOCK

INTERLACE {O = OFF
1 = ON

T SPEED 19161 0

AUTO XON XOFF {O = OFF
1 = ON

J:.

~_ ANSI/VT52 {O = VT52
KEYCLICK

rO = OFF

R SPEED

9161 0

0 1

POWER{O = 60 Hz*
1 = 50 Hz*

L-.._ _ _

t1 = ON
1-..._ _ _ _

' - - _ BITS PER CHAR {O = 7 BITS
. 1 = 8 BITS

1 = ANSI

L-.._ _ _

PARITY {O = OFF
1 = ON
O= ODD
PARITY SENSE { 1 = EVEN

MARGIN BELL{O = OFF
1 = ON
*SET TO 1 FOR VT100-WB,
SET TO 0 FOR ALL OTHERS

NOTE: SETTINGS SHOWN ARE FOR
VT100-WA & -WB
MA~1988A

Figure 6-4

,~,'

'---...--

VT100-AA, -AB, -WA, -WB SET-UP B Summary

'~.

''---'/

-....-'~'

'~~

.--

.'--"

I~£

SCROLL{O = JUMP
\..1 = SMOOTH

GROUPS FOR
VT1 XX-AC ON L Y
I(SHIFTEDI{O=#
1=£

A~OUND {O = OFF

. AUTOREPEAT{O = OFF
1 = ON

WRAP

SCREEN {O = DARK BACKGROUND
1 = LIGHT BACKGROUND

NEW LINE {O = OFF
1 = ON

CURSOR{O = UNDERLINE
1 = BLOCK

INTERLACE {

. - - - - } RESERVED

1", ON

0 = OFF
1 = ON

PRINTER PORT (1=ON
TEST MODE
!FOFF

r - i i - I -:

r,;;;;;;:,

I
I
I
'P=~ T=196001 R=~
L_L ..1_I_J

5,
0\
,
VI

AUTO XON XOFF(O= OFF
1 = ON

POWER {O = 60 Hz*
1 = 50 Hz*

ANS INT52fO = VT52
1 = ANSI

LOCAL ECHOeO = OFF
1 = ON

'--_ _ KEYCLICK

SET-UP B FOR:
VT100-WC THRU WK
VT100 WITH VT1XX-AC
VT132

0= OFF
1 = ON

WPS TERMINAL

MARGIN BELL{O= O. FF
1 = ON

RECEIVE PARITY (0 = IGNORE
1 = CHECK

*SET TO 1 FOR VT100-WD,-WF,-WH,-WK
SET TO 0 FOR ALL OTHERS

NOTE: SETTINGS SHOWN ARE FOR
VT100-WC THRU-WK.
MA-3576D

Figure 6-5

VT100-WC, -WK, VT132, VTIXX-AC SET-UP B Summary

SET-UP B
--_,' == ==

:--=-./
-=,.6.;-;,,4
i

PRES""'"
_~

== c:::~ T -UP I I
iri~

SCROLLING MODE
AUTOMATIC REPEAT
BACKGROUND SHADE
CURSOR MODE
RIGHT MARGIN TONE
KEYCLICK
ESCAPE SEQUENCE (0 = VT52. 1 = ANSI)
XON/XOFF
CHARACTER CODE
AUTOWRAP
LINE FEED INTERPRETATION
INTERLACING
} TRANSMIT AND RECEIVE PARITY
NUMBER OF BITS/CHARACTER
LINE FREQUENCY SELECTION

MA-4748

Figure 6-6

VTI05 SET-UP B Summary

)

\1
;

6-6

"'~?.,...v

'~'

r----- U.S.lU.K.

...--_ _ _ SCROLL {O = JUMP
1 = SMOOTH

CHARACTER SET

,~/

{O = U.S.
1 = U.K.

SCREEN {O = DARK BACKGRND
1 = LIGHT BACKGRND

LINE FEED/ {O = OFF
NEW LINE
1 = ON

CURSOR{O = UNDERLINE
1 = BLOCK

INTERLACE {O = OFF
1 = ON

...-----RESERVED = 0
AUX PORT BITS{ 0 =7 BITS
"""'---PER CHAR.
1 =8 BITS

...----AUTO WRAP {O = OFF
1 = ON

AUTO REPEAT {O = OFF
1 = ON

}

00 = 300
AUX PORT { . 01 = 1200
SPEED
10 = 2400
11 = 9600

I I I II I I I I I I J
R=

9"-

-.J

MARGIN BELL{O = OFF
1 = ON
KEY CLlCK{O = OFF
1 =ON
ANSIIVT52{01 =
= VT52
ANSI _ _ _.J
AUTO XON XOFF {O1 =ON
OFF _ _ _ _.oJ

0 = ODD
1 = EVEN

PARITY SENSE {

0 = OFF
1 = ON

PARITY {
BITS PER CHAR. {

0 = 7 BITS
1;= B BITS

POWER {

0 = 60 Hz
1 = 50 Hz
MA-6652A

Figure 6-7

VT125 SET-UP B Summary

SET-UP C

TO EXIT PRESS "SET-UP"

))

DISPLAY

(LANGUAGE)

KEYBOARD

(LANGUAGE)

MA-9339

Figure 6-8

VT100-We, -WK SET-UP e

6-8

))

SET-UP C
HSET-UP··

)

F~INTER PORT:

MA5677

)

}

Figure 6-9

VTIXX-AC SET-UP C

)
6-9

) )

SET-UP
C
TO El(IT PRESS "SET-UP"
))

MA-3574

Figure 6-10

VT132 SET-UP C

6-10

-'---'""

'~'

""--,,,

}

RESERVED

~ESERVEO

RESERVED
PRINT
TERM.,CHAR.
PRINT
EXTENT

-A

D
_

......-a...........---I

= NONE
1 = F.F.

= SCROLL REG.
1 = FULL SCREEN

PRINTERPORT:

T/R=

MA·5674B

Figure 6-11

VTIXX-AC SET-UP C Summary

BOLD
(0= OFF
, . . . - - - - PROTECTED 1 = ON

EDIT MODE(O = OFF
1 = ON
EDIT KEY (

r -_ _ UNDERLINE (0 = OFF
PROTECTED 1 = ON

0 = DEFER
1 = IMMEDIATE

CHAR.SENT{O = UNPRO
1 = ALL

BLINK
(0 = OFF
PROTECTED 1 = ON

SP. COMPRES (0 = OFF
1 = ON

REVERSE (0 = OFF
PROTECTED 1 = ON

0\
I
N

TRANSMIT (0 = DEFER
1 = IMMED.

PRINT
(O=SCROLLREG.
EXTENT
1 = FULL SCREEN

TRANSMIT (0 = NONE
TERM.CHAR.
1 = FF

PRINT
(0 = NONE
TERM.CHAR. 1 = FF

'-----

fo=

'--_ _ ERASURE
UNPRO
MODE
\.1 = ALL

TRANSMIT
}
EXTENT

o 0 PARTIAL PAGE

'--_ _ _ NORMAL
( 0 = OFF
PROTECTED
1 = ON

o 1 PAGE
o LINE
1 1 LINE

MA·3573

Figure 6-12

',--,-,'

VTl32 SET-UPC Summary

"-.--/

'~.:'

6.2

ADV ANCED VIDEO OPTION

The advanced video option (AVO) provides the extra memory required for the VT100 to display 24
132-character lines, an extra 4 bits of memory per character for character attributes, plus space for
extra program ROM. Additional hardware in the AVO are the address decoders for memory expansion,
and the 4-bit wide set of character-clocked latches for the attribute data. Refer to the video processor
functional diagram (Figure 4-6-3) to see how the AVO functions with the basic video circuitry.

6.2.1

Advanced Video Option Installation

Use the following procedure to install the advanced video option.

,
)

Remove the terminal access cover.

Remove the terminal controller board.

Place the terminal controller board on a flat surface with the component side up.

Locate the four mounting holes drilled in the terminal controller board; mount a standoff in
each. (Figure 6-13).

Grasp the advanced video board by the edges and carefully align the connector pins with the
connector on the terminal controller board. Gently but firmly mount the advanced video
board onto the terminal controller board.

Reinstall the terminal controller board. You must install the terminal controller board in the
leftmost slot in the card cage.

Reinstall all cables removed.

Reinstall the access cover.

6.2.2

Ad\'anced Video Option Checkout

Use the following procedure to check out the operation of the advanced video option.

Turn the terminal power on and verify that no error was detected during the power-up selftest.

Press the SET-UP key. The words "SET-UP A" should blink in boldface, the words "TO
EXIT PRESS SET-UP" should be underlined, and the ruler should contain alternating normal and reverse video fields.

Place the terminal in 132-column' mode and then in LOCAL mode.

Exit SET-UP and type the following sequence.

<ESC> < <ESC>#8
The screen should now display 24 lines X 132 columns.

6-13

STANDOFF
COMPONENT SIDE
OF ADVANCED
VIDEO BOARD
COMPONENT
SIDE OF
TERMINAL
CONTROLLER
TERMINAL
CONTROLLER
BOARD

J2 GRAPHICS
CONNECTOR

J520mA
CURRENT LOOP
CONNECTOR
J3 STP
CONNECTOR

))
J4 EIA
COMMUNICATIONS
CONNECTOR

J8 VIDEO IN
CONNECTOR
J9 VIDEO OUT
CONNECTOR
J7 KEYBOARD
CONNECTOR
MA-1995

Figure 6-13

Advanced Video Option Installation

6-14

}

6.2.3 Program Memory Expansion
Basic program memory resides on the basic video board and may be patched in any or all of its 2K
segments, or it may be overlayed by one 8K X 8 ROM on the advanced video board as follows.
Patch
Address
(Hex)
0000-07FF
0800-0FFF
1000-17FF
1800-1 FFFt
Overlay
Address
(Hex)
0000-1 FFF

ROM
Type
ROMA
ROMB
ROMC
ROMD

Jumpers
Used
WI
W2
W3
W4,W8,
W11,W14

ROM
Type
ROMS A,
B,C,D

Jumpers
Used
W1,W2,W3
W4,W5,WIO,
WI2,W13

)

Patch
PROM
Loc
El9
E17
E13
E8

ICType*
Intel 2716 or 2316E
Intel 2716 or 2316E
Intel 2716 or 2316E
Intel 2716 or 2316E

ROM
Loc
E8

IC Type*
Signetics 2664

You can expand program memory by up to 14K bytes, using one 8K X 8 ROM and up to three 2K X 8
EPROMs, if·no 2K X 8 patches were implemented. ROMs may both patch and expand; however,each
socket may either patch the address range given above or expand in the range given below, but not
both.

)

Expansion
Address
(Hex)
8000-87FF
8800-8FFF
9000-97FF
98FF-9FFFt
AOOO-BFFF:j:

ROM
Type
2K X 8
2K X 8
2K X 8
2K X 8
8K X 8

Jumpers
Used

W8,11,14
W6,10,12,13

Expansion
Program
Memory
Loc
E19
E17
E13
E8
E8

IC Type*
Intel 2716 or 2316E
Intel 2716 or 2316E
Intel 2716 or 2316E
Intel 2716 or 2316E
Signetics 2664

Or for addressing convenience use the following range.
Expansion
Address
(Hex)
8000-9FFF:j:
AOOO-A7FF
A8000-AFFF
BOOO-B7FF
B800-BFFFt

ROM
Type
8K X 8
2K X 8
2K X 8
2K X 8
2K X 8

Jumpers
Used
W7, I 0,12, 13

Expansion
Program
Memory
Loc
E8
E19
E17
E13
E8

IC Type*
Signetics 2664
Intel 2716 or 2316E
Intel 2716 or 2316E
Intel 2716 or 2316E
Intel 2716 or 2316E

May be used only if 8K overlay or 8K expansion has not been implemented.

These parts must have an access time of 350 ns maximum.

:j:

If basic ROM has been overlayed by 8K X 8 RAM, program memory cannot be expanded by
another 8K X 8, but only by three of the 2K X 8 patches as shown above.

6-15

-You must install either W5, W6, or W7 in all configurations, if none of these jumpers is required by the
above specification. Install one that enables the 8K ROM in an unused address space.

-\
) )

You can use jumper 15 in place of jumper 10, or jumper 16 in place of jumper 9, if a high asserted chip
select is desired on an 8K ROM.
6.2.4 Alternate Character Set
If the AVO is present and an alternate character ROM is installed on the terminal controller, you can
select an alternate character set on a character-by-character basis. The alternate character set can provide an additional 127 characters. If the AVO is installed without the alternate character RO~, any
character cell in which the alternate set is selected appears white (black if reverse video). See Appendix
A (select character sets) for selection of alternate characters.
6.2.4.1

Alternate ROM Description

Size:

2048 words X 8 bits

Speed: 300 ns access time if 132 column operation is desired; 550 ns access time is suf) )
ficient for 80 columns o n l y . ,

Pin-out (Intel 2316E equivalent)
Pin
No.
1
2
3
4
5
6
7
8
9
10
II
12
13
14
15
16
17
18
19
20
21
22
23
24

Pin
Name
A7
A6
A5
A4
A3
A2
Al
AO

00
01

02
GNO

03
04
05

06
07
CS2
AIO
CSI
CS3
A9
A8
Vcc

Use for Alternate Character ROM
Character address 3
Character address 2
Character address I
Character address 0
Scan address 3
Scan address 2
Scan address I
Scan address 0
Fill bit
Rightmost character bit/character data I
Character data 2
Ground
Character data 3
Character data 4
Character data 5
Character data 6
Leftmost character bit/character datil 7
Chip select - asserted low
Character address 6
Chip select - asserted low
Chip select - asserted low (Paragraph 6.2.4.2 below)
Character address 5
Character address 4
+5 volts

6-16

))

6.2.4.2

Character ROM Programming Instructions

Chip selects must always be asserted low.
(To allow use of pin compatible UV erasable PROMs, pin 21 may be tied to + S V by removing W4 and inserting WS. The speed restrictions listed above still apply.)

A high coming from a character data pin produces a dot on the screen. A high from the fill
bit causes the right space, between the character in which the bit is asserted and the following character, to be filled on the scan(s) for which the bit is asserted. One-dot-wide horizontal
spaces in a character are filled by the dot stretcher.
The following scan and character addresses are in hex.

Scan addresses
Scan
Address

o
1
2
3
4

)
4.

)

Character Scan Number
(Figure 4-6-17)
I
Normally blank in VTl 00 main ROM
2
Top of normal uppercase character
3
4

S
6
7

7
8
9-E

Bottom of normal uppercase character
Normal descenders (underline scan)
Normal descenders
Not displayed

Character address 7FH is not displayed and should be left blank. Character addresses
20H- 7EH are accessed by ASCII codes 20H-7EH respectively when the VT100 character
set is set to "alternate character ROM" (Appendix A, select character sets). Character
addresses OH-l FH are accessed by ASCII codes 5FH-7EH respectively, and character
addresses 20H-S EH respectively, when the VT100 character set is set to" alternate special
graphics" (Appendix A, select character sets).

6.2.5 AVO Technical Description
This section provides a technical description of the advanced video option board.
6.2.5.1 Extended Character and Attribute Memory - The complete address and data buses from the
microprocessor enter the A va through the circuit board connector. Bits AOO through A09 directly access I K of 8-bit RAM '(E18 and E20) when AIO, All, and SEL 8-12K enable the RAM chip selects.
The Select Attribute RAM (SEL ATT RAM) signal, decoded on the basic video board, enables the 4K
X 4 memory (E2, E7, ElO, E14) for writing and reading attributes. When read by the microprocessor,
these 4K words appear in address space 3000H to 3FFFH and the four bits of each word appear on
00-03. The attribute bits are read and written by the microprocessor through buffer Ell which is
configured to drive the four bits in one direction or the other. One half of E9 and a pull-up resistor form
an OR gate, so that data is output from the board when MEM RO and SEL AIT RAM are both
asserted. Ouring a OMA by the video processor, the four attribute bits are read in parallel with the
seven character address bits and one attribute bit from the memory on the terminal controller. This is
done by enabling the RAM outputs to the attribute latches whenever an address in the range of 2000H
through 2FFFH is accessed during a OMA.

6-17

6.2.5.2 Character Attribute Latches - The attributes have their own set of character-clocked latches
to provide complete synchronization of attribute and character data in the video processor. At the assertion of Hold Request and DMA Enable, data passes from the attribute RAM to the input of latch ES.
As with the characters in the character latches the attributes shift through DI-4 of ES, then through E9
to the attributes line buffer E4 for storage, and again through D5-8 of ES to the attribute inputs to the
video processor on the basic video board. The attributes line buffer (E4) is addressed by the same LBA
signals as the line buffer on the basic video board.

))

6.2.5.3 Program ROM Decoding - The AVO has sockets for four 2K or one 8K and three 2K ROMs.
You can select the address blocks that these ROMs operate in with wire jumpers (or switches in newer
models) on the AVO board. Flexible decoders can address the ROMs as extended program memory or
a ROM can overlay an existing block of program. When existing memory is being overlayed or memory
above 4000 H is being addressed, the Memory Disable signal is asserted to turn off memory on the
basic video board.
6.2.6 Troubleshooting the AVO
This is mostly an extension of the character latches in the video processor. You may suspect problems
in the AVO if character attributes will not work (check with the SET-UP Screen Test). Also, a portion
of the screen RAM resides on the AVO. Problems are evident if the screen is full in 132 column mode.
(Type <ESC> # 8 in local mode to get the E test pattern.) Check the power connections, memory
decoders, and the memory disable control line.

}

Self-test tests the AVO RAM. If you suspect errors in the AVO, but it passed self-test, the problem
may be on the attribute latch (video processor) side of the AVR RAM control circuits.
6.3 20 mA CURRENT LOOP ADAPTER
The VT100 current loop adapter converts the Electronic Industries Association (EIA) standard serial
voltage input and output from the basic video board to 20 rnA current signals. You can separately
switch both receiver and transmitter to operate either passively or actively.
6.3.1 20 mA Current Loop Option Installation
Use the following procedure to install the 20 rnA Current Loop Option.
1.

Remove the terminal access cover.

On the new terminal access cover with the 20 rnA current loop card, set the TRANS switch
to the NORMAL position (Figure 6-14). (If the terminal must provide current on the receiver line set the switch to the ACT position.)

Set the REC switch to the NORMAL position (Figure 6-14). (If the terminal must provide
current on the receive line set the switch to the ACT position.)

Connect PS to 1S on the terminal controller board (Figure 6-13).

Install the terminal access cover containing the 20 rnA current loop option in place of the old
access cover.

Connect the communications line to the Mate-N-Lok connector on the bottom of the access
cover.

Perform the 20 rnA current loop option checkout.

6-18

)

a
CAPTIVE
SCREWS (4)
......_ _ _ ACCESS
COVER
P5

20mA
CURRENT
LOOP BOARD

)

TRANSMIT
LINE
SWITCH
RECEIVE
LINE
SWITCH

)

20mA
COMMUNICATIONS

C~~~~~E§~~====~~-------CONNECTOR
--

Figure 6-14

)
/

MA·1996

20 rnA Current Loop Option

6.3.2 Configurations
In most current loop applications, the VT100 is connected in a passive configuration - that is, current is
supplied to the VT100. In this mode, the transmitter and receiver are both passive, both optically isolated, and the transmitter goes to the mark state when power is turned off.
Conversion from active to passive mode (or vice versa) for either transmit or receive side is switch.
selectable.
In active mode either the transmitter, or the receiver, or both may be connected. so that the VT100
sources the 20 rnA of current. In active mode, isolation is not present, and the transmitter goes to the
space state when power to the VT100 is turned off.

6-19

6.3.3 20 mACurrent Loop Option Checkout
The VT100 contains an internal test called the data loop back test. In the data loopback test the VT100
transmit and receive lines are connected to each other via a special external connector. A predefined set
of characters are then transmitted. The terminal receives the characters and compares them to the
characters transmitted. If the characters do not match an error is then flagged.

) )

Use the following procedure to check out the operation of the 20 rnA current loop option.
1.

Disconnect the terminal from the communications line.

Remove the terminal access cover containing the 20 rnA current loop back board and set both
switches to the NORMAL. Reinstall the access cover.

Connect the 20 rnA loopback connector (PN 70-15503-00) to the Mate-N-Lok connector on
the bottom of the access cover.

Place the terminal in ANSI-compatible mode (in SET-UP B, group 2 switch 3 = 1).

Type <ESC> [2;2y to perform the data loopback test.
After the test is complete, the screen clears and the message "WAIT" appears in the upperleft corner of the screen. The test takes about six seconds to run.

A loopback error is shown by an 8 in the upper-left corner of the screen. If an error is detected, check the 20 rnA board connectors and switch settings and then repeat step 5.

After the test is complete, return the 20 rnA current loop board switches to the original positions, remove the loopback connector, replace the access cover, and reconnect the terminal to
the communications line.
NOTE
The t~rminal can use either 20 mA or EIA communications. If EIA is used on a terminal containing
the 20 mA option, the cable connecting the 20 mA
option board to the terminal controller board at JS
must be disconnected.

6.3.4 Current Loop Principles
The current loop system was designed for teletypewriter communications between remotely located stations. The 20 rnA current was designed to operate the selector electromagnet that decodes data in teleprinters. All teletypewriters on a circuit were in series so that any station could signal all the other
stations simultaneously by passing and interrupting the current through all stations' selector magnets.
Today, decoding of the current signal is performed electronically, rather than mechanically, but the
good noise immunity of the system has kept it around as a reliable way to run lengthy in-house terminal
drops at low cost. Figure 6-15 defines the states of the interface line.
6.3.4.1 Current Loop Adapter Description - This section describes both the transmitter and receiver
sides of the current loop adapter.
6.3.4.1.1 Transmitter Side - An EIA-ta-TTL level converter (E3) inverts the VT100 output signa\' but
a second stage reinverts it. The TTL signal switches a pair of paralleled open-collector drivers that
drive the LED inside an opta-coupler. Resistor Rl1 limits the LED current. The LED output controls
the conductance of the phototransistor inside the opta-coupler. The transistor's base is tied to its emitter
by R 13 and its base is tied to its collector by 05 to improve its switching speed.
6-20

) )

0- 2 rnA = OFF = SPACE = 0
18 - 50 rnA = ON MARK = 1

10 rnA

MA-4277

Figure 6-15

Interface States

When the phototransistor is conducting, the base drive for Q3 is diverted and Q3 and Q4 are cut off to
put the transmitter in the space state. The only current that can flow in the space state passes through
03, which is a constant current diode rated for 1 rnA. When the phototransistor is cut off, D3 provides
base drive to Q3 and forces the transmitter to the mark state. Use of the constant current diode D3
allows sufficient base drive to Q3 to keep the transmitter in the mark state when the phototransistor is
cut off or when power is removed from the VT100. But D3 still limits the current through the transmitter in the space state to less than 2 rnA even with 50 volts across the circuit.

)

In passive mode the transmitter can control the flow of current from an external current source. In
active mode the transmitter is placed in series with current supplied from the + 5 volt and :.... 12 volt
power supplies and limited by series resistance.
6.3.4.1.2 Receiver Side - Passive: The optional capacitor on the input gives line transient immunity.
The LED in opto-coupler El accepts current from a remote source limited by R2 and Ql. To ensure
proper recognition of the off state, Rl draws 3 rnA to bypass low level currents around the LED. If the
line current rises to make the voltage drop across R2 greater than 0.6 volts, transistor Q 1 turns on to
shunt excess current around the LED. Besides protecting the LED, this also improves speed by reducing saturation of the LED and transistor. D 1 protects the circuit from reverse connection of the signal
line.
Active: With switch SW2 in the up position, the adapter provides power for the line: + 5 volts and - 12
volts through similar resistors R8 and R9. The remote device passively switches current to activate the
circuit.
The phototransistor in the opto-coupler controls the base current to Q2. R5 speeds up Q2's turn-off by
pulling the base down faster. C 1 speeds Q2 turn-on. R3 prevents the phototransistor from saturating by
allowing the base and emitter voltages to rise until excess base charge can be removed by conduction
through R4.
The output is pseudo EIA (RS-232) because it assumes that a particular receiver device is being used
which happens to accept a 0 as a mark. The normal RS-232-C specification calls for +6 = space and
-6 = mark. 04 shunts -12 in case someone connects the EIA signal to the exposed connector while
the current loop is in"t~ lien. It !lfotects E2 pin 3. R 7 pulls up to give high drive to the 1489EIA receiver
- TTL converter on the basic video board.

6-21

6.3.5 Interface Signals
The currrent loop option interfaces to external communication equipment through an 8-pin Mate-NLok connector as described below.
Pin
No.

) )

Description
Active Mode

Signal Name

Passive Mode

Negative Test
Voltage

V - with a series resistance of 480 ohms
±S%.

Transmit negative
(T-)

Current flows away
from this terminal.

Current flows into
this terminal which
is connected to V via a series
resistance Rs.

Receive negative
(R-)

Current flows away
from this terminal.

Current flows into
this terminal.

(Not used)

Transmit positive
(T+)

Current flows into
this terminal.

Current flows away
from this terminal.
The equivalent
voltage source is
(V+)-Vd with a
series resistance
Rs.

(Not used)

Receive positive
(R+)

Ground

VV=
RS
Vd

-12 Vdc ±S%
+S Vdc ±S%
330 ohms ± S%
2.0 V maximum

6.3.6

Interface Specifications

Current flows into
this terminal.

})

))

Current flows away
from this terminal
which is connected
to V + via a series
resistance Rs.

))

Transmitter
Open circuit voltage
Voltage drop marking
Spacing current
Marking current

Min
S.OV
O.OV

Receiver
Voltage drop marking
Spacing current
Marking current

Min

20 rnA

l;mA
6-22

Max
SO V
2.0V
2.0 rnA
SOmA
Max
2.3 V
3.0 rnA
SOmA

jl
I

6.3.7 Troubleshooting the Current Loop Adapter
Except for incorrect setting of the normal (active) and passive selection switches and connector problems, malfunction is probably due to bad semiconductors.
6.4 VTI05 GRAPHICS PROCESSOR
This section describes the VTI05 waveform generator module, M7071. The M7071 complements the
VT100 alphanumeric terminal by adding graph drawing capabilities. Together they comprise the
VTl05 alphanumeric and graphic terminal. This section describes the following items.

)

•
•
•
•
•
•
•
•
•
•
•
•

the graph module in a block diagram format
decoding the input
establishing mode of operation
phase lock loop timing
rectangular aspect ratio graph drawing field
square aspect ratio graph drawing field
loading the registers
loading graph memories
generating baselines
loading vertical and horizontal lines
generating strip charts
combining video out

6.4.1 Enabling Graphic Information
On initializing the VTI05, the M7071 forces the GRAPHICS FLAG low so the VT100 terminal controller module recognizes that the graphics option is installed.

)

To enter the graph drawing mode, an escape sequence ESC 1 is needed. The terminal remains in this
mode until ESC 2 is received. In this mode, graphics data is passed to the M7071 on parallel data lines.
When data is passed, the GRAPHICS FLAG goes high and stays high until the data is stored in the
static random access memories (RAMs), This signal is sensed by the VT100 terminal controller, and
does not pass another character to the M7071 until this signal goes low.
The M7071 also uses the following signals from the VT100.
DO 00 H - 00 06 H - The parallel 7-bit graphic information in the form of a character to the M7071.

)

DO 07 H - This signal is used by the VTI05 for hard copy.
RESET H - The M7071 uses this signal on power up to initialize its registers. Pressing the RESET key
during SET-UP mode also generates this signal.
HORIZ BLK H - Defines the horizontal blank time of 11.4 ILS. The horizontal active time is 52.1 ILS;
the total period is then 63.5 ILS.
VERT BLK L - Vertical blank time is 1.016 ms. Vertical active time is 15.24 ms for a total period of
16.256 ms.
WR GRAPHICS L - A pulse passed to the M7071 to strobe the parallel data (DO 00 H - DO 06 H)
into the input latch.
GRAPHICS 1 IN L and GRAPHICS 2 IN L - The video output from the M7071.

)

The following paragraphs provide a brief discussion of the waveform generator block diagram, Figure
6-16.

6-23

GRAPHICS FLAG L

LOAD X

WRITE
LOGIC

DATA

WRGRAPHOL
WR GRAPH I L
WR DATA L
WR HORIZ L

CUR 0
OBI
STRIP
ENABLE

TEST
LOGIC

YO-Y7
LOAD
MODE

512 x
B-BIT
RAM

9'
N

DBO

512 x 4

BBIT
COM-

RAM

PARATOR~--~Ht'-~

BASELINE 0

BASELINE

HORIZ LINE 1
HORIZ (1) H
HORIZ
LINE
MEMORY

L-

r--

8 BIT
COMPARATOR
BASE~ BASELINE

1')..,.......;>0,.1 256. 1
RAM

TIME 11 I H

COUNT TIME H

TIME(1I H
......".. - vIDeo H (GRAPHICS 1 IN LI

....... 737

Figure 6-16

----/

M7071 Waveform Generator Block Diagram

--~

,,--/

-~-

;

6.4.1.1 Writing Data to the Waveform Generator (block diagram discussion) - Data in the form of 7bit ASCII characters is received by the data input latch. The first character must be a control character to give instructions to the waveform generator module as to where to place the next data information. Is the next data to be loaded into the registers or into the graph memories? Is it horizontal line
data or vertical line data? The control character is loaded into a binary to decimal decoder that enables
I of 10 load modes. (See Paragraph 6.4.1.3 to decode the input character.)
One of these modes is load enable register 0; another is load enable register 1. These two registers store
the information as to the type of graph desired (line graph, shaded graph, or strip chart) and the desired
graph features (horizontal lines, vertical lines, and graph markers). Both registers use either one or two
data characters to enable the desired graph features. The First Data flip-flop keeps track of these two
data characters. Two data characters form a data word.

)

The second data character following a load enable register 1 command enables the mode select logic.
Data bit 0 of this character determines the aspect ratio of the graph to be displayed on the video screen.
Wfienlhis bifis-seito 1, thesqiiare aspec:Cfatio is eria15lea. This aspect ratio isa-rfe-w featufe--of-fhe
VTI05 and uses all 240 scan lines on the terminal. When this bit is set to 0, the rectangular aspect ratio
is selected. This aspect ratio is compatible with previous DECgraphic video terminals (i.e., VT55) and
uses only 230 scan lines of the terminal for the graph display. This later aspect ratio is also enabled
during the power-up sequence of the terminal (by RESET H).
The outputs of the mode select logic go to a phase-lock-loop to establish the two timing signals necessary to set up the rectangular and square aspect ratios. These are 10.4 MHz and 12.4 MHz, respectively. The phase-lock-loop has a phase detector to monitor the horizontal blank signal from the VT100 and
divides the frequency of this signal by a set number for each aspect ratio.

)

Another function of the decoder is to select the memory into which the data is to be loaded. This may
be Graph 0 memory, Graph I memory, vertical line and graph marker memory, or horizontal line memory. Data to these memories must always be in the form of two data characters (one data word). A 5-bit
data register holds the first character while waiting for the second. Then they are both parallel loaded
through a 2-line to I-line multiplexer, becoming RAM DATA to the inputs of each of the memories.
The memory enabled to "write" at this time is determined by the decoder. The load enable signal from
the decoder is synchronized by the First Data flip-flop and the Load X-Data multivibrator. One of the
following signals is enabled to the memories.
WR GRAPH 0 L
WR GRAPH I L
WR DATA L
WR HORIZ L
Data is then written into the appropriate memory, as shown in Figure 6-16.
Data for the shade lines (base lines) is loaded into Baseline 0 register and/or Baseline I register.
Data for the starting X-address is loaded into the initial X-address counter. The final X-address counter
is loaded to this starting X-address at the end of the current scan (by the 512-bit counter). Data for
successive X-address positions in memory only require one control character. The final X-address
counter increments the position in memory for placing the new data.
When a strip chart is enabled (from register 0), the clear input to the final X-address counter is disabled. This counter does not reset to zero, but rather it follows the initial X-address counter. New data
increments the X-address and is added to the end of the graph. This new data is the last data to be read
from memory. Previous data is read one position earlier causing a shift in the video graph to the left.

6-25

6.4.1.2 Reading Data from the Waveform Generator (block diagram discussion) - As long as the terminal is in graph mode, the enabled graph data is read continuously from the waveform generator module.
The V-address down counter is initially preset to one of two values: 240 lines for the square aspect ratio,
or 230 lines for the rectangular aspect ratio. The outputof this counter is enabled through the 2-to-l
multiplexer, becoming RAM DATA to strobe the row address of the memories. As each scan proceeds
across each row, the X-address counter strobes the column address of the memories.

) )

The output of each memory is strobed into a corresponding D-type data latch. This provides buffering,
as well as an input source for rewriting .the data back into memory so it is not lost. The data from each
memory is sent to a respective 8-bit comparator where the data is compared to the V-address (YO-Y7).
If both inputs are high at the same time, a high output is sent to the video data latch to intensify that
position on the screen. If a histogram is enabled, all points below the graph position are also intensified.
If a cursor (graph marker) is enabled, 16 points are intensified on the screen creating a short vertical
line on the graph at a specific graph point. If a shade line (base line) is enabled at a specific Y-address:
(1) all X-address positions on that line are ·intensified on the screen; and (2) all positions between the
graph data and the shade line are also intensified. If a vertical line is enabled at a specific X-address,
that X-address is intensified on each scan creating a vertical line on the screen.
All data is presented to the video data latch. At alternate times Histogram 0 and Histogram 1 data are
gated out. This allows two shaded graphs to be displayed on the screen and still be discernible in overlapping areas. Graph markers, horizontal lines, and vertical lines are clocked to the output every TIME
(1) H pulse. This timing comes from the phase-lock-loop previously mentioned ..
The blanking gates keep track of both horizontal and vertical blank times from the VT100 and create
the signal HORIZ + VERT H. This signal is used to clear the X-address counter and a 512-bit
counter. Every COUNT X 00 (1) H from this counter clock the window flip-flop at the proper time to
gate the video to the VT100 terminal controller. These signals are VIDEO H (GRAPHICS 1 IN L in
the VT100) and VIDEO 2 (GRAPHICS 2 IN L).

))

6.4.1.3 Decoding the Input- When the data received is one ofthe control characters listed in Table 6..:1,
bits 0-3 of the character are used to determine which control signal to enable. Only one set of control,
characters is used for one M707 L The extended character set is primarily used for a second M7071
installed in the same terminal to enable up to four graphs or strip charts. These bits are sent to a binary to
decimal decoder (Figure 6-17) that enables only one of the outputs at a time.
Table 6-1

Char
@

A
B
C
D
H
I
J
K
L

Octal
Code

Enable Signal

100
101
102
103
104
110
111
112
113
114

Load Baseline
LDENABLEOL
LDGRAPHOL
LDCURSOROL
LDHORIZL
LDXL
LDENABLE 1 L
LDGRAPH 1 L
LDCURSOR 1 L
LD VERT L

))

Control Characters
Unit 0
Ext
Char

Unit 1
Octal
Code

P
Q
R
S
T
X
Y
Z
[

120
121
122
123
124
130
131
132
133
134

6-26

)

GRAPHICS FLAG L

o~--~~~~~------------------------------~
PREWRITE

FROM
VT100
TBl

---Ir-""""

o ~D~O~0~7..'::H~____--'-_ _ _ _ _ _

COpy L

WR GRAPHICS L
o I--r-;';';';"-=":";::"':;"':"':":"::::":::...a

DB6 H
LD CONTROL L

)
FIRST
DATA
F/F

DB5 H

LOAD
MODE
MV
50ns

LOAD
LD X DATA H
X-DATA
MV
TO LOAD X
lOOns
ADORESS
DATA
AND
WRITE
LOGIC

LD MODE H

)

UNIT 1 H
UNITO H

W3
W2

LD VERT L
LD HORIZ L
LD CUR 1 L

LD CUR 0 L

)

LD GRAPH 1 L
DB4 H
FROM
DATA
INPUT
LATCH

DB 2 H
DB 1 H
DB 0 H

LD GRAPH 0 L

TO
REGISTERS

LD ENABLE 1 L

D-TYPE
FLlPFLOPS

LD ENABLE 0 L
LD XL
NOP L

DB 3H

(LD BASELINE)
MA-4735

Figure 6-17

Decoding the Control Character

)
6-27

The extended character set is reserved for future expansion. Current software does not support the extended character set.- Discussion in this chapter is limited to unit 0 to avoid confusion.

) )

Data bit 4 is used to load X-data and select Graph 1.
Two ASCII characters are necessary to transmit a complete data word. Data bits 5 and 6 monitor these
data bits. The first flip-flop keeps track of the first and second data characters.
Data bit 7 (DO 07 H) forces the GRAPHICS FLAG high to halt further transmission from the VT100
terminal controller while the video screen is being copied by a hard copy unit. Signal COpy L is sent to
the hard copy unit on TB 1. The hard copy unit must detect and hold this signal low while it copies the
screen.
604.1.4 Selecting Mode of Operation - There are two formats or modes for setting up the display: rectangular format and square format. The rectangular format uses a 10.4 MHz signal to display graphic
data on the video screen in an area 230 dots high by 512 dots wide, as shown in Figure 6-18. The square
format increases the rate of transmission to 12.4 MHz. The same graphic information is displayed in a
more compact area, as shown in Figure 6-19.

))

Rectangular Aspect Ratio - In the rectangular aspect ratio graph drawing field, (Figure 6-18) space is
provided in the left margin for one character. Under the graph field, there is space for one line of characters. To set up the left margin, a delay is needed before starting the graph. Two signals (1.2 IlS and
320 ns) are added together to create a 1.52 jJ.s delay after horizontal blank time. This time allows space
for one character in the left margin before starting the first X-address of the graph.
The horizontal line counter is initially loaded to 230 scan lines. Twenty-three lines of characters, each
having a 10-line scan cell, can print within the graph drawing field. A 24th character line can print
under the graph field.
Square Aspect Ratio - I n the square aspect ratio graph drawing field, (Figure 6-19) additional delay
must be achieved before starting the first X-address. A 640 ns pulse clocks a counter loaded to divide
by 8. A 5.12 IlS delay occurs before starting the graph centering the graph field on the screen.
The square aspect ratio uses the full vertical screen area of 240 scan lines; 24 lines of alphanumeric
data can be placed on the graph field. Up to eight characters may be placed on the screen to the left of
the graph field; up to seven may be placed to the right of the graph field.
6.4.1.5 Decoding Field Selection - Selecting the field is accomplished by data bit 0 (DB 0) in the
second data character following a Load Enable Register 1 instruction. The first flip-flop (Figure 6-20)
keeps track of the first and second data characters. The second data character enables FIRST (1) L
which clocks DB 0 into the VTI05 mode select flip-flop. If DB 0 is high, the square field is enabled. If
DB 0 is low, the rectangular field is selected.
The signal RESET H (lNIT L) from the VT100 terminal controller clears the mode select flip-flop and
also enables the rectangular field.

6-28

) )

(XO. Y22910)

10.9CM
(43 18 IN)

(

230 DOTS

512 DOTS
A
_'---- A

r.. . -________

20.32
(8IN)

CM~_ _ _ _ _ _ _... (XS111O' YO,)
MA-4732

Figure 6-18

Rectangular Aspect Ratio Graph Drawing Field

(XS11. Y239)
(XO. Y 239)

I
I
I
I
I

240 bOTS

)

480 DOTS

11.5 CM
(4518 IN)

I
I

512 DOTS

'.1

EIGHT
CHARACTER

I
I

(XS11. YO)

1~:~------l:.~~~------~J
15.2 CM

7 CHARACTERS

(6.5 IN)

Figure 6-19

MA-4731

Square Aspect Ratio Graph Drawing Field

)
.'

6-29

FIRST (1) L

FROM

TO PHASE
LOCK LOOP
f":V~T~1:-:0~5"& Y-ADDR COUNTER
D
t----.
DBOH
MODE VT105 (1) H
SEl.E.-C (SQUARE FORMAT)
FLIP~_~CFLOPt--_ _•
VT105 (1) L
~~--(RECTANGULAR
FORMAT)

LDCONTROLL

DECODER LD ENABLE 1 L
FROM
VT100
MA-4730

Figure 6-20

Selecting Mode of Operation

)
/

6.4.1.6 Pbase-Lock-Loop Timing - A 10.4 MHz signal and a 12.4 MHz signal are needed to achieve
the rectangular and square aspect ratio, respectively. A phase-lock-loop (Figure 6-21) generates and
maintains these frequencies. The HORIZ BLK L signal from the VT100 terminal controller is used at
one input to a phase detector. The phase detector monitors the output of the voltage controlled oscillator (VCO) to detect any frequency drift. The' output frequency is divided by the number N in a
counter to approximate the HORIZ BLK L input. The value for N is 662 for the rectangular aspect
ratio and 800 for the square aspect ratio.
Should the frequency drift, the difference between the leading edge of the inputs to the phase detector
is presented as a voltage change to the VCO and either raises or lowers the output frequency to compensate for the drift.
6.4.1.7 Establishing Desired Display - Selecting the desired graph, shading the graph, single or dual
strip chart operation, and adding vertical and horizontal lines is initially stored in registers, shown in
Figure 6-22. The signals LD ENABLE 0 Land LD ENABLE 1 L from the decoder enable register 0
and register 1, respectively. The first data flip-flop monitors the characters to enable the proper register
to store the type of graph desired.
Loading Register 0 - The letter A, when typed on the keyboard or coded in a program, enables register
O. One or two data characters may follow this letter. The first data character is clocked into aD-type
flip-flop to determine the graphs or histograms desired (Figure 6-23). DB 0 H must be set to display the
graphic features enabled in register 0 and register 1; if 0, the graphic display is turned off.
A second data character may be used to set up baselines (shade lines) or strip charts (Figure 6-24). The
state of the signal BASELINE CTRL determines whether Baseline 0 or Baseline 1 is to be loaded. If
low, Baseline 0 may be loaded; if high, Baseline 1 may be loaded. See Paragraph 6.4.1.10 for more
details on baselines.
DB 3 H enables the signal STRIP H to allow Graph 0 or Graph 1 to be a strip chart. DB 4 H enabies
STRIP H and DUAL STRIP H. Both signals are required to display both Graph 0 and Graph 1 as strip
charts. Refer to Paragraph 6.4.1.15 for more information on strip charts.

6-30

HO R IZ ,...-_ _ _-,
FROM
VT100

BLKL

~~~~~TOR r- G~~~~R

~ TIME (1) H

OSCILLATOR

'------~

VT105
(1)H
_
(N=BOO)
VT105
(1)L
(N=662)

VOLTAGE
CONTROLLED _

TO OUTPUT
CIRCUIT

PHASE
LOCK
LOOP

N
COUNTER
CLK

TIME (1) L

TIME (1) H

COUNT
ENABLE
FROM
HORIZ +
B LAN KI NG _V_E_RT_L---,_ _ _ _...c C

COUNT
TIME H
TO
X ADDRESS
COUNTERS

GATES

VT105 (1) L
FROM MODE
SELECT F/F
VT105

DELAY
+3 V-,-- COUNTER

RECTANGULAR
.,. 1
SQUARE

(1) H

"'8
LD

CUP

1.2 JJS

)

320 NS'----

n- >-

t---il---""""L__
_

VIDEO DELAY TIME

HORIZ BLK L

640 NS

1 . . __. . 1.

RECTANGULAR
MODE

1.52 JJs-j
~
SQUARE MODE _ _~_~Ir -

~ 5.12 JJs-j
MA-4734

Figure 6-21

Phase-Lock-Loop Timing

6-31

FIRST (1) H

LD MODE H
DB 6
DB 5

LOAD
ONTROl
GATES
LD CONTROL

FIRST
DATA
FIRST
F/F
1...(0) H

)

r+CUR GRAPH 1
f+VERTICAL LINE
r+HORIZ LINE

(2ND CHAR)

LOAD
MODE IMV

WR GRAPHICS L
FROM
VT100

j+HISTOGRAM 1

SOns

....~~----~

CLK

FROM ~
VT100 000

r+GRAPHl
r+GRAPH 0

OCTAL
INPUT
DATA
LATCH

(2ND CHAR) f+DISPLAY (1) H

DB4
DB 0

. .______

STRIP
f+DUAL STRIP
AND
BASELINE ~STRIP
ENABLE
I+BASELiNE ENA 01
REGISTER
I+BASELINE ENA 00
REG 0
I+BASELINE
(3RD CHAR)
CTRL (1l H

~
~

HEX
~ D·TYPE
F/F

I--

CONTROL
I-ENABLE

I--

DECODER

LD ENABLE 1 L

....-+---....

LD ENABLE 0 L
~:...::.:..:..:....;;::..:;,;:...;;.~--l~...J

1ST
DATA

BUFFER

I-(1ST CHAR) I-FIRST (0) H~
2ND DATA CHARACTER DBQ-+3 H

Figure 6-22

Loading the Registers

6-32

B DATA

~CHARACTER . . .Q-+4 (1) H
TO X.ADDRESS
COUNTER,
STATIC
MEMORIES,
AND BASE LINE
REGISTERS

))

)

DB 4 H

HISTOGRAM 1 (1) H

DB 3H

FIRST
DATA
CHARACTER

DB 2 H
DB 1 H

HISTOGRAM 0 (1) H
GRAPH 1 (1) H
GRAPH 0 (1) H
DISPLAY (1) H

DBO H
CLK

)

FIRST (1) H
LD CONTROL L
LD ENABLE 0 L - - L _ - ' "

Figure 6-23

MA.... 728

DUAL STRIP H
DB 4 H

)

STRIP H

DB 3 H
DB 2 H

BASELINE ENA 01 (1) H

DB 1 H

BASELINE ENA 00 (1) H

DBOH

BASELINE CTRL (1) H

FIRST(1) L
LD CONTROL L
LD ENABLE 0 L

MA-4722

Figure 6-24

6-33

Loading Register 1 - The letter I produces the signal LD ENABLE 1 L to enable register 1. One or two
data characters may follow this letter. The first data character enables horizontal lines, vertical lines,
and graph markers, as shown in Figure 6-25.

) )

A second data character following the letter I enables the desired field and the interactive graphics test
(Figure 6-26). The square field is established when data bit 0 is set to 1; the rectangular field is selected
when DB 0 equals o. The rectangular field is also selected during the initial power-up sequence of the
terminal by the signal RESET H (lNIT L) from the VT100 terminal controller.
The interactive test is used to check the M7071 in LOCAL mode of operation. See Paragraph 6.4.2.
6.4.1.8 Loading X-Address Information - In order to load the horizontal address of a dot or line, two
data characters must be transmitted. The first character contains the lower five bits of the binary Xaddress and is stored in a register while waiting for the second character with the upper bits. When both
characters are received, the signal LD XcDATA H loads the initial X-address counter, (Figure 6-27).
When the current scan is complete, this data is loaded into the final synchronous X-address counter and
presented to the random access memories.
During a write data program, the signal LD X DATA L clocks the pre-write flip-flop which triggers the
write monostable multivibrator. The write multivibrator enables one of four write pulses to the RAM
memories, either WR GRAPH 0 L, WR GRAPH 1 L, WR HORIZ H, or WRITE DATA L. This
latter pulse is the write input signal to the vertical line and graph marker static RAMs. If either of the
graph write pulses or WRITE DATA L is present, the initial X-address counter is incremented to be
ready for the next data point.
When the pre-write flip-flop is cleared, GRAPHICS FLAG L is sent to the VT100 terminal controller,
confirming that the data has been stored in the static RAMs, and they are ready to receive a new data
word.
6.4.1.9 Loading Graph Memories - Characters Band J produce signals LD GRAPH 0 Land LD
GRAPH 1 L from the decoder. When enabled by the write multivibrator, these signals become WR
GRAPH 0 L and W~ GRAPH 1 L that determine into which memory to write the data.
The second and third characters of a load graph instruction form an 8-bit Y-value. This data passes
through a two-line to one-line multiplexer becoming RAM DATA 0-7 (1) H, (Figure 6-28). The data is
then written into memory in the address, XADD 0-7, from the X-address counter.
Each RAM memory has 25610 X 4 bits of space available. Four of these are connected together to
provide 512 X 8 bits of memory for each graph. The signal X ADDR 8 L enables the upper memory
addresses, location 256 to 511.
The M7071 uses a down-counter for a Y-address monitor. For the rectangular field, the down counter is
initially loaded with address 23010 each time the scan reaches the bottom of the screen. The scan then
starts at the top-left corner of the screen. After each row of X-address information is processed, the Yaddress monitor is counted down one increment. (For the square field, the down counter is initially
loaded with address 24010.)
In order to read memory, only the X-address must be available. The position of the graph data is (;ompared with the current Y-address of the scan. If they are equal and the graph is enabled, the position on
the screen is intensified.

6-34

) )

))

)
REGISTER 1

DB 3 H
DB 2 H

CUR GRAPH 0 (1) H
CUR GRAPH 1 (1) H

FIRST
DATA
CHARACTER

DB 1 H
DB 0 H

VERTICAL LINE (1) H
HORIZ LINE (0) H

ClK
lD CONTROL l

MA·4723

Figure 6-25

)
/

VT105 (1) H
(SQUARE)

DB 0 H

)

DB 5 H
DB 6 L
LD MODE

D·TYPE
FLIP·
FLOP

TO PHASE LOCK LOOP
VT105 (1) L
(RECTANGULAR)

LD ENABLE 1 L
CONTROL
ENA H

INIT L

TEST (1) H
DUAL
MONO·
STABLE
MULTI·
VIBRATOR

DB 1 H

)

NOTE:
DB 2"'" DB 4 NOT USED
MA·4724

Figure 6-26

)
6-35

2ND DATA
CHARACTER

1ST DATA
CHARACTER
LD XL

DB 3H
t
DBO H

B DATA 4

t
B DATA 0

INITIAL
X
ADDR

ADD 08
t
ADD 00

INCREMENT
COUNTER

LD
FROM
DECODER LD VERT L
LD CUR 1 L
LD CUR 0

LOAD
DATA"""--

FINAL
X
ADDR
SYNC
COUNTER

X ADDR 8 L
XADD7
t
X ADD 0,
TO
RAM
MEMORIES

CUP
COUNT TIME H
COUNT UP

LDXDATAH
ODD EVEN (1) H
X STOP (1) L

WR
HORIZ H
LD HORIZ L
WR
DATA L
LD X
DATA

MV
12 liS

PREWRITE (1) H

LD X DATA L

PREWRITE
FLIPFLOP
GRAPHICS FLAG L

WRITE DATA L

TO
VT100

DIS CLR + INIT L
MA·4725

Figure 6-27

Loading Address Data

6-36

))

•

WR GRAPH 0 L

1ST DATA
CHARACTER
B DATA 0-4 H
2 NO DATA
CHARACTER DB 0-2 H

RAM DATA 0-7

•

0-255

•

: 256-511
I

'0 DATA 0-7

GRAPH 0
MEMORY
I

2-L1NE
TO
1.L1NE
MUX

X AjD B L
WR GRAPH 1 L

0-255

I 256-511

I
I

FROM
X-ADDR
COUNTER

X ADD 0-7

Y1 DATA

GRAPH 1
MEMORY
I
I

0-7

X AJD 8 L

GRAPH 0

AO<BO

YADDR
COMPAR..... B ATOR

AO= BO

~ A

)

AO>B 0
TO OUTPUT
CIRCUIT
HISTOGRAM 0 (1) HAND
_---.....
BASELINE
A1 < B1 GENERATOR
....t -... A GRAPH 1

)
INITIAL
INPUT
23010
OR
24010*

~B
Y
ADDR
MONITOR
DOWN
COUNTER
LD

CON

VERT BLANK H ,
HORIZ + VERT L

YADDR
COMPARATOR
~

YO (1) H - Y7 (1) H

A1 = B1

A1 > B

HrSTOGRAM 1 (1) H
TO BASELINE
COMPARATORS

·SQUARE FORMAT

MA-4733

Figure 6-28

Graph Memory and V-Address Monitor

6-37

6.4.1.10 Generating Baselines (Shade Lines) (Figure 6-28) - Data bit 0 in the second data character
following a load register 0 creates a signal called BASELINE CTRL (I) H. If high, this signal loads a
baseline for Graph I; if low, it loads a baseline for Graph O. (See Paragraph 6.4.1.7 for loading register

'. ))
.

0.)

The position of the baseline is determined by two data characters following the character @ (NOP). As
the scan proceeds from top to bottom, its V-address is compared with the position of each baseline.
When they are equal, the baseline is intensified on the screen.
6.4.1.11 Enabling a Histogram (Shading a Graph) - This section explains how to enable a histogram,
with or without a baseline.
Without a Baseline (Shade Line) Enabled"- Enabling a histogram intensifies points below a graph to the
bottom of the graph drawing field Data bits 3 or 4 in register 0 enable HISTOGRAM 0 (1) Hand
HISTOGRAM 1 (1) H, respectively. These signals, shown in Figure 6-28, enable all points below the
graph determined by the V-address comparator.
With a Baseline (Shade Line) Enabled - When its baseline is enabled, a graph is shaded between the
graph line and its baseline. To do this, two sets of comparators are used. One comparator, shown in
Figure 6-28, monitors the position of the graph with respect to the current scan. The other(Figure 6-29)
compares the position of the baseline with that of the current scan. Together, they determine the points
to be intensified. The baseline can be moved up or down, and the intensified area changes accordingly.
Bits 3 and 4 in register 0 enable Histogram 0 and Histogram 1, respectively. These bits should not be
enabled if the baseline is enabled for that particular graph. Shading the histogram to line 0 eliminates
the visual effect of shading only those areas between the graph and the movable baseline.
6.4.1.12 Loading Vertical Lines - Vertical lines are loaded with two data characters following the letter L. Bit 4 of the second data character [B DATA 9 (1) H] with LD VERT H from the decoder provide the data input (DB) to the vertical line and graph marker memory. The other bits of the two data
characters transmit the X-address in memory where the vertical line is to be stored. (See Figure 6-30.)
If BDATA 9 (1) H is high, the vertical line is loaded; if low, the line is erased at that address.
Memory is continuously read by the X-address counter. Data in the current address is monitored by a
D-type latch and is enabled only if VERTICAL LINE (1) H is present in register 1. This causes VERT
LINE L to be sent to the output circuit. Every other point at a particular X-address is intensified to
create a vertical line. Up to 512 vertical lines can be enabled on the graph field.
As the vertical line is read from the static RAM, its position is fed back (item C in Figure 6-30) and
rewritten into the RAM so as not to lose the data.
When the strip chart feature is enabled, vertical lines follow the graph address in a wraparound fashion;
that is, vertical lines move from right to left with the strip chart graph. When the current address exceeds address 51110, the vertical line data is taken again from the beginning, address 0, I, 2 ... , etc.

6-38

.."...../

I'---./

""---.--!

'- .... -~

,~.'

DB 2

t
DB 0

2ND DATA
CHARACTER'

1ST DATA
CHARACTE

B DATA 4
,
B DATA 0

BASELINE
1
STORAGE
REGISTER

BASE 17(IIH
,
BASE 10 (1) H

Al < Bl
NOPH
BASELINE CTRL (11 H
LD XDATA H

SCAN<BASELINE
A<B

BASE·
LINE 1
BASELINE 1
A=B

IIA
Y

ADDRESS
COMPAR- SCAN> BASELINE
ATOR
A>B
AI> Bl GRAPH 1 > V·SCAN
FROM Y-ADDRESS
MONITOR
DOWNCOUNTER

YO (11 H - Y7 (1) H

AO < BO GRAPH 0 < V·SCAN

9'
w

'~C~N <. BASELINE

BASELINE

CIRCUIT

o
L----_I~I A

V
ADDRESS
COMPAR·

,AT~IR

DB 2
2ND DATA
CHARACTER

1ST DATA
CHARACTER

'SCAN> BASEUNE

A:> B

GRAPH 0 > Y·SCAN ,

DB 0

BASE 07 (1) H
BASELINE

B DATA 4
I
BDATAO

BASELINE 0
A=B

AO> BO

BASE 00 (1) H

STORAGE
REGISTER

NOP L
BASELINE CTRL (1)
LD XDATA L
MA·,c'J6

Figure 6-29

Baseline (Shade Line) Registers and Comparators

(FROM WRITE MV)
WRITE DATA ~

LO VERT H
BDATA9111H

~)
""\J

C
LD VERT L

VERT DATA H

DI2

H-J1

LD CUR 1 L

VERTICAL
LINE
CS
AND
GRAPH MARKER
I-MEMORY
011

XADD8

FROM
X-ADDR
COUNTER
LD CUR 1 H

DI3

X ADD 0-7

CUR 1 DATA H

LD CUR 0 H

))

A
LD CUR 0 L

CUR 0 DATA H

~
'J

(FROM REGISTER 1)
VERTICAL LINE (1) H
YO 1 H
HORIZ (1) H TO
"-OUTPUT _

HORIZ LINE L
FROM HORIZ
LINE STATIC
RAM

QUAD
D-TYPE
LATCH

VERT LINE (1) H

' - VERT LINE L

C
CUR 1 (1) H
CURO(1)H

TO
OUTPUT
CIRCUIT

B
A

CUR 1 ENA H ___
CUR GRAPH 1 (1) H
TO Y-ADD R
COMPARATOR

FROM
REGISTER 1

CUROENAH

CUR GRAPH 0 (1) H
MA-4718

Figure 6-30

Vertical Line and Graph Marker Memory

6-40

6.4.1.13 Adding Graph Marker - The graph marker in the VTI05 is a short vertical line that straddles
the graph. For each graph marker, 16 additional dots on the screen are enhanced above and/or below
the graph point. The number of dots above or below the graph point is hardware-dependent according to
the current graph position. If the graph is at the top of the screen, the 16 dots are all below that of the
graph point; if at the bottom, they are all above the graph point.

(

Loading a graph marker is accomplished by two data characters following the letters C or K. These
letters are decoded as LD CUR 0 Hand LD CUR 1 H, shown in Figure 6-30. When bit 4 of the second
data character [B DATA 9 (l) H] is a one and WRITE DATA L is present, a graph marker is loaded in
the address formed by the two data characters. If bit 4 is a zero, the graph marker is erased.
When the graph marker is enabled and read from memory, an equal condition is forced from the lower
four bits of the V-address comparator (Figure 6-31). The upper 4-bit comparator holds this equal condition until one of its bits changes. This does not happen until the V-address has decremented 16 lines.
The graph data point may be on anyone of these 16 lines causing the graph marker to vary in number
of dots above and below the graph point;
As the X-address is incremented, the output of the vertical line and graph marker memory is monitored
by a D-type latch. As with the vertical line data, the graph marker data is also fed back into memory
(items A and B in Figure 6-30) and wraps around when in strip chart mode.

)

6.4.1.14 Generating Horizontal Lines - A horizontal line is loaded with the character D. The two data
characters following the D represent an 8-bit V-position of a horizontal line. Data bit 4 in the second
data character enables the signal DIS CLR + INIT H that allows the A inputs to the two-line to oneline multiplexer (Figure 6-32) to be gated out. The data, or address of the line, is then presented to the
256 X I static RAM memory. Data bit 4 [B DATA 9 (l) H] is also the data to b!! written into memory.
If it is a one, a horizontal line is stored; if zero, a line is erased at that address.
A down-counter is used for tracking the V-address when reading data from memory. To read from
memory, only the V-address is needed. A complement of the data in memory is the output. From register I, HORIZ LINE (1) L enables all points at that Y-address creating a horizontal line. The signal 320 ns
allows only every other dot to be displayed to decrease its intensity.

)

6.4.1.15 Generating Strip Charts - Strip charts are enabled by the second data character following a
load enable register 0 instruction, as shown in Figure 6-33. Data bit 3 (DB 3 H) of this register enables
the single strip chart feature; either Graph 0 or Graph 1 can be incremented from right to left across
the screen.
Data bit 4 (DB 4 H) of this register enables the dual strip chart feature; both Graph 0 and Graph I data
can be incremented from right to left across the screen. This must be done in the following sequence.
Data Entry
Enable Graph 0
Enter Graph 0 data
Enable Graph 1
Enter Graph I data

Character(s) Transmitted

B
Two data characters
J
Two data characters

)

6-41

GRAPHO(1) H

CUR 0 ENA H - 1

A=B
TO OUTPUT
CIRCUIT

)

A<B I--

A
A= B

VERTICAL YO-Y3 (1) H
SCAN
POSITION

A>B

))

CUR 0 ENA H
MA-4726

Figure 6-31

Graph Marker V-Address Comparator

))
6-42

,',

2ND DATA
CHARACTER
1ST DATA
CHARACTER

DB 0-2 H
B DATA 0-4 (1) H

Y
ADDRESS
LOAD TO 23010
DOWN
(OR 24010*)
COUNTER

A 2 LINE
TO
1 LINE
MUX

YO (1) H ~ Y7 (1) H

B
SELECT'"

LDCDN
VERT BLANK H
HORIZ + VERT L

DIS CLR + INT L
X STOP (1) L

)
X STOP (1) H
DIS CLR + INT H

160NS

)

~>-

WR HORIZ H)

)

HORIZ LINE (1) L
(FROM REGISTER 1)

()

B DATA 9 (1) H
RAM DATA
0-7 (1) H
(Y-ADDRESS)

WE
DIN
256 X 1
STATIC
RAM

,...

HORIZ
LINE
RAM
MEMORY
RAM DATA
0-7 (1) H

TO
GRAPH
DATA
INPUTS

~HORIZ
LINE L
TO
OUTPU T
CIRCUI T

320 NS

* SQUARE FORMAT
**LOW: OUTPUT = A; HIGH: OUTPUT = B
MA-4727

Figure 6-32

Horizontal Line Memory

6-43

DB 3 H
FROM
DATA
REGISTER

t
INITIAL
OVERFLOW (1) H
2ND DATA
DB 0 H
X ADD
CHARACTER
INCREMENT
ADDR 00-08 (1) H
B DATA 4 (1) H COUNTER
1ST DATA
t
CHARACTER B DATA 0 (1) H

LD CUP CON
(,)
LOX H

FINAL
XADDR
COUNTER

TO GRAPH
MEMORIES
CLR CLKLD

COUNT TIME H )

""'"

FRO::.:M~D~E~C~O~D~E~R1-_~~~----~~
LD X DATA H
./

X ADD

0-8 H

~--...,

HORIZ + VERT......
H _ -.....
-)r
+3VH

WR GRAPH 1 L-d

....

WR GRAPH 0 L-o)

~:::~;r::..:..:I:-':~~-~~~:~'----}r------.
REGISTER

'- 0

STRIP
ENA
F/F

LD X DATA H

I-

STRIP

~-y"';"~ ENA (1) L

BIT 4
STRIP H
BIT 3

)
r--

2ND
DATA
CHARACTER

f-f-f-f-f-f--

I--

X STOP (1) H
512
BIT
COUNTER

f--

I--

TlME ___ C

F/F

(1) H

f--

DUAL STRIP H ~~

X STOP

CLK

HORIZ + VERT L '
COUNT TIME H
MA-4721

Figure 6-33

Generating Strip Charts

6-44

Single Strip Chart Operation - Two X-address counters are used to create the strip chart motion. Without the strip chart feature enabled, the final X-address counter is cleared to zero at horizontal blank
time (by HORIZ + VERT H). With a strip chart enabled, this clear signal is disabled allowing the
final X-address counter to follow the initial X-address counter counting in a wraparound manner.
Data is loaded into the graph as usual for addresses up to 511. At address 512, an overflow occurs in the
initial X-address counter. OVERFLOW (1) H clocks the strip enable flip-flop disabling the clear signal
to the final X-address counter.
A 512-bit counter monitors the horizontal scan position and enables the load signal for the final Xaddress counter only after completing the current line. New data increments the X-address and is
added to the end of the graph. This new data is then the last data to be read from memory. Previous
data is read one position earlier causing a shift in the video graph to the left.
When the strip chart is disabled, the final X-address counter is cleared at horizontal (or vertical) blank
time regardless of its current count. This causes the displayed video graph to reposition itself as the
counter is reset to zero.

Dual Strip Chart Operation - For dual strip chart operation, data bit 4 in register 0 must be set, creating
the signal DUAL STRIP H. Data for both Graph 0 and Graph 1 must be placed at the same X-address
and shifted simultaneously. Refer to the timing diagram (Figure 6-34) and the following sequence of
events.

)

An instruction to load Graph 0 is received. WR GRAPH 0 L is placed on the count-up input
to the initial X-address counter. The data is written into Graph o. After 50 ns, WR GRAPH
o times out, and the counter increments one address.

When the command to load Graph 1 is received, the Initial X-address counter counts down
one address.

WR GRAPH 1 L is placed on.the count-up input to the initial X-address counter. Data is
written into Graph 1. After 50 ns, the initial X-address counter increments one address.

When the initial X-address counter is incremented, this new address is loaded into the final X-address
counter at the end of the current graph line [as X STOP (1) H goes loW]. The last data word entered in
each graph is the last X-address read.
)

Incrementing the initial X-address counter, by adding a new data word to each graph, causes the final
X-address counter to access that data last in both memories. This causes both graphs on the screen to
shift simultaneously one unit to the left for each new set of data words.
The 512-bit counter that monitors the graph drawing field is initially loaded to a 1 in dual strip chart
mode. A count of 512 is reached one count time earlier, eliminating the last bit position on the graph.
This removes the switching motion that would be visible on the screen while alternately plotting data on
Graph 0 and Graph 1.

6-45

DUAL STRIP H

LD GRAPH 0 H
WR (iRAPH 0 L ~

LD GRAPH 1 H
WR GRAPH 1 L _ _ _ _ _......,

COUNTUP-~

COUNT DOWN - - - - - -...

STRIP ENA (1) L

-INITIAL X-ADDR
COUNTER
MA·4729

Figure 6-34

Dual Strip Chart Timing Diagram

6.4.1.16 Combining Video Out and Timing - All data signals generated within the M7071 are combined to form one video output to the VTl 00 terminal controller (Figure 6-35)
The timing signals are developed by the phase lock loop using HORIZ BLK L from theVTIOO terminal controller: (See Paragraph 6.4.1.6.)
Graph 0 and Graph I data, horizontal lines, vertical lines, and Graph 0 and Graph 1 markers are pre-.
sen ted to the video output every TIME (1) H. At COUNT TIME H the 5J2-bit horizontal counter
clocks the window flip-flop to gate the data out.
Histogram 0 or Histogram 1 data is alternately enabled to the video output with TIME (0) Hand
TIME (1) H. This allows both shaded graphs to be discernible in areas on the screen where they overlap.

)1
6-46

TIME
(1 H

BASE LINE ONE H
WITH SHADED GRAPH
HISTOGRAM 1
(IF NO BASELINES)

)

D2
QUAD
D·TYPE

BASE LINE ZERO H
WITH SHADED GRAPH
HISTOGRAM 0
(IF NO BASELINES)
GRAPH 0
OR GRAPH 0 MARKER

Dl
FLIP-FLOP

TIME (0) H

HORIZ (1) H
DO

GRAPH 1 QR GRAPH 1 MARKER

CLK

VERT LINE L

TIME (1) H

--)

VIDEO

H
109

___ J
WINDOW

512
BIT
HORIZ
DOT
COUNTER
DUAL
STRIP H

TO
VT100

CLK
VIDEO
FLlP·F LOP

L::S:...:.T.:...A:..:..R:...:.T...),(~l):...:H~...... K
LD CL.
COUNT XOO (1) H

HORIZ + VERT L
COUNT TIME H

FROM
VT100

DELAY
COUNTER

HORIZ
BLK L
PHASE
LOCK
LOOP

MA·4719

Figure 6-35

Combining Video Out

6-47

6.4.2 VTI05 Graphic Test Procedure
The interactive test feature of the VTI05 is a series of displayable test patterns. These patterns verify
that all waveform generator features are operating correctly. Perform these data tests in the order indicated.
NOTE
The tests described in this procedure are run in the
rectangular format. The tests may also be run in the
square format, but the test patterns are slightly different.
Do not use the SPACE BAR unless the word
<SPACE> is spelled out.

Remember to use the SHIFT key for uppercase symbols; the CAPS LOCK key is only used for uppercase letters.
If at any time· the wrong character is entered, initialize the registers and memories by typing the following sequence.

A <SPACE> <SPACE> I 0 I <SPACE> "
Then reenter the test data.
6.4.2.1

Test Setup

Place the terminal LOCAL.

Turn on the auto repeat feature (in SET-UP B group I, switch 2 = I).

Type the following sequence.
<ESC>II<SPACE><SPACE> I<SPACE> "

6.4.2.2 Test Graph 0, Histogram 0, and Graph 0 Markers
1.

Type the following sequence.
A#

The graph test pattern in Figure 6-36 is now on the screen.
2.

Type the following sequence.
A)

The histogram test pattern in Figure 6-37 is now on the screen.

6-48

))

Type the following sequence.

IS
The graph marker test pattern iI1 Figure 6-38 is now on the screen.
4.

Type the following sequence.

I <SPACE> A<SPACE>
The graph 0, histogram 0, and graph

°markers are now disabled.

. . --1
J

)

MA·4741

)

Figure 6-36

Graph Test Pattern

6-49

MA-4742

Figure 6-37

Histogram Test Pattern

MA-4743

Figure 6-38

Graph Marker Test Pattern

))
6-50

6.4.2.3
1.

Test Graph 1, Histogram 1, and Graph I Markers
Type the following sequence.

A%
The graph test pattern in Figure 6-36 is now on the screen.
2.

Type the following sequence.

Al
The histogram test pattern in Figure 6-37 is now on the screen.
3.

Type the following sequence.
I(

The graph marker test pattern in Figure 6-38 is now on the screen.
4.

Type the following sequence.

I<SPACE> A<SPACE>
The graph 1, histogram 1, and graph 1 markers are now disabled.

)

6.4.2.4
1.

Test the Horizontal Lines
Type the following sequence.

A!I!
The horizontal line test pattern in Figure 6-39 is now on the screen. Note that the test pattern
appears to be a series of vertical lines. This is the correct test pattern.
2.

Type the following sequence.

I <SPACE>
The horizontal line test pattern is now disabled.

6.4.2.5
1.

Test the Vertical Lines
Type the following sequence.

A!I"
The vertical line test pattern in Figure 6-40 is now on the screen. Note that the test pattern
appears to be a series of horizontal lines. This is the correct test pattern.
2.

Type the following sequence.

I <SPACE>
The vertical line test pattern is now disabled.
6-51

))
MA-4744

Figure 6-39

Horizontal Lines Test .Pattern

MA-4745

Figure 6-40

Vertical Lines Test Pattern

))
6-52

6.4.2.6 Test Shade Line 0 (Baseline 0)
1.

Type the following sequence.

A#"
The shade line test pattern in Figure 6-41 is now on the screen.
2.

Type the following sequence.

@1l 2233445566 <SPACE> <SPACE>
The shade line shifts upward with each pair of numbers typed.
3.

Type the following sequence.

A<SPACE> <SPACE>
The shade line 0 test pattern is now disabled.

)

MA·4746

Figure 6-41

Shade Line (Baseline) Test Pattern

6-53

6.4.2.7 Test Shade Line 1 (Baseline 1)
1.

))

Type the following sequence.
A%%

The shade line test pattern in Figure 6-41 is now on the screen.
2.

Type the following sequence.

@11 223344 SS 66 <SPACE> <SPACE>
The shade line shifts upward with each pair of numbers typed.
3.

Type the following sequence.

A<SPACE> <SPACE>
....

The shade line 1 test pattern is now disabled.

-))

6.4.2.8 Test Str:ip Chart 0
1.

Type the following sequence.

I<SPACE>"A+(
The histogram test pattern shown in Figure 6-37 is nQ~ on the screen.
2.

Type the following sequence.

"??B
The strip chart test pattern shown in Figure 6-42 is now on the screen.
3.

Type any sequence of two numbers. This moves the graph to the left. The SPACE BAR enters a 0 as in Figure 6-42, 11 enters data at line 49, 22 enters data at line 82, etc. Hold down
the number key if in auto-repeat mode.
Type the following sequence.

A<SPACE> <SPACE>
The strip chart test pattern is now disabled.

6-54

»)

.. -1
/

MA-4747

Figure 6-42

Strip Chart Test Pattern

6.4.2.9 Test Strip Chart 1

)

Type the following sequence.

1< SPACE>"A5(
The histogram test pattern in Figure 6-37 is now on the screen.
2.

Type the following sequence.

H??J
')

The strip chart test pattern in Figure 6-42 is now on the screen.
3.

Type any sequence of two numbers. This moves the graph. The SPACE BAR enters a 0 as in
Figure 6-42, 11 enters data at line 49, 22 enters data at line 82, etc. Hold down the number
key if in auto-repeat mode.

Type the following sequence.

A<SPACE> <SPACE>
The strip chart test pattern is now disabled.

6.4.2.10

Exit the Graphic Test Mode

Type the following sequence.

IO<SPACE> A<SPACE> <SPACE> I <SPACE> <SPACE> <ESC>2
The terminal is now in interactive mode.

6-55

6.5 VTIXX-AC PRINTER OPTION
The VTIXX-AC option provides an interface between a VT100jVT132 terminal and a serial printer.
6.5.1 Option Installation
Use the following procedure to install the VTIXX-AC option in a VT100jVT132 terminal.
I.

Turn terminal power off and remove the terminal access cover (Paragraph 5.4.1).

Remove the terminal controller board (Paragraph 5.4.2).

If installed, remove the advanced video board (AVO) from the terminal controller board
(Paragraph 5.4.3).

Reconfigure the VT100 terminal controller board. This consists of removing the ROMs 0, 1,
2, and 3 from the controller board, and replacing them with the ROMs shipped with the
printer option. Figure 5-32 shows the ROM locations on the terminal controller board. The
VT132 does not use the ROMS sent with the printer option kit. Table 5-7 lists the ROMs
used in each product.
......

))

NOTE
If the terminal controller board ever fails, remove
the printer port ROMs. Then, move the ROMs that
come with the replacement terminal controller board
to the defective board before returning it for repair.
5.

Reconfigure the advanced video board (AVO). This consists of removing any ROMs
mounted on the board and replacing them with the ROMs shipped with the printer option.
Also check, and change if necessary, the jumpers or switches on the AVO board. Figures 533 and 5-34 show the new ROMs installed on the AVO board. Tables 5-8 and 5-9 show the
correct switch positions and jumper locations for the AVO board. Table 5-7 lists the ROMs
used in each product.
NOTE
Do not discard the ROMs removed in step 6 until after the terminal and option have been successfully
tested. If the AVO board ever fails, remove all
ROMs from the board before the board is returned
for repair.

Install the AVO board on the terminal controller board (Paragraph 5.4.3).

Install the printer interface option board in the STP connector, 13, of the terminal controller
board (Paragraph 5.4.5).

Reinstall the terminal controller board in the terminal.

If installed, remove the 20 rnA current loop option board and connector from the access cover. Reinstall the option board and connector in the new access cover shipped with the printer
option (Paragraph 5.4.8).
NOTE
Discard the old access cover after the terminal and
printer option have been successfully tested.

6-56

) )

10.

Attach the option designation label to the rear of the terminal. Attach the new SET-UP label
to the bottom of the keyboard. If the terminal is a VT132, transfer the SET-UP settings to
the existing label.

11.

Reconnect all cables you removed from the rear of the terminal in step 1.

12.

Perform the printer option checkout procedure.

6.S.2 Printer Port Option Checkout Procedure
Two different test programs check out the printer interface; the power-up and the printer interface data
loop back test.

NOTE
The terminal will fail the NVR section of the powerup test (error code 2) when first powered up after
the printer option is installed. Correct the condition
by performing a save operation on the current contents of the NVR.

--)
6.S.2.1

•
•
•

Power-Up Test - You can start the power-up test in any of the following ways.
Turn the terminal power switch on
Reset the terminal in SET-UP mode
Type the following sequence in LOCAL.
<ESC>[2;ly

)

The terminal displays Wait while executing the test. When successfully completed, the screen clears.
NOTE
The continuously running test ends only if it finds an
error, or you turn off power. The VTIXX-AC is not
tested during a continuous test.
The error indications for this test are as follows.
•
•
•

Lighted keyboard indicator (other than ON LINE or LOCAL)
Character displayed on screen under the blinking cursor
Unlighted VTIXX-AC indicator (located on the printer interface board)

Table 6-2 shows the keyboard indicator error codes and their meanings.
Table 6-3 shows the error character displayed on the screen under the blinking cursor. Any errors detected by the test appear on the terminal screen as a character in the upper-left cotner.
NOTE
If character 4 appears, check that the keyboard is

plugged in.

6-57

Keyboard Indicator Error Codes

Table 6-2
Error

ROM 0 checksum
ROM 1 checksum
ROM 2 checksum
ROM 3 checksum
Main data RAM

Off
Off
Off
Off
Off

Off
Off
Off
On
On

Off
On
On
Off
Off

On
Off
On
Off
On

Table 6-3
Character

Fault Detected
AVO
NVR

Displayed Error Codes

KBD

DATA

1
2
3

4
5
6

.....

X
X

7
8

X
X

))

X
X

)

X
X
X
X

X
X
X
X·

X
X
X
X

X
X
X
X
X
X
X
X

6.5.2.2 Printer Interface Data Loopback Test - Use the following procedure to perform the printer
interface data loopback test.

Turn the power switch off. Install the data loop back connector (PN 12-15336) onto the printer interface connector (the upper EIA connector). The data loopback connector is also used
to perform the EIA communication interface data loop back test.

Turn the power switch on. The terminal performs the power-up test.

Enter SET-UP B and place the terminal in the ANSI-compatible mode (group 2 switch 3 =
1). Perform a save operation.

Place the terminal in printer port test mode (SET-UP B group 5 switch 4 = 1).

Enter SET-UP C. Select the proper printer transmit/receive speed. Use either the 70r 8
key on the main keyboard to select the speed.

6-58

))

Place the terminal ON-LINE and exit SET-UP mode. The screen now displays: PRINTER
PORT TEST MODE

Type one of the following sequences.
(single pass test)

<ESC>[2;2y
or
<ESC>[2;lOy

(continuously running test)

NOTE
The continuously running test ends only if it finds an
error, or you turn off power.

---)

Any error detected by the test appears on the terminal screen as a character in the upper-left corner.
Table 6-3 shows the possible error codes. A printer interface data loopback test error is shown by an X
in the Data column.
6.6 VTIXX-CB, -CL KIT
TheVTl XX-CB, -CL kit includes all the parts needed to install a VTl25 Graphics Processor in a
VTl 00 or VT105. This section explains the procedure.

)

NOTE
Before installing the graphics processor, check the
ROMs and the etch revision level on the terminal
controller board (step 3 of this procedure). Compare
the numbers of the ROMs on the board with· the
numbers in Figure 6-43. They should be equal or
larger.

Check the etch number on the solder side of the
board. Look for the number SO 13008 followed by a
letter. If the letter is D, E, etc., you can use the
board. If the letter is A, B, or C, you cannot use the
board. Use the EIA line filter connector (included
in kit) with a Revision F board.
If the board does not meet either of these prerequisites, consult your Field Service branch office.

)
6-59

,J)

J2
GRAPHICD
OPTION
CONN.

ADVANCED
VIDEO
OPTION
CONN.

20 Ma
J5
OPTION
CONN. -

1== I

8b~NECTOR J3

EIA
I
CONN. J41
I

J8
J6

CHARACTER
GENERATOR

D
23·061 E2

J9
KEYBOARD
CONN.

NUMBER SHOULD BE 061 OR LARGER

NOTE: ROMS MAY BE
INSTALLED IN ANY
ORDER ON TERMINAL
CONTROLLER BOARD.
MA·4582A

Figure 6-43

Check ROM Numbers

6.6.1 Installation
, You need the following tools to install the VT125 graphics processor.
•
•
•
•

1/ 4-inch nutdriver
2 Phillips screwdrivers
Needlenose pliers
3/ 16-inch blade screwdriver

Use the following procedure to install the VT125 graphics processor in a VTl 00 or VTI05.
I.

Turn off power and remove the access cover (Paragraph 5.4.1).

Remove the top and bottom covers (Paragraphs 5.4.14 and 5.4.15).

Remove the terminal controller board (Paragraph 5.4.2).

6-60

))

For a VT100: Remove the terminal controller edge connector by performing one of the following two steps.
a.

Release the two retaining rings from the edge connector with needlenose pliers.

Lift the clips at the top and bottom of the edge connector and discard.

For a VT105 or VT125: Disconnect the power cable from the VTI05/VTI25 expansion
backplane, and remove the backplane (Paragraph 5.4.23).
5.

The new expansion backplane has two capacitors on it, and its terminal controller edge
connector has an alignment key in it. Remove this key with needlenose pliers unless installing
the option in a terminal with a Revision F or higher terminal controller board. Install the new
expansion backplane with four 12.7 mm(l/2 in) X 4-40 screws andkepnuts andfour6.4 mm
( 1/4 in) spacers. Install the screws at the connector positions marked J 1 and J 5 on the board
(Figure 6-44). The dc power cable must be flat against the chassis or the backplane will not fit.

---J

)

EXPANSION BACKPLANE

SPACER

TERMINAL
CONTROLLER
EDGE
CONNECTOR

DC POWER CABLE
MA·45BOA

Figure 6-44

Installing Expansion Backplane

6-61

Bend the dc power cable around so the opening faces the CRT. Use needlenose pliers to
remove the alignment key (if installed) from the connector opening in the dc power cable.
Install the connector onto the board edge connector, passing the cable around the wire frame
(if present).

Install card guides into the top and bottom of the card cage at the VTl25 connector position.

At the fourth hole from the left on top of the chassis (over the BNC bracket), press the
ground clip with a wire onto the chassis (Figure 6-45).

)
)

NOTE: '
INSTALL CLIP BEFORE
INSTALLING SHIELD.
MA-9460A

Figure 6-45

Installing Ground Clip

6-62

--) )

Install the FCC RF shield (Paragraph 5.4.21).

10.

Put the chassis into the bottom and top covers (Paragraphs 5.4.14 and 5.4.19).

11.

Slide the terminal controller board partially into the chassis.

12.

Install the VT100 end of the 16-pin flat cable connector to the lower 16 pins of the terminal
controller board's graphics connector (marked J2 on the terminal controller board), with the
cable entering from the right (Figure 6-46).

D
D
D

o
MA-9343

Figure 6-46

16-Pin Flat Cable on Terminal Controller

6-63

13.

Install the STP paddle board in the STP connector. Lift the 16-pin cable slightly if needed t o , ) )
clear the spacer when inserting the STP board. Attach the STP board to the terminal controller board with the supplied screw and washer (Figure 6-47).

14.

Find the end of the 24-pin cable that has pin 1 on the cable side. Fold the cable under itself to
the right approximately 30 mm (1 in) from the connector. Install that end into the socket on
the STP board with the cable down (Figure 6-48).

STANDOFF

NYLON WASHER

4-40 X" SCREW

//~~~
~

STP CONNECTOR

MA·9347

Figure 6-47

Installing STP Board

OJ )
/

6-64

1" .

----)

)

MA-I:M2

)
Figure 6-48

24-Pin Flat Cable on STP Board

)
6-65

15.

Install the VT125 board into the chassis so that it sticks out approximately 5 cm (2 in) more'
than the terminal controller board.

16.

Arrange the 16-pin graphics connector with the cable entering from above and install it into
the connector at the top edge of the VT125 board (Figure 6-49).

_))

---} I

MA·9344

Figure 6-49

Graphic Cable Connections

6-66

)

17.

Arrange the 24-pin connector with the cable entering from the right and install it into the 24pin socket at the right edge of the VTl25 board (Figure 6-49).

18.

Check the switches on the STP board and set switches 1, 3, and 4 off, with all the other
switches on (Figure 6-50).

19.

Connect the ground wire from the top of the chassis to the quick-connect terminal on the
BNC connector bracket (Figure 6-51).

20.

Slide both boards into the chassis together, taking care not to stretch the cables. Seat both
boards in their sockets one at a time.

21.

If the 20 rnA adapter is installed on the VT100, remove the adapter board from the VT100
access cover and install it into the new access cover. (Refer to Paragraph 6.3.1.)

22.

Install the 20 rnA connector cable (if present) to the red connector on the terminal controller
board.

23.

Adjust the cables in the access opening and install the access cover. Tighten the four screws
carefully.

24.

Ifusing the EIA interface on a Revision F etch board, make sure to install the EIA line filter
connector (Figure 6-51 A).

25.

Attach the new SET- UP label to the bottom of the keyboard.

6.6.2 Graphics Option Checkout Procedure
Perform the test procedures described in Paragraph 5.2.8. If you have display problems at power-up,
recheck the flat cables for correct positions. Refer to Figures 6-45, 6-47, and 6-48.
NOTE
The terminal will fail its power-up test with a 2 indication the first time you turn it on after installing
the option. Check the SET-UP features and save
them by typing SHIFf /S in SET-UP.

,
1

6-67

J
)

)

))

MA-7256B

Figure 6-50

STP Board Switch Types

6-68

))

\.
GROUND WIRE
'"CLIP
IUlI~--- QUICK·

CONNECT
TERMINAL

t.r\.ro---- BNC BRACKET
ON GRAPHICS
PROCESSOR
BOARD

)
l

Figure 6-51

Connecting BNC Bracket Ground Wire

)

EIA
LINE FILTER
CONNECTOR

TIGHTEN

EIA
CABLE
CONNECTOR

Figure 6-51 A

EIA Line Filter Connector

6-69

6.7 VT125 GRAPHICS PROCESSOR TECHNICAL DESCRIPTION
The VT125 is a graphics display terminal with medium resolution graphics for business and scientific
applications. The VT125 combines bit map architecture with the alphanumeric capability of the VT100
video terminal. The VT125 is a smart terminal that directly executes DIGITAL's general-purpose
graphics descriptor, ReGIS (Remote Graphics Instruction Set). ReGIS lets you create and store
pictures as ASCII text and efficiently send those pictures to remote displays. The VTl25 also has an
auxiliary data port for making hard copies of the display with the DECwriter IV Graphics Printer. For
users who have VT105 programs, the VT125 includes a VTI05 emulator.

Il)

The VT125 is a VT100 video text terminal and a graphics processor in one package. The graphics
processor processes graphics commands from a computer to generate an image in its own memory. Then
it sends a video representation of that image to the VT100 text terminal's internal monitor screen and to
an optional external color monitor. It can also send a bit map representation of the image to a graphics
printer. If the VT125 receives data that is not graphics commands or other commands to the graphics
processor, the graphics processor sends the data to the VT100 for processing and display as text or
VT100 control functions.
The VT125 graphics processor is also available as an option that uses the VT100 chassis, power supply,
and display to add graphics to a VT100.

))\

The VT100 is described in detail in Chapter 4. This chapter is a detailed description of the graphics
processor and its interface to the VT100. This chapter has two sections: an overview description that
explains the parts of a block diagram, and a detailed description that uses detailed diagrams and the
VT125 Field Maintenance Print Set.
6.7.1 Block Diagram Description
The VT125 graphics processor (Figure 6-52) is a bit map graphics display system. The system has a bit
map memory and devices for writing and reading data in memory. The system writes into the bit map
under control of the graphics descriptor ReG IS. (See the VT125 User Guide.) The system reads data out
of the bit map under the timing control of the VT100 terminal, for display on the VT100 screen or an
external color monitor. The next paragraphs describe the parts of the system.
The bit map is at the center of the block diagram. The bit map memory has one data storage address for
each pixel on the display screen. The number of addresses available controls the resolution of the bit
map. A collection of addresses with one bit at each address is called a plane. The bit map in the VTl25
has two bits of memory at each address, so it has two planes. This lets the VT125 display up to four
colors or intensities at one time. See the VT125 User Guide for more details.
The 8085 microprocessor ( lower right) controls most of the graphics processor. This microprocessor has
a read only memory (ROM) for its instructions and dynamic read-write memory for its operations. It
connects to the graphics processor and to other systems, through its input/output bus (I/O BUS).
The STP board is part of the graphics processor option, but it plugs into the STP connector on the VT100
terminal controller board. It connects to the graphics processor board (mono board) via a 24-wire cable.
The STP board connects the graphics processor to the serial data path between the VT100 text terminal
and the external host computer. The board also has the serial interface for the auxiliary port, often used
with a graphics printer.
The graphics processor also connects to the VT100 terminal controller board via a 16-wire cable through
the graphics connector. This connector brings the graphics processor's monochrome video output to the
VT100 for display on the VT100' s screen. The graphics connector also brings the VT100 display timing
signals to the graphics processor to synchronize the two devices so their outputs can appear on the same
screen.

6-70

)

'\
I'
i

)

rSmO~D---'

TO COLOR
MONITOR

IL
I
I
I

r
DIGITAL TO
ANALOG
CONVERTER

COMPUTER PORT
SERIAL INTERFACE
VT100 INTERNAL
SERIAL INTERFACE
AUXILIARY PORT

I
I
II
II

L ~E::INTER':E_ J

OUTPUT
MAP

SHIFT
REGISTER

BIT MAP
MEMORY

WRITE
CONTROL

TO VT100
DISPLAY

)

VT100
GRAPHICS
CONNECTOR

TIMING
AND
CONTROL

ADDRESS
SELECTOR

SCREEN
REFRESH
ADDRESS
COUNTERS

VECTOR
GENERATOR

{'f

MICROPROCESSOR

8085
ROM AND DRAM

I/O BUS
MA-8694

Figure 6-52

VT125 Graphics Processor Block Diagram

6-71

The output map is a small memory loaded by the microprocessor. It holds four sets of values that represent colors or intensities on video monitors. Two bits for each pixel in the bit map select one of the four
sets of values at display time. ReGIS commands from the user set up the output map to correspond to
the desired appearance of the display.
.

))

The digital to analog converter (DAC) takes a value from the output map at display time and converts
the value to three analog voltage levels that an external color video monitor can use. (Another value
from the output map passes unconverted to the DAC on the VT100 terminal controller for the internal
video monitor.)
The timing and control section takes signals from the VT100 and uses them to synchronize the graphics
processor's operations. This section produces many signals to control the vector generator and screen
refresh processes.
A pair of counters provide the screen refresh addressing for the display. All the bit map locations appear on the screen every 60th of a second in synchronization with 'the scanning beam in the video monitor cathode ray tube (CRT).
For each address produced by the refresh counters, 12 bits are read out of the bit map. The shift register moves the 12 bits out to the video monitor (through the output map) one at a time, shifting at the
pixel rate.

})

The vector generator computes the locations of pixels for drawing straight lines (called vectors in graphics) into the bit map much faster than the microprocessor.
The write control reads the contents of the location that the vector generator computes, and then writes
a new value into the location according to user specifications.

))

The address selector controls the addressing of the bit map. Display refresh happens all the time, so the
vector generator must share the bit map address bus with the refresh address counters. Timing signals
switch the address selector between the two address sources.

NOTE
The following discussion is keyed to the VT125
Field Maintenance Print Set. All device numbers
(for example, E45, Q6, R19) refer to components in
that print set.
6.7.2 Microprocessor and Memory
The 8085 microprocessor supervises or controls all VT125 graphics processor activities. The microprocessor operates with firmware stored in a 24K X 8 read only memory (ROM), and with a 16K X 8
read/write memory (RAM) that serves four functions.
•
•
•
•

scratch pad memory for the microprocessor computations
buffer memory for the six transmit and receive data ports
dynamically allocated macrograph storage
storage for three loadable character sets

6-72

6.7.2.1 Memory, 110, and Bus Operation - The microprocessor has two types of data transfer
operations, memory and input/output (110). Paragraph 6.7.2 describes the use of memory.
I/O devices in the VT125 include serial data interiaces (UARTs), a status buffer, and data registers.
Figure 6-53 is a block diag~am of the microprocessor address and data paths.
The microprocessor uses a partially multiplexed bus to transmit data and addresses. The memory and
I/O data use 8 bits. The memory address bus uses 16 address bits. The lower 8 address bits share the
bus with the 8-bit data in the system. The shared bits are also the I/O address bus (but the same I/O
address data also appears on the high 8 bits). A signal from the 8085 called address latch enable (ALE)
appears briefly while an address is present on the bus. ALE strobes the low 8 address bits into a latch.
The latch supplies the memories with their low 8 address inputs while data is being transferred. When
the latch holds the address, the bus is cleared of address information. If the 8085 is reading, it sends a
read signal (RD L), and a data byte appears on the bus from the accessed location. If the 8085 is writing into memory, it sends data and a write signal (WR L) to the RAM, which stores the data on the bus
in the accessed location. Another signal from the 8085 called IO/M selects the I/O or memory buses so
that the lower 8-bits of the 16-bit memory address do not conflict with the 8-bit I/O address.

)

The read only memory (ROM) holds the instructions that run the microprocessor. The ROM is three
8K X 8 ICs in three of four available IC positions. Each IC has 13 address inputs, plus a chip select
line that enables the IC's outputs. A separate decoder selects one of the chip select lines according to
the decoder's higher address line inputs.
The dynamic read/write memory (DRAM) is eight 16K X 1 ICs that are all addressed and read at the
same time. Each IC holds all 16K locations for of the 8 data bits. Each DRAM needs 14 address bits to
address all 16K bits, but the IC saves pins by dividing the 14 bits into two groups of 7 bits that it uses
sequentially (Figure 6-54). An IC (E21) under control of the refresh controller selects between the two
groups. One row of 128 locations is addressed during the row address strobe (RAS) portion of the address cycle. Then one column is addressed during the column address strobe (CAS) portion of the cycle.
Any data in that location appears at the output during CAS. If the WRITE signal is low at any time
during CAS, the data on the input is stored.

6-73

TO STP BOARD

TRANSCEIVER
E30

I/O DATA

DRIVER

~ -

DRIVER

- k:
J?
~

ADDRESS/DATA . ) TRANSCEIVER
E29

,......
8085
E63

Cf'
-..J

- l

8 BIT
LATCH
E26

ADDRESS

I- 8

STATUS
SIGNALS
E35

I/O DATA
BUS
E32

MEMORY DATA

- k:

' ' ; ADDRESS f-J
MUX
7 ....
&
6.1\.. REFRESH
MEMORY ADDRESS
COUNTER

rV
<7

E39
LATCH

.--

I/O
ADDR
DECODE
E40,81
8."

ADDRESS/REFRESH
TIMING
E135, 136, 137

J,)

7
16K X8
DRAM
El,2,3,4,5,6,7.8

E21

ADDRESS
DECODER
E92,95,32.55

WAIT STATE'
GENERATOR
E140.95

Jl
~

CHIP
SELECTS

24KX8ROM
E22.23.24,25

......

,/
MA·8760

Figure 6-53

',~'

''-.--"

Microprocessor Address and Data Paths

-......

...

6.7.2.2 Refresh Control- The graphics processor has only dynamic read-write memories (DRAM).
These memories store more data and use less power than the static RAMS used in the VT100. They are
also more complicated to use. A static memory circuit has a two-transistor bistable latch at each memory
location. This circuit has two states, with one or the other transistor turned on. The circuit holds either
state, without new inputs, as long as power is on.
The dynamic memory circuit has one transistor and a capacitor at each memory location. It stores data
as a charge on the capacitor. This charge leaks away over time, so the memory location must be read
and rewritten before the charge becomes unmeasurable. This process is called refresh, and must happen
within 2 milliseconds of the last write.
The IC includes some of the circuitry for a refresh. When any row of 128 columns is addressed during
the RAS portion of the address cycle, all 128 bit locations are refreshed. But the microprocessor does
not always access to all 128 rows within every 2 milliseconds. A 7-bit counter is needed to address the
rows consistently. The clock is not on the IC because extra pins would be needed to control the refresh
process, to prevent it from interfering with accesses to the DRAM. Therefore, two external circuits .
control the addressing and the timing of the refresh.

)

The first external refresh control circuit is the microprocessor memory control timing sequencer (Figure 6-55). This circuit (with the WAIT state generator) controls the timing of refresh; it also controls
reading and writing to the DRAMs. The timing sequencer consists of a ROM (E135) and a latch
(E 137). The ROM has eight address inputs. The addresses point to groups of data that, when latched,
are the control signals that drive the refresh process. The latch is strobed by a 6 MHz clock made from
the 3 MHz 8085 clock to provide synchronization to the 8085 times states. (The time relationship between the 6 MHz clock input to the 8085 and the 3 MHz 8085 CLK output is too loosely specified to be
useful in this circuit. Generating one signal from the other provides the tighter tolerance that is
needed.) E12 and E50 perform the doubling. The sequencer produces five signals: REF EN/CNT (Refresh Enable/Count), ROW/COL, RAS, CAS, and WE L (Write Enable Low).

ADDRESS
INPUTS
AD

A2 A3 -

ROW
DECODER
1 OF 128

MEMORY
ARRAY

DATA
IN/OUT

A6 -

)

COLUMN DECODER
1 OF 128
MA-8729

Figure 6-54

DRAM Block Diagram

6-75

))
8085 CLOCK

ADDRESS
LATCH
ENABLE
ALE FROM
8085)

----

3MHZ

•

WAIT STATE
GENERATOR
(ONLY RUNS
DURING DRAM
ACCESS)
E95, 140

CLOCK RATE
DOUBLER
SYNCHRONIZED
TO 8085 TIME
STATES.
E12,50

READY
(ROY TO 8085.
ROY L HALTS 8085 FOR ONE 8085
CLOCK WHILE SEQUENCER RUNS.)

ALE HIGH
FOR ONE 3 MHZ·
PERIOD. (TWO
LATCH STROBES)

6MHZ
LATCH
STROBE

...1.

IO/M (SELECTS MEMORY)

A15 (SELECTS ROM OR RAM)

Sl
SO

}

8085 CONTROL
STATES
01 WRITE
10 READ
11 FETCH

{

4
3

ROM
E135

0
r--

ROW/COL

~ REFEN/CNT
"""" WE L
8BIT
LATCH
E136

CAS L
RAS L

!--

Y ROM ENABLE L

~'REDI
NOR
E137

, ALL O's DISABLE ROM

MA·8759

Figure 6-55

Microprocessor Memory Control Timing Sequencer

6-76

-) )
-

}

The second external refresh control circuit is the address multiplexer and refresh counter (E2I) (Figure
6-56). When the REF EN/CNT signal from the control timing sequencer goes high, a refresh cycle
begins. Refresh counter E2I puts the contents of its internal 7-bit counter on the DRAM address lines.
The RAS signal from the control timing sequencer strobes the 7-bit count into the DRAMs, refreshing
the addressed row of 128 data bits in each ·DRAM IC.
When the REF EN/CNT signal from the control timing sequencer goes low, the refresh cycle is completed and the refresh counter increments by one. The new counter contents are used in the next refresh
cycle. Now the address multiplexer part of E21 passes microprocessor address data to the DRAM if
needed. There are 14 address bits from the microprocessor to address the 16K DRAM. The
ROW ICOL signal from the control timing sequencer controls the multiplexer to select one of two
groups of 7-bits. While the ROW ICOL signal is high, the RAS signal from the control timing sequencer strobes the row address bits into the DRAM. Then ROW ICOL goes low, and CAS from the control
timing sequencer strobes the column address bits in. WE L goes low for a write memory access.
6.7.2.3 DRAM Control Timing Sequencer Details
There are three groups of address inputs to the ROM.

)

The signals on ROM inputs 3-6 come from the 8085. They indicate the type of cycle needed.
For example, inputs 3, 4, and 5 high, and 6 low indicate an opcode fetch from RAM. These
signals stay fixed for the entire cycle.

The signal on ROM input 7 is ALE from the 8085 (extended to two clock periods in the
WAIT state generator). ALE indicates the beginning of an address bus transaction. The control timing sequencer starts its outputs at this signal. ALE goes low after two clocks, but the
data is arranged in the ROM to handle the change in addresses.

The signals on ROM inputs 0-2 are outputs from the ROM itself. Each ROM data location
provides a 3-bit address for the next location in the cycle. The latch holds this address (along
with the control signal outputs) for one clock period. The data from the ROM includes another address, so at the next strobe the latch provides a new address to 0-2 on the ROM. Figure
6-57 shows the ROM outputs for the opcode fetch from DRAM cycle. Note that the octal
value of any time state's Next Address 0-2 outputs is the same as the least significant digit of
the next time state's ROM location. The last location in a cycle has all Os in the low 3 bits.
When the Os are latched on the next clock, the wired-NOR output of EI37 goes high and
disables the ROM. Disabling the ROM forces the ROM's outputs to all Is.

Address latch enable (ALE) is the signal the 8085 puts out at the beginning of every machine cycle.
ALE is always defined; it never becomes tristated or random. However, all the other inputs to the ROM
address lines may vary. AI5 from the microprocessor address bus, for example, may become tristated
in the last state of a machine cycle (according to the 8085 system timing diagrams). Therefore, the
ROM is disabled for the last clock of a machine cycle to prevent errors.
The ROM has all I sin all locations that can be addressed with ROM inputs 7 low and 0-2 all I s. So the
outputs do not change state until ALE goes high. Then the ROM address inputs point to a location that
has some value other than Is in the low 3 bits. On the next clock, the ROM address changes to the new
location. The WAIT state circuit extends ALE to two clock cycles; ALE is the most significant bit of
the ROM address, so the first two states in the ROM are in the upper half of the ROM address range.
On the next clock, ALE goes low, so the ROM is addressed in the lower half of its range.

6-77

ROW/COLUMN SELECT
(FROM TIMING SEQUENCER)

ADDRESSES O. 6 (ROW)
FROM
8085
8US

MULTIPLEXER
7
ADDRESSES 7.13 (COLUMN)

MULTIPLEXER

TO MEMORY
ADDRESS
LINES

7-BIT ROW
ADDRESS
COUNTER
FOR REFRESH

ADDRESS/REFRESH
SELECT

COUNT
(ADDS ONE TO COUNT)
. REFRESH ENABLE/COUNT _ _ _ _ _ _.L-_ _ _ _ _ _ _ _ _ _--J
(FROM TIMING SEQUENCER)

MA·8726

Figure 6-56

\'-""""'-)

Address Multiplexer and Refresh Counter

'~'

'-,---,,,,/

}

6.7.2.4 The Microprocessor DRAM Memory Cycle
This section refers to the timing diagram for an opcode fetch from DRAM (Figure 6-57) to illustrate a
DRAM memory cycle with refresh. During opcode fetch from DRAM, the 8085 reads the DRAM at
one location. Then the refresh control refreshes the DRAM at 1 of 128 locations controlled by E21 's
internal counter.
Durin~e first clock, ALE points the ROM to a location that sets ROW/COL high and sets REF
EN/CNT low, incrementing E21 's 7-bit counter (ROM location 277). The address multiplexer and refresh counter (E2I) puts the low 7 bits of the 8085 bus address on the DRAM address lines. Next
Address 0-2 point to ROM location XXI (octal). The other inputs still point to 27X(octal). The following events occur in order on the next seven latch strobes.

)

•

ROM location 271 has RAS go low to strobe the ROW bits into the DRAM. Next Address
0-2 point to location XX2. ALE goes low, but the new address contains the correct data.

•

ROW /COL goes low and E21 puts the high 7 bits of the 8085 bus address on the DRAM
address lines. Next Address 0-2 point to location 073.

•

CAS goes low and the DRAMs put their data on the bus. Next Address 0-2 point to 074.

•

RAS goes high (data is still on the DRAM outputs while CAS is low) and REF EN /CNT
goes high. E21 puts the 7-bit count on the DRAM address bus. (ROW/COL is ignored.)

•

CAS goes high and RAS goes low again and refreshes 1 of the 128 rows in the DRAMs, as
selected by the 7-bit count. Next Address 0-2 point to 076.

•

Next Address 0-2 all are Os and they disable the ROM, forcing all its outputs high.

•

All signals go high.

Similar ..£!:£.cesses occur in other memory cycles. The read from DRAM is an RAS-CAS cycle with
ROW /COL selecting the address bits. The write to DRAM is the same cycle, but with write enable
(WE L) low before CAS. CAS then performs the write. This early write keeps the data outputs tristated. (Tristating would allow the input and output lines to connect. However, for bus loading reasons, a
data latch isolates and buffers the input and output lines.) The opcode fetch from ROM and I/O microprocessor cycles doesn't access the DRAM, so the timing sequencer adds a quick refresh by holding
Refresh Enable high, setting RAS low, then ending the cycle.
6.7.3 Timing
Timing for the VT125 comes from several circuits (Figure 6-58). The VT100 signals are Horizontal
Blank (HORIZ BLANK H), Vertical Blank (VERT BLANK L), and a 24 MHz clock. Two dividers
make slower clocks from the 24 MHz for the graphics processor and the microprocessor. HORIZ
BLANK H controls blanking in the VT100, but it is not the actual VT100 blank time. (Another signal
provides the signal's end time, in synchronization with VT100 characters.) On the graphics processor
board, HORIZ BLANK H goes through a synchronization circuit to align it with the graphics processor
clock(16 MHz). VERT BLANK Lgoes to the state sequencer as a control input. VERT BLANK Lalso
combines with HORIZ BLANK H in the synchronization circuit. The output is a complete video
blanking control signal that blanks the display through the output map.
The state sequencer controls all bit map operations, both reading and writing. Different operations use
different combinations of sequencer outputs, but all operations take the same number of states. Signals
from the microprocessor, vector generator, and other sources select the sequence to use. The sequencer
counter provides the clock for the states of each sequence.

6-79

DATA FOR THIS
TIME STATE COMES
.~ FROM DISABLED ROM
(ALL 1'S)

) )

ROW/COL
REF EN/CNT

WE L
CAS L
RAS L

/II""---

..------REFRESH RAS

......

---~

)

NEXT ADDR 2

NEXT ADDR 1
NEXT ADDR 0
ROM LOCATIONS

277

271

OPCODE FETCH FROM DRAM
MA-8727

Figure 6-57

Opcode Fetch from DRAM

The vertical and horizontal counters provide bit map read addresses for display refresh. The microprocessor can load the counters with offsets from the I/O bus. Offsets can make the image appear in a
different place on the screen from where it was created. The horizontal counter counts only in 12-pixeJ
groups, so a separate, smooth offset counter lets the image move by 1 to 12 pixels at a time. A pause
signal from the smooth offset counter synchronizes the state sequencer at the beginning of each horizontal scan.
Several small synchronization circuits adjust microprocessor and vector generator control signals to the
timing of the state sequencer.
6.7.3.1 Clocks - The master clock for the VT125 is the 24.0734 MHz video processor clock from the
VT100 graphics connector. This clock keeps the graphics processor video output synchronized with the
VT100 video processor output However, the 24 MHz clock is not used directly, but divided into several
smaller clocks. It is divided by 1.5 to give a 16 MHz pixel rate clock. The 16 MHz clock is used in several
places, including inputs to other dividers for pixel-related processes. The 24 MHz clock is also divided
by 4 and 8. The 6 MHz signal is the 8085 clock source, and the 3 MHz signal is the clock for the internal
operation of the UARTs on the STP board. The phase ofthe internal 8085 clock is too loosely specified
relative to the 6 MHz input for timing that must be synchronous with the 8085. A multiplier makes 6
MHz from the 8085 3 MHz clock output to drive the refresh controller.

6-80

) )

'~------ '

,-""

.'~

'-- /

-~,/

VERTICAL BLANK
HORIZONTAL BLANK
VT100
SIGNALS

24 MHz
CLOCK

----r-

DIVIDE
BY 1.5

I
I

.16 MHZ

L--I DIVIDE 16 MHz
BY4

I CLOCK

J----~~ BLANK SIGNAL

~ TO 8085

READ ADDRESSES TO
BIT MAP ADDRESS MUX

VERTICAL
COUNTER

1---- - - LATCH

<f'
00

TO OUTPUT MAP

SYNCHRONIZER

SEQUENCE
SELECTION
SIGNALS

CONTROL
SIGNALS
TO BIT MAP
AND VECTOR
GENERATOR

STATE
SEQUENCER

VECTOR

~ 8085 BUS

~ GENERATOR
CONTROL
SIGNAL
SYNCHRONIZERS

SEQUENCER
COUNTER

f--,

READ ADDRESSES TO
BIT MAP ADDRESS MUX

PAUSE

HORIZONTAL
COUNTER

SMOOTH
OFFSET
COUNTER

~-----LATCH

~
I/O BUS

f---.'

<'
MA-B725

Figure 6-58

VTl25 Timing Block Diagram

6.7.3.2 Horizontal and Vertical Counters - The horizontal and vertical counters provide the addresses
for reading the bit map memory to the raster-refreshed screen. The vertical counter has 8 bits to count
each of the 256 scans of memory. The HORIZ PAUSE L signal is the clock for the vertical counter,
because that signal occurs once each horizontal scan. Thus, the vertical counter increments once each
scan. The horizontal counter has 5 bits to count each of the 64 groups of 12 pixels in each scan. Each
time a read from the bit map occurs, 1 bit from each of the 12 RAMs of each memory plane is loaded
into a 12-bit-Iong shift register. The first input bit appears at the shift output of each shift register
during the load signal. When the load signal ends, the next clock moves the next bit to the output. The
shift process continues while the vector generator reads or writes to the bit map. The LD/SHIFT
(Load/Shift) signal from the state sequencer controls the operation of the shift register and clocks the
H position counter.
6.7.3.3 Offsets - Assuming the default address axes, offset is the distance between the top left corner of
the screen and the address space origin (location [0,0]). An offset of [100,100], for example, means that
the display location [0,0] moves to the right and down by 100 pixels each. Vertical offsets are produced
by loading some new number into the vertical offset latch (LD V OFFSET). (Both latches are always
loaded with standard offset values that adjust the display relative to the blank signals.) The V STROBE
signal loads the offset value into the vertical counter which then counts as usual from that value. Hori- .
zontal offsets are more complicated. (See Figure 6-59.)

\1
/\

The horizontal c:ounter provides addresses to groups of 12 pixels at a time. (This is because each horizontal counter address accesses 12 DRAMs in each plane at the same time. These are the 12 bits that
are shifted out.) Therefore, a horizontal offset that changes the horizontal counter output by 1 changes
the display position by 12 pixels. To provide more control of the offset, a separate smooth offset circuit
delays the start of the count by 1 to 12 counts. The count actually begins before BLANK L ends, so the
effect of the delay is to move the display by 1 to 12 pixels. Larger changes, of course, are made with tho;:
offset latch.
The HORIZ PAUSE L signal is the output of the smooth offset circuit. The signal loads the rough
horizontal offset value from the horizontal offset register, but it does not return to a high state until the
smooth offset counter completes its count of 1 to 12. The low state keeps the H position counter from.
counting, so the HORIZ PAUSE L signal both loads the rough offset and provides control of the
counter by the smooth offset circuit.
HORIZ PAUSE L also stops the state sequencer. When HORIZ PAUSE L goes high, the state sequencer starts. HORIZ PAUSE L can go high only on the rising edge of the shift clock, because of the
smooth offset circuit. Therefore, LDjSHIFT comes from the state sequencer, some integral number of
16 MHz clocks after HORIZ PAUSE L. LDjSHIFT is ANDed with the second half of the 8 MHz
clock to make LD/SHIFT CLK at the end of the LDjSHIFT pulse. If LD/SHIFT is delayed by one
16 MHz clock shift, the anding happens with the wrong phase of 8 MHz and LD /SHIFT CLK. After
two clock shifts, the 8 MHz clock is the correct phase again. To prevent LD /SHIFT CLK from appearing in the wrong half of LD jSHIFT, the 8 MHz clock is inverted every other pixel offset; this gives the
correct signal level for ANDing with LD jSHIFT. When the display is scaled, the Shift Clock is 8 MHz
and each smooth offset delay is an integlal number of 8 MHz clocks. So the phase of LDjSHIFT does
not change relative to the Shift Clock, and the 8 MHz clock is not inverted.

6-82

,...-~--

'-~'--~

SYNC UP L
(E36 P9 LD) NOTE 2

"-~

. '---J,./

SYNC UP L SETS E36 LD; CARRY OUT L CLEARS E36 LD

E36P15
CARRY OUT
3 CHARACTER
DELAY E67 P13

I-

-,-II'-' _ _..,J,--'\~_ _ _ _ _ _ _ _ __

1100
NOTE 1

COUNT ENABLE H
E50 P13
PAUSE ENABLE L E69 P12
(END OF 12TIME STATES)
HORIZONTAL PAUSE L
E69 P4

~~::sCLOCK
LD/SHIFT
E65 P16 SCALE=l
LD/SHIFT
E65 P16 SCALE=2
BLANK L
E82 P12

________
1:-}{_ -1-·

~n
13IJs

'=. PRODUCED BY SYNCHRONIZER-COUNTER AT END OF PREVIOUS SYNC UP CYCLE.

NOTE 1: ALL TIMES IN NS EXCEPT WHERE SPECIFIED.
NOTE 2: TIMING SIGNAL FROM START OF H BLANK TIME.

Figure 6-59

MA-8730

Smooth Scroll Offset Timing

6.7.3.4 Scaling - Scaling means changing the size of an object in the display. In the VT125, the complete display changes at the same time, and there are only four size choices. The display can be times 1
(Xl, default), X2 in horizontal direction, X2 in vertical direction, or X2 in both directions. The VT125
performs scaling by changing the rate that data moves from memory; this means a given pixel appears
for a longer period of time, while the video raster scanning operates without change. Vertical scaling is
simple. A signal from the 8085 adds a divide-by-2 circuit to the count input of the vertical address
counter. Then. only every second HORIZ PAUSE L increments the vertical address, so each pixel appears in two sequential scans. Scaling only affects the counter-incrementing rate, so offsets are not
scaled.

))

Horizontal scaling is more complicated. It begins with a change in the pixel-shifting rate (SHIFf CLK)
from 16 MHz to 8 MHz. Now each pixel appears twice as wide in the display, which is scanning without change at 15.7 kHz. If not for write circuit time limits, all of the bit map timing could be slowed
from 16 MHz to 8 MHz. But LD/SHIFf loads the shift registers every 12 counts at 16 MHz, which
would load new data with only 6 of the pixels displayed. So LD /SHIFf changes to load every 24 counts
at 16 MHz. And alternate pixel offset inversion of the 8 MHz clock for LD /SHIFf is not performed.

6.7.3.5 State Sequencer - The state sequencer has three main parts: a counter, a bit map control section made of two ROMs and their latches, and a vector generator section made of one ROM and its
latch.
The counter (E62) has an 8 MHz clock input, four count outputs, and a carry output that loads the
counter with a starting value to make the count self-repeating and equal to 12 (modulo 12). The counter
counts down from 1011 (decimal 11, the starting value) and carries at 0000 (decimal 0). The counter
changes output states once each 8 MHz cycle, so the fastest state sequencer change is at a 4 MHz rate
(250 ns period). The clock comes from the smooth offset circuit that turns the clock off for a portion of
the horizontal blank period (Figure 6-59).
The bit map control ROMs and latches (E64, E65, E66, E83) provide control signals that the bit map
needs to refresh the screen continously. They also provide the control signals that the vector generator
needs to access the bit map in synchronism with the screen refresh process.
ROM E64 and its latch E65 provide the following signals.
•

SYNC to synchronize microprocessor control signals

•

X SCALE to indicate horizontal scaling to other circuits

•

V STROBE to load the vertical offset into the vertical refresh address counter

•

LD/SHIFf to load the bit map shiftregisters and then shift the data to the screen

•

RA and RB signals that address the register file to select the data for the vector generator in
synchronism with the refresh process

ROM E66 and its latch E83 provide the following signals.
•

VG STROBE to control the vector generator read from the bit map and latch the carry bit
from the adder

•

WR/RD to switch the bit map address multiplexer between the vector generator (write) and
the refresh (read) address circuits

6-84

) )

'))
.

•

CAS to the bit map DRAMs

•

RAS TIME to the RASjCAS selector for the bit map

•

four RAS signals to four groups of 3 bit map DRAMs. All four RAS signals cycle for each
screen refresh read, but one of the four doubles its rate when the vector generator is accessing its group (determined by the XO and Xl inputs to the ROM).

The vector generator control ROM and latch (E71, E70) only provide signals when the microprocessor
sends GO L to enable the ROM when a vector is needed. The signals are as follows.

)

•

ERASE L, which continuously forces the WRjRD signal from ROM 66 to the read state, so
the timing circuit can write the background value to every location in the screen

•

SHIFf ENA, which is not used but could lock the pattern multiplier to a single bit output if
used

•

C In, which is a carry in to the adder to convert aI's complement number into a 2's complement number

•

D LOAD, which is synchronized with VECTOR CLK in another circuit to control the error
register

•

WRITE L, which writes a pixel into the bit map at the computed location

•

DONE L, which signals the end of the vector after the down counter sends DC NT DONE L.
DCNT DONE L is synchronized with D LOAD to make the FINISH signal

•

VECTOR CLK, which increments the position registers after new data is written in the current position

•

STROBE ERROR L, which loads the adder's output into the error register

6.7.3.6 Blanking and Clock Synchronization - The VTI25 gets its timing signals from the VT100.
They are the 24 MHz clock, and Horizontal Blank, and Vertical Blank. But the graphics screen area is
different from the text screen area, so the VT100's blanking time is not correct for the VTI25. There is
a 2.3 character difference between the start of the text area and the start of the graphics area (more in
VTlO5 compatibility mode). Therefore, the VTI25 creates its own blanking signal, using Horizontal
Blank from the VT100 for synchronization.
First, HORIZ BLANK H goes through a latch and gate circuit to produce a single clock-wide pulse
(SYNC UP L) at the beginning of the VT100's horizontal blank. After horizontal blank starts, the first
clock latches the highsignal on pin 2 of E120. The second clock latches it on pin 19, which connects to
one input of a NAND gate. The third clock latches it on pin 16, which connects to one input of an XOR
gate. The XOR gate, with only one of its inputs high, outputs high to the other input of the NAND
gate. The NAND gate outputs SYNC UP L. The fourth clock latches HORIZ BLANK H on pin 15,
which connects to the other input of the XOR gate. The XOR gate, with both inputs the same, outputs
low to the NAND gate. The NAND gate output goes high and ends SYNC UP L, which lasted one
clock time.

6-85

In the VT100, HORIZ BLANK H from the DCOll that enters the DCOl2 ends a partial character
time early. The actual end here is not critical. The DCO 12 extends the signal with the character clock to
make blanking end on a character boundary. The DCOII HORIZ BLANK H end time can vary as
much as one VTI25 pixel time due to temperature changes and long term drift in gate delays within one
VT100, or component differences between VT100s. If the actual end time is between two pixel times,
synchronization of the graphics horizontal blank can jitter between the two pixel times from one vertical interval to the next. A microprocessor-controlled circuit prevents the annoying appearance of the
first pixel in each horizontal scan, blanking and unblanking randomly at speeds up to the vertical scan
rate.
The problem with drifting signals is that the clock might rise at the same time as HORIZ BLANK l
causing race conditions. Latch EI20 is the place where this is most evident. If HORIZ BLANK L goes
high in time before the next rise of 16 MHz, then it gets latched properly. But if H BLANK l is late
enough, the latch may miss it and have to catch it on the next clock tick. This would cause SYNC UP L
to occur one clock time late. At the same time the counters E84, E67, and E68 are counting normally
from the 16 MHz clock. They expect to count to 768 each scan. If HORIZ BLANK L is normal, then
at SYNC UP pin 14 has a high level during that count. But if HORIZ BLANK L is missed at latch
E120, then when SYNC UP occurs a clock tick later, the count at pin 14 is low. Then SYNC UP
strobes the inverted count into flip-flop E52, which latches itself up. The microprocessor reads E52's
output, ClK OUT H, to determine if the rising edge of HORIZ BLANK L is too close to another
transition to give clean results. Then the microprocessor waits until a safe time (relative to RAS signals
occuring in the memories) and commands the clock to invert with CLK SHIff. The inversion moves
the rising edge of the clock away from the rising edge of H Blank. CLK SHIff is a toggled, rather than
set, signal. It only changes when drifts cause the rise times to coincide, and its result. is always a move
away from a critical state, in whatever direction. A few scans later, the microprocessor clears the ClK
OUT flip-flop to allow the monitoring to continue.
SYNC UP L, derived from HORIZ BLANK L, loads Os into counter E84. (The complete counter E84, E67, E68 - is loaded with a value that provides a three-character delay to the start of the graphics
display area, relative to the VT100 text area.) SYNC UP L goes away on the next high state of the 16
MHz clock. A few scans after the Vertical interrupt signal, before vertical blank ends, the microprocessor reads a signal on its status register that indicates whether the clock is synchronized correctly
or not. If not, the microprocessor commands the circuit to use the inverted clock with CLK/SHIFf H.
(This signal comes from an unused modem control line on the internal communication VT100 UART
on the STP board.)
The V STROBE signal loads the vertical offset into the vertical address (Figure 6-60). V STROBE
occurs at the end of vertical blank and is synchronized with enable vertical sync (ENA VERT SYNC)
from the synchronization circuit. The offset value loaded into the counter compensates for the difference in time between the start of the screen and the end of blanking.

V STROBE

V BLANK

---I 4/Js I--

====-==-==::i.I.....;.......:.--------------------- .

- - ,....- - - - - - 1 . 4 ms
...

ENA VERT SYNC
MA-B763

Figure 6-60

V STROBE Timing

6-86

....
))

)!
.

>
)!

6.7.4 Vector Generator
The vector generator is a special circuit that writes straight lines (called vectors in graphics) into the bit
map much faster than the microprocessor can. A vector can be specified with three items: starting
point, direction, and length. The microprocessor puts that information in a fast, dedicated circuit that
performs a process called Bresenham's algor~~hm to draw the vector.
6.7.4.1

Circuit - The vector generator is made of several parts (Figure 6-61).

The X and Y position counters address the bit map for most operations. The microprocessor loads the
position counters with the starting point coordinates for the vector to be drawn.
The down counter stops the vector generator when the vector is completed. The microprocessor loads
the counter with the length of the major axis of the vector to be drawn. Then, as the vector generator
computes and draws each pixel, the counter decrements by one. When the counter overflows, it outputs
the Done signal, which stops the vector generator.

)

The register file holds the X and Y lengths of the end point (Figure 6-62) and the direction of the
vector. (The fourth word in the file is not used.) The microprocessor loads these values before starting
the process. The vector generator's data path is 8 bits wide, so the longest vector that can be drawn in a
single step is 256 pixels. The microprocessor has to reload the vector generator with more segments to
complete a long vector.
The adder uses hinaryarithmetic methods to subtract the length of the shorter axis of the vector from
the length of the longer axis.

)

The direction control ROM has 5 input bits: 3 bits of direction (the same as the 8-pixel vector directions), I bit that indicates whether the direction is one of the pixel vector directions or between them,
and the carry output from the adder. If the direction is between the pixel vector directions, that bit
makes the ROM accept the carry output to decide when to increment or decrement the minor axis. The
ROM ignores carry if the direction is one of the pixel vector directions. Then one or the other or both X
and Y counters increment(s) or decrement(s) every time. The ROM's four outputs select the direction
and specify the count of each position counter.
The error latch and multiplexer hold the results of the adder's operation to subtract from in the next
cycle. The multiplexer selects the input for the latch. The input comes from the adder during a vector
operation, but it comes from the microprocessor during setup for the vector. The microprocessor loads a
calculated value into the latch to make vectors skip from scan to scan in an orderly manner.

)

6-87

TO BIT MAP
ADDRESS MUX

______- ,
r-:~~~~~~~~.EC~O~U~N!T
UP/DOWN

LOAD X LO (8 BITS)----....... X POSITION COUNTER
LOAD X HI (4 B ITS) _ _ _.....~_12_B_I_T_S---::~_ _ _~

TO BIT MAP
ADDRESS MUX

COUNT

LOAD Y LO (8 BITS)

))

UP/DOWN

LOAD Y HI (4 BITS)

DIRECTION
CONTROL ROM

DONE

CA R RY &.....~--::~---'
,......---"--~---., OUT

DOWN
COUNTER

t-------'

ADDER

8 REGISTER FILE
ERROR
LATCH + MUX

MAJOR AXIS

MINOR AXIS
LOAD SIGNAL
FROM 8085 OR
VECTOR
GENERATOR

RA + RB
} READ
ADDRESSES
FROM TIMING

(NOT USED)
}

WRITE
ADDRESSES
FROM 8085

MA-8724

Figure 6-61

Vector Generator

6-88

DVM

DillS ALWAYS THE LONGER (MAJOR) AXIS
DVM IS ALWAYS THE SHORTER (MINOR) AXIS

Oil

UNIT OF MEASURE = PIXELS
MA-B762

Figure 6-62

Derivation of D#L and DVM

6.7.4.2 Vector Generator Timing - The microprocessor sends an Execute signal to the vector generator after it loads parameters into the vector generator (Figure 6-63). EXEC L from the microprocessor
is synced to the state sequencer by SYNC. The result is GO L, a signal that switches the state sequencer from bit map reading only (for screen refresh) to a shared read/write cycle. The vector generator X
and Y counters share the address lines to the bit map with the H and V counters.

)

Before GO L, only H and V addresses are accessed. (See RAS TIME and CAS Lon Figure 6-63.)
LD/SHIFT loads twelve data bits from the bit map memories into the shift register. LD/SHIFT also
increments the H counter. After 12 pixel clocks (6 transitions of the 4 MHz state sequencer clock, or
halfway through the sequence) another LD/SHIFT loads new data. Each H and V bit map read takes
half of the time available, and one quarter of a state sequence.
GO L is synchronized to occur while the bit map contents from an H and V read are loaded into the
shift registers. After GO goes low, the bit map address multiplexer, under control of the WR/RD signal, selects the initial X and Y values (loaded by the microprocessor) to address the bit map. A citcuit
described in Paragraph 6.7 .5 selects the addressed pixel bits and creates a new pair of bits to write back
into the bit map at the same location. While the circuit is preparing data for writing, the bit map address multiplexer returns to the H and V counters and writes the modified data into the bit map. To
shift out, the multiplexer switches back to the H and V counters for 12 more pixels.

)

At the same time, the vector generator computes the next location to draw a pixel. The computation
makes the X, Y, or both counters increment or decrement by one count. Then the X and Y counters
provide an address to read, modify, and write the next pixel in a vector.

6-89

NOTE 1

4 MHZ CLOCK
SYNC

EXEC
GO L

WR/RD

H+V BIT MAP ADDRESSING

X+Y

H+V

X+Y

RAI
RB

H+V

WRITE

~
o

!1'1

1!1

'---_ _~I--

RASTIME
CASL

LD/SHIFT

LI--~-~~----=-rL.J~-~

--'

n,-__

NOTE 1 SIGNALS ARE LATCHED BY 8 MHZ. 1/2 CYCLE SHIFT NOT SHOWN.
NOTE 2 DOTTED SIGNALS SHOW ADDED BIT MAP MEMORY ACCESSES WHEN VECTOR GENERATOR IS RUNNING.
MA·B728

Figure 6-63

'~'

,---,,'

Vector Generator and Bit Map Address Timing

'--./'

'---.........'

'--.../:

5 (225°)

6 (270°)

MA-8198

Figure 6-64

)

Basic Vectors

In this method, any vector starts at some X and Y coordinate in the bit map plane (the current value in
the position counter), and ends at another X and Y coordinate_ The differences between the two XS and
two Ys are the axes of the vector_ The signs of the differences determine the direction of the vector_
The direction controls incrementing or decrementing of each position counter. The longer axis is called
the major axis and the shorter axis is called the minor axis. (References to DJ.L and DVM are from the
original mathematical analysis.)
The length of the major axis loads into a counter that counts down after each pixel is written. The
counter stops the vector generator when the vector is done. The length of the major axis also loads into
an arithmetic device (the adder) where the length of the minor axis is subtracted from the major. After
each subtraction, one pixel is written. Then the position counter that controls the major axis is incremented or decremented. If the value remaining in the adder before the subtraction is smaller than
the value of the minor axis, the next subtraction causes the adder to carry. Then the minor axis counter
is incremented or decremented and a pixel is written.
The next series of figures and tables shows the computation and writing of pixels in more detail. The
rules listed refer to the changes needed in the algorithm to correct for odd-Y simulation. The microprocessor computes start and end values with 512 Y addresses, but the vector generator only has 256 Y
addresses to write in.
6.7.4.3 Drawing Vectors - The vector generator produces basic and arbitrary vectors. The basic vectors are shown in Figure 6-64). These are the pixel vector directions referred to in the VT125 User
Guide. There are eight basic vector directions, 45 degrees apart. These vectors are the easiest for the
vector generator to draw. From any position, a vector along one of these directions changes one or both
coordinate values with every pixel write. For example, in the 0 direction, any vector is drawn by incrementing X, writing a dot, incrementing X, writing a dot. Once the direction is specified, the vector
generator circuit does not make any decisions about when to change a position counter's value.

)

For arbitrary vectors, the vector generator must decide whether to change one counter after writing a
pixel. For example, an approximately 15 degree angle vector increments its Y value only once for every
four X value increments. The vector generator method for deciding when to increment is Bresenham's
algorithm.

6-91

Calculation 1
Error register
DVM register
Carry in

002 8

Error register

3778
No carry

3748 (l's complement of 3)

001 8

ODD
EVEN~+-~

ODD
EVEN~4-~~~-4~~

ODD
EVEN~+-4-~-+-4~~

Decrement

major and minor axes.
MA-9760

Strobe
Figure 6-65
Error register
D#L register

005 8

Error register

004 8

3778

Carry
Strobe
Direction - Decrement X and Y,
and do a Pixel Write.
Decrement the down counter.

5-1=4
Down counter

Draw Vector. (Use these rules.)
1.

Do not write in direction
2 from an odd line or
6 from an even line.

Do not write if the direction
is 5 or 7, the scan line is
even (YO), and the last
direction was 6.

Do not write if the direction
is 1 or 3, the scan line is
odd, and the last direction
was 2.

6-92

Calculation 1

)

Calculation 2
Error register
DVM register
Carry in

0048
3748
001 8

Error register

001 8

EVEN~~~~-+-+~~

ODD
EVEN~~4-~~-+~~

ODD
EVEN
ODD
EV EN L-..I..-....L...-1....--L..---I..---L---J

Carry

Decrement

major axis.

MA-9758

Strobe -> Set Carry flip-flop.
Figure 6-66
Error register
DIL register

001 8
005 8
006 8

Error register

0018 (No Error CLK)

Strobe
Direction - Decrement X
and do a Pixel Write.
Decrement down counter_

)

4-1=3
Draw vector_

)

6-93

Calculation 2

Calculation 3
Error register
DVM register
Carry in
Error register

')
001 8
374 8

001 8
376 8
No carry

Decrement
major and minor axes.

ODD

Y=1

EVEN

Y=O

ODD

Y=1

EVEN

Y=O

ODD

Y=1

EVEN

Y=O

)

MA-9759

Strobe

Figure 6-67

Error register
DIL register

005 8

Error register

003 8

Calculation 3

3768

Carry
Strobe
Direction - Decrement X
and Y. Doa Pixel Write.
Decrement down counter.

3-1=2
Draw vector.

)

6-94

)

Calculation 4
Error register
DVM register
Carry in

003 8
3748
001 8

Error regi~ter

0008
Carry

Decrement

major axis.
MA-9756

Strobe
Figure 6-68
Error register
D#L register

0008
005 8
005 8

Error register

0008

Strobe
Direction - Decrement X
and do a Pixel Write.
Decrement down counter.

)

2 - 1= 1
Draw vector.

)

6-95

Calculation 4

Calculation 5
Error register
DVM register
Carry in
Error register

0008
374 8

001 8
375 8
No carry

Decrement

major and minor axes.
MA-9757

Strobe
Figure 6-69
Error register
Dp. register

005 8

Error register

002 8

Calculation 5

375 8

Carry
Strobe
Direction - Decrement X
and Y, and do a Pixel Write.
Decrement down counter-

1-1=0
Draw vectorvector is complete_
6.7.4.4 Vector Calculation Timing - Twelve time states are needed to perform a calculation (Figure 670). These 12 time states are divided into 4 groups. The 4 groups are repeated until the vector is drawn.
The four sequential groups are as follows.
1.

DVM time

Dp. time

WOPS time

Direction time

))

The following paragraphs describe the separate functions that take place in each group.

•

DVM Time - The error register is added to the DVM register plus carry in. The sum is
loaded into the error register by error clock. The carry output of the adder is strobed into the
carry flip-flop. (The error register is loaded at the start of the vector with one half the major
axis value, ignoring the remainder.)

•

Dp. Time - The contents of the error register are added to the Dp. register. If the carry flipflop is reset, the vector ROM allows an error clock pulse that loads the result of the add into
the error register. If the carry flip-flop is set, the vector ROM does not allow an error clock
pulse to occur. This means the contents of the error register remain unchanged.

6-96

..-'
,,~

'--'

-J.

TWELVE TIME STATES

u-r---r

OUTPUT
DVM
DIRECTION
DVM
D/l
D/l
OF
FILE REGISTER ......_ _ _ _ _-'-_ _ _ _ _- L_ _ _--1_ _ _ _ _ _ _ _.L_ _ _ _ _...L_ _...::..:~_

LDERROR
0'1
I
\0
-...J

STROBE L

CALCULATION
ER+DVM-(ER)

CALCULATION
BR+D/l-ER

MODIFY
SCREEN
RDM

EFFECT X ARRAY
MAJOR & MINOR AXIS

TWELVE TIME STATES

CALCULATION
ER + DVM + l-ER

DIRECTION
___1L=:..:.._L_....:=::..:.:.:=__J

MODIFY
SCREEN
RAM

CALCULATION
ER+D/l- ER

EFFECT X ARRAY
MAJOR AXIS

na...______________

------,
CARRY (1) H

_____ ~I----------------------------------------------------~

------

WRT L

VCLK

CLKS X & Y REGISTER
CLKS DOWNCOUNTER

1
~

CLK MAJOR AXIS
MA·8331A

Figure 6-70

Arbitrary Vector Timing

•

WOPS (Write Operations) Time - The modified data bits are written into the bit map location specified by the X and Y counters.
The direction ROM circuit produces INHIBIT WRITE L, which must be high for the write
control to write to the bit map. There are three conditions that prevent a write operation from
occurring.

•

Do not write in direction 2 from an odd line or 6 from an even line.

Do not write if the direction is 5 or 7, the scan line is even (YO), and the last direction
.
was 6.

Do not write if the direction is 1 or 3, the scan line is odd, and the last direction was 2.

Direction Time - The outputs of the direction register, bits F3 - FO, are inputs to the direction ROM. An arbitrary vector is drawn in direction 3 when the input bits of the direction
ROM equal B (hex). The direction ROM input bits FO - F2 determine the direction affected.
Direction 3 is a negative direction. If the carry bit is ai, only the major axis counter is decremented. If the carry bit is a 0, both the major and minor axis counters are decremented.

) I

The vector sequence ends when the V clock signal clocks the down counter to O. This resets
the GO flip-flop.
6.7.5 Bit Map Write Control
The vector generator computes the series of memory locations that make a vector. The bit map write
data generator controls the data stored for each pixel in the vector. The appearance of a pixel is controlled by two groups of data.
1.

Information stored in two planes at the pixel location

Information stored in four locations in the output map (section 6 ... ).

Each pixel has 2 bits of information, and those 2 bits select one of the four output map locations during
screen refresh.
A multifunction circuit under microprocessor control selects one of the four combinations of two bits to
write for each pixel. Briefly, after the vector generator computes a pixel location, the current contents
of the location are read. Depending on the user's write mode, the contents are logically combined with
one bit of a pattern and the results are written back to the current location.
6.7.5.1

))

Block Diagram - The bit map write control circuit has the following parts (Figure 6-71).

Two l-oJ-12 selectors (one Jor each plane). Data always comes from each bit map plane 12 pixels at a
time. When the vector generator addresses the bit map, the bottom four bits of the address select the
desired pixel data.
Write control RAM. Eight locations hold 2 bits each. The microprocessor loads the RAM before the
vector operation according to the user's write mode. The RAM addresses are 2 bits of the current 'pixel
from the bit map, and 1 bit from the pattern register. See the VTI25 User Guide for a complete description of the operation of the Custom Writing Control. (The IC has 16 locations of 4 bits each. The
VT125 uses only 8 locations and stores only 2 data bits. Each data bit is in 2 bit locations so the IC can
drive all 24 bit map ICs.)

))

6-98

WRITE DATA TO
PLANE 1
VECTOR
GENERATOR
RUNNING _ _...... ADDRESS MUX
E90
(SELECTS
PLANE & PATTERN ~1Il""'"--""""_ _- - '
INPUTS)

PLANE 2

WRITE CONTROL
RAM
16 X 2
E89
(8 X 2 USED)

2
8085 ADDRESS
BUS

)

I/O DATA BUS
PLANE 2

WOPS BIT------~
(ALWAYS 0)
PATTERN

PLANE 1

1 OF 12
SELECTOR
PLANE 1

MEMORY
DATA

)

CLOCK
(WRITE SIGNAL
FROM VECTOR
GENERATOR)

1 OF 12
E 116,
SELECTOR
113
PLANE 2

E117,
97

ADDRESS
MEMORY DATA
FROM
VECTOR GENERATOR

PATTERN SHIFT
REGISTER
E28
(LOADED BY 8085)

PATTERN
MULTIPLIER
COUNTER
E87

)
MULTIPLIER
E88
REGISTER

I/O BUS
DATA

Figure 6-71

Bit Map Write Control Block Diagram
6-99

MA-B765

Address multiplexer. When the vector generator is not running, the multiplexor connects the low 4
address bits of the microprocessor bus to the write control RAM. The microprocessor uses the bus addresses to load the RAM according to the user's write mode. When the vector generator is running, the
multiplexor connects 2 bit map data bits and 1 pattern register bit as addresses; the multiplexer uses
these addresses to read data from the write control RAM to write to the bit map.

))

Pattern register. The microprocessor loads a pattern into an 8-bit shift register. Each pixel that the
vector generator addresses uses 1 pattern register bit as part of the write control process. When the
pixel is written, the write pulse to the bit map shifts the register (or increments the mUltiplier counter,
below). The default pattern for vectors is all Is. The output of the shift register goes toits input so a
vector longer than 8 pixels gets a wraparound version of the starting pattern. When a character is
drawn, the pattern loaded into the shift register varies according to which scan of the character is
drawn.
Pattern multiplier counter. Each bit map write pulse is a clock input for the multiplier counter. The
microprocessor loads the counter with a starting count when it loads the pattern register; the pattern
register reloads itself with the starting count at the end of each count. If the pattern multiplier is 1, then
each count gives an output and shifts the pattern register. Larger pattern multipliers delay shifting the
pattern register until the count finishes. This makes the displayed pixel appear wider.

.'J )

Multiplier register. The microprocessor loads this register with the multiplier value. The register holds
the value for loading into the pattern multiplier counter after every finished count and at every pattern
register load.
6.7.5.2 Character Writing - Writing a character into the bit map is like writing a vector with the
pattern register and pattern multiplier. The ReGIS text command translates an ASCII character code
into a set of dot patterns. To do this, the microprocessor reads a character look-up table located in
memory. The look-up table consists of a series of character cells, one cell for each ASCII code. Each'
character cell contains ten 8-bit bytes of pattern data to write as vectors. These pattern vectors compose
the character in the bit map and on the screen.

))

Figure 6-72 shows both the vector generator and pattern register operations for a size 1 character. Each
character scan is one short vector drawn with pattern data loaded from the stored information for that
scan. When a character is larger than size 1, Figure 6-73 shows the pattern multiplier action. For a size
2 character, the pattern multiplier counts twice before sending a shift clock to the pattern register.
Therefore, two sequential pixels have the same data. The vector generator draws lines that are twice as
long so all the data can be displayed. The microprocessor counts the number of scans drawn with the
same pattern data. For size 2, it loads the pattern register with the next scan of character cell data after
two identical scans. Each of these parameters (vector length, pattern multiplication, pattern repetition),
plus others such as vector direction, can be controlled with ReGIS commands. The VTl25 User Guide
has complete descriptions of the commands.

6-100

)

00000000
00010000

•

• •

•

•
•
• •••• ••
•
•

•

00101 a 0 a

a 1 000 1 a a
1 1 1 1 1 110
1 a aa aa 1 a
1 a aa 0 a 1 a
1 a a0 0 a 1 a
00000000
00000000

)
I

DATA LOADED
BACKWARDS

PATTERN REGISTER
1

aaaa aaa

VECTOR
START

VECTOR MOVEMENT IS:

)

WRITE

~O"~ - ~O"~ -

.. -

~
-

~<v/

-WRITE

o"~

a --

SCAN 2

•I

•

... - - -•
• --- •
....- ---

- .... ~o/

WRITE

SCAN 1

~O~---

.. _ -WRITE
10
SCANS

)

aaaaaaaa

•

SCAN 10
VECTOR ENDS

MOVE = LOAD X AND Y COUNTERS WITH NEW START VALUE.
MA·8761

Figure 6-72

Draw a Character

)

6-101

PATTERN MEMORY

MULTIPLIER = 2

PATTERN STARTS HERE AT
BEGINNING OF WRITING COMMAND
OR P [J.

DARK AREAS REPRESENT PIXELS THAT ARE CONTROLLED BY ONES.
LIGHT AREAS REPRESENT PIXELS THAT ARE CONTROLLED BY ZEROS.

Figure 6-73

MA-9437

Pattern Memory and Multiplier

6.7.6 Bit Map Memory
The bit map has the following parts (Figure 6-74)

Two planes of memory. Each plane is twelve 16K X 1 DRAMs arranged in four rows of 3 DRAMs
each. One RAS signal controls each row. For any bit map read, all 4 RAS signals go low at the same
time to access all 12 DRAMs. (One CAS signal is common to all the DRAMs.) For a bit map write,
only the RAS for the addressed row goes low. There are three write signals to drive three columns of
DRAMs. Only the write signal for the addressed DRAM goes low. The intersection of one RAS signal
and one write signal is only one DRAM.
Write select!disable circuit. This is a ROM that decodes address inputs to select the group of DRAMs
that receive a write signal. If an address would write to a location that is not in the plane, the circuit
disables all write outputs. (Such addresses occur when the graphics processor computes a graphic object
that overlaps the margins of the display.)

)

Row/column address multiplexer. This multiplexer is controlled by the WR/RD signal and selects the
source of addressing for the bit map. For most write operations, the bit map addresses come from the X
and Y multiplexer, supplied by the X and Y position counters in the vector generator. For read operations, the bit map addresses come from the H and V mUltiplexer, supplied by the horizontal and vertical counters in the timing circuits. Each set of counters provides a total of 14 address bits. RAS TIME
high selects one group of seven address signals, while RAS TIME low selects the other group.
The address lines on the multiplexers are scrambled to guarantee refreshes often enough for all the
DRAMs. Each raster takes 16.7-ms, but refreshes must be less than 2 ms apart. Therefore, the address
lines are arranged so that sequential addresses jump among all DRAMs at least eight times per raster.
Then the vector generator address lines are scrambled in the same pattern so a given auur\!ss on I,;ii.ill!r
side of the multiplexer always accesses the same bit. This scrambling method means the RAMs themselves do not have to reflect the H and V orientation of the counters, provided thai. th~rc arc enough
unique addresses to cover the screen area. So a bad memory IC does not give a simple pattern on t h e ) , I
screen, although a bad shift register line does.

6-102

)
TO
OUTPUT
RAM

{

12 BIT SHI FT REGISTER

~
y

I
4

RAS
RASGOEST OALL DRAMS
DURING RE AD TO
SHIFT REGI STER.
RASGOES T 010F4
GROUPS OF 3
DURING
CASWRITE.
CASGOEST OALL
DRAMS

)

WRITE

.......

16K X 12
BIT MAP
PLANE 1

WRITE
CONTROL

I
12 BIT SHIFT REGISTER

PLANE 2
WRITE DATA

(ISAME AS

'---WRITE

... '-

7 BITS

DISABLE WRITE IF
ADDRESS IS OUTSIDE
OF SCREEN AREA
ALSO SELECT FROM
3 GROUPS OF 4 DRAMS.

....

ADDRESSES

'CD=
....

'CD=

READ SELECTS H+V
WRITE SELECTS X + Y

RD/WR ADDRESS MUX

H+V MUX

.(~
Vl
'=

I-rl

X+Y MUX

.('\..
Vl

'CD=

H+V COUNTERS

I I

X+Y COUNTERS

RASTIME
SELECTS
1 GROUP OF 7

NOTE: RAS, CAS, AND WRITE SIGNALS
GO TO BOTH PLAN ES.

MA-8695

Figure 6-74

6-103

Bit Map

Bit map output shift registers. With each bit map read cycle, each memory IC puts 1 bit on its output
The IC outputs for each plane connect directly to the inputs of three 4-bit shift registers. When
lO/SHIFT is high, it strobes the inputs into the registers. The right bit in each register appears at the
shift output while lO /SHIFT is high. lO /SHIFT is high for one pixel time; this means the first bit
appears on the screen at lOAO time, then the other 11 bits shift onto the screen with the shift clock
while lO/SHIFT is low.

6.7.7 Output Map
The VT125 displays color and brightness by addressing one of four preset values for each pixel on the
screen. The VT125 can display each pixel on the screen with a different hue, lightness, or saturation.
However, it can only use four different colors at one time. That is, any pixel can differ from its neighbors, but only four different colors can appear on the screen at one time. This is the tradeoff that the
VT125 makes in its memory usage: rather than displaying many different colors at one time but only
allowing changes at fixed boundaries that are larger than the pixel size (as in the VKlOO, which can
change only at 12-pixel horizontal boundaries), the VTl25 displays a limited set of colors at anyone
time but allows changes to occur at any boundary down to the individual pixel dimension.

The VTI25 displays color and brightness with a bit map memory and an output map. The bit map
memory has the same number of addressable locations in it as the writable addresses of the screen
display (768 X 256). However, for each addressable location, there are two bits of data. Think ofthese
pairs as existing in two separate but closely connected planes of addressable locations (Figure 6-75).
The pairs of bits can be written either one at a time or together. (See Writing Controls in the VT125 User
Guide.) They represent the four numbers 0, 1,2, and 3 when their binary values are decoded.

))

The four numbers are the addresses offour output map locations. Each output map location is I byte of a
4-byte RAM. When the graphic memory is displayed, each pixel in the bit map requests that the
information in one of the output map locations be sent to the digital to analog converters (DAC). The
DACs convert binary data into drive levels for cathode ray tube (CRT) gun drives. Each output map
byte has 2 bits for the monochrome monitor in the VT100 part ofthe terminal (representing four levels of
intensity on the monitor screen: dark, dim, normal, bold). Each output map byte also has 2 bits for each
of the color guns in the external or alternate RGB color monitor. These bits represent four levels for each
of the colors, or a total of 64 different values of hue, lightness, and saturation (HLS). (RG B is required
because the National Television Standards Committee (NTSC) color standard does not provide enough
bandwidth to display color changes on individual pixels.) You can set the information in each output map
by using the mapping command with either RG B , letters, or HLS specifiers. (See the Screen Output Map
Definition in the VT125 User Guide.)
6.7.8 Output latches and DAC Converters
Each time a pixel is output from the bit map shift registers, its 2 bits (SHIFT OAT A and B) address a
location in the output RAM (E58 and E46). The next SHIFT ClK strobes the data from the addressed
location into latches E91 and E94. The data stays in the latch while the RAM is addressed by the next
pair of data bits. The latch provides a stable signal to drive the digital to analog converters (OACs).
The OACs are open collector inverters that provide paths to ground resistors in the base circuits of the
video drive transistors.
Using the green output as an example, reducing the voltage on Q4's base causes more current to pass
through Q4 and R 77. R 77 is the output load resistor; as more current passes through R 77, the voltage
across it increases. A CRT connected to that voltage gets brighter as the voltage increases. One way to
reduce Q4's base voltage is to connect a resistor from the base to ground. The added current through
R75 causes a voltage drop at the base. The DAC has two different resistors connected in parallel between the base and common. These resistors are in series with switches (the inverter output transistors)
that can disconnect the resistors, or connect one or both resistors, to common. The resistors have different values, so there are four possible voltages at Q4's base. Two bits from the output RAM are inputs to
the two inverters (ElOO) for each color plus the monochrome output.

6-104

-\'
j

ADDRESS IS SAME FOR \
BOTH PLANES

P~ANE

X,Y

BINARY

DECIMAL

X,Y+l

LOW
BIT

0
(LOW BIT}

P ~ANE
1

X,Y

X,Y+2

(H IGH BIT}

X,Y+l

X,Y+3

X,Y+2

)

OUTPUT MAP

HIGH
BIT

X,Y+3

MONO

COLOR

WHITE

GREEN

LIGHT
GREY

RED

DIM
GREY

BLUE

DARK

SET BY SIMI
DEFAULT VALUES
SHOWN

i f
I

PLANE WRITING
CONTROL BITS

)

0
OR

PLANE

OR
1

W
COMMAND

W(FO)
W(F1)
W(F2)
W(F3)

CONTROL
BITS
0
0
1
1

0
1
0
1

PLANES THAT CAN
BE WRITTEN
NONE
PLANE 0
PLANE 1
BOTH 0 AND 1

WRITING
CONTROLS
(PATTERNS, ETC)

)

WRITING
COMMANDS
(V,C,T)
MA-9468

Figure 6-75

Bit Map Planes and the Output Map

)
6-105

6.7.9 STP Board
The STP board (also called paddle board) holds the serial interfaces for the terminal. The interfaces
are made of UARTs, a baud rate generator, address decoders, and EIA line drivers and receivers. A
switch pack on the board selects the connectors used for the computer port and baud clock sources for
two of the UARTs.

')
.)

6.7.9.1 UARTs - UART E3 communicates between the VT100 terminal controller and the graphics
processor. The typical switch setting configuration has the clock for both receive and transmit on E3
coming from a fixed rate signal on the terminal controller (A 18-1). The same fixed rate signal returns
to the terminal controller UART on A14-2 and A 15-2 to drive the internal communications lines at the
same fixed rate.
UART E8 communicates between the VTl25 graphics processor and the communications port. Its
receive and transmit clocks come from the terminal controller through the STP connector (AI4-1 and
AI5-1). These clocks are controlled by the bud rate generator on the terminal controller. If the STP
connector does not have a board plugged in, these clocks return to the UART on the terminal controller.
But with the STP board installed, these clocks control communications with the host computer and can
be set with the SET-UP B controls.
UART Ell communicates between the VTl25 graphics processor and the auxiliary port( also called the
printer port). This port is more flexible than a printer port because the graphics processor allows full tw~
way communications over it. For example a digitizing tablet can be connected to it, given host software
support; for this reason the words Auxiliary Port are used in the VTl25 documentation. A baud rate
generator provides the baud rates for UART Ell on the STP board. The baud rate generator is the same
type on the terminal controller. Switches are set to drive both UART clock inputs from the same
generator output, but firmware controls the use of the dual generator. The UART speed and number of
bits are set in SET-UP B, switch group 5.
All three UARTs have their receiver ready lines ORed together, and their transmitter ready lines ORed
together. The two ORed outputs go to microprocessor interrupt inputs on the graphics processor board.
6.7.10 Firmware
The VTI25 firmware includes several sections. The next paragraph describes some of them.
6.7.10.1 Memory and 1/0 Addresses - The VT125 has 24K bytes of firmware in ROM in addresses
OOOOH to 5FFFH, with 8K more unused ROM space from 6000H to 7FFFH. The RAM has 16K bytes
in addresses 8000H to BFFFH.

))

The I/O addresses are listed in Table 6-4.
6.7.10.2 Self-Test Processes - The VT125 has verification and diagnostic tests available. ANSI control functions invoke the tests. Some of the tests are VT100 specific, and others are VT125 specific.
This section describes the actions of the tests.
VT100 Self-Tests
The VT100 performs all power-up tests (CPU, ROM, RAM, AVO) as usual when you power up or
reset (SET-UP 0) the terminal. The VT100 power-up/reset sequence invokes the VT125 power up-test.
You cannot run the VT100 data loopback and EIA modem loopback tests unless you remove the VT125
STP board.
If the "infinite loop" VT100 self-test is invoked from the VT100 keyboard in Jocn.) mode or if the
VT100 escape sequence is sent from a host through the VT125, only the VT100 tests are repeated.
Under these conditions, any resulting VT125 tests are meaningless.

6-106

)

Table 6-4

I/O Addresses and Data

Command

Address

Load X Position Register Low Byte
Load X Position Register High Byte
Load Y Position Register Low Byte
Load Y Position Register High Byte
Load Error Register
Load Pattern Register
Load Pattern Multiplier
Clear CLOCK SHIFT Glitch Flag
Load DJ.! Register
Load DVM Register
Load Direction Register
Load WOPS Register
Execute Move Operation
Write Dot
Execute Vector Write
Execute Screen Erase
Load Horizontal Minor Offset
Load Horizontal Major Offset
Load Vertical Offset
Load Output RAM !four starting at:)
Load Write Control RAM (S starting at:)
Read System Status
VT100 UART Data (R/W)
VT100 UART Control (R/W)
Computer UART Data (RjW)
Computer UART Control (RjW)
Auxiliary Port UART Data (RjW)
Auxiliary Port UART Control (RjW)
Load Auxiliary Port Baud Rates

SOH
90H
AOH
BOH
COH
13H
I4H
ISH
FOH
FIH
F2H
F3H
OOH Start Data = 01 H
OIH
02H
03H
IOH
llH
I2H
ISH
DOH
20H
OEH
OFH
OCH
ODH
OAH
OBH
OSH

VT125 Self-Tests
Power-Up Tests - These tests occur whenever you power up or reset the terminal. See Paragraph 6.S.2
for an explanation of error reports.
1.

CPU register test - This test verifies that the basic CPU register operations work. If this test
fails, the microprocessor tries to flash an error code on the monochrome monitor.

ROM checksum test - A checksum is stored in each 4K bytes of ROM (2 for every SK
bytes). The checksum is computed by this procedure: Shift and XOR each byte, starting with
an initial value of high ROM address byte + I; the result should be zero. If this test fails, the
microprocessor tries to flash an error code on the monochrome monitor and an encoded
ROM number on the color monitor.

RAM test - This test performs a worst-case readjwrite check of all 16K bytes of program
RAM. If this test fails, the monochrome monitor flashes an error code and the color monitor
flashes an encoded RAM bit number.

6-107

Vertical sync timeout - This test checks that the vertical sync signal appears repeatedly within approximately 25 ms. If ihis test fails, the monochrome monitor flashes an error code.

Vector timeout - This test checks that none of the vectors drawn in either the bitmap RAM
test or the vector generator test take longer than 100 ms. If this test fails, the monochrome
monitor flashes an error code.

Partial video bitmap RAM test - This test checks that every bit tested in both video bitmap
planes can be written to both 1 and O. This partial test only tests the first eight scan lines of
video bitmap RAM; you can test the entire RAM on request. If this test fails, the microprocessor tries to display the message "VT125 BM Error".

Vector generator tests - This tests the basic vector functions by drawing a test pattern into
the upper-left corner of the display, then verifying it against a predetermined pattern. If this
test fails, the microprocessor tries to display the message "VTl25 VG Error".

Communications test - This test verifies the VT100-to-VTl25 communications link by requesting the terminal status from the VT100. If this test fails, the microprocessor tries to
display the message "VT 125 I C Error". If the VT 100 is LOCAL, the message "VT 125
Offline" appears on the display.

)

Requested Tests - The following tests require operator participation, and can only be invoked with the
DECTST escape sequence sent to the VT125. (See Paragraph 6.7.)
I.

Data loopback test - This test requires an external loopback connector on the host communications port. The test only verifies data leads, and only at the speed selected by the VT100.
If this test fails, the message "VT125 EC Error" appears.

Printer port loop back test - This test requires an external loopback connector on the printer
EIA port. This tests data lines on the printer EIA port. If this test fails, the message "VT125
SC Error" appears.

Display test - This test requires operator visual verification. The test cycles through the four
intensity levels of each of the three primaries and white, thereby testing the color output
RAM.

Entire video bitmap RAM test - This test checks that every bit in both video bitmap planes
can be written to both one and zero. If this test fails, the microprocessor tries to display the
message "VT125 BM Error".

6.7.10.3 Graphics Protocols - The VT 125 has three different graphics protocols. Control sequences in
the data select the protocol.
ReG IS (remote graphics instruction set) is a graphics descriptor that uses high-level commands to construct basic graphics objects such as lines and curves. Except for the ANSI device control string mechanism for turning graphics on and off, all ReGIS commands use printable ASCII characters and decimal numerical values for option selection and pixel location specifications.
VT105 emulation provides most of the functions of the VTl05 graphics terminal for compatibility with
existing software. The VTI05 has a few built-in processes such as shading and strip charting. Its pixel
locations need binary specifications from software in groups of ASCII characters selected for their
combined bit patterns.

6-108

))

DECwriter graphics is a protocol for transmitting the complete contents of a bit map. It uses ASCII
characters selected for their bit patterns. In the VT125, DECwriter is two different protocols. One protocol converts the VT125 bit map into ASCII characters for transmission to a DECwriter graphics terminal. The other protocol writes incoming DECwriter graphics data into the bit map.
6.7.10.4 Communication Protocols - The VT125 is a complex device that has several separate processes operating at the same time to provide graphics and text over a simple terminal communication
line. Figure 6-76 shows a general block diagram of the VT125, divided into two main functional blocks:
the graphics processor and the VT100 text terminal that holds the graphics processor. The figure shows
two important facts.
1.

The keyboard communicates only with the VT100, so LOCAL operation can only cause actions in the VT100, not in the graphics processor.

The graphics processor manages all communications between the computer and the VT100,
and always uses XONjXOFF. (This SET-UP feature cannot be turned of0

-)
EXTERNAL
COLOR
MONITOR

INTERNAL
MONOCHROME
MONITOR

GRAPHICS
AND
TEXT

)

VT100
TEXT
TERMINAL

VIDEO SYNC·

f
R G B

MONO VIDEO

~ONLINE-'
I

GRAPHICS
PROCESSOR

f-~

CURRENT
LOOP
ADAPTER
OPTION

~
~

COMPUTER

L ___ J

r - - - -,
I

)

LOCAL·

•L. _ _ _ _ .JI

EXTERNAL
GRAPHICS
PRINTER

MA·9436

Figure 6-76

VT 125 General Block Diagram

)
6-109

Figure 6-77 is a more detailed look at the communication structure in the graphics processor itself. This
shows the buffers included for each of the three bidirectional communication ports in the graphics
processor. The dispatcher is many processes. It controls the butTers with XON/XOFF. It sets up the
data paths from each sending port to each receiving port. And it passes all communications according to
ANSI X3.64-1979. That is, it examines data from all ports for instructions to itself. It passes anything
that does not apply to the graphics processor to the ports set to receive the data from that source. Among
the ports controlled by the dispatcher are the selection of the graphics protocol under ANSI sequence
control. The choices in that block of the diagram are ReGIS, VT105 emulation, and. DECwriter
graphics. The following five figures show the internal connections that are set up for different
applications.

- II
---,

V T100
T EXT
T ERMINALI

. I
I I

64 CHAR
RECEIVE
BUFFER

f f f .,
------ ...-.1.....1""""""...,

I GRAPHiCs'

PROCESSOR

MONO
VIDEO
OUTPUT

BIT MAP
'--_ _M_E_Mr-0_R_Y_ _-'

I I

COLOR
VIDEO
OUTPUT

I
I

16 CHAR
TRANSMIT
BUFFER

GRAPHICS
PROTOCOLS

I I
7-9 CHAR
TRANSMIT
BUFFER

))

r--

DISPATCHER
128CHAR
RECEIVE
BUFFER

XON/XOFF CONTROL
ANSI PARSER

128 CHAR
RECEIVE
BUFFER

COMPUTER

16 CHAR
t---~ TRANSMIT. f--+
BUFFER

f
16 CHAR
TRANSMIT
BUFFER

128CHAR
RECEIVE
BUFFER

)

I
I

-- ...J L - - - - .-._..,.......... - -.....,r--..... ___ ...1
EXTERNAL
GRAPHICS
PRINTER

)
MA-9472

Figure 6-77

VTl25 Data Paths

6-110

Figure 6-78 shows the VT 125 operating as a text-only terminal. If you send XOFF from the keyb()ard,
either by pressing NO SCROLL (if AUTO XON/XOFF is on) or by holding down CfRLand pressing
S, the receive buffer in the VT100 fiUs to the 32-character mark. Then the VT100 sends XOFF to the
graphics processor. When the 16-character transmit buffer in the graphics processor fills, it sends
XOFF to the dispatcher; then the 128-character receive buffer fills to its 48-character mark. At that
point (80 characters later) the graphics processor sends XOFF to the computer.

- -......,I
V Tl00
T EXT
T ERMINALI

'GRAPHICS
IPROCESSOR

I I
64 CHAR
RECEIVE
BUFFER

16 CHAR

~ TRANSMIT
BUFFER

MONO
VIDEO
OUTPUT

----- ~

BIT MAP
MEMORY

f f f ...,
COLOR
VIDEO
OUTPUT

GRAPHICS
PROTOCOLS

128 CHAR
RECEIVE
BUFFER

I I
7-9CHAR
TRANSMIT
BUFFER

128 CHAR
RECEIVE
BUFFER

---

)

I I

....J

L ----

DISPATCHER
--------~
XON/XOFF CONTROL

-------

f
128 CHAR
RECEIVE
BUFFER

I
I
~

I
--""

ANSI PARSER

16 CHAR
TRANSMIT
BUFFER

...

COMPUTER

I
I

___ ...1

EXTERNAL
GRAPHICS
PRINTER
MA-947S

Figure 6-78

VTI25 as a Text-Only Terminal

6-111

Figure 6-79 shows the VT125 operating as a graphics terminal. A device control string is being sent to
the terminal from the computer. The terminal may be in any protocol: ReGIS, VT105, or DECwriter
graphics. The keyboard can communicate with the computer, but any screen response to keyboard commands is under the control of the computer's programming.

-noo- -.,I
V

T EXT

T ERMINAq

I GRAPHiCS

------ f f f -,

I PROCESSOR MONO
VIDEO

.I I
64 CHAR
RECEIVE
SUFFER

OUTPUT

GRAPHICS
PROTOCOLS

SUFFER

....

12SCHAR
RECEIVE
SUFFER

'----- .....

DISPATCHER
12SCHAR
RECEIVE
SUFFER

-"'

L ----

16 CHAR
TRANSMIT
SUFFER

XON/XOFF CONTROL

--------ANSI PARSER

16 CHAR
TRANSMIT
SUFFER

12SCHAR
RECEIVE
SUFFER

- t

I
I
I
~

I
I I
--I

COLOR
VIDEO
OUTPUT

16 CHAR

i-- TRANSMIT

I I
7-9CHAR
TRANSMIT
SUFFER

BIT MAP
MEMORY

...

COMPUTER

I
I
___ ...I

EXTERNAL
GRAPHICS
PRINTER

MA 9471

Figure 6-79

VT 125 in a Graphics Terminal

6-112

).)

Figure 6-80 shows the VT125 operating as a graphics terminal. A device control string is being sent to
the terminal from the computer. The graphics protocol commands are being displayed on the screen at
the same time. This is a feature of VT125 ReGIS and is not available with the other protocols. The
keyboard can communicate with the computer, but any screen response to keyboard commands is under
the control of the computer's program.

V Tl00

'GRAPHiCS'
PROCESSOR

I I
I

T EXT
T ERMINALI

I I
64 CHAR
RECEIVE
BUFFER

MONO
VIDEO
OUTPUT

)

BIT MAP
MEMORY
j~

f f f ..,
COLOR
VIDEO
OUTPUT

~
16 CHAR
TRANSMIT
BUFFER

GRAPHICS
PROTOCOLS

......

.
•

_ _DISPATCHER
_ ...1 _ _ _ _
128CHAR

XON/XOFF CONTROL
------ANSI PARSER

r-. RECEIVE
BUFFER
I

I I

- - ....J L - - - -

14-

16 CHAR
TRANSMIT
BUFFER

128CHAR
RECEIVE
BUFFER

I
I

...

12BCHAR
RECEIVE
BUFFER

I I
7-9 CHAR
TRANSMIT
BUFFER

----- -

COMPUTER

....

16 CHAR
TRANSMIT
BUFFER

I
I
_ _ _ ...J

EXTERNAL
GRAPHICS
PRINTER
MA·9474

)

Figure 6-80

VTl25 in ReGIS Graphics with Commands on Screen

6-113

Figure 6-81 shows the VT125 printing from the screen to the optional graphics printer in DECwriter
graphics protocol. This is the result of the screen hardcopy command in ReGIS. The keyboard can
communicate with the computer,but the computer cannot communic.ate with the dispatcher until the
print operation is complete. However, if a graphics off command (string terminator: ESC \) immediately follows the hardcopy command, the computer can communicate with the dispatcher during the
print operation.

Figure 6-82 shows the VT125 printing from the computer to the optional printer. This is the result of
the ANSI media copy command from the computer. The media copy command can turn the VT100
and auxiliary (printer) ports on and off. Therefore, the screen can display the data going to the printer
if desired. To print a stored DECwriter graphics protocol file, display it on the screen and use the
ReG IS hardcopy command to print it.

- --,

V TlOO
T EXT
T ERMINAL I

'GRAPHics
PROCESSOR

I I
I
I I

64 CHAR
RECEIVE
BUFFER

- - - - - - f f f ...,
MONO
VIDEO
OUTPUT

16 CHAR
TRANSMIT
BUFFER

GRAPHICS
PROTOCOLS

I I
7-9 CHAR
TRANSMIT
BUFFER

....
I
I I

_-J

COLOR
VIDEO
OUTPUT

BIT MAP
MEMORY

, t
,- t

128CHAR
RECEIVE
BUFFER

DISPA CHER

16 CHAR
TRANSMIT
BUFFER'

IXON/XOFF CONTROL

L ----

16CHAR
TRANSMIT
BUFFER

128CHAR
RECEIVE
BUFFER

....

Ie.... -'--1------1 ANSI PARSER

128CHAR
RECEIVE
BUFFER

I
I
I

COMPUTER

r-I
I

___ ...J

EXTERNAL
GRAPHICS
PRINTER

MA·g.76

Figure 6-81

VT125 Printing from Screen

\)
J
6-114

V T100
T EXT
T ERMiNAL

64 CHAR
RECEIVE
BUFFER

IGRAPHiCs'
PROCESSOR

I I
I I
I I

-..----- f f f -,
MONO
VIDEO
OUTPUT

16CHAR
TRANSMIT
BUFFER

I I
7-9 CHAR
TRANSMIT
BUFFER

....

128 CHAR
RECEIVE
BUFFER

)

I I

-1

L ----

BIT MAP
MEMORY

GRAPHICS
PROTOCOLS

DISPATCHER

;X0N/XOFF CC;N-:;:-R~L

...

COLOR
VIDEO
OUTPUT

16 CHAR
TRANSMIT
BUFFER

128 CHAR
RECEIVE
BUFFER

EXTERNAL
GRAPHICS
PRINTER

I
I
~

ANSI PARSER

16 CHAR
TRANSMIT
BUFFER

128 CHAR
RECEIVE
BUFFER

...

COMPUTER

I
I

___ .J

t
MA·9473

Figure 6-82

)

VTI25 Printing from Computer

6.S VT125 TESTING AND TROUBLESHOOTING
This section explains the VTl25 testing and troubleshooting procedures.
6.S.1 Preparing for Self-Tests
Use this procedure to prepare the terminal for all self-tests except a single power-up test.

)

Turn the power switch off.

Disconnect the communication cables from the computer and auxiliary data ports. The computer port communication cable may be either EIA or 20 rnA.

If a cable is connected to the EIA computer port connector, install an ElA loopback connector on the EIA connector. See Figure 6-83. The EIA loopback connector part number is
12-15336. One loopback connector is shipped with the terminal or upgrade kit.

If a cable is connected to the 20 rnA connector, the 20 rnA current loop adapter option is
installed. Install the loopback connector (PN 70-15503-00) included with the option. Do not
use the EIA loopback connector at the same time as the current loop connector.

To perform the auxiliary port tests, install an EIA loopback connector on the auxiliary port
connector. If the communication port is EIA, you need a second EIA loopback connector.

Turn the power switch on.

Make sure the terminal is ON LINE, with the ANSI/VT52 SET-UP B feature set to ANSI
(SET-UP B switch 2-3 = 1).

6-115

)

))

))
TOP CONNECTOR
USED ON VT125
AND VTIXX·AC

oo}

VT125
VT132

BOTTOM
CONNECTOR

CONNECTORS
FROM PIN

TOPIN

*DATA LEADS ONLY
NOTE: ACCESS COVER
SHOWN IS VT125
MA·7868A

Figure 6-83

EIA Loopback Connector
-

)

6-116

)

6.8.2 Error Reporting
This section explains how the VT125 reports VT100 and VT125 errors.
6.8.2.1 VT100 Errors - VT100 errors are reported in the usual VT100 manner, in the LEOs or as a
character in the upper-left corner of the display.
6.8.2.2 VT125 Errors - The VT125 reports its self-test results with the graphics display. It reports
fatal errors by flashing the monochrome display according to a coded intensity pattern (Table 6-5). Use
an oscilloscope to measure the monitor outputs.
On ROM or RAM errors, the color display provides additional information in the form of an encoded
color output. Table 6-6 shows the color combinations for each of the possible values from 0 to 7.
You do not have to terminate the video outputs for troubleshooting. However, unless 75-ohm loads are
attached, the output voltages are higher than specified. Table 6-7 shows the voltages for both the monochrome composite video output and the RGB outputs when they are not terminated.

)

The nonfatal errors (terminal still functions partially) try to display an error code as text. This requires
a large amount of working code and hardware, and thus may be somewhat meaningless. If, however,·
enough of the bardware is working, one of the following messages will appear in the center of the display.
VTI25 xx Error
VT125 OK
VT125 Offline
xx is an error code representing one of the following error(s).
BM - Video bitmap RAM error
VG - Vector generator error
IC - Internal (VT125-to-VTI00) communications error
SC - Serial (auxiliary) port communications error
EC - External (host) communications error

)

The terminal clears this status message when the next character sent from the host arrives. The VT125
is operational after self-test jf no fatal errors occurred. If a nonfatal error occurred, then the VT125 will
work as well as possible considering the failure.

Table 6-5

VT125 Fatal Error Indication

Error

Monochrome Display
Alternates Between

1
2

oand 1
o and 2

3
4

Oand 3
1 and 2

I and 3

Color Display
Shows
ROM address
B14-12
RAM bit number

)
6-117

Explanation
CPU register error
ROM checksum error
Program RAM error
Vertical Sync
timeout
Vector timeout

Table 6-6

ROM and RAM Failure Indications

)

Number

Color

ROM

RAM

0
1
2

0
0
I
1
2
2
3
3

0
2
0
2
1
3
1
3

0
1
2
3
0
1
2
3

Dark
Pink
Blue
Violet
Orange
Light brown
Pale blue
White

E22
E22
E23
E23
E24
E24
E25*
E25*

E4
E3
E2
EI
E8
E7
E6
E5

3
4
5
6
7

*E25 is not used in the VT125.

1
J

Table 6-7
Numbered
Level

Unterminated
Mono
RGB

3
2
1

2.0
1.7
1.3
0.6

Error Output Signal Levels

2.0
1.3
0.5

)

Note: Mono composite sync tip is at 0 volts.

'}
I

6-118

CHAPTER 7
STANDARD TERMINAL PORT

7.1 INTRODUCTION
The standard terminal port (STP) is intended to be a standard interface for terminal options. These options may include communications, graphics, a terminal processor, and mass storage devices.

)

The STP was originally developed as part of the VT100. Attempts have been made to ensure the generality of the STP, but some compromises toward the VT100 may have been made. Special VT100 considerations are noted in this chapter where applicable.
7.2 DEFINITIONS
The following definitions apply in this discussion:

)

Active device - A device that either: 1) wishes to take over control of the host communication line; 2) requires all received and transmitted data passed between the host communication line and the terminal
controller to be looped through the device; or 3) needs to communicate with the terminal controller
directly. One example of an active device is a terminal processor doing local editing, standalone computing, etc.
Host - That which connects to the serial line connector on the outside of the terminal. Normally, this is a
computer system (via direct line or otherwise), but might in some cases be another terminal or other device.
Host link - The serial line between the option and the host. The option's line UART is on one end of this
link, and the host is on the other.

)

Local link - The serial line between the terminal controller and option when an option is present. The terminal controller's UART is on one end of this link, and the option's,Iocal UART is on the other.
Passive device - Those devices that do not need the capabilities of an active device. A passive device normally only wants to listen to the data stream from the host and to pass data back to the host, but not to
interfere with normal terminal or communications line operation. Terminal mass storage, such as a tape
cartridge, would fall into this class.
Receive only device - A special case of a passive device that never needs to transmit data to the host, but
only to listen to data from the host. Certain types of graphics options fall in this category.
Terminal controller (TC) -The processor in the basic terminal that controls normal terminal operation.
This term is used to distinguish it from any other processor that might be packaged into a terminal.
Terminal processor (TP) - An additional processor, not necessary for basic terminal operation, but to add
other functions, such as local editing or computing.
SET-UP mode - The terminal mode that allows user to set terminal characteristics.

7-1

7.3 OVERVIEW
The STP provides a means of interfacing terminal options. Logically, this interface is similar to that of a
serial line splice (Figure 7-1) which is accomplished by breaking the serial line from terminal to host and
inserting the option. An internal interface connector is provided for reasons of packaging aesthetics, and
additional signals are present to reduce the interface cost. Part of the terminal controller firmware is also
dedicated to support the STP, since some information needed by the option is passed as data across the
serial line, rather than assigning additional pins on the interface connector. This firmware also allows the
option to use the terminal characteristics specified by the user in SET-UP Mode. The STP provides a reasonably cost-effective interface for a large class of terminal options, and is general enough to allow for
decoupled evolution of terminalS and terminal options.

TERMINAL

I_ ·1

DEVICE

I_ ·1

HOST

1
MA-4288

))
Figure 7-1A Serial Line Splice Connection

TERMINAL
TC
CONTROLLER UART
(TC)
(+EIA)

EIA
CONN

STP
CONN

HOST
MA-4289

Figure 7-1B

STP with No Option Present

TERMINAL
TC
CONTROLLER UART
(TC)
(+EIA)

EIA
CONN

EIA
CONDX

LOCAL
UART

LINE
UART

r-"

HOST

... --,

OPTION
MA-4290

7-1C

STP with Option Present

7-2

7.4 FUNCTIONAL SPECIFICATION
7.4.1 Interface Signal Lines
The signal lines that make up the STP are shown in Figure 7-2. The lines are accessed through a standard
20-pair shorting connector, described in Paragraph 7.6. In general, the signals are standard EIA serial
line and modem controls, though some special signals are also present. See Paragraph 7.5 for complete
electrical specifications.

SHORTING
PINS

TO TERMINAL
CONTROLLER
TC RXD _
TC TXD
TC TCLK
INTERNAL TCLK _
TC RCLK
INTERNAL RCLK _
LOCAL CLK
OPTION PRES _

-,-,,-

TO EIA LINE
CONNECTOR
LINE RXD
- LINE TXD

-,I_

SIGNAL GROUND

-I

TC INIT

)

TC RTS
TC DTR
TC SRTS/SPDS

_
_

TC DSR
TC CTS
TC CD
TC SCD/SPDI _
TC RI

)

+5 V
+12 V
-12 V

-,
-I

,-tj4

I_
tI,_

-,.

_ LINE RTS
_ LINE DTR
- LINE SRTS/SPDS
LINE DSR
LINE CTS
LINE CD
LINE SCD/SPDI
LINE RI
EXT TCLK
EXT RCLK

-I
MA.4287

Figure 7-2 STP Signal Lines

Each pair of signals is connected together when no option is present. When an option is plugged in, each
connection is broken, though the option may choose to short through any signal it does not wish to control.
Note that some signals are used between the terminal controller and option only or between option and
external connector only, and so are defined on only one side of the connector. The signals and their meanings are as folllows.

"
7-3

Signal
TCRXD

Type
EIA

Meaning
TC received data. Serial,
asynchronous data is passed from
host or option to the TC on this
line.

LINERXD

EIA

Line received data. Serial data is
passed from host to option or TC
on this line.

TCTXD

. EIA

TC transmitted data. Serial data
is passed from TC to host or
option on this line.

LINETXD

EIA

Line transmitted data. Serial data
is passed to host from option or
TC on this line.

TCTCLK

TTL

TC transmitter clock. This signal
from the TC bit rate generator
normally drives the UART that
talks to the host. Its frequency
is 16 times the bit rate.

INTERNAL TCLK

TTL

Internal transmitter clock. This
signal drives the UART associated
with the TC. When no option is
installed this signal is driven by
TC TCLK. With an option installed
this signa1.would normally be
driven by either TC TCLK or LOCAL
CLK. (Refer to Paragraph 7.4.2.2
for further information.) The baud
rate applied to this line may not
be more than 19,200 for a VT100.

TCRCLK

TTL

TC receiver clock. Just like TC
TCLK.

INTERNAL RCLK

TTL

Internal receiver clock. Just like
INTERNAL TCLK (19,200 max. for
VT100).

LOCALCLK

TTL

Local bit rate clock. This is a
clock signal provided by the TC
which is 16 X, a "convenient" data
rate for the local (TC to option)
link. Normally, this bit rate
would be near the highest rate
that the TC can handle
efficiently. It need not be a
standard bit rate, so this' clock
can be sourced'by some timing

7-4

)

'~
/

)

Signal

Type

Meaning
signal that happens to be
available in the terminal. The
equivalent X 16 baud rate supplied
by the VT100 is 15,734 baud
(251.744 kHz).

OPTION PRESENT

TTL

Option present. A low on this line
means that no option is present,
and normal terminal operation is
in order. It must be passively
pulled up on the TC board. A high
on this line indicates the
presence of an option and informs
the TC to make appropriate changes
in its operating modes.

TC INIT

TTL

Initialize. This line notifies the
option that a power-up clear or
terminal reset has occurred. (See
electrical specifications in
Paragraph 7.5.)

TCRTS
TCDTR
TC SRTSjSPDS

EIA

Terminal controller modem control
signals. These signals are the
standard EIA modem control
signals: request to send, data
terminal ready, and secondary
request to send (or speed select).
The option may ignore them, use
them itself, or pass them through
to the corresponding "LINE"
signals.

LINERTS
LINEDTR
LINE SRTSjSPDS

EIA

Line modem control signals. EIA
modem control signals that go to
the host line may be fed from
option or from the corresponding
"TC" signals. In the VT100, LINE
SRTSjSPDS is strapped to pins 11,
19, and 23 on the RS-232-C
connector mounted on the terminal.

TCDSR
TCCTS
TCCD
TCSCDjSPDI
TCRI

EIA

TC modem status signals. These
lines are the standard EIA status
signals: data set ready, clear to
send, carrier detect, secondary
carrier detect (or speed
indicator), and ring indicator.
The option may source these
signals to the TC, or pass through
the corresponding "LINE" signals.

7-5

Signal
LINEDSR
LINECTS
LINE CD
LINESCD/SPDI
LINERI

Type
EIA

Meaning
Line modem status signals. The EIA
modem status signals received from
the host line connector. May be
used by the option, or passed
through to the corresponding "TC"
signals. In the VT100, LINE
SCD /SPDI is connected to Pin 12 of
the RS-232-C connector mounted on
the terminal. This pin is used for
different functions by different
modems.

EXT TCLK

EIA

External transmit clock. This
signal is provided by a modem or
other external device. This pin is
connected to pin 15 on the
RS-232-C connector mounted on the
terminal. It is intended to be
used by an option that needs this
signal.

EXT RCLK

EIA

External receive clock. This
signal is provided by a modem or
other external device. This pin is
connected to pin 17 on the
terminal's RS-232-C connector.

+5 Y, + 12 V,
-12V

Power

Interface power. (See Paragraph
7.5.2 for further specification.s.)

SIGNAL GROUND

Ground

Signal ground.

7.4.2 Protocol Specification
Proper operation with the STP requires some firmware support in the terminal controller. This section
specifies the protocol used to communicate between option and terminal controller.

))

7.4.2.1 Terminal Operation with No Option Present- In standalone terminal operation, all signals on
the STP are passed througb by the shorting connector. The terminal controller senses the absence of an
option and performs its normal function. In SET-UP mode, when the user makes changes to terminal
characteristics, these changes take effect in the normal manner. Any escape sequences from the host destined for a nonexistent device are ignored by the terminal controller.
7.4.2.2 Standard Set- Up for Local Link - When an option is present and breaks the serial line as it
passes through the STP connector, the option and terminal controller must agree on the line characteristics for the local link. To this end, the assertion of OPTION PRESENT causes the following changes:

_.. _.

'." ,-., ~,n;,u

Set to 8 bits per :.;h.1i a",(t,l

))

Sea LU 1 ;)LUl' UlL

7-6

Parity

Set to no parity

XOFF

Line synchronization (XON /XOFF) must be supported in both directions on the
local link. Note that the TCmust accept XOFF as well as source it.

Normally, it is assumed that the option will use the TC TCLK and TC RCLK signals to drive its line
UART, while using LOCAL CLK to drive INTERNAL TCLK and INTERNAL RCLK. The option
may, however, choose to source another set of clocks to drive the TC UART instead. INTERNAL
CLOCKS should not be driven at a frequency higher than 19,200 baud for the VT100. It is recommended that INTERNAL TCLK and INTERNAL RCLK be set to the same rate to avoid XON/XOFF timing problems.
7.4.2.3 Control Sequences for Terminal Parameters - Only two control sequences are specified for STP
operation. The first of these reports terminal parameters of interest to the option, while the latter is used
to request this information. The formats of these sequences are:
DECREPTPARM - Report Terminal Parameters
ESC [ <sol>;<par>;<nbits>;
<xspeed>;<rspeed>;<clkmul>;
<flags> x
DECREQTPARM - Request Terminal Parameters
ESC [<sol> x

)

The final character for both strings has been chosen as lowercase x (octal code 98, column/row specifier
7/8). This is a DEC Private sequence.
DECREPTPARM is sent by the terminal controllerto notify the option (or host) of the status of selected
terminal parameters. It may be sent when requested by the host or option, or at the terminal's discretion.
DECREPTPARM will be sent upon receipt of a DECREQTPARM, or voluntarily upon leaving SETUP mode when the terminal sees option present asserted and it has been asked to report unsolicited by a
previous host request.
The option is responsible for sensing a terminal power-up or reset, which it may note by the assertion of
TC INIT by the terminal.
The meanings of the parameters are as follows:
(* indicates the assumed value if the parameter is not specified.)

Parameter
<sol>

Value
Oornone

2
3

Meaning
This message is a request (DECREQTPARM) and the terminal
will be allowed to send unsolicited reports.
This message is a request, and henceforth, the terminal may only
report in response to a request.
This message is a report (DECREPTPARM) and the terminal
thinks it may report at will, although this particular report may
not be voluntary.
This message is a report, and the terminal is only reporting on
request.

7-7

Parameter
<par>

Value
1*

Meaning
No parity set
Parity is always space (not a VT100 state)
Parity is always mark (not a VT100 state)
Parity is set and odd
Parity is set and even

1*
2

8 bits per character
7 bits per character

2
3
4

<nbits>

Because some terminals have limitations in
handling numeric values, baud rates cannot be
sent as decimal strings (e.g., 2400). This
list of baud rates is basically encoded on
multiples of 8 so further speeds can be
specified later and left in baud rate order.

Bit
Per
Sec

Rate
50
75
110
134.5
150
200
300
600
1200
1800
2000
2400
3600
4800
7200
9600
19200

8
16
24
32
40
48
56
64
72
80
88
96
104
108
112*
120

(nota VT100 speed)

1*
2

The bit rate multiplier is 16
The bit rate multiplier is 64

<flags>

0-15

This value communicates 4 bits of user defined information as a
decimally encoded binary number. These bits may be assigned in
an STP device dependent fashion. Bit weights are 842 1 from left
to right. The default value for these four bits is O. In the VT100,
these bits are switch group 5 of SET-UP B.

If any parameter (except for <sol» is zero or not present, it should be interpreted as the default value
for this parameter. The default for each parameter is marked with an asterisk (*) in the above list. Any
parameter out of its defined range must be treated as an error. Additional parameters (beyond these seven) must be ignored without affecting the interpretation of the other parameters. This last restriction allows for future standardization of the additional parameters.

7-8

))

-1 )

)

7.4.2.4 Initialization - The option must sense the signal TC IN IT from t~e terminal controller and
reset itself to initial state upon reception of this signal. Timing for this signal is specified in Paragraph
7.5.1.2. This signal may be used to reboot a terminal processor if desired. It will be generated at
terminal power-up, and in response to a "reset to initial state" or confidence test request from the host
or operator.
7.4.2.5 BREAK - The BREAK signal will be transmitted over both the local and host links in the
normal manner. If an option which passes characters from the terminal controller to the host without
modification, the option must detect the break on the local link and generate a corresponding break on
the host link.

If a terminal processor treats the terminal controller as a terminal, BREAK from the terminal CO,ntroller may be used for a standard purpose such as causing entry into ASCII console microcode. This
may require the option to use a UART with an external "framing error" line, or to design in a
hardware break detector (counter and gate).
)

7.5

ELECTRICAL SPECIFICATIONS

7.5.1 Signal Lines
Specifications. for the signal (nonpower) lines are a follows.
7.5.1.1 Signal Levels - Two classes of signals are present on the STP connector. The first class are
standard EIA levels, and include transmitted and received data, modem control signals, etc. The second class are TTL signals used to control interface operation, and include bit rate clocks, OPTION
PRESENT, etc.
)

For VT100:
All EIA outputs
Mark State -6.0 to -12.0 V
Space State +6.0 to 12.0 V
All EIA inputs
Mark State +0.75 to -25.0 V, -8.3 rnA max.
Space State +2.25 to +25.0 V, +8.3 rnA max.

)

All TTL inputs
VIH = 2.0 V max.
IIH = 20 JLA max. @ VI = 2.7 V
VIL = 0.8 V max.
IlL = 0.5 rnA @ VI = 0.4 V
All TTL outputs
VOH = 2.7 V @ IOH = 0.1 rnA
VOL = 0.5 V@IOL = 3.2mA
7.5.1.2 Signal Timing - The INIT H signal goes high when the VT100 starts an initialize sequence and
goes low when the VT100 is ready to operate. The minimum high time is 100 ms.
LOCAL CLK has the following period for the VT100 as shown in Figure 7-3.

7-9

I WHERE T = 1.86928 jJsec
MA-4298

Figure 7-3 Local Clock
7.5.2 Power Supply Lines
Voltage regulation specifications and base product power requirements for the VT100 may be found in
Chapter 4. All options drawing power through the STP or 20 rnA connectors on the VT100 terminal
controller board may draw no more than O.S A from anyone voltage. Options drawing power from
any connector in the VT100 must be sure that their requirements, the needs of other options, and the
needs of the base VT100 are consistent with the capability of the VT100 power supply.
7.5.3

Connector Pinout

)).
,

TCTXD
TCRXD
TCRTS
TCCTS
TCDSR
TCDTR
TCCD
TCRI
TCSPDS
TC SPDI
-12 V
TCTCLK
TCRCLK
+12 V
OPTION PRESENT
'LOCALCLK
TCINIT
+5V
7.6

2
-4
6
8
-10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40

I LINETXD
3 EXT TCLK
SLINERXD
7 LINERTS
9 EXT RCLK
II LINECTS
13 LINE DSR
IS LINE DTR
17 LINE CD
19 LINE RI
21 LINESPDS
23 LINESPDI
25-27 INTERNAL TCLK
29 INTERNAL RCLK
31-33 SIGNAL GROUND
35-37-39--

MECHANICAL SPECIFICATIONS

7.6.1 Shorting Connector
The STP connector, DEC part number 12-14829 consists of 20 pairs of shorting bifurcated contacts,
spaced on 0.125 inch centers, with 0.4 inch penetration (Figure 7-4).
7.6.2 STP Connector Card
The card that plugs into the STP connector may serve as just a paddle card with connection to a
separate module, or may actually contain circuitry such as a minimum terminal processor. The terminal provides room for a card at least as large as and with finger and connector placement similar to
those in Figure 7-5. STP interfaces to be dedicated to a particular terminal family may depart from this
specification, but at penalty of future generality.

7-10

)

MA-4286

Figure 7-4 STP Connector
7.7 GUIDELINES FOR THE DESIGNER
This section is not, strictly speaking, part of the STP specification. It is meant to provide some indication of how the STP may be used to implement some different terminal configurations. It is not
intended to be an exhaustive study of all possible options, nor to dictate particular design decisions. It
is hoped, however, that it will be useful to the architect and option designer, and provide some insight
into the choices made in specifying the STP.

)

7.7.1 Use with Receive Only Device
There are two basic ways to implement a receive only device on the STP. One method requires only
one UART, the other two. The first method is more limited, however, while the second is more
flexible.
7.7.1.1 Single UART Method - In this case, the option shorts through all STP signals and does not
assert OPTION PRESENT. It picks up the receive bit rate clock from the STP signal lines and uses
only the receive side of its U AR T to tap off the TC RXD JLINE RXD line. It also watches for TC
INIT and may, if desired, monitor the state of the modem control and status lines.
The limitations of this method are:
1.

The option may not send XOFF to the host to control the received data rate.

The option gets no information or parity status or sense from the terminal controller. It may
try to deduce lhi~ information from the data it sees, or specify that it may only be used in
certain modes, however.

7-11

The option gets no information on number of bits/char or bit rate multiplier from the
terminal controller. Similar to parity problem.

The advantage of this method is its low cost for implementing a receive only device.

10999 mm

-.":r.:====:::I.._ 2.03 mm

.....-I.----S-.5-1-m-m--_-_(_4_.3_2_5_in_.)_-_-_-_-_-_-_-_
.....- - - (3.350 in.)

(.SOO in.)

I
:

J
2.94 mm
(1.156 in.)

I
I

l------------------l-----U
I

HOLE LOCATIONS ON
TERMINAL CONTROLLER BOARD

105.66 mm
(4.68 in.)
COMPONENT SIDE

))
--..j I--- .127 mm

(.05 in.)

CTi~;3~~.)-...-~;~~~7.:.I=:j------r-f

.76 mm
(.300 in.)

693 mm
(2.730 Ill.)

/
p-

T1T1TT1T1T1T I TTTTTI!

mR/ I I I I I I I I I I I I I I I I I I I I
I

15 m
(.06 in.)
MIN

L .127mm
.05

I---- .31S~m
(.125

(

m.)

in.)

KEYSLOT

TYP
19 PLACES

l·20mm
(.080 in.) FINGERS
20 PLACES
MA-4296

Figure 7-5 STP Option Card
-~
/

7-12

7.7.1.2 Two UART Method - In this case, the option breaks the serial data and clock lines on the
STP, while passing through the modem control and status signals without change. It asserts OPTION
PRESENT, normally ties LOCAL CLK to INTERNAL TCLK and INTERNAL RCLK as well as to
its own local UART, and uses TC TCLK and TC RCLK to drive its own line UART.
In this mode, the option is responsible for implementing XON jXOFF on the host link, if that feature
is enabled. It may also XOFF the terminal controller if the data is coming too fast, since the data rates
on the local and host links may not be the same. It must, of course, also respond to an XOFF from the
terminal controller.
7.7.2 Use with Passive Device
Unlike the receive only device, the passive device option wishes to transmit data on the host link, and
must therefore break the serial line as it crosses the STP connector.
The passive device connects to the STP in the same way as the two UART receive only device. It
asserts OPTION PRESENT and drives INTERNAL TCLK and INTERNAL RCLK (with either
LOCAL CLK or a clock generated by the option itself).
In this mode of operation, the option must parse all valid ANSI sequences it passes from the terminal
controller to the host. First, it must recognize DECREPTPARM in order to get the current set of
terminal characteristics. Second, it must parse all other ANSI sequences so that it never sends data to
the host in the middle of a sequence generated by the terminal controller. Sequences that must be
handled correctly are specified in ANSI X3.64-1977, and include: escape sequences, control sequences,
control strings, and single shifts.

)

Since a passive device does not need to talk to the terminal controller, it need not parse ANSI sequences from the host before passing them on to the terminal controller. Note that the terminal
controller must accept an XOFF in the middle of a (escape or control) sequence without error. The
option need only recognize those sequences which belong to it. As long as these are valid ANSI
sequences they will be ignored by the terminal controller.
Because a single terminal may have more than one option, it is required by this specification that any
passive device must replicate the standard STP shorting connector. This shorting connector should
break the host link on the option before that link returns to the STP connector of the terminal controller (or option) that the device plugs into. (For more detail on these requirements, see Paragraph
7.7.6).

)
7.7.3 Use with Active Device
There are at least two types of active devices: (I) the terminal processor acting as the CPU in a
standalone system packaged in a terminal box, and (2) the terminal processor acting to enhance the
terminal functionality, to provide local editing, format and value checking, etc. In the former case, the
terminal processor (the active device) views the local link and host link sides of the STP as two separate
entities and may, in fact, not even make use of the host link. In the latter case, by contrast, the terminal
processor (again the active device) is seen only as augmenting the basic terminal operation, and (in
some abstract sense) passes through all the data it handles between host and terminal controller.
7.7.3.1 Terminal Processor as Standalone CPU - In this configuration, the terminal processor treats
the local and host links of the STP as two separate devices. It asserts OPTION PRESENT to set the
local link to a standard state. It retains control of the host link, though it may listen to DECREPTPARM sequences sent by the terminal controller if this is appropriate to the application. It may use
or ignore TC RCLK and TC TCLK as it wishes. If the TP needs "boot" and "console entry" signals,
these needs may be met by INIT and local BREAK, respectively.

7-13

7.7.3.2 Terminal Processor Augmenting Basic Terminal Operation - -In this case, the terminal processor is used to add functionality to the terminal. Logically, communication is between the host and the
"terminal" (terminal controller plus terminal processor), rather than between a CPU and two devices
as in the standalone CPU case. The terminal processor may intercept the normal data stream to and
from the terminal controller, rather than just listening for (and interjecting) special control sequences,
as in the passive device case. One example of such an option is a printer graphics option, which would
receive graphic instructions, convert them into a bit map representation, and transmit that bit map
serially to the printer terminal controller. Another example is that of a multiplexer which might make a
CRT terminal into three logical terminals by maintaining cursors in each of three screen areas, routing
keyboard input to the proper host task, etc.

))

7.7.4 Use with Communications Option
This case is similar to the active device configurations discussed in Paragraph 7.7.3. In addition, the
option may use the EXT TCLK and EXT RCLK signals. Communication options would not normally
need to replicate the STP connector, since there would be no further options between the terminal and
the communications line.

7.7.5

Use with an External Processor
As long as control sequences used by the terminal controller to pass information to the terminal
processor are present in the terminal controller firmware, there is no reason to prevent an external
processor (host) from using them. This applies when the host CPU wishes to act as an enhancement to
the terminal, (Paragraph 7,7.3.2), but does not fit into the terminal cabinet. Such a capability is also
useful in writing and debugging programs that eventually rUn in a terminal resident, terminal processor. To allow this operation, a small interface card plugged into the STP does the following:
1.

Asserts OPTION PRESENT.

Provides a standard bit rate (known to the host) to INTERNAL RCLK and INTERNAL
TCLK. TC RCLK and TC TCLK would not be used directly, though DECREPTPARM
would advise the host of this information.

Passes through RXD, TXD, and all modem control signals. If the host needed to sense and
drive all modem signals, a special host interface might be required.

In this way, the host CPU can do a fairly good job of emulating the terminal processor and may
replace it in some applications.
7.7.6 Use with More than One Option
While it is believed that mUltiple options can be supported on the STP, no detailed specifications for
.such operation are given at this time. All options which plug into the STP must, however, replicate the
STP connector on the "host" side of their interfaces, to allow for the addition of other options. The
only exceptions to this rule are those devices which must, by their very nature, be the "last" device in
the terminal. Examples of such devices are a current loop adapter or modem.
.

7-14

))

}

CHAPTER 8
GRAPHICS CONNECTOR

8.1

INTRODUCTION
An 18 pin dual-in-line connector is provided in the VT100 for internal options that generate digital video
signals, and may additionally require 8-bit ASCII data forwarded by the VT100 from the communications line as specified below. An option not requiring all 18 signals may use a shorter connector
plugged into the appropriate set of holes on the graphics connector.

All signals below are identified as "to the VT100" (T) or "from the VT100" (F).
07 (F) 1
06(F) 2
05 (F) 3
04(F) 4
03 (F) 5
02(F) 6
01 (F) 7
00 (F) 8
SignalGnd 9

)

Pin Numbers
18 Signal Ground
17 (T) Graphic Video 1 L
16 (F) Vertical Blank L
15 (F) Graphic Write L
14 (F) Horizontal Blank H
13 (F) INITH
12 (T) Graphic Video 2 L
11 (T) Graphic Flag L
10 (F) 24.0734 MHz Clock

Signal
07-00

Description
Ouring the VT100 "Graphic Processor On" mode (see Appendix A for
entry and exit), all characters transmitted to the VTI 00 on the
communications line are transferred to these data lines with a graphic
write strobe. The only characters not transmitted are the "Graphic
Processor Off' escape sequence. (Maximum load = two 74XX loads
plus 50 pF.)

24.0734 MHz

VT100 Master Video Clock. (Maximum load = one 74SXX load.)

)

)
8-1

Signal
Graphic Flag L

Description
On initialization (between the time INIT H goes low and the first
character is transmitted to the graphics connector in "Graphic
Processor On" mode) the GRAPHIC FLAG will be checked. If the
flag is low, the VT100 assumes the presence of an option on the
graphics connector and enters "Graphic Processor On" mode normally
upon receipt of the appropriate control. If the flag is high, the VT 100
assumes that no option is installed and refuses to enter "Graphic
Processor On" mode. After initialization a low on the GRAPHIC
FLAG indicates that the option is prepared to receive characters from
the VT100. A high GRAPHIC FLAG indicates the option is busy, the
VT100 stores incoming characters in its buffer and responds to the
host in its normal fashion if its buffer becomes full (see Appendix A
Communications Protocol). (Input load = one 74LSXX load plus
4.7Kohmsto +5 V.)

INITH

The VT100 asserts this pin high whenever it begins an initialize
sequence (power-up or on command) and asserts INIT low whenever it
is ready to begin normal operation. The minimum high pulse width is
100 ms. (Maximum load = two 74XX loads plus 50 pF.)

Horizontal Blank H

Asserted high during VT100 blank time. Used for horizontal sync and
to indicate the start of VTl 00 display.
Pulse period = 63.556 IlS
Pulse width = 11.465 Ils/80 column mode; 12.088 IlS/ 132 column

Delay Falling Edge of HORIZONTAL BLANK H to First Dot from
VT100 = 500ns/80column mode; 250ns/132column mode
(Maximum load = one 74XX load plus 50 pF.)
Vertical Blank L

Asserted high only during displayed scans on the VT100. In all modes
the VT 100 displays 240 complete scans /field. The VERTI CAL
BLANK signal transitions between 1.2 IlS and 1.5 IlS after the rising
edge of HORIZONTAL BLANK H. (Maximum load = one 74XX
load plus 50 pF.)

Graphic Video 1 Land 2 L

A truth table for the intensity of the display for different values of
these inputs is shown below. The table applies only to areas of the
screen that are otherwise black. When graphic data overlaps other
data in the VT100, the display is as bright Or brighter than the values in
the table.
GVl
H
L
H
L

GV2
H
H
L
L

Intensity
Black
Dim
Normal
Bright

(Input load = one 74XX load plus 4.7K ohm to + 5 V.)

8-2

)

Signal
Graphic Write L

Description
This pin is pulsed low by the VTl 00 during valid data on D7-DO. Data
(D7-DO) is stable 25 ns before the falling edge of GRAPHIC WRITE
and remains stable for 25 ns after the rising edge of GRAPHIC
WRITE. The pulse width low of GRAPHIC WRITE is 350 ns ± 50
ns. (Maximum load = two 74XX loads plus 50 pF.)

8.2 HARDCOPY ENABLE
Provision has been made in the VT100 for the future addition of an external hardcopy option to record the
screen. Such a device is assumed to monitor the graphics connector to receive the "copy" command and
to obtain its picture information from the composite video output. Upon receipt of the escape sequence
DECHCP (see Appendix), the VT100 will stop updating the screen but will not cease blinking operations. Following the freeze, the VT100 will write an 8-bit character of all ones (FFH) to the graphics
port. The GRAPHIC FLAG should go high when this character is transmitted; and the VT100 will main'tain the screen frozen until the flag is seen to be low, at which time the VTl 00 will resume normal operation.

)

8-3

APPENDIX A
PROGRAMMING INFORMATION

This appendix summarizes the VT100 series video terminal transmitted characters and received character processing. The VT100 terminal normally performs a two-part function. It is an input device to a
computer - information entered through the keyboard is sent to the computer. It is simultaneously an
output device for the computer; that is, data coming in from the computer is displayed on the video
screen. Figure A-I shows the data flow.
This appendix discusses data flow between the VT100 and the host. Included are the codes generated
by the keyboard; the transmission protocol followed by the terminal; and the actions and reactions of
the terminal to control codes and escape sequences, in both ANSI and VT52 modes of operation.

)

The VT100 is an upward and downward software compatible terminal; that is, previous DIGITAL video
terminals have DEC private standards for escape sequences. The American Standards Institute
(ANSI) has since standardized e'scape sequences in terminals. ANSI standards allow the manufacturer
in implementing each function. This appendix describes how the VT100 responds to the implemented
ANSI control functions.

VT100

r---------,

COMPUTER
(HOST)

RECEIVER

TRANSMITTER

_______ .JI
~
COMMUNICATION
LINES

Figure A-I

Terminal Data Flow

A-I

MA·1994

The VT100 is compatible with both the previous DEC standard and ANSI standards. Customers may·
use existing DIGITAL software designed around the VT52 or new VT100 software. The VT100 has a
VT52 compatible mode in which the VT100 responds to escape sequences like a VT52. In this mode,
most of the new VT100 features cannot be used.

\
))

Throughout this discussion references are made to VT52 mode or ANSI mode. These two terms indicate the VT100's software compatibility. All new software should be designed around the VT100
ANSI mode. Future DIGITAL video terminals will not necessarily be committed to VT52 compatibility.
VT100 KEYBOARD
The VTl 00 keyboard (Figure A-2) has a key arrangement similar to an ordinary office typewriter. In
addition to the standard typewriter keys the VT100 has keys and indicators that generate escape and
control sequences, cursor control commands, and show the current terminal status.
You use the keyboard to trarismit codes to the host. Some keys transmit one or more codes to the host
immediately when typed. Other keys such as CfRL and SHIff do not transmit codes when typed. but
modify the codes transmitted by other keys. When you press a code-transmitting key. the terminal
makes a clicking sound to verify it processed the keystroke. If you press two code-transmitting keys
together, two codes are transmitted in the order typed. The terminal does not wait for the keys to be
lifted, but transmits both codes as soon as possible after the keys are typed. If you press three such keys
together. the codes for the first two keys are transmitted immediately; the code for the third is transmitted when one of the first two keys is lifted.
LED Indicators
The keyboard has seven light emitting diodes (LEOs); two are committed to the complementary ONLINE/LOCAL function. The power-on condition is implicitly shown by one of the two LEOs being on;
that is. if the keyboard is connected and power is on, one of these LEOs is on.

)
' )

\. )
)

A third LED indicates a "keyboard-locked" condition. In this condition the keyboard has been turned
off automatically by the terminal due to a full buffer, or by the host through the transmission of an
XOFF to the terminal.
You can program the four remaining LEOs and assign any meaning for specific applications. The code
sequences to turn these LEOs on or off are discussed later.
SET· UP Key
The SET· UP key is at the upper-left corner of the main key array. Operations performed in SET-UP
mode can be stored in nonvolatile memory (NVR) so that turning terminal power off does not, by itself,
alter the terminal configuration.
The procedures to change the SET·UP features are provided in the operator's information section of
this manual. SET·UP features that may be modified by the host are listed in Table A-I and described
in detail under the escape sequences:
Alphabetic and Nonalphabetic Keys
The VT 100 transmits lowercase code unless either or both SHIFf keys are down. or unless the CAPS
LOCK key is down. Pressing CAPS LOCK locks only the 26 alphabetic keys in the shifted (uppercase)
mode. Figure A-3 shows the codes generated by the alphabetic keys.
Each nonalphabetic key can generate two different codes. One code is generated if neither SHIFT key
is pressed. The other code is generated if either or both SHIff keys are down. Unlike the shift Jock key
of a typewriter, CAPS LOCK does not affect these keys; it affects only the alphabetic keys.

A-2

\)
}

""."-",,,,-..---

r:::-l

ON LINE

LOCAL KBDLOCKED L 1

lIN~

C?0I132

0
B
Itl ~ . . 0

crmnnriflm~~mr7lmm8r+lI-l~a
L=:J
SETI

CLEAA2L

SE8

TOgLE

TA2SMlT 2EIVE

LJL!.JLULU~~~L2J~~~LJL::JLJ SPACE

G000G~~0800rnrnnB~

~ B@CJ0000ffi000~CJ[J[::Jrn
~I

SHIFT

1000~000CJDwL:J~
"
r

[JBBE]

0000
0000
~000

lolDO
MA·'",

Figure A-2

VT I00 Keyboard

Table A-I

SET-UP Features and Machine States

SET-UP Feature
or Machine State

Changeable
from Host
Computer *

Saved in NVR
and Changeable
inSET-UP

Alternate keypad mode
ANSIjVT52
Auto repeat
Auto XON XOFF
Bits per character
Characters per line
Cursor
Cursor key mode
Graphics mode
Interlace
New Line
Keyclick
Margin bell
Origin mode
Parity
Parity sense
Power
Receive speed
Screen
Scroll
Tabs
Transmit speed
Wraparound

Yes(DECKPAMjDECKPNM)
Yes(DECANM)
Yes(DECARM)
No
No
Yes(DECCOLM)
No
Yes(DECCKM)
Yes(DECGON jDECGOFF)
Yes(DECINLM)
Yes(LNM)
No
No
Yes(DECOM)
No
No
No
No
Yes(DECSCNM)
Yes(DECSCLM)
Yes(HTSjTBC)
No
Yes(DECAWM)

No
Yes
Yes
Yes
Yes
Yes
Yes
No
No
Yes
Yes
Yes
Yes
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes

f#£l
l2J (shifted)

Yes

*The appropriate control or escape sequence mnemonic is indicated in parentheses.

OCTAL CODES GENERATED BY KEYBOARD
(SHIFTED CODES SHOWN ABOVE KEYCAP LEGENDS
UNSHIFTED CODES SHOWN BELOW KEYCAP LEGENDS)

0000_
~ ~~
~~ I ~3:
~5~ I' 071
°95~ I' 005111'
O L..Qill
062 '~4:"
063 064~4: /I 065
066 I' ~4:067 /I 070
060 055:~ II 075!5~ " 140~7~ 100
DI
;~; I :~; I :!: I ;~ I :~ " :~: I ;:; I :~; " ;~: I
10
~rfl~llg41r¥lryl~~rJ3l~f0721, 0421
fT7fl
0 ~UillUill
144 ~l!ill~LillJLillJUhJ~ 047
wu
IChJ
01
!7232 [!]30
170 [!]03
143 [!]26
166 Ch]02
142 [ZJ16
156 []J15
155 c:054EJ74 CiJ76
056 DZJ77
057 B §

:l: " :p

SHIFT

LINE

FEED

SE~CE

I
MA·7418C

Figure A-3

Standard Kt:y Codes

A-4

Table A-2

Function Key Codes

.\ .

)

Key

Octal Value
of Code Sent
or Received
by VT100

RETURN*
LINE FEED
BACKSPACE
TAB
SPACEBAR
ESC

015
012
010
011
040
033

DELETE

177

Action Terminal Would
Take if Host Sent That
Code
Carriage return function
Line feed
Backspace function
Tab function
Deposit a space on the screen erasing what was there before
The initial delimiter of an escape sequence - interpret the
following character string from the host as a command, rather
than displaying it
Ignored by the VT100

*You can use the new line SET-UP feature to redefine the RETURN key so that it issues 0158 0128
(carriage return - line feed).

Function Keys
Several keys on the keyboard transmit control codes. Control codes do not produce displayable characters but are codes for functions. If these codes are received by the terminal, the VT100 performs the
associated function described in Table A-2.

)

NO SCROLL - Pressing NO SCROLL generates a single XOFF code and inhibits further scrolling.
When pressed again the same key generates XON. In practice, if the software recognizes XOFF, the
host stops transmitting until you press NO SCROLL again to allow scrolling. If the XOFF jXON feature is disabled (SET-UP function), NO SCROLL causes no action.
BREAK - Pressing BREAK forces the transmission line to its space state for 0.2333 seconds ± 10 percent. I f either SHIFT key is down, the time increases to 3.5 seconds ± 10 percent.

)

Pressing SHIFT and BREAK together provide the long-break-disconnect function. Used with properly
configured modems with RS232-C levels, it disconnects both the local and remote data sets. For
modems connected via the 20 rnA loop, issuing the long space may disconnect the remote data set only.
Pressing CTRL and BREAK together transmits the answerback message.
BREAK does not function when the VT100 is in LOCAL.

A-5

CTRL (Control) - Figure A-4 shows the keys that generate control characters. You can generate control characters in two ways.
I.

Hold down CTRL. and press any of the unshaded keys in Figure A-4.

Press any of the shaded keys in Figure A-4 without using CTRL. The shaded keys are dedicated keys that generate control characters without using CTRL.

NOTE
You can change the RETURN key character code
with the Iinefeed/new line feature. When off, this
feature causes RETURN to generate a single control character (CR). When on, this feature causes
RETURN to generate two characters (CR, LF).
Also, depending on the auxiliary keypad mode selected, RETURN and ENTER may generate the
same control codes.
Auto-Repeating
All keys auto-repeat except: SET-UP, ESC, NO SCROLL, TAB, RETURN, and any key pressed with
CTRL. Auto-repeating may be disabled (SET-UP function). Auto-repeating works as follows: when you
press a key, its code(s) is sent once, immediately_ If you hold down the key for more than one-half
second, the code(s) is sent repeatedly at a rate of approximately 30 Hz (less if low transmit baud rates
are used) until you release the key.
Cursor Control Keys
The keyboard contains four keys labeled with arrows in each of four directions. These keys transmit
escape sequences. If the host echoes these escape sequences back to the terminal, the cursor moves one
character up. down, right, or left. Table A-3 describes the escape sequences generated by each key.

DD
··DDDDDDDDDDDD=

))

MNEMONICS SHOWN ABOVE KEYCAP LEGENDS
OCTAL CODES SHOWN BELOW KEYCAP LEGENDS

.. , XgN

021

IIE~B " E~Q " D~211 D~41~1 N~K I~ml D~E I~~
027
005
022
024 ~ 025 ~Lill1J 020 LQllJl2ill

Dr--l~~I¥lfA?lrs!ll~MM~DD
uL---.J~l£hJ~~l2hJ~~~LlliJ

))

~fCeNl~~~~~DDI9nD
l ~~U>hJ~~~~
LQ{J

I
Figure A-4

Function Key Control Codes

A-6

MA-741!JC

Table A-3

Cursor Control Key Codes

Cursor
Key

VT52
Mode

ANSI Mode and Cursor
Key Mode Reset

ANSI Mode and Cursor
Key Mode Set

Up
Down
Right
Left

ESC A
ESCB
ESCC
ESCD

ESC[A
ESC[B
ESC[C
ESC[D

ESCOA
ESCOB
ESCOC
ESCOD

AUXILIARY KEYPAD
The keys on the auxiliary keypad normally transmit codes for the numerals, decimal point, minus sign,
and comma. ENTER transmits the same code as RETURN. The host cannot tell if these keys were
pressed on the auxiliary keypad or on the main keyboard. Therefore, software that requires considerable numeric data entry need not be rewritten to use the keypad.

)

However, if software must distinguish between pressing a key on the auxiliary keypad and pressing the
corresponding key on the main keyboard, the host can give the terminal a command to place it in keypad application mode. In keypad application mode all keys on the auxiliary keypad are defined to give
escape sequences that may be used by the host as user-defined functions.
The codes sent by the auxiliary keypad for the four combinations of the VTS2/ ANSI mode and numeric keypad/application mode is listed in Table A-4. None of the keys are affected by pressing SHIFT,
CAPS LOCK, or CTRL.

NOTE
In ANSI mode, if the codes are echoed back to the
VT100, or if the terminal is in LOCAL, the last
character of the sequence is displayed on the screen;
for example, PFI displays a "P."
SPECIAL GRAPHICS CHARACTERS
If you select the special graphics set, the graphics for ASCII codes 1378 through 1768 are replaced
according to Table A-S. (See the SCS escape sequence.)

)

NOTE
Codes 1528-1568, 1618 and 1648-1708 are used to
draw rectangular grids. Each piece of this line drawing set is contiguous with others so that the lines
formed are unbroken.
Codes 1578-1638 give better vertical resolution than
dashes and underlines when drawing graphs. Using
these segments 120 X 132 resolution may be obtained in 132 column mode with the advanced video option installed.
.

)
A-7

Table A-4

Auxiliary Keypad Codes
\

Key
0
1

3
4

5
6

Numeric
Keypad
Mode

))

VTS2Mode

ANSI Mode
Alternate
Keypad
Mode

ESC

060

033

ESC

061

033

ESC

062

033

ESC

063

033

4
064

033

ESC

065

033

ESC

066

033

Numeric
Keypad
Mode

Alternate
Keypad
Mode

0
117

ESC

160

060

033

077

160

0
117

ESC

161

060

033

077

161

0
117

ESC

162

062

033

077

162

0
117

ESC

163

063

033

077

163

0
117

ESC

164

064

033

077

164

0
117

ESC

165

065

033

077

165

ESC

166

066

033

077

166

0
117

)1
/

8
9

ESC

067

033

ESC

070

033

ESC

071

033

t (minus)
055

ESC

• (comma)

ESC

033

054

033

. (period)

ESC

056

033

0
117

ESC

167

067

033

077

167

0
117

ESC

170

070

033

077

170

0
117

ESC

171

071

033

077

171
m

155*

0
117
0
117
0
117

- (minus)*

ESC

155

055

033

077

, (comma)*

ESC

077

054t

154

054

033

• (period)

ESC

056

033

077

156

A-8

}

)

TableA-4

Auxiliary Keypad Codes (Cont)
VTS2Mode

ANSI Mode

Key

Numeric
Keypad
Mode

ENTERt

CR or CR

PFt
PF2

)

PF3
PF4

01S

ESC
033

117

0
0

ESC
033

117

ESC
033

117

ESC
033

117

0
0

Numeric
Keypad
Mode

Alternate
Keypad
Mode

ESC
033

?
077

lIS

P
120

ESC
033

?
077

P
120

ESC
033

Q
121

ESC
033

077

Q
121

R
122

ESC
033

R
122

ESC
033

?
077

R
122

S
123

ESC
033

S*
123

ESC
033

?
077

S*
123

LF
012

ESC
033

CRor CR

117

lIS

01S

P
120

ESC
033

0
117

P
120

ESC
033

Q
1"21

ESC
033

117

Q
121

R
122

ESC
033

11.7

S
123

ESC
033

117

0
0
0

Alternate
Keypad
Mode
LF
012

*These sequences were not available in the VTS2. Do notuse the PF4, "-" (minus), or "," (comma)
keys with VTS2 with VTS2 software.

tWhen numeric keypad mode is selected (alternate keypad mode off), the ENTER character code can
be changed by the line feed/new line feature. When off, this feature causes ENTER to generate a
single control character (CR, octal 0 IS). When on, this feature causes ENTER to generate two characters (CR, octal 01S and LF, octal 012).

)

)
A-9

Table A-S

Special Graphics Characters
\

Octal
Code

US or
UK Set

137
140
141
142
143
144
145
146
147
150
151
152
153
154
155
156
157
160
161
162
163
164
165
166
167
170
171
172
173
174
175
176

,
a
b
c
d
e
f
g
h

))

Special Graphics Set

•

~
~

i
~
0

!
~

r
m
n

p
q
r
s
t
u
v
w
x

z
(

i
J..

I..
:::.

11
~

}

Blank
Diamond·
Checkerboard (error indicator)
HT (horizontal tab)
FF (form feed)
CR (carriage return)
LF (line feed)
Degree symbol
Plus/minus +
NL (new line)
VT (vertical tab)
Lower-right comer
Upper-right comer
Upper-left comer
Lower-left comer
Crossing lines .
Horizontal line - scan 1
Horizontal line - scan 3
Horizontal line - scan 5
Horizontal line - scan 7
Horizontal line - scan 9
LeftT
Right T
Bottom T
TopT
Vertical bar
Less than or equal to
Greater than or equal to
Pi (mathematical)
Not equal to
UK pound sign
Centered dot

A-I0

))

COMMUNICATION PROTOCOLS
Full-Duplex
The terminal can operate at transmission speeds up to 19,200 baud. However, the terminal may not be
able to keep up with incoming data. The terminal stores incoming characters in a 64-character buffer
(128 characters in later model terminals) and processes them on a first-iil/first-out basis. When the
content of the buffer reaches 48 characters, the terminal transmits 0238 (XOFF or DC3). On this signal
the host should suspend its transmission to the terminal. Eventually, if the host stops transmitting, the
terminal depletes the buffer. When 16 characters remain in the buffer the terminal transmits 0218
(XON or DC 1) to signal the host that it may resume transmission.

If the host fails to respond to an XOFF from the terminal in a timely manner, the buffer continues to
fill. When the 64-character capacity of the buffer is exceeded, a condition occurs called buffer overflow. To determine if the buffer will overflow use the following formulas.
No. of chars to overflow = 16 -

[3 X (receiver speed/transmit speed)]

Time to respond to XOFF = No. of chars to overflow X (bits/char + parity bit + 2)/receiver speed
Example
The VT I 00 is transmitting 8-bit characters with no parity at 1200 baud and receiving at 1200 baud.
The terminal has just sent an XOFF which the host must respond to within 0.1083 second to avoid a
buffer overflow.
No. of chars to overflow = 16 -

[(3 X 1200/1200)] = 13 chars

Time to respond to XOFF = 13 X (8+0+2)/1200 = 0.1083 sec

Example
The VT 100 is transmitting 7-bit characters with parity at 300 baud and receiving at 1200 baud. The
terminal has just sent an XOFF which the host must respond to within 0.0333 second to avoid a buffer
overflow.
No. of chars to overflow = 16 -

[(3 X 1200/300)] = 4 chars

Time to respond to XOFF = 4 X (7 + I + 2) /1200 = 0.0333 sec
)

If the buffer overflows, the VT100 begins to discard incoming characters and the error character is
displayed.
Software that does not support receipt of XOFF /XON signals from the terminal can still use the
VT100 provided the software never sends the ESC code to the terminal, the baud rate is limited to 4800
or less, and the software does not use smooth scrolling or split screen features.
Two of the terminal functions, Reset and Self-Test, reinitialize the terminal and erase the buffer. Thus
if characters are received after the commands to perform these two functions and the characters are
placed in the buffer, the characters are destroyed without being processed.

A-ll

To compensate for this, the host may act in one of two ways.
1.

Immediately after sending the terminal the commands to perform either the Reset or SelfTest functions, the host may act as if it had received XOFF from the terminal, thus sending
no more characters until it receives XON. The terminal transmits XON only after it completes the specified operation and the XOFF /XON feature is enabled.

When the first method cannot be implemented, a delay of no less than 10 seconds may be
used to allow the terminal time to complete the invoked function. This method, however, does
not guarantee against the loss of data when an invoked function has detected an error. And
while this delay is currently adequate, future options may require a change in the time delay.

»)

The XOFF /XON synchronization scheme has an advantage over requiring the host to insert delays or
filler characters in its data stream. Requiring a minimum of software support, XON/XOFF ensures
that every character or command sent to the VT100 is processed in correct order. It frees interface
programs from all timing considerations and results in more reliable operation.
In addition to the buffer-filling condition, there are two other means of transmittingXOFF and XON:
the NO SCROLL key, and CTRL-S/CTRL-Q. If XOFF mode is enabled, the VT100 coordinates these
three sources of XOFF and XON so that the desired effect occurs. For example, if the buffer-filling
condition causes an XOFF to be sent and then you press NO SCROLL, a second XOFF is not sent.
Instead of sending an XON when the buffer empties, the VT100 waits until you press NO SCROLL
again before sending XON.
Also, entering SET-UP mode causes the VT100 to temporarily stop taking characters from the buffer.
An XOFF is sent if the buffer becomes nearly full.
CTRL-S and CTRL-Q are also synchronized with NO SCROLL.
If the XOFF feature is disabled, the buffer-filling condition does not send an XOFF, NO SCROLL is
disabled, and CTRL-S and CTRL-Q are transmitted as typed.

The VT100 also recognizes received XOFF and XON. Receipt of XOFF inhibits the VT100 from
transmitting any codes except XOFF and XON. From three to seven keystrokes on the keyboard are
stored in a keyboard buffer (some keys transmit two or three codes, e.g., cursor controls). If the keyboard buffer overflows, keyclicks stop and the KBD LOCKED LED lights. Transmission resumes upon
receipt of XON.
If the user transmits an XOFF to the host (by CTRL-S or NO SCROLL), the host should not echo any
further type-in until the user types XON. This places the burden of not overloading the host's output
buffer on the user.
Entering and exiting SET-UP clears the transmit and keyboard locked modes.
TERMINAL CONTROL COMMANDS
The VT100 has many control commands that cause it to take action other than displaying a character
on the screen. In this way, the host commands the terminal to move the cursor, change modes, ring the
bell, etc. The following paragraphs discuss terminal control commands.
Control Characters
Control characters have values of 0008-0378, and 1778. The control characters recognized by the
VT100 are listed in Table A-6. All other control codes have no effect.
Escape Sequences
Table A-7 summarizes the VT100 escape sequences .

. A-12

))

Table A-6

)

Control Characters

Octal
Char

Code

Action Taken

NUL

000

Ignored on input (not stored in input buffer; see full duplex protocol).

ENQ

005

Transmit answerback message.

BEL

007

Sound bell tone from keyboard.

010

Move cursor left one character position, unless it is at left margin in which case no action occurs.

011

Move cursor to next tab stop, or to right margin if no further tab
stops are present on line.

012

Causes line feed or new line operation. (See new line mode.)

013

Interpreted as LF.

014

Interpreted as LF.

015

Move cursor to left margin on current line.

016

Select G 1 character set, as selected by ESC) sequence.

017

Select GO character set, as selected by ESC( sequence.

XON

021

Causes terminal to resume transmission.

XOFF

023

Causes terminal to stop transmitting aU codes except XOFF and
XON.

CAN

030

If sent during an escape or control sequence, the sequence is immediately terminated and not executed. It also causes the error
character to be displayed.

SUB

032

I nterpreted as CAN.

ESC

033

Introduces an escape sequence.

DEL

177

Ignored on input (not stored in input buffer).

)
i

A-13

Table A-7

VT100 Escape Sequences Summary

Feature

VTS2 Compatible Mode

Cursor up
Cursor down
Cursor right
Cursor left
Select special graphics character set
Select ASCII character set
Cursor to home
Reverse line feed
Erase to end of screen
Erase to end of line
Direct cursor address
Identify
Enter alternate keypad mode
Exit alternate keypad mode
Graphics processor on
Graphics processor off
Enter ANSI mode

ESC A
ESCB·
ESCC
ESCD
ESCF
ESCG
ESCH
ESC I
ESC]
ESCK
ESC YIc*
ESCZt
ESC =
ESC>
ESC}:j:
ESC2:j:
ESC <

* Line and column numbers for direct cursor address are single character codes whose values are the
desired number plus 378. Line and column numbers start at 1.
tResponse to ESC Z is ESC / Z.
:j:Ignored except in VTlOS.
ANSI Compatible Mode
Cursor Movement Commands
Cursor up
Cursor down
Cursor forward (right)
Cursor backward (left)
Direct cursor addressing

ESC [Pn A
ESC [Pn B
ESC [Pn C
ESC [Pn D
ESC [ PI; Pc H * or
ESC [PI; Pc f *
ESCD
ESCM
ESC7
ESC 8

Index
Reverse index
Save cursor and attributes
Restore cursor and attributes
*PI = line number; Pc = column number.

NOTE
Pn refers to a decimal parameter expressed as a
string of ASCII digits. Multiple parameters are separated by the semicolon character (0738). If a parameter is omitteu or spt:cified to be 0 the default
parameter value is used. For the cursor movement
wmmA...!;, the default parameter value is 1.

A-14

))

Line Size (Double-height and Double-width) Commands
Change this line to double-height top half
Change this line to double-height bottom half
Change this line to single-width single-height
Change this line to double-width single-height

ESC#3
ESC#4
ESC#5
ESC#6

Character Attributes
ESC [ Ps;Ps;Ps; .... ;Ps m
Ps refers to a selective parameter. Multiple parameters are separated by the semicolon character
(0738)' The parameters are executed i.n order and have the following meanings.

o or none
1
4

5
7
)

All attributes off
Bold on
Underscore on
Blink on
Reverse video on

Any other parameter values are ignored.
Erasing
From cursor to end of line
From beginning of line to cursor
Entire line containing cursor
From cursor to end of screen
From beginning of screen to cursor
Entire screen

)

ESC [ K or ESC [ 0 K
ESC [ 1 K
ESC [2 K
ESC [ J or ESC [ 0 J
ESC [ 1 J
ESC [2 J

Programmable LEDs
ESC [ Ps; Ps; ... Ps q
Ps are selective parameters separated by semicolons (0738) and executed in order, as follows.

o or none
1
2
3
4

All LEDs off
LED 1 on
LED 2 on
LED 3 on
LED 4 on

Any other parameter values are ignored.
Character Sets (GO and G 1 Designators)
The GO and G 1 character sets are designated as follows.
Character Set
United Kingdom (UK)
United States (USASCII)
Special graphics characters
and line drawing set
Alternate character ROM
Alternate character ROM
special graphics characters

GO
ESC(A
ESC ( B
ESC (0

Gl
ESC)A
ESC) B
ESC)O

ESC ( 1
ESC(2

ESC) 1
ESC)2

A-15

Scrolling Region
ESC [ Pt ; Pb r
Pt is the number of the top line of the scrolling region. Pb is the number of the bottom line of the
scrolling region and must be greater than Pc.
Tab Stops
Set tab at current column
Clear tab at current column
Clear all tabs

ESCH
ESC [g or ESC [0 g
ESC[3g

Modes
Mode Name
Line feed/new line
Cursor key mode
ANSI/VT52
Column mode
Scrolling mode
Screen mode
Origin mode
Wraparound
Auto repeat
Interlace
Graphics processor
option
Keypad mode

To Set
Mode
New line
Application
ANSI
132 column
Smooth
Reverse
Relative
On
On
On
On

Sequence
ESC [20h
ESC [?Ih
N/A
ESC [?3h
ESC [?4h
ESC [?5h
ESC [?6h
ESC [?7h
ESC [?8h
ESC [?9h
ESC 1

To Reset
Mode
Line feed
Cursor
VT52
80 column
Jump
Normal
Absolute
Off
Off
Off
Off

Sequence
ESC [201
ESC [?H
ESC [?21
ESC [?31
ESC [?41
ESC [?51
ESC [?61
ESC [?71
ESC [?81
ESC [?91
ESC 2

Application

ESC =

Numeric

ESC>

Reports
Cursor Position Report
Invoked by
Response is ,

ESC [6 n
ESC [ PI ; Pc R (PI = line number and
Pc = column number)

Status Report
Invoked by
Response is

ESC [5 n
ESC [ 0 n (terminal ok)
ESC [ 3 n (terminal not ok)

What Are \' ou
Invoked by

ESC [c
or
ESC [0 c

Response is

ESC [? 1 ; Ps c

A-16

))

Ps is the "option present" parameter with the following meaning.
Ps

o
1
2
3
4
5
6

Meaning
Base VT100, no options
Processor option (STP) .
Advanced video option (A Va)
AVO and STP
Graphics processor option (GO)
GO and STP
GO and AVO
GO, STP, and A va

Alternately invoked by ESC Z (not recommended). Response is the same.
Reset
Reset executes the power-up reset routine.
ESC c
Confidence Tests
Fill screen with "Es"
Invoke teste s)

)

ESC# 8
ESC [2; Ps y

Ps is the parameter indicating the test to be done and is a decimal number computed by taking the
"weight" indicated for each desired test and adding them together.
Test
Power-up self-test
(ROM checksum, RAM, NVR, keyboard
and A va if installed)

Weight
1

Data loopback

2 (Loopback connector required)

Repeat selected tests(s)
indefinitely (until failure or
power off)

Valid ANSI Mode Escape Sequences (Detailed)
Definitions - The following listing defines the basic elements of the ANSI mode escape sequences. (See
Appendix D for a detailed explanation.)
Control Sequence Introducer (CSI) - An escape sequence that provides supplementary controls and is
itself a prefix affecting the interpretation of a limited number of contiguous characters. In the VT100
the CSI is ESC [.
Parameter - (I) A string of one or more decimal characters that represent a single value; (2) The value
so represented.
Numeric Parameter - A string of characters that represent a number, designated by Pn. Pn has a range
of 0 to 9 (608 to 718).

A-I7

Selective Parameter - A string of characters that selects a subfunction from a specified list of subfunctions, designated by Ps. In general, a control sequence with more than one selective parameter
causes the same effect as several control sequences, each with one selective parameter; for example,
CSI Psa; Psb; Psc F is identical to CSI Psa F CSI Psb F CSI Psc F.

))

Ps is a string of zero or more characters with a range of 0 to 9 (608 to 718) with each selective parameter separated from the other by a ; (semicolon, 738).
Default - A function-dependent value assumed when no explicit value, or a value of 0, is specified.
Final Character - A character whose bit combination terminates an escape or control sequence.
Escape sequences - All of the following escape and control sequences are transmitted from the host
computer to the VT100 unless otherwise noted. All of the escape sequences are a subset of those
specified in ANSI X3.64-1977 and ANSI X3.41-1974.
CPR

Cursor Position Report - VT100 to Host

Format: ESC [ Pn; Pn R

default values: 1

The CPR sequence reports the active position by using the parameters. This sequence has two parameter values, the first specifying the line and the second specifying the column. The default condition with
no parameters present, or parameters of 0, is equivalent t6 a cursor at home position.
Numbering of lines depends on the state of the Origin mode (DECOM).
This control sequence is solicited by a device status report (DSR) sent from the host.
CUB Cursor Backward - Host to VT100 and VT100 to Host
Editor Function; format: ESC [ Pn D

default value: 1

The CUB sequence moves the active position to the left. The distance moved is determined by the parameter. If the parameter value is 0 or 1, the active position moves one position to the left. If the parameter value is n, the active position moves n positions to the left. If an attempt is made to move the
cursor to the left of the left margin, the cursor stops at the left margin.
CUD Cursor Down - Host to VT100 and VT100 to Host
Editor Function; format: ESC [ Pn B

default value: 1

The CUD sequence moves the active position downward without altering the column position. The
number of lines moved is determined by the parameter. If the parameter value is 0 or 1, the active
position moves one line downward. If the parameter value is n, the active position moves n lines downward. If an attempt is made to move the cursor below the bottom margin, the cursor stops at the bottom
margin.
CUF Cursor Forward - Host to VT100 and VT100 to Host
default value: 1

Editor Function; format: ESC [ Pn C

The CUF sequence moves the active position to the right. The distance moved is determined by the
parameter. A parameter value of 0 or 1 moves the active position one position to the right. A parameter
value of n moves the active position n positions to the right. If an attempt is made to move the cursor to
the right of the right margin, the cursor stops at the right margin.

A-18

_,)'
,'

CUP Cursor Position
Editor Function; format: ESC [ Pn; Pn H

default value: I

The CUP sequence moves the active position to the position specified by the parameters. This sequence
has two parameter values, the first specifying the line position and the second specifying the column
position. A parameter value of 0 or 1 for the first or second parameter moves the active position to the
first line or column in the display, respectively. The default condition with no parameters present is
equivalent to a cursor to home action. In the VT100, this control behaves identically with its format
effector counterpart, HVP.
The numbering of lines depends on the state of the Origin mode (DECOM).
CUU

Cursor Up - Host to VT100 and VT100 to Host

Editor Function; format: ESC [ Pn A

)

default value: I

This sequence moves the active position upward without altering the column position. The numberof
lines moved is determined by the parameter. A parameter value of 0 or 1 moves the active position one
line upward. A parameter value of n moves the active position n lines upward. If an attempt is made to
move the cu~sor above the top margin, the cursor stops at the top margin.
DA

Device Attributes
default value: 0

Format: ESC [ Pn c
I.

The host requests the VT100 to send a device attributes (DA) control sequence to identify
itself by sending the DA control sequence with either np parameter or a parameter of O.

Response to the request described above (VT100 to host) is generated by the VTl 00 as a DA
control sequence with the numeric parameters a~ follows:

Sequence Sent
ESC [?I;Oc
ESC [?l;lc
ESC [?1;2c
ESC [?1;3c
ESC [?1;4c
ESC [?1;5c
ESC [?1;6c
ESC [?1;7c

Option Present
No options
Processor option (STP)
Advanced video option (AVO)
AVO and STP
Graphics option (GO)
GO and STP
GO and AVO
GO, STP, and AVO

)

DECALN Screen Alignment Display (DEC Private)
Format:

ESC #8

This command fills the entire screen area with uppercase Es for screen focus and alignment. This command is used by DIGITAL manufacturing and Field Service personnel.
DECANM
)

ANSI/VTS2 Mode (DEC Private)

This is a private parameter applicable to set mode (SM) and reset mode (RM) control sequences. The
reset state causes only VT52 compatible escape sequences to be interpreted and executed. The set state
causes only ANSI "compatible" escape and control sequences to be interpreted and executed.
A-19

DECARM

Auto Repeat Mode (DEC Private)

This is a private parameter applicable to set mode (SM) and reset mode (RM) control sequences. The
reset state causes no keyboard keys to autorepeat. The set state causes certain keyboard keys to autorepeat.
DECAWM

Autowrap Mode (DEC Private)

This is a private parameter applicable to set mode (SM) and reset mode (RM) control sequences. The
reset state causes any displayable characters received when the cursor is at the right margin to replace
any previous characters there. The set state causes these characters to advance to the start of the next
line, doing a scroll up if required and permitted.
DECCOLM

Column Mode (DEC Private)

This is a private parameter applicable to set mode (SM) and reset mode (RM) control sequences. The
reset state causes a maximum of 80 columns on the screen. The set state causes a maximum of 132
columns on the screen.
DECCKM

Cursor Keys Mode (DEC Private)

This is a private parameter applicable to set mode (SM) and reset mode (RM) control sequences. This
mode is only effective when the terminal is in keypad application mode (see DECKPAM) and the
ANSI/VT52 mode (DECANM) is set (see DECANM). Under these conditions, if the cursor key mode
is reset, the four cursor function keys send ANSI cursor control commands. If cursor key mode is set,
the four cursor function keys send application functions.
DECDHL Double-Height Line (DEC Private)
Format: Top Half: ESC #3, Bottom Half: ESC #4
These sequences cause the line containing the active position to become the top or bottom half of a
double-height double-width line. The sequences must be used in pairs on adjacent lines and the same
character output must be sent to both lines to form full double-height characters. If the line was singlewidth single-height, all characters to the right of the center of the screen are lost. The cursor remains
over the same character position unless it would be to the right of the right margin, in which case it is
moved to the right margin.
DECDWL
Format:

Double-Width Line (DEC Private)

ESC #6

This sequence causes thdine that contains the active position to become double-width, single-height. If
the line was single-width, single-height, all characters to the right of screen center are lost. The cursor
remains over the same character position unless it would be to the right of the right margin; in which
case, it is moved to the right margin.
DECGOFF Graphics Processor OFF (DEC Private)
Format:

ESC 2

Turn off the VTI05 graphics processor.

)I
A-20

NOTE
Some DIGITAL hardcopy terminals interpret this
private escape sequence as CLEAR TABS.
DECGON
Format:

Graphics Processor On (DEC Private)
ESC 1

All subsequent characters are interpreted as commands or data to the VTl 05 graphics processor option.
The terminal remains in this mode until the graphics processor off command is received This command
is ignored if no graphics processor option is installed.
NOTE
Some DIGITAL hardcopy terminals interpret this
private escape sequence as SET TAB.
DECHCP

)

Format:

Hard Copy (DEC Private)
ESC #7

This sequence causes the screen to cease updating and freeze while the hardcopy output is enabled. The
screen resumes normal operation when the hardcopy has been completed. This command is ignored if
no hardcopy option is installed.
.

)

DECID

Identify Terminal (DEC Pr.vate)

Format:

ESC Z

This sequence causes the same response as the ANSI device attributes (DA). DECID will not be supported in future DIGITAL terminals, therefore, DA should be used by any new software.
DECINLM

)

Interlace Mode (DEC Private)

This is a private parameter applicable to set mode (SM) and reset mode (RM) control sequences. The
reset state (noninterlace) causes the video processor to display 240 scan lines per frame. The set state
(interlace) causes the video processor to display 480 scan lines per frame. There is no increase in character resolution.
DECKPAM
Format:

Keypad Application Mode (DEC Private)

ESC =

Auxiliary keypad keys and cursor control keys transmit escape sequences.
DECKPNM
Format:

Keypad Numeric Mode (DEC Private)

ESC>

Auxiliary keypad keys send ASCII codes corresponding to the characters on the keys. Cursor control
keys send cursor controls.

A-21

DECLL

Load LEDs (DEC Private)

Format: ESC [ Ps q

default value: 0

Loaa the four programmable LEOs on the keyboard according to the parameter(s).
Meaning
Clear LEOs 1 through 4
Light LED 1
Light LED 2
Light LED 3
Light LED 4

Parameter

o
1
2
3
4

LED numbers are indicated on the keyboard.
DECOM

Origin Mode (DEC Private)

This is a private parameter applicable to set mode (SM) and reset mode (RM) control sequences. The
reset state causes the origin to be at the upper-left character position on the screen. Line numbers are
therefore independent of current margin settings. The cursor may be positioned outside the margins
with a cursor position (CUP) or horizontal and vertical position (HVP) control.

')
/ )

The set state causes the origin to be at the upper-left character position within the margins. Line numbers are therefore relative to the current margin settings. The cursor cannot be positioned outside the
margins.
The cursor moves to the new home position when this mode is set or reset.
Lines and columns are numbered consecutively, with the origin being line I, column 1.
DECRC

Restore Cursor (DEC Private)

Format:

ESC 8

This sequence causes the previously saved cursor position, graphic rendition, and character set to be
restored.
DECREPTPARM
Format:

Report Terminal Parameters

ESC [ <sol>; <par>; <nbits>; <xspeed>; <rspeed>; <c1kmul>; <f1ags>x

These sequence parameters are explained below in the DECREQTPARM sequence.
DECREQTPARM
Format:

Request Terminal Parameters

ESC [ <sol>x

This sequence is sent by the terminal controller to notify the host of the status of selected terminal
parameters. The status sequence may be sent when requested by the host or at the terminal's discretion.
DECREPTPARM is sent upon receipt of a DECREQTPARM.

))
A-22

The meanings of the sequence parameters are:

}
Value

Parameter
<sol>

oor none

Meaning
This message is a request (DECREQTPARM). The terminal is
allowed to send unsolicited reports.
This message is a request. From now on the terminal may only
report in response to a request.

<par>

This message is a report (DECREPTPARM).

This message is a report and the terminal is only reporting on
request.

No parity set
Parity is set and odd
Parity is set and even

4
5

)

<nbits>

2
<xspeed>
<rspeed>

0
8
16
24
32
40
48
56
64
72

80
88
96
104
112
120

)

8 bits per character
7 bits per character
Bits per second
50
75
110
134.5
150
200
300
600
1200
1800
2000
2400
3600
4800
9600
19,200
The bit rate multiplier is 16.

<clkmul>
0-15

<flags>

DECSC

Save Cursor (DEC Private)

Format:

ESC 7

This value communicates the four switch values in block 5 of
SET UP B, which are only visible to the user when an STP option
is installed. These bits may be assigned for an STP device. The
four bits are a decimal-encoded binary number.

This sequence causes the cursor position, graphic rendition, and character set to be saved. (See
DEeRe.)

A-23

DECSCLM

Scrolling Mode (DEC Private)

This is a private parameter applicable to set mode (SM) and reset mode (RM) control sequences. The
reset state causes scrolls to "jump" immediately. The set state causes scrolls to be "smooth" at a maximum rate of six lines per second.
DECSCNM

Screen Mode (DEC Private)

This is a private parameter applicable to set mode (SM) and reset mode (RM) control sequences. The
reset state causes the screen to be black with white characters. The set state causes the screen to be
white with black characters.
DECSTBM

Set Top and Bottom Margins (DEC Private)
default values: (see below)

Format: ESC [Pn; Pn r

This sequence sets the top and bottom margins to define the scrolling region. The first parameter is the
line number of the first line in the scrolling region. The second parameter is the line number of the
bottom line in the scrolling region. Default is the entire screen (no margins). The minimum size of the
scrolling region allowed is two lines, that is, the top margin must be less than the bottom margin.

DECSWL Single-Width Line (DEC Private)
Format:

ESC #5

This causes the line that contains the active position to become single-width, single-height. The cursor
remains on the same character position. This is the default condition for all new lines on the screen.
DECTST

Invoke Confidence Test

Format:

ESC [ 2 ; Ps y

Ps is the parameter indicating the test to be done. Ps is computed by taking the weight indicated for
each desired test and adding them together. If Ps is 0, no test is performed but the VT100 is reset.
Test
Power up self-test (ROM checksum,
RAM, NVR keyboard and AVO if
installed)

Weight

Data Loopback

2 (loopback connector required)

Repeat selected test(s)
indefinitely (until failure or
power off)

A-24

DSR

Device Status Report
default value: 0

Format: ESC [ Ps r

Requests and reports the general status of the VT100 according to the following parameter(s).

Parameter

Meaning
Response from VT100 - ready, no malfunctions detected (default)

Response from VT100 - malfunction - retry

Command from host - please report status (using a DSR control sequence)

Command from host - please report active position (using a CPR control sequence)

DSR with a parameter value of 0 or 3 is always sent as a response to a requesting DSR with a parameter value of 5.

)

Erase In Display

Editor Function; format: ESC [ Ps J

default value: 0

This sequence erases some or all characters in the display accor9ing to the parameter. Any complete
line erased by this sequence returns that line to single-width mode.
Parameter

Meaning
Erase from the active position to the end of the screen, inclusive (default).
Erase from start of the screen to the active position, inclusive.
Erase all of the display - all lines are erased, changed to single-width, and the cursor
does not move.

Erase In Line
default value: 0

Editor Function; format: ESC [ Ps K
)

Erases some or all characters in the active line according to the parameter.
Parameter

Meaning
Erase from the active position to the end of the line, inclusive (default).
Erase from the start of the screen to the active position, inclusive.

HTS

Erase all of the line, inclusive.
Horizontal Tabulation Set

Format Effector; format: ESC H
Set one horizontal stop at the active position.

A-25

HVP

Horizontal and Vertical Position
default value: 1

Format Effector, format: ESC [ Pn; Pn f

Moves the active position to the position specified by the parameters. This sequence has two parameter
values; the first specifies the line position; the second specifies the column. A parameter value of either
zero or one causes the active position to move to the first line or column in the display, respectively. The
default condition with no parameters present moves the active position to the home position. In the
VT100, this control behaves identically with its editor function counterpart, CUP. Numbering lines and
columns depends on the reset or set state of the origin mode (DECOM).
IND

Index

Format Effector; format: ESC D
This sequence causes the active position to move downward one line without changing column position.
If the active position is at the bottom margin, a scroll up is performed.
.
LNM

))

Line Feed/New Line Mode

This is a parameter applicable to set mode (SM) and reset mode (RM) control sequences. The reset
state causes the interpretation of the line feed (LF), defined in ANSI Standard X3.4-1977, to imply
only vertical movement of the active position and causes the return key (CR) to send the single code
CR. The set state causes the LF to imply movement to the first position of the following line and causes
the return key to send the two codes (CR, LF). This is the new line (NL) option.
This mode does not affect the index (IN D), or next line (NEL) format effectors.
NEL

Next Line

Format Effector; format: ESC E
This sequence causes the active position to move to the first position on the next line downward. If the
active position is at the bottom margin, a scroll up is performed.
RI

Reverse Index

Format Effector; format: ESC M
Moves the active position to the same horizontal position on the preceding line. If the active position is
at the top margin, a scroll down is performed.
RIS

Reset to Initial State

Format:

ESC c

Resets the VT100 to its initial state, i.e., the state it has after it is powered on. This also causes the
execution of the power-up self test and signal IN IT H to be asserted briefly.
RM

Reset Mode
default value: none

Format: ESC [ Ps ;Ps ; ...; Ps I

Resets one or more VT100 modes as specified by each selective parameter in the parameter string.
Each mode to be reset is specified by a separate parameter. (See modes and set mode (SM) following
this section.)

A-26

1)
j

)

SCS

Select Character Set

The appropriate GO and G 1 character sets are selected from the five possible character sets. GO and G 1
are selected by codes SI and SO (shift in and shift out) respectively.
GO Sets
Sequence
ESC(A
ESC ( B
ESC(O
ESC ( 1

Gl Sets
Sequence
ESC)A
ESC)B
ESC )0
ESC) 1

ESC ( 2

ESC) 2

Meaning
United Kingdom set
ASCII set
Special graphics
Alternate character ROM
Standard character set
Alternate character ROM
Special graphics

The United Kingdom and ASCII sets conform to the "ISO international register of character sets to be
used with escape sequences." The other sets are private character sets. Special graphics means that the
graphic characters for the codes 1378 to 1768 are replaced with other characters. The specified character set will be used until another SCS is received.
SGR

Select Graphic Rendition

Format Effector; format: ESC [ Ps; ... ; Ps m

default value: 0

Invoke the graphic rendition specified by the parameter(s). All following characters transmitted to the
VT100 are rendered according to the parameter(s) until the next occurrence of SGR.

Parameter

o
1
4

5
7

Meaning
Attributes off
Bold or increased intensity
Underscore
Blink
Negative (reverse) image

All other parameter values are ignored.

)

Without the advanced video option only one character attribute is possible as determined by the cursor
selection. In that case specifying either the underscore or the reverse attribute activates the currently
selected attribute.
.
SM

Set Mode

Format: ESC [ Ps; ••. ; Ps h

default value: none

Causes one or more modes to be set within the VT100 as specified by each selective parameter in the
parameter string. Each mode to be set is specified by a separate parameter. A mode is considered set
until it is reset by a reset mode (RM) control sequence.

)
A-27

TBC

Tabulation Clear
default value: 0

Format Effector; format: ESC [ Ps g

Parameter

Meaning
Clear the horizontal tab stop at the active position (the default case).

Clear all horizontal tab stops.

Any other parameter values are ignored.
MODES
You can change the following VT100 modes with set mode (SM) and reset mode (RM) controls.
ANSI Specified Modes
Parameter

Mode
Mnemonic

LNM

Function
Error (ignored)
Line fee.djnew line mode

DEC Private Modes

If the first character in the parameter string is ? (778), the parameters are interpreted as DEC private
parameters according to the following modes.
Parameter

o
1
2
3
4

5
6

7
8
9

Mode
Mnemonic
DECCKM
DECANM
DECCOLM
DECSCLM
DECSCNM
DECOM
DECAWM
DECARM
DECINLM

Function
Error (ignored)
Cursor key
ANSIjVT52
Column
Scrolling
Screen
Origin
Auto-wrap
Auto-repeat
Interlace

Any other parameter values are ignored.
The following modes, specified in ANSI X3.64-1977 standard, may be considered to be permanently
set, permanently reset, or not applicable, as noted. Refer to that standard for further information concerning these modes.

A-28

Mnemonic
CRM
EBM
ERM
FEAM
FETM
GATM
HEM
IRM
KAM
MATM
PUM
SATM
SRTM
TSM

TIM
VEM

)

Function
Control representation
Editing boundary
Erasure
Format effector action
Format effector transfer
Guarded area transfer
Horizontal editing
Insertion-replacement
Keyboard action
Multiple area transfer
Positioning unit
Selected area transfer
Status reporting transfer
Tabulation stop
Transfer termination
Vertical editing

State
Reset
Reset
Set
Reset
Reset

NA
NA
Reset
Reset

NA
Reset

NA
Reset
Reset

NA
NA

Valid VT52 Mode Escape Sequences (detailed)
Graphics Processor On
Format:

ESC 1

All subsequent characters are interpreted as commands to the graphics option until the graphics processor off command is received. This sequence is ignored if no graphics processor is installed.

Graphics Processor Off
Format:

ESC 2

Turn off the graphics processor.
Cursor Up
Format:

)

ESC A

Move the active position upward one position without altering the horizontal position. If an attempt is
made to move the cursor above the top margin, the cursor stops at the top margin.
Cursor Down
Format:

ESC B

Move the active position downward one position without altering the horizontal position. If an attempt
is made to move the cursor below the bottom margin, the cursor stops at the bottom margin.
Cursor Right
Format:

)

ESC C

Move the active position to the right. If an attempt is made to move the cursor to the right of the right
margin, the cursor stops at the right margin.

A-29

Cursor Left
Format:

ESC D

Move the active position one position to the left. If an attempt is made to move the cursor to the left of
the left margin, the cursor stops at the left margin.
Enter Graphics Mode
Format:

ESC F

Selects the special graphics character set.
NOTE
The special graphics characters in the VT100 are
different from those in the VT52. (See Table A-S.)
Exit Graphics Mode
Format:

ESC G

Selects the standard ASCII character set.
Cursor to Home
Format:

ESC H

Move the cursor to the home position.
Reverse Line Feed
Format:

ESC I

Move the active position upward one position without altering the column position. If the active position
is at the top margin, a scroll down is performed.
Erase to End of Screen
Format:

}

ESC J

Erase all characters from the active position to the end of the screen. The active position is not changed.
Erase to End of Line
Format:

ESC K

Erase all characters from the active position to the end of the current line. The active position is not
changed.

A-30

Table A-8

Special Character and Line Drawing Set and Graphics Mode Comparison

Octal
Code

US/UK
Set

137
140
141
142
143
144
145
146
147
150
151
152
153
154
155
156
157
160
161
162
163
164
165
166
167170
171
172
173
174
175
176

)

Special Character and
Line Drawing Set

•
I

a
b
c
d
e
f
g
h

~
~

i
0

±
~
~

r
m
n
0

p
q
r
s
t
u
v
w

r
i
J.

y
z
{

~
~

11
~

}

Blank
Diamond
Checkerboard (error indicator)
Horizontal tab .
•
Fonn feed
Carriage return
Line feed
Degree symbol
Plus/minus
New line
Vertical tab
Lower-right comer
Upper-right comer
Upper-left comer
Lower-left comer
Crossing lines
Horizontal line - scan 1
Horizontal line - scan 3
Horizontal line - scan 5
Horizontal line - scan 7
Horizontal line - scan 9
LeftT
Right T
Bottom T
TopT
Vertical bar
Less than or equal to
Greater than or equal to
Pi
Not equal to
UK pound sign
Centered dot

*Not available in VT125

)
A-31

VTS2 In
Graphics Mode*
Blank
Reserved
Solid rectangle

1/
3/
5/
7/
Degrees
Plus or minus
Right arrow
Ellipsis (dots)
Divide by
Down arrow
Bar at scan 0
Bar at scan 1
Bar at scan 2
Bar at scan 3
Bar at scan 4
Bar at scan 5
Bar at scan 6
Bar at scan 7
Subscript 0
Subscript 1
Subscript 2
Subscript 3
Subscript 4
Subscript 5
Subscript 6
Subscript 7
SUbscript 8
Subscript 9
- Paragraph

Direct Cursor Address
Format:

ESC Y line column

Move the cursor to the specified line and column. The line and column numbers are sent as ASCII
codes whose values are the number plus 0378. For example. 0408 refers to the first line or column. 0508
refers to the eighth line or column. etc.
Identify
Format:

ESC Z

Causes the terminal to send its identifier escape sequence to the host. The sequence is: ESC / Z.
NOTE
Information regarding options must be obtained in
ANSI mode, using the device attributes (DA) control
sequence.
Enter Alternate Keypad Mode
Format:

ESC =

Auxiliary keypad keys send unique identifiable escape sequences for use by applications programs.
Exit Alternate Keypad Mode
Format:

ESC>

Auxiliary keypad keys send ASCII codes for functions or characters on the key.
Enter ANSI Mode
Format:

ESC <

All subsequent escape sequences will be interpreted according to ANSI Standards X3.64-1977 and
X3.41-1974. The VT52 escape sequence in this section will not be recognized.

A-32

)

VT125 PROGRAMMING SEQUENCES
This section repeats the information on the VT125 Programming Reference Card (EK-VT125-RC).

CONTROL CHARACTERS RECEIVED

)

Name

Character
Mnemonic

Octal
Code

Null

NUL

000

Ignored when received and
used as a fill character.

Enquire

ENQ

005

Transmits the answerback
message.

Bell

BEL

007

Generates a bell tone.

Backspace

010

Moves the cursor to the left
one character position.

Horizontal
Tab

011

Moves the cursor to the
next tab stop.

Line Feed

012

Causes a line feed or a new
line operation. (Refer to
Linefeed/ New Line mode.)

Vertical
Tab

013

Processed as LF.

Form Feep

014

Processed as LF.

Carriage
Return

015.

Moves the cursor to left
margin on the current line.

Shift Out

016

Selects the G1 character
set. as designated by a
Select Character Set
sequence.

Shift In

017

Selects the GO character
set. as designated by a
Select Character Set
sequence.

Device
Control 1

DC1

021

Processed as XON. Causes
the terminal to continue
transmitting characters.

)

I
j

A-33

Function

Character
Mnemonic

Octal
Code

Device
Control 3

DC3

023

Processed as XOFF. Causes
terminal to stop transmitting all characters except
XOFF and XON.

Cancel

CAN

030

If received during an
escape or control sequence. the sequence is
cancelled and substitution
character (~I is displayed.

Substitute

SUB

032

Processed as CAN.

Escape

ESC

033

Processed as a sequence
introducer.

Delete

DEL

177

Name

Function

))

Ignored when received.

ANSI COMPATIBLE SEQUENCES
Set Mode
Name

Mnemonic

Mode

Sequence

Line feed/new line
Cursor key
ANSI/VT52
Column
Scrolling
Screen
Origin
Auto wrap
Auto repeat

LMN
DECCKM
DECANM
DECCOLM
DECSCLM
DECSCNM
DECOM
DECAWM
DECARM

New line
Application
ANSI
132 column
Smooth
Reverse
Relative
On
On

ESC [ 20 h
ESC [ ? 1 h
N/A
ESC [ ? 3 h
ESC [ ? 4 h
ESC [ ? 5 h
ESC [? 6 h
ESC [ ? 7 h
ESC [ ? 8 h

))

Reset Mode
Name

Mnemonic

Mode

Sequence·

Line feed/new line
Cursor key
ANSI/VT52
Column
Scrolling
Screen
Origin
Auto wrap
Auto repeat

LMN
DECCKM
DECANM
DECCOLM
DECSCLM
DECSCNM
DECOM
DECAWM
DECARM

Line feed
Cursor
VT52
80 column
Jump
Normal
Absolute
Off
Off

ESC [ 20 I
ESC [ ? 1 I
ESC [ ? 2 I
ESC [ ? 3 I
ESC [ ? 4 I
ESC [ ? 5 I
ESC [ ? 6 I
ESC [ ? 7 I
ESC [ ? 8 I

• The last character of each sequence is lowercase L (1548 )

A-34

Cursor Key Codes Generated

Cursor Key
(Arrow)

Reset
(Cursor)

Up
Down
Right
Left

ESC [A
ESC [ B
ESC [ C
ESC [ D

ANSI Codes
Set
(Application)
'ESC O' A
ESC 0 B
ESC 0 C
ESC 0 D

VT52 Codes

ESC A
ESC B
ESC C
ESC D

Keypad Character Selection

)

Name

Mnemonic

Sequence

Alternate
Numeric

DECKPAM
DECKPNM

ESC
ESC>

Keypad Codes Generated

Key

ANSI Mode
Numeric
Alternate
Keypad
Keypad
Mode
Mode

VTS2 Mode
Numeric
Alternate
Keypad
Keypad
Mode
Mode

0
1

0
1
2
3
4
5

3
4
5

6
7
8
9

- (minus)
, (comma)
. (period)
ENTERt
PFl
PF2
PF3
PF4

- (minus)
, (comma)
. (period)
CR or CRLF
ESC 0 P
ESC a Q
ESC OR
ESC a S

t
t

ESC 0 p
ESC 0 q
ESC 0 r
ESC 0 s
ESC 0 t
ESC 0 u
ESC 0 v
ESC 0 w
ESC a x
ESC a y
ESC a m
ESC a 1*
ESC a n
ESC a M
ESC a P
ESC a Q
ESC a R
ESC a S

ESC? P
ESC? q
ESC? r
ESC? S
ESC? t
ESC? U
ESC? v
6
ESC? W
7
ESC? x
8
ESC? Y
9
- (minus)t ESC? mt
, (comma)t ESC? 1* t
ESC? n
. (period)
CR or CRLF ESC? M
ESC? P
ESC P
ESC? Q
ESC Q
ESC? R
ESC R
ESC? St
ESC St

The last character of the sequence is lowercase L (154 8 )
These sequences were not available in the VT52. Do not use the
PF4, "-" (minus), or "," (comma) keys with VT52 software.
Line feed/new line off causes ENTER to generate CR (01 5 a). On
causes ENTER to generate CRLF (0-15 8 01 2s).

A-35

Select Character Sets SCS
Character Set

GO Designetor

G1 Designator

United Kingdom (UK)
United States (USASCII)
Special characters
and line drawing set
Alternate character ROM
Alternate character ROM special characters

ESC (A
ESC ( B
ESC (0

ESC) A
ESC) B
ESC) 0

ESC ( 1
ESC (2

ESC) 1
ESC) 2

Name

Mnemonic

Sequence

Single Shift 2
Single character shift
to G2 (ASCII)
Single Shift 3
Single character shift
to G3 (ASCII)

SS2

ESC N

SS3

ESC 0

NOTE: The VT725 generates the fol/owing control characters
differently from previous DIGITAL terminals.

Code

vr125 Keys

Previous Terminal Key.

NUL
RS
US

CTRL - Space bar
CTRL- CTRL- ?

CTRL- @
CTRL- A
CTRL-_

Character Attribute.
Name

Mnemonic

Select Graphic Rendition
No attributes
No attributes
Select bold attribute
Select underline attribute
Select blink attribute
Select reverse video.
attribute

SGR

Sequence

ESC [m
ESC [0 m
ESC [ 1 m
ESC [4 m
ESC [5 m
ESC [7 m

NOTE: Without advance video option (AVO). only underline orreverse
attribute is available.

A-36

))

US/UK Character Set

. . . , ,, , ,

.,TS

.. IlI)"';o;,

0
1
3
,. SP2
, , , , NUL ,,,
..
"» 0 "
". 1 ..
DC1 ""
,,,
!
·
,.
""
·
·
,
.,""
,,
,, , ,
".,., " " 2 "»
·
,, DC3
"
,, , ,
"" "Yc " 3 ""
,
·
"
"
, ,,
"". S ,. 4 ",.
·
,
, , , ENO ,,
."
,".",.. "" " 5 I"
·
·
"
II
I
, ,
""
"
·
· , IEL •,,
"
"
,
·, ,· ,· IS .,, CAN ,.""" ( "" 87 """,.
· , , , HT ",• ,." ) "",... ""
• "
,
""
· ·
"
LF "., SUI ",. * .,
, ,
",. : "
· , .· " VT ESC • " ;
..",. ,"". "
· , ,· " FF .."
.. < I;i"
"
"
·· "
,.
, ., CR .,
· · ·, SO ..."c ,I "., - "" ..> ..,."
· · ,· .,.. 51 I", !I "~:" I I .."" ? """
I I:
I ,.
""
I "
·· ·

.
lD

'.OH!

I_OH,

···

..
..
..
..
.
..
,H

· ·
I

C
D
E

G
H
I

..
...

OC1A~

DECIMAL

H(X

)

)
A-37

..
'..,"
"

.,..,

."",.

",
"OJ

'Do

'""
"
'""
.,."

• ...

•

" I'"
• ..." q ","
b

...",

•
.,"
......""

>0'

!'".

• '"

>0'

.... ...., I""."
.".. • .,.
.... ....""
...,.
•..
,.
'".
." w .".. • .",.,
."
.,"
"
.... • .".,."
'"
•
." .,.... .,.. .".,.
".
.".,
.,"
... .",.".. ....., '"'",."

.
'"...
...."
II

."
"."
'"..

".."
'"

'lD

]

.",.
""

'J!.
OJ

...

." - I ::

OJ'

", {
."
.'" I

...

",
."
"

.,.
,
." } '".""
."
."
."
.,."
6C

>3,

1116

'Do

[

DENNDSON THE CI'IA"ACnlt SET SE LfCn: 0 US·. U It·(

KEY

ASCII CHA."CfEIII E i l l l J

...

..
..

°NOTE

.. • .".,..

)

tOLUM ...

n
0

..
..." - '""
'"

"... DEL '"

Special Characters and Line Drawing Set

)
...

IITS::::
....1.1.• ,

"'*'

o 0 0 0

('I

tIn u..,.

NUL: :
!~
' o r 10

".eo

0010

aD'

o I 0

o I

",' ::a:!I'Q'R
",
.,

DC3"
'I

IIlOH,

."
",.
"..
""

E"Q

•

o 0

•

.,."

I::

BEL'
I 0

o I 0 0
I

litO""

SUI

32
"

l:;

• ..,.,. • I::..

.
7

",.,

.,".,,.
...
."

... Y .,...

I'"
'!t!,

"
,.

...
'0' w ",
"
.." x '""
"
'"

...

<I'

I,.

o 0

FF I "

CR I"

I ,.

so ..,

Ir.

I""

III

I.'
61
It3

N. 116
I

11/

I fiJ

KEY
ASCIll.M..... C f [ R E i l l l l OCU.I
OfClllU...

'1 .011_

A-38

I'JI

,i 9!t
J.'!IJ SCA'" I

lI'ILA....

'"IG
'6'

III
SCA~.· II
I

,.L1
II•

KA. .. ': "
III

...
.".,

III

II!,

SCAN'

...,. ",."
. ..
.'"., r ....,
.".. .,...
j '"

'ID

I~'

I II

1m.

1!'1

Ui'

'01

,,.,

lor

1!pI

" ..

101

,,.

"
'"
! '",,.

l!or

DEL ; :;~

I I.
, !II

11/

110

stAhl

!oC

IIU

,.,

!ot

,,<l

1M)

•

!,.

""
"
"
* "",.

'0'

•

..
""
." ';='--Z--I-;f,
..
.. ..
..
. "," ,,. ..
• ""
"
< .
"
..
,."
.. , ! "
."
..
.,
.
,. > .',u '",.
",.
. -.j.2[ot". ' . t "."..
~

, CAN :-;
"
"

•

.,
..

ir.,."

1---I-+---+-;....lr.:D,",c",,:-+ "
o 0'0 I

- -.-

_. --- - r---'~ .. r-" -

- - - -...... -r-

i "

Scrolling Region

)

Name

Mnemonic

Sequence

Set top and bottom margins

DECSTBM

ESC [ Pt; Pb r

Name

Mnemonic

Sequence

Cursor up
Cursor down
Cursor forward (right)
Cursor backward (left)
Cursor position
Cursor position (home)
Horizontal and vertical position
Horizontal and vertical position
(home)
Index
Reverse index
Next line
Save cursor (and attributes)
Restore cursor (and attributes)

CUU
CUD
CUF
CUB
CUP
CUP
HVP
HVP

ESC [ Pn A
ESC [ Pn B
ESC [ Pn C
ESC [ Pn 0
ESC [ PI; Pc H
ESC [H
ESC [ PI; Pc f
ESC [ f

IND
RI
NEL
DECSC
DECRC

ESC 0
ESC M
ESC E
ESC 7
ESC 8

Name

Mnemonic

Sequence

Horizontal tab set
(at current column)
Tabulation clear
(at current column)
Tabulation clear
(at current column)
Tabulation clear (all tabs)

HTS

ESC H

TBC

ESC [ g

TBC

ESC [0 g

TBC

ESC [ 3 g

Name

Mnemonic

Sequence

Double- height top half
Double-height bottom half
Single-width single- height
Double-width single- height

DECDHL
DECDHL
DECSWL
DECDWL

ESC.3
ESC • 4
ESC.5
ESC.6

Cursor Movement Commands

)

Tab Stops

Line Attributes

A-39

Erasing
Name

Mnemonic

Erase in line
Cursor to end of line
Cursor to end of line
Beginning of line to cursor
Entire line containing cursor

Erase in display
Cursor to' end of screen
Cursor to end of screen
Beginning of screen to cursor
Entire screen

Sequence

ESC [K
ESC [0 K
ESC [ , K
ESC [ 2 K

ESC [J
ESC[OJ
ESC [ 1 J
ESC [2 J

Communication and Graphics Protocol Commands
Name

Mnemonic

Device control string
Enter ReGIS at previous
command level
Enter ReGIS at highest
command level
Enter ReGIS at previous
command level with
commands to screen
Enter ReGIS at highest
command level with
commands to screen
Enter DEClivriter graphics
Enter VTl05 emulator

DCS

String terminator
Exit graphics

Media copy
Turn off computer to auxiliary
port .
Turn on computer to auxiliary
port
Turn off computer to screen
Turn on computer to screen
Select auxiliary port for
ReGIS hardcopy output
Select computer port for
ReGIS hardcopy output

Sequence

))

ESC P p
ESC P' p
ESC P 2 p

ESC P 3 p

))

ESC P q
ESC P t

ESC \

A-40

,)
/

ESC [4 i
ESC [5 i
ESC [6 i
ESC [ 7 i
ESC (? 0 i
ESC [? 2 i

Reports

)

Name

Mnemonic

Sequence

Device status report
(request status of VT125)
Response:
Terminal OK
Terminal not OK

DSR

ESC [ 5 n

DSR
DSR

ESC [0 n
ESC [ 3 n

Device status report
(request cursor position)
Cursor position report

DSR

ESC [6 n

CPR

ESC [ PI; Pc R

Device attributes (what are you)
Device attributes (what are you)
Identify terminal (what are you)

DA
DA
DEClO

ESC [ c
ESC [0 c
ESC Z

NOTE:

ESC Z is not recommended.

Device attributes
response: VTl 25

See Note.

NOTE: Format is ESC [ ? 12 ; <vt100> ; <vt125> ; <version> c

no AVO, 7
AVO
1 = printer, 0 = no printer
Graphics firmware

Reset
Name

Mnemonic

Sequence

Reset to initial state

RIS

ESC c

A-41

VT100 Tests and Adjustments
NOTE: Do not use VT100 loopback tests with the graphics processor
installed. Loopback tests require test connector. Continuous tests end
at failure or power-off.

Name

Mnemonic

Screen alignment display
Fill screen with "Es"

DECALN

Sequence

ESC # 8

Invoke confidence test
DECTST
Power-up test
Data loopback test
Power-up and data loopback
tests
EIA modem control loopback
test
Power-up and EIA loopback
tests
Data loopback and EIA
loopback tests
Power-up, data loopback,
and EIA loopback tests
Repeat power-up test
continuously
Repeat data loopback test
continuously
Repeat power-up and data
loopback tests continuously
Repeat EIA test continuously
Repeat power-up and EIA
tests continuously
Repeat data loopback and EIA loopback tests continuously
Repeat power-up, data
loopback, and EIA loopback
tests continuously

A-42

ESC [ 2 ; 1 y
ESC [ 2; 2 y
ESC [ 2 ; 3 y
ESC [ 2 ; 4 y
ESC [ 2; 5 y
ESC, 2; 6 y
ESC [ 2 ; 7 y
ESC [ 2; 9 y
ESC, 2 ; 10 y
ESC, 2 ; 11 y
ESC, 2 ; 12 y
ESC, 2 ; 13 y
ESC [ 2; 14 y
ESC, 2; 15 y

))

VT125 Tests and Adjustments
NOTE: All tests require loopback connector. Always include power-up
test for correct display of error indications.
Name

Mnemonic

Invoke confidence test
OECTST
VT125 power-up test
VT125 computer port
data loopback test
VT125 auxiliary port
data loopback test
VT1 25 display test
VT1 25 video bit map
memory test
Repeat any selected
tests continuously until
power-off or failure

Sequence
ESC [ 4 ; 1 ; Ps ... ; Ps y
Ps = 1
Ps= 2
Ps= 3
Ps= 4
Ps = 5
Ps= 9

Keyboard Indicators
Name

Mnemonic

Load LEOs
All off
L1 on
L2 on
L3 on
L4 on

OECLL

Sequence

ESC [ q
ESC [ 1 q
ESC [2 q
"ESC [ 3 q
ESC [4 q

VT52 COMPATIBLE MODE

)

Mode

Sequence

Enter ANSI mode

ESC <

Keypad Character Selection
Name

Sequence

Enter alternate keypad mode
Exit alternate keypad mode
(numeric keypad mode)

ESC =
ESC>

NOTE: VT52 alternate keypad and numeric keypad modes are different
from ANSI.

A-43

Character Sets
Name

Sequence

Special graphics character set
Select US/UK character set
(as determined by the US/UK
character SET-UP feature)

ESC F·
ESC G

Same as special character and line drawing set in ANSI mode.

Cursor Position
Name

Sequence

Cursor up·
Cursor down·
Cursor right·
Cursor leftCursor to home
Direct cursor address
Reverse line feed

ESC A
ESC B
ESC C
ESC D
ESC H
ESC Y PI Pct
ESC 1*

•

Same when sent from the terminal.

Line and column numbers for direct cursor address are single
character codes whose values are the desired number plus 31,0.
Line and column numbers start at one.
The last character of the sequence is an uppercase i (111 B).

Erasing
~ame

Sequence

Erase to end of line
Erase to end of screen

ESC K
ESC J

Reports
Name

Sequence

Identify (what are you)
Response: VT52

ESC Z
ESC / Z

A-44

))

ReGIS COMMAND SUMMARY
Position Command Summary
Command

Function

[<position>]
<pixel vector> or <pv>

Reset pattern memory.
Move to <position>.
Move <multiplier> pixels in

(B)
(5)
(E)
(W «temp. writing controls»)

<pv> direction.
Save current location.
Save dummy location.
Move to last saved location.
P (W (M<multiplier»).

[]

Vector Command Summary
Command

Function

v []

Draw dot at current position.
Draw vector to <position>.
Draw <multiplier> pixels in
<pv> direction.
Save current position.
Save dummy position.
Draw to last saved position.

[<position>]
<pixel vector> or <pv>
(B)
(5)

(E)

(W «temp. writing controls»)

Curve Command Summary

)

Command

Function

Circle with center at current
position. circumference at
<position> .
Circle with center at
<position>. circumference at
current position.
Arc with center at current
position. starting at <position>
for <degrees>.
Arc with center at <position>.
starting at current position for
<degrees>.
Bounded (closed) curVe
Unbounded (open) curve

[<position>]

(A<degreeS» [<position>]

(A<degreeS>C) [<position>]

(B) [<pos.>] ... [<pos.>] (E)
(5) [ ] [<pos.>] ... [<pos.>] [ ] (E)
(W«temp. writing controlS»)

A-45

Text Command Summary

))

Command
T

(5 <size number»
(H <height»

[<spacing> )
(5 [<width in pixelS>.<height in pixelS>))

(M [<width pixel multiplier>.<height pixel multiplier>))
(0 <direction angle»

(0 <string tilt> 5 <size> 0 <char tilt»
(T <italic degrees»
(A <pattern set number»
((B) <temporary attributes block> (E))
(W«temp. writing controls»)

Writing Controls Summary
Command

Function

W (C)

Complement
Erase
'Replace
Overlay
0= no planes
1 = plane 1
2 = plane 2
3 = planes 1 and 2

(E)
(R)

(V)
(F <foreground planes»

Foreground intensity:
(I

or (D)

1
2
3

(R)

Dark
or . Dark
Dim grey
Red
Light grey
Green
White
Blue
Cyan
Yellow
Magenta
White

(G)
(B)

(W)
or
(I (H <hue angle>
L <lightness percent>
5 <saturation percent»)
(M <multiplier»
(N 1)
(N 0)
(51)
(SO)
(S [. shading reference))
(S . shading character)
(P <binary pattern»
(P <pattern number»
(P (M <pattern multiplier>))
(W<i>(P<j>.N<k>))

A-46

Pixels per <pv>
Negative on
Negative off
Shading on
Shading off

Enter pattern,
Use VT125 pattern.
Cusfom writing control

Screen Controls Summary
Command

Function

Scroll.

<pixel vector>
[<position> J
(A [<position> J [<position»)
(E)
(H [<position» [<position»
(H( P[< position> )))

Background intensity:
Dark
or Dark
Dim grey
Red
Light grey
Green
White
Blue
Cyan
Yellow
Magenta
White

(D)

(R)

2
3

(B)

Display addressing.
Erase screen.
Hardcopy (corner positions
optional)
Set hardcopy offset.

(G)

(M)
(W)

or
(I (H <hue angle>
L <lightness percent>
S <saturation percent»)
S(M<n>«mono HLS>)(A<color HLS>)) Output mapping
(S <scale»
(5 (X<scale>Y<scale»
(T <tickS»

Time delay

Macrograph
Command

Function

Clear. all macrographs.
Define macrograph.
Invoke macrograph.

)
!

:keyletter character_string @;
keyletter
Character Cell Control Summary
Command

Summary

Select for loading.
Give name to set.
Load cell .

(A<integer»
(A"<name>")
.. <ASCII char>" <hex pair> ...
<hex pair>;

A-47

Report Command Summary
Command

Function

Set selected for loading.
Contents of macrograph.
Use of storage.
Reply to use.
Cursor position.

(L)

(M «keyletter»)
(M(=))
"<free> .<total> "
(P)

))

)

A-48

)

APPENDIX B
RECOMMENDED SPARES LIST (RSL)

Table B-1 lists the recommended spares for the basic VTl 00 and all variations.
Table B-1

VT100 Recommended Spares

V
T
1
0
0
W

V
T
1
0
0

V
T
1

x
x
x
x
x
x

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

V
T
1
0
0

!-J

Qty

Part Number/Description
30-14590-02 Monitor PCB (Ball)
30-14590-0 I Flyback transformer (Ball)
70-17362-00 Monitor PCB (Elston)
70-17363-00 Flyback assembly (Elston)
70-17364-00 CRT and yoke assembly
54-13009-00 Terminal controller PCB
(not FCC-complying)
54-13009-03 Terminal controller PCB
(FCC-complying)
54-13097-00 Advanced video PCB
70-1527320 rnA adapter assembly
70-1 5506-0B 20 rnA internal cable
BC05 F-15 20 rnA cable (15 ft)
BC05D-IO EIA cable (M-F) (10 ft)
BC03M-IO EIA cable (null modem)
70-15765-00 LK keyboard
70-14652 Cable assembly, keyboard
12-15050 Speaker
12-14333-72 LK07 keycap set
12-14333-91 LK08 keycap set
74-16355 Keycap removal tool
70-14979 Power supply assembly
12-15232 Power switch
12-16901-00 Voltage select switch

~I
l

B-1

V
T
1
0

V
T
1
2

V
T
1

x
x
x
x

x
x
x

x
x
x
x

x
x
x
x
x
x

x
x
x
x
x
x
x
x

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

X
X

Table B-1

V
T
1

0
0
W

0
0

X
X

x
x
x
x

x
x
x

x
x
x
x

x
x

x
x
x
x
x

x
x
x
x

x
x

x
x
x

x
x

x
x.
x

x
x
x

x
x

V
T
1
0
0

Qty

Part Number IDescription

10
I
1
I
1
2
5
5
5
15
15
5
15
15
5
15
5
5
1
5
5
1
1
1
1
1
1
I
1
1
1
1
1
1
1
1
I
1
1
I

90-07217 Fuse 3 Amp
17-0083-09 Line cord, 115 V
17 -0083-10 Line cord, 230 V
70-14978-01 DC power dist. cable
70-14978-03 DC power dist. cable
12-12405-00 Card guide
90-10007 Retainer ring*
90-09747-01 Support*, chassis
90-10016-00 Cable clamp*
90-09964 Plunger*, chassis mounting
90-09966-01 Grommet *, chassis
12-14811 Captive screw*
12-14740-00 Plunger*, base mounting
90-09965-00 Plunger*, base mounting
90-09747-03 Standoff*, AVO
90-09966-02 Grommet*, base
12-14817 Mounting screw*
90-09624-00 Feet*
12-12893 Fuse holder*
90-09680-04 Screw*, tap 6 X 1/2 hex
90-09701-00 Screw*, 6-32 X 5/16
94-03220-03 CRT mask, alignment
94-03270-03 CRT mask, char. alignment
29-23189-00 Shaft extender
29-23190-00 Alignment tool, monitor
29-23187 Kit, carrying case
99-05812 IC container
12-15336 Loopback connector, RS232C
70-15503-00 Loopback connector, 20 rnA
70-15765t- Word processor keyboard
54-14260-00 STP board
M7071 Waveform generator module
70-08612-0F Cable
54-14277 Mono graphics board
54-14275 STP paddle board
70-16165-1 E 24-pin flat cable
70-18396-YA 16-pin flat cable
12-14333-U3 Keycap, DELETE
12-14333-U4 Keycap, INSERT LINEjPFI
12-14333-U5 Keycap, DELETE LINEjPF2
12-14333-U6 Keycap, CHAR INSERTjPF3

)

VT100 Recommended Spares (Cont)

B-2

x
x

x
x
x
x
x

V
T

V
T
1
0

V
T

V
T
1

x
x
x

x
x

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

x
x

x
x
x

x
x
x
x

x
x

x
x
x
x
x
x
x
x

x
x
x

x
x
x
x

x
x
x

x
x
x
x

))

x
x
x
x

x
x
x

x
x
x
x
x
x
x
x
x
X

x
x

'))
j

)

x
x

x
x
x
x

x
x
x

x
x

))

)
APPENDIX C
GLOSSARY OF TERMS AND ABBREVIATIONS

20 mA - Value of current used for current loop communications option; also, the name of the option.
75 ohm - Source and terminating impedance for video inputs and outputs.
7FH - Hexadecimal value representing 0111 1111 in binary; the terminator at the end of each line of
characters; also, the last keyboard address, always returned at the end of a scan.
8080 - The microprocessor device.
8224 - The clock generator device.
8228 - The system controller and bus driver device.

)

8251 - The PUSART.
AO.Al •..•• Ax - Address bus.
Active - For the current loop option, the interface supplies switched current to transmit, and to receive,
detects changes as the remote sender switches the current supplied by the active dev.ice. For the STP,
the STP device intercepts the interface signals, acts on them, and then can either withhold them or pass
them on to the terminal controller or the host.
Active Position - The active column and active line position in which the next displayable character will
be placed. The active line is the line in which the cursor is presently located. The active column is the
cursor location on the active line.

ADDR - Address
ADDR CNT - Address count. Signal that drives the address counters in the video processor.
ADDR CNT ON - Address count on. Signal inside the DCOll that controls the output of Address
Count signals.
ADDR LD - Address load. Signal that loads the address bytes from the end of a line into the presettable address counters; also stores line attributes.
Advanced video option - Optional circuit board that contains extra screen RAM, extra attribute RAM,
and program ROM sockets and decoders.

ALT CHAR SET - Alternate character set. If the AVO is present, a ROM with a different character'
set can be installed in a socket on the terminal controller, and selected with a control function.

C-l

ANSI - American National Standards Institute
ANSI mode - A mode in which the terminal recognizes and responds only to control functions whose
syntax and meaning follow ANSI specifications.
ASCII - American Standards Committee for Information Interchange
Asynchronous - For serial data transmission; method allows sender and receiver to operate with nonidentical clocks.
Attribute - A display feature such as blinking characters, double width lines, or reverse screen.
AVO - Advanced Video Option
Base attribute - The one attribute that characters on the screen can have when the AVO is not present.
Selectable at SET-UP to be either reverse or underline.
Baud rate - Rate of data exchange on a serial interface.

))

Bipolar - A kind of transistor construction used in TTL and high speed LSI.
Bit map - A large memory with as many addresses as the display has cartesian coordinates. There is a
one-to-one correspondence between pixels on the screen and addresses in the memory.
Blink - An attribute that makes a character blink.
Bold - An attribute that makes a character brighter.
Bottom half - In a double height line, this is asserted for the bottom half of the line.
Bus - A group of w,ires carrying several separate but related signals.
Byte - Eight bits treated as a unit.
C/O - Command/Data. Control line to the PUSART.
..~

Caps lock - A key that forces alphabetics to uppercase without affecting numeric and symbolic keys.
Cathode - The element that is driven to control intensity of the electron beam in the CRT.
CHAR CLK - Character Clock. A clock in the video processor with a period equal to the time between
characters on the display. Varies according to 80 or 132 column mode.
Character - A pattern of dots on the CRT screen representing an ASCII character; a pattern which
represents an element of written language or mathematics; a group of 7 or 8 bits representing a control
or graphic entity. In serial-by-bit transmission, a character is transferred from low-order bit to high.
order bit.
Character generator - A ROM that translates character codes into patterns for display.
Character position - That portion of a display that is displaying or is capable of displaying a graphic
symbol.
Checksum - A number created specifically to detect errors in ·stored or exchanged data.

C-2

Column mode - The number of characters positions provided on a line - 80 or 132.
COMP SYNC - Composite Sync signal
Composite sync - The signal that coordinates the motion of the electron beam in a video monitor that is
external to the VT100.
Control- The term "control" refers to a "control function." A control function is implemented through
the use of a control character, escape sequence, control sequence, or control string.
Control (key) - Produces control characters when pressed with other keys.
Control chip - The DCO 12
Control character - A single character whose occurrence in a particular context initiates, modifies, or
stops a control function. The value of a control character isin the range 0 through IFH and 7FH in a 7'bit environment.
.

)

Control function - A special action the terminal can perform to affect recording, processing, transmitting, or interpreting data. Also, the sequence of characters that cause the action.
Control Q - An ASCII control character meaning XON.
Control S - An ASCII control character meaning XOFF.

Control sequence - A sequence of characters used for control purposes to perform a control function. A
control sequence is a string of characters that begins with the control sequence introducer (CSI) and
ends with the first occurrence of a final character (40H - 7EH). A control sequence may contain 0 or
more parameter characters (30H - 3FH) and/or intermediate characters (20H-2FH).
Control sequence introducer - A prefix to a control sequence that provides supplementary control functions. The CSI for the VT100 series is ESC [ (1 BH SBH).
Control string - A string of characters used to perform a control function and delimited by an opening
and closing delimiter character.
CPU - Central processing unit. Generally means host.
CRT - Cathode ray tube. The display device in a monitor.
CSI - Control sequence introducer
Current loop - The 20 rnA interface option.
Cursor - A blinking underline or blinking reverse field indicating the active position.
DO,Dl, ...,Dx - Individual lines on the data bus.
DB bus - The bidirectionally buffered data bus.

DCOll - Timing chip for the video processor. Produces timing signals, clocks, and line buffer addressing.
')

DC012 - Control chip for the video processor. Controls DMAs and generation of character video .

C-3

DCA - Direct cursor addressing
DEC - Prefix for DEC private control functions.
Default - Parameters provided when no choice is made by the user.
Demultiplexer - Routes a signal to one of sever~l outputs according to control signals.
DH - Double height
Direct drive - Controls movement of electron beam in the CRT with separate horizontal and vertical
signals rather than combining the control signals into the single composite video signal.
Direct memory access - An action by which the video processor can read data directly from the microprocessor's memory.
DMA - Direct memory access .
DMA ENABLE - A signal allowing a DMA to occur.

)

DO bus - The buffered write-only (output-only) data bus.
Dot - The smallest displayable unit of information on the screen.
DOT elK - The fastest clock in the video processor; clocks the video shift register. Varies with 80 or
132 column mode.
Dot stretcher - In the DCO 12. adds one dot to each row of contiguous dots. Ensures that bandwidth
requirement of CRT is met.

Double height - Makes each line display in 20 scans instead of the regular 10.
Double width - Makes each character display in 20 dots instead of the regular 10.
DSR - Data Set Ready
DTR - Data Terminal Ready

'II

)

DW - Double Width
E4 - Normal character set ROM
E9 - Alternate character set ROM
Ell - Line buffer 04-07
El5 - Character latch buffer
El6 - Character generator latch
El7 - Line buffer 00-03
E20 - Screen RAM character latch

C-4

E21 - Address counter AS-All
E25 - Address counter A4-A 7
E30 - Address counter AO-A3
E60 - Programmable baud rate generator
EAROM - Electrically alterable read-only memory - the NVR.
EIA - Electronic Industries Association; used to refer to the EIA standard voltage I/O interface, RS232-C.
ERl400 - Nonvolatile RAM
ESC - Escape character (ASCII IBH)
-

)

Escape character (ESC) - A control character that provides code extension and that is itself a prefix
affecting the interpretation of a limited number of contiguous characters.
Escape sequence - A sequence of characters used for control purposes to perform a control function.
The sequence begins with an (1 BH) control character and ends with the fir~t occurrence of a final character (30H-7EH). An escape sequence may have 0 or more intermediate characters (20H-2FH) preceding the final character.
Exclusive-OR - A logical function that provides an output only when one or the other input signal IS
present but not when both or neither are present.
EXT - External
Field - That part of a video image displayed during one vertical sweep of the CRT beam from the top
of the screen to the bottom.
.
Fill line - A terminator and a pair of address bytes. Synchronizes the DMA process in the video processor without requiring a full line of memory.
Firmware - The microprocessor program.
Flag - An internal signal to the microprocessor.
Flyback transformer - Generates high voltages in the CRT monitor.
Frame - A complete video image.
Graphics - A kind of display showing lines and shapes rather than alphanumerics.
Halt - A condition in the microprocessor when no instructions are performed.
Hard copy - Computer output that can be carried away from the output device in human-readable
form.
Hardware - The electrical and mechanical structure of a device.
HOLD REQ - Hold request. A signal asking the microprocessor to give up use of the address, data, and
control buses to the video processor.

C-5

HORIZ BLANK - Horizontal blank. Signal that turns the electron beam off during the horizontal retrace interval.

))

HORIZ DRIVE - Horizontal drive. Direct drive signal from the video processor that synchronizes horizontal deflection in the internal monitor.
HORIZONTAL TIME - Gating signal inside the DCOI2.
Host - The computer that the VT100 communicates with.
Hysteresis - A characteristic of detection circuits that makes the threshold of detection different for
different directions of change of the input signal.

Ie - Integrated circuit.
Idle - On a communication line, a state when no information is being exchanged.
INIT - Initialize. Signal that informs options and internal devices that the VT100 has performed a reset
operation.

INTA - Interrupt acknowledge. Signal the 8080 produces to indicate it is ready for a RST instruction
containing an interrupt vector. The signal enables the outputs on the interrupt vector buffer.
Interlace - A kind of video display where the information from two fields is displayed by offsetting the
vertical position of one field slightly from the other so the scans of one field appear between the scans of
the other.
Interrupt - A signal to the microprocessor to get it to set aside its current work to take care of a high
priority task. Such tasks include getting data from a communication line before it disappears.

I/O RD - Input/output read. A microprocessor control bus signal.
I/O WR - Input/output write. A microprocessor control bus signal.
I/O - Input/output. A general word that refers to data transfers between the microprocessor and devices un~er microprocessor control, like sensors and indicators.
ISO - International Standards Organization
Jump - In firmware, a movement to a nonsequential instruction. In a display, a form of scrolling where
lines of characters on the screen move up or down by the height of a character line in one operation
(compare to smooth scrolling).
KEY DOWN - Signal in the keyboard that causes the transmission of a key address to the terminal.
Keypad - Generally refers to the cluster of 18 numeric and special function keys on the right side of the
keyboard.
Latch - A device that can store ·data.
LATOFS - Line address offset table. Used by the firmware to determine the display order of the lines.
LBA - Line buffer address. Output from the DCOII to address character lotation in the video. proces-')I
sor's line memory.
,

C-6

LC - Inductor-capacitor
LED - Light emitting diode. A light source (usually red). There are seven at the top of the keyboard.
Line - A line of either 80 or 132 characters. Characters do not have to be present for the line to exist in
memory.
Line attribute - An attribute that affects the entire line, such as double width.
Line buffer - Memory that stores the current line for display during the nine non-DMA scans.
Local - A condition in which the output from the keyboard goes directly to the screen without going to
the host.
LSI - Large scale integration. Very complex circuitry packed into small packages. The DCO I I,
DCO I 2, and 8080 are examples.

)

Mark - One state of a communication line. Generally defined as a low signal level or the presence of
current flow. See also Space.
Mask-programmed - Programmed as part of the manufacturing process and not changeable.
Matrix - An arrangement that allows addressing of many individual points with few address lines. Used
in the keyboard switch array.
MEM DISABLE - Memory disable. Signal used by the AVO to disable main memory outputs when
other memory is being addressed.

)

MEM RD - Memory read. A microprocessor control bus signal.

MEM WR - Memory write. A microprocessor control bus signal.
Microprocessor - The 8080 and associated devices. Controls VT100 operation.
MNOS - Metal nitride oxide semiconductor. The semiconductor technology used in the nonvolatile
RAM.
Mode instruction - Command to the PUSART that sets up the basic operating protocol.
Modem - A device that converts the VT100's EIA output to audio tones that can pass over telephone
lines.
Modulus - The larges~ unique value in a counter. If incremented beyond its modulus, a counter returns
to 0 and counts up again.
MOS - Metal oxide semiconductor. A class of devices distinct from bipolar.
ms - Millisecond
Multiplexer - A device that selects from several inputs to give one output.
MUX - Multiplexer

C-7

New scroll zone - A signal inside the DCO 12 that indicates the beginning of a scroll zone and commands a new DMA.
NMOS - N-channel metal oxide semiconductor. Class of device used in the 8080 and 8251 and 2114
(memory chip).
NO SCROLL - A key that stops new data from entering the screen.
Nonvolatile RAM - Writable memory that does not lose its data when power is off.
NTSC - National Television Standards Committee
NVR - Nonvolatile RAM
Offset - The number of scans a line is moved from the normal display position in a given frame during a
smooth scroll. Also, the rearrangement of line display order according to LATOFS.
On line - A condition in which all keyboard information passes to the host computer and the screen
receives its data from the host.
Overrun error - Occurs in the PUSART if the microprocessor did not read a character before the next
one arrived on top of it.
Parallel - Data path where all bits travel simultaneously on separate wires.
Parity - An error detection system based on the number of bits set in each data byte.
Parity error - An error condition indicating that at least one of the bits in a byte changed.
Parser - A process that separates a sequence into its component parts.
Passive - For the current loop option, the interface accepts current from outside and passes it or blocks
it to transmit: to receive, it detects changes in the incoming current. For the STP, the option plugged
into the STP examines the data on the interface and can add messages of its own, but cannot change the
data passing between the terminal controller and the host.
Pn - Parameter
Pop - The microprocessor retrieves data from the stack.
Port - A place where data can enter or exit a device.
Pulse width modulation - Encoding of data by varying the duty cycle of a continuous clock.
PUSART - Programmable universal synchronous/asynchronous receiver/transmitter. The communication device in the VT100.
Push - The microprocessor puts data in the stack.
PWM - Pulse width modulation
RAM - Random access memory (also known as read/write memory).

C-8

))

Raster - On a CRT screen, the effect of continuous vertical and horizontal deflections of the electron
beam covering the full height and width of the screen. If the beam is turned off, the raster is not visible.
RC - Resistor-capacitor
RCLK - Receive clock
Ready - A control line that forces a wait state in the 8080. Not used in the VT100.
Recall - A routine that sets SET-UP data from the NVR.
Refresh - The process of repeatedly rewriting the screen with data so it appears to to be constantly lit.
Reset - Setting a device to a starting condition, often for clearing errors.
Restart - Instruction the 8080 performs when interrupted.
Retrace - Rapid movement of the turned-off CRT beam from the end of one pass to the beginning of
another.
Reverse video - A character attribute: characters are seen as dark areas in fields of light.
Reverse screen - A screen attribute: the entire screen is normally rendered as black characters on a
white background.
RO - Receive-only
Rollover - Ability to accept more than one key pressed at the same time.

)

ROM - Read only memory
Routine - Set of instructions to the microprocessor that makes it perform a particular function.
RS-170 - An E I A standard that dictates television signal characteristics.
RS-232-C - An EIA standard that dictates data interface characteristics.
RSL - Recommended spares list
RST - Restart instruction
RTS - Request to send - modem control signal.
RxD - Receive data - PUSART
RxRDY - Receiver ready - PUSART
Save - Process of storing SET-UP data in the NVR.
Scan - One horizontal pass of the CRT beam; also, the character information displayed in that pass.
Scan count - The video processor keeps count to help decide when to initiate a DMA and to provide
addresses for the character generator ROM.

C-9

seD -:- Secondary carrier detect. A modem control signal.
Schmidt trigger - A device that accepts a slowly varying signal to an input with hysteresis in its threshold of detection. It outputs clean transitions when the input passes the threshold of detection in either
direction.
Scratch RAM - That portion of RAM used for microprocessor operations.
Screen - Face of CRT on which data are displayed.
Screen attribute - Applies to the entire display area: reverse screen, smooth scrolling.
Screen RAM - That portion of RAM used for display storage.
Scroll - Upward or downward movement of data on the screen.
SEL 8-t2K - Select memory in the range 2000H to 2FFFH.
SEL ATT RAM - Select attribute RAM. Signal for reading and writing data in the attribute RAM on
the A YO under microprocessor control.
Serial - Transmission of data bit-by-bit over a single data line.
SET-UP - Special mode of terminal operation for entering operating parameters from the keyboard.
SET-UP specifications - Those terminal operating parameters entered from the keyboard or changeable from the host.
SHUF AD - Shuffle address. The pointer address location that changes during a shuffle.

)

SHUFDT- Shuffle data. The pointer address stored in SHUFAD during a shuffle.
Shuffle - The process of quickly rearranging pointer addresses when a line scrolls off or on the screen.
SILIN - The address where the next character enters the SILO.
SILO - A Scratch RAM area where data coming from the communication port is buffered on a firstin/first-out basis.

')'
,

SILOUT - The address where the next character exits the SILO.
Smooth scroll - Scrolling in which the data on the screen moves only one scan per frame.
Soft copy - Computer output that only exists as light on a screen.
Space - One state of a communicatiqn line. Generally defined as a high signal level or the absence of
current. (See also Mark.)
SPDI - Speed indicator. A modem control signal.
SPDS - Speed select. A modem control signal.
Split screen - Display operation where one part of the screen can scroll while another part remains
stationary.

'1
/

CIO

)

SR - Shift register. The device that performs parallel to serial conversion of data from the character
generator ROM to the CRT. Also, the parallel-to-serial (or s-to-p converter) in UARTs and PUSARTs.
SRTS - Secondary request to send. A modem control signal.
Stack - Area of scratch RAM where the microprocessor places its current status while processing an
interrupt or subroutine call.
Start bit - The first bit in a serial, asynchronous byte transmission, always a space.
I

Status byte - A byte of information about the operation of the 8080 that it outputs during the first
machine cycle of an instruction. The byte is latched and decoded into control signals by the 8228 system controller.
Stop bit - The last bit in a serial, asynchronous byte transmission, always a mark.
STP - Standard terminal port

··l

Strobe - A signal (usually brief) that commands a device to perform a function.
STSTB - Status strobe. Signal output by the 8080 to latch the status byte into the 8228.
Sync - Any signal that allows one device to operate precisely in step with another. Particularly applies
to synchronization of the electron beam in a monitor to the video data that modulates the beam.
Synchronous - Processes that occur in synchronism.
TBMT - Transmit buffer empty. A signal from the keyboard UART.
TC - Terminal controller. The main VT100 circuit board.
TCLK - Transmit clock
Terminator - A character mixed in with screen data that signals the end of a line and causes the video
processor to advance to the next line.
Threshold of detection - That value of input that causes a detector's output to change state.

)

Timing chip - The DCa 1 1
Toggle - To alternate the state of a device between two values.
Top half - In a double height line, this is asserted for the top half of the line.
TP - Terminal processor. A hypothetical nonvideo processor built into the VT100 cabinet.
Tristate - A device output that can sink current, source current, or become high impedance and not
affect the circuits connected to it.
Tx ENable - Transmitter enable. A bit in the PUSART command instruction.
TxD - Transmitter data. PUSART serial data output.

)

TxEmpty - Transmitter empty. PUSART control output.

ell

TxRDY - Transmitter ready. PUSART control output.
UART - Universal asynchronous receiver-transmitter. Wire-programmed device used in the keyboard
interface.
Underline - A character attribute that forces scan 9 to show during a character.
IJ.S -

Microsecond

Vector - The address of the first instruction for an interrupt handling routine.
VERT BLANK - Vertical blank. Signal that turns off the CRT beam during the vertical interval.
VERT DRIVE - Vertical drive. Direct drive signal from the video processor that synchronizes vertical
deflection in the monitor.
VERT RESET - Signal that occurs at the bottom of the screen to start the video processor at the top.
Vertical interval - The portion of a raster when the beam is turned off and returning to the top of the
screen.

\)
_-

VID WR 1 - Video Write I. Signal to load control data into the DCOlt.
VID WR 2 - Video Write 2. Signal to load control data into the DCOl2.
Video processor - The circuitry that converts character codes stored in RAM into video signals that
display as graphic characters on the screen.
VID IN - Video input (to the DCOI2).

)

VID OUT - Video output (two outputs from the DCOI2).
VSR LD - Video shift register load. A signal that determines whether the video shift register performs
a parallel load or a shift when clocked.
VT52 - The DIGITAL video terminal that preceded the VT100. It responded to escape sequences conforming to an internal DIGITAL standard.

)

Wait - Displayed message for operator during NVR operations. Also, an indefinite state in the 8080
controlled by Ready (and not used in the VT100).
Wire-ANDed- TTL devices with open-collector outputs can be tied together to form a logical AND
gate at the outputs.
WRITE LB - Write line buffer. Signal that writes a character into the line buffer.
XMIT flag - Transmit flag. Signal to microprocessor from TxRDY at PUSART.
XOFF - Control character that asks the sender to stop sending.
XON - Control character that asks the sender to resume sending.

)
C-12

)
APPENDIX D
ANSI CODE EXTENSION TECHNIQUES

GENERAL
This appendix describes the ANSI code extension techniques as defined in ANSI standards X3.411974 and X3.64-1979 (ISO 2022 and 6429). (Refer to Chapter 1 for ANSI standards ordering information.) The description is based on the functions used in the VTl 00 and LA 120 families of terminals.
There are many special cases and details in the specifications that are not described here.
CLASSES OF CHARACTERS
The ANSI system is based on the use of classes of characters for specific purposes. The classes are
determined by the character position in the ASCII table (Figure 0-1). This table and the ANSI system
can work for either a 7-bit or an 8-bit character environment. Current terminals support only 7-bit characters.

)

CONTROL FUNCTIONS
Control functions are all control characters and groups of characters (strings) that control terminal operation but are not displayed on the screen. Not all control functions perform an action in every device
that understands ANSI, but each device can understand all control functions and discard any that do
not apply to it. Therefore, each device is said to perform a subset of the ANSI functions.
ANSI COMPLIANCE
Different devices use different subsets. Therefore, compliance with ANSI does not mean compatibility
between devices. Compliance only means that a given action, if defined in the ANSI standard, is
caused by the same control function in all devices. If an ANSI device does not perform an action that
has a control function defined in the ANSI standard, it cannot use that control function for any other
purpose.
For example, ESC c is the Reset sequence for devices meeting ANSI and having a remote reset function. If a device does not have this function, it may not use ESC c for any other purpose. ESC 7 (Save
Cursor Position), however, is a private sequence and may be used for other purposes by devices from
other manufacturers. But within DIGITAL each private sequence is registered in an internal standard
so that all DIGITAL products use each sequence for only one purpose.
CONTROL CHARACTERS
A control character is a single character which (when received by the terminal) starts, modifies, or
stops a control function. The value of a control character is in the octal range of 0 through 37 and 177.
Appendix A of this document explains the control characters understood by the terminal. All other
control character codes are ignored.

This terminal can perform some actions usually caused by control character codes from the 8-bit ASCII environment, which this terminal does not understand. It does this by understanding certain cornbinations of 7-bit codes, which other sections of this appendix will explain.

0.1

0
0

BITS; - -

COLUMN

o 0 0 0

NUL

0
0
0

DLE

20
16
10

1
1
1

DC1
(XON)

0
1

40
32
20

60
48
30

21
17
11

41
33
21

42
34
22

o 0

STX

2
2
2

DC2

22
18
12

ETX

3
3
3

DC3

23
19
13

43
35
23

100
64
40

61
49
.31

101
65
41

102
66
42

63
51
33

120
80
50
121
81
51

122
82
52

103
67
43

123
83
53

64
52
34

104

1
1

.SP

o 0 0

1
0

SOH

0
1

84838281 ROW

0
0

6
140

160
112
70

141
97
61

161
113
71

142
98
62

162
114

143
99
63

163
115
73

124
84
54

144
100
64

164
116
74

o 0

EOT

4
4
4

DC4

24
20
1.4

ENQ

5
5
5

NAK

25
21
15

45
37
25

65
53
35

105
69
45

125
85
55

145
101
65

165
117
75

ACK

6
6
6

SYN

26
22
16

66
54

126

146
102
66

166
118
76

ETB

67
55
37

106
70
46
107
71
47

27
23
17

46
38
26
47
39
27

127
87
57

50
40
28

130
88
58

150
104
68

167
119
77
170
120
78

)

51
41

71
57
39

131
89
59

151
105
69

,
1

o 1

(XOFF)

36
24

BEL

7
7
7

10
8
8

CAN

11
9
9

31
25
19

SUB

32
26
lA

52
42
2A

53
43
28

;

54
44
2C

001

24
18

12
10
A

o 1 1

13
11
8

ESC

33
27
18

o (j

14
12
C

34
28
lC

1 0

o 1

15
13
0

35
29
10

16
14
E

17
15
F

37
31
IF

30
IE

(

147
103
67

171
121
79
172

111

112
74
4A

132
90
5A

152
106
6A

73
59
38

113

[

133
91
58

153
107
68

{

173
123
78

74
60
3C

114
76
4C

174
124
7C

75
61
3D

115
77
40

]

154
108
6C
155
109
60

55
45
20

134
92
5C
135
93
50

}

175
125
7D

76
62
3E

116
78
4E

77
63
3F

117
79
4F

58
3A

57
47
2F

ASCII C H A R A C T E R E33
i l l I OCTAL
27
DECIMAL
HEX

Figure D-I

ASCII Table

D-2

73
49

KEY
18

86
56

110
48

•

68
44

75
48

136
94
5E
137
95
5F

m
n
0

156
110
6E
157
111
6F

DEL

122
7A

176
126
7E
177
127
7F

ESCAPE SEQUENCES
The Escape or ESC character (0338) is a control character that causes the terminal to wait for more
characters that are not in the control character numerical range. This character is defined by ANSI
standard X3.4-1977 as Introducer. If the terminal receives this character, it waits for more characters
to follow within certain numerical ranges to form an escape sequence as defined in ANSI X3.41-1974
and ANSI X3.64-1979.
The format of an escape sequence is:
ESC
033
Escape
sequence
introducer

1...1
040-057
Intermediate
characters
(Any number of
codes - 0 or more)

060-176
Final
character
(One code)

If following characters are in the range 040 - 057(8) (column 2), they are called intermediate characters. The device accepts and stores them.
If a following character is in the range 060 - 176(8) (columns 3 to 7), it is a final character. The final
character signals the end of an escape sequence which the device then analyzes. Final characters from
column 3 are for private control functions for use in a specific device. Final characters from columns 4
- 7 are for ANSI standardized control functions.

Some two-character escape sequences perform the same actions as some 8-bit single-character control
functions. The VT100 family supports six of these. (Refer to Appendix A.)
1.
2.
3.
4.
5.
6.

ESC [ is CSI
ESC D is IND
ESC E is NEL
ESC M is RI
ESC N is SS2
ESC 0 is SS3

The VTI25 also supports ESC P. which is DCS and ESC \, which is ST. (Refer to Communication and
Graphic Protocol Controls in the VTJ 25 User Guide.)

)

The intermediate and final characters are taken together to define the function of the sequence. Then
the device performs the action and accepts more data. If the action defined by the escape sequence
does not apply to the device, the device ignores the complete sequence and accepts more data.

)
D-3

Escape sequence examples (all examples have added spaces between characters to make them easier to
read).
ESCH
ESC ( B
ESC#6
ESC ( 0

( )

Set tab at active position
Designate GO character set as ASCII
Double width line (VT100)(6 = DIGITAL private)
Designate GO character set as DIGITAL private
special graphics character set (0 = private)

. Example sequence: Designate GO character set as ASCII
Select Character Set (SCS) = ESC ( B
- Sequence
_ESC

Octal representation
of sequence
(

. " Escape
character
':.'

Final
character

033

050

( )

Final
character

Escape
character

Intermediate
character

102

I nterinedia te
character

CONTROL SEQUENCES
The string ESC [ is a two-character escape sequence and represents the 8-bit control character Control
Sequence Introducer (CSI). CSI precedes all control sequences in the same way that the ESC introducer precedes all escape sequences. ESC [ allows the extended functions of the 8-bit control sequence environment to work in the 7-bit environment of current terminals. The control sequence is defined in ANSI X3.64-1979.

c:
_

The format of a control sequence is as follows .
. CSI
033 133
Control
sequence
introducer

P... P
060-077
Column 3
parameter
(0 or more
codes)

1•. .1
040-057
Column 2
intermediate
(0 or more
codes)

F
100-176
Column 4-7
final
(One code)

(

A device parses this sequence without considering its meaning. That is, characters are stored in classes
only according to their range of values. Then. the device interprets these characters by value according
to their classes. The intermediate and final characters are taken together to define the function of the
sequence. In the range of final characters, 100 - 157 (columns 4 - 6) are reserved for standardization
by ANSI, while 160 - 176 (column 7) are reserved for private use.

(
D-4

Parameters
Parameters modify the action or interpretation of the function. The parameters are from column 3 and
may be any combination of the characters 0 - 9 (060 - 071) with each parameter separated from the
others by; (073). (The other characters in column 3 are: (072) which is reserved, and <=>? (074077) which are assigned for private use and mean that the following parameters have a private interpretation.) Any leading zero in a parameter is ignored; this also applies to the parameter value o. Therefore, a sequence with no parameter is the same as a sequence with a parameter of 0 and both are understood as having the default value for that parameter in the sequence.
A single parameter that modifies the action of a control function is called a numeric parameter and has
the abbreviation Pn. (Example: Cursor Up, ESC [ Pn A, where Pn is number of lines.) A parameter
that defines the action of a control function by selecting from a list of possible actions is called a selective parameter and has the abbreviation Ps. (Example: Set Mode, ESC [ Ps h, where Ps selects the
mode to be set.) Control functions that have selective parameters can accept multiple parameters to
allow several actions to be commanded with a single control function.

(

A sequence with multiple parameters has several Ps separated by; characters (Ps;Ps;Ps). This is called
a parameter string. If the parameters apply to the screen image, their abbreviations indicate this: Pt;Pb
for top and bottom, and PI;Pc for line and column.
Character 077 (?) at the beginning of a parameter string means that the parameters are private parameters. That means the control sequence is standardized but the function that it controls is private, for
example, set and reset mode control functions. Some control functions are defined to have a default
value for a parameter. The default value is assumed when no parameter character is included in a sequence.

(

Examples with Octal Equivalents
ESC [
033 133

063

147

The above sequence clears all tabs.

(

ESC [
033 133

g
147

The above sequence clears all tabs at active position (default value = 0).

ESC [
033 133

1
061

6
066

073

3
063

2
062

165

The above sequence sets tabs at columns 16, 32 (LA 120) (u = private).

(
0-5

ESC [
033 133

077

062

073

063

h
150

The above sequence sets modes 2 and 3 (? = private).

ESC [
033 133

062

060

h
150

The above sequence sets line feed/new line mode (parameter = 20).

ESC [
033 133

21
062 073 061

171

The above sequence runs power-up self-test (VT100) (y = private).
NOTE
There. are no examples of control sequences with intermediate characters because current terminals do
not have any control· functions in that format. However, new software written to understand the ANSI
syntax should be able to parse sequences with intermediate characters. Future DIGITAL terminal
products may use intermediate characters.
': Example Sequence: Control sequence to set modes for 132 column mode, smooth scrolling, and reverse
. screen
ESC[?3;4;Sh

)
Indicates private
parameter

ESC [

CSI

(

Delimiters

Parameters

Final
character

' - - - - - - Parameter string

)
0-6

}

Indicates private
parameter

(Delimiters

033

063

133

077

073

064

073

065

Lparameters - - - - I

CSI

150

Final
character

'---_ Parameter string _ _...J
Alternate sequences that will do the same thing:
ESC[?3hESC[?4hESC[?5h

Parameters can be split into separate control sequences.

,
"

ESC[?03;004;5h

Leading zeroes are ignored.

ERROR RECOVERY
The ANSI standards do not define error recovery techniques for incorrect control functions. Theseerrors include out of range parameters, invalid control functions. and control characters embedded in
control functions. The VT100 family recovers from errors with as much correct function as possible
rather than discard any error. For example, jf the VT100 receives a sequencej:lsking it to move the
cursor beyond the right margin. it moves the cursor to the right margin. In the LA120, a command to
move beyond the right margin is ignored and the active position stays unchanged.

If a control character appears within a sequence, the VTI 00 performs the function of the control character (for example, a carriage return) as if it had been received before the beginning of the sequence.
However, CAN and SUB appearing in a sequence stop the processing of the sequence at that point. The
terminal returns to regular character processing and displays any characters remaining inthe sequence.
;,

An unrecognizable control function is ignored. Unsupported control functions (any apparently valid sequences that are not listed in this document) are generally ignored but may produce unpredictable,re..;.
sponses.

NOTE
Some programmers have used error condition actions in a given terminal to get the actions they
wanted. This is not a safe practice in the ANSI environment because there is no guarantee that different
ANSI-complying terminals will handle an error the
same way. Using error conditions would limit the
transportability of code.

CHARACTER SETS AND SELECTION
GO, Gt, CO, Ct Character Sets
The ANSI and ISO standards provide extensions to the range of graphic and control character sets in a
terminal, in addition to the extension of control functions described in the preceding section.
A typical terminal transmits and receives the 7-bit ASCII character set. This character set has an
eight-column chart, and:in it, columns 0 and 1 are control characters, while the rest of the set is graphics (except SP and DEL). SP (space) and DEL (delete) are always the same control characters with the
same codes, regardless of character set and so they are independent of character set selection.
The ANSI standards provide a system to allow the use of larger character sets in any terminal, without
increasing the number of bits that tHe terminal must use to describe each unique character. Refer to
Figure D-2. The left side of the figure represents the familiar 7-bit ASCII character set. GO and Glare
labels attached to character sets to indicate how the sets can be substituted. The ANSI word for this is
designate. There are escape sequences "that designate character sets as either. GO or G 1. The control
characters shift out (SO (016» and shift in (SI (017», when included with 7-bit ASCII data, switch the
display of a terminal from one character se\to the other. The ANSI word for this is invoke.

))

I n the VT 100, any character set whose display patterns are stored in the terminal can become either GO
or G I. SO always invokes the G 1 set and SI invokes the GO set. Sets can be invoked or designated at
any time and in any order. Some character sets have been internationally registered, while others are
private for use in a given terminal.
In the VT100, the CO control character set is normally available. There are escape sequences that cause
the actions of single control functions of the C 1 set on a one time basis. The ESC character followed by
a final character from columns 4 or 5 causes the action of a control function that is also caused by a
single 8-bit character in the C 1 set. Figure D-2 shows how ESC [ causes the CSI function and ESC D
causes the IND function from the Cl set. The other Cl characters that are supported in the VT100
family of terminals are also shown.

)

Figure D-3 shows a schematic representation of the Shift Out and Shift In concept. This figure shows
how GO and G I character sets are designated by escape sequences and invoked by the SO and SI characters ..

)

0-8

"'COLUMN "~;;.
-01' n :2;' 3 4 5
6, 7 1 a 11 1 2' ' 1 3" 14' 1'5 ' 1 6 1 7
ROWr-~~__-+~~~~__~~~__~~~~~~~~~

a
1

, ~,:

" ,SET
. :'
~

" 4"

CO
·,SET.

,"6

. (

.. )

10
[

.1'2,
14
15}

,'"

;,; ,,'''j,e

'ST

)

, . :-: c"'~J);:" ',)(J

E---+-? C,SI
:

.';. l' (~

,:.;j ~,,,

~.":

r; " .

:,"'

17: .:-',""",...
, -,~_ _ _......;:O~""-_""""""",,,,,,,,,,_-,,,,,,,,,,~,,....-_..,.,'""'":""....-_-.-1-____
~\~':..

CO,Q1 = CONTROL CHARACTER SETS
GO, G1 = GRAPHIC CHARACTER SETS

,'1..' ;',

. •... ,

.'
.'

DCS, ST=VT125,VK1000NLY
,

:d· ,,' '

"./
.'.

:,'(

MA-6660
',r

Figur~ D-2 .8-:Bit ASCII Chart'

0----- ESC (A,;--7~GO SET 1 (UK)
SI

)

. - -,ESC(B--:-- ~GO

SET 2 (ASCII)

o---ESC (0 ---~ GO

SET 3 (SPECIAL GRAPHICS)

~----ESC)A ---~ Gl

0-- 0

ESC)B ---~ G1

SET 1 (UK)
SET 2 (ASCII)

figure D-3

SET n

Shift Out and Shift In

)

0.9

...,.'-

.!,.'

"0--- ESC)O ----+ Gl SET 3 (SPECIAL GRAPHICS)
0----------+ Gl SET 4
o-----------~ Gl

i'-

'';,

0----------+ GO SET 4
0------------+ GO SET n

7-BIT
DATA

. ,

MA-6659

VT100 VIDEO TERMINAL SERIES TECHNICAL MANUAL

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