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Document:	VT55-E,F,H,J DECgraphic Scope Users' Manual
Order Number:	EK-VT55E-TM
Revision:	001
Pages:	94
Original Filename:	http://www.mirrorservice.org/sites/www.bitsavers.org/www.computer.museum.uq.edu.au/pdf/EK-VT55E-TM-001%20VT55-E%2C%20F%2C%20H%2C%20J%20DECgraphic%20Scope%20User's%20Manual.pdf

OCR Text

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, Users' Manual
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1976

EK·VT55E·TM·001

VT55-E, F, H, J DECgraphic Scope
Users' Manual

digital equipment corporation • maynard, massachusetts

1st Edition, December 1976

The material in this manual is for informational
purposes and is subject to change without notice.
Digital Equipment Corporation assumes no responsibility for any errors which may appear in this
manual.
Printed in U.S.A.

This document was set on DIGITAL's DECset-8000
computerized typesetting system.

The following are trademarks of Digital Equipment
Corporation, Maynard, Massachusetts:
DEC
DECCOMM
DECsystem-10
DECSYSTEM-20

DECtape
DECUS
DIGITAL
MASSBUS

PDP
RSTS
TYPESET-8
TYPESET-II
UNIBUS

CONTENTS
Page
CHAPTER 1

INTRODUCTION

1.1
1.2
1.2.1
1.3
1.4
1.5
1.5.1
1.5.2
1.5.3
1.6

GENERAL DESCRIPTION
HARDWARE DESCRIPTION
VT55 Technology
VT55 MODELS . . . . . . .
RELATED DOCUMENTS . .
TECHNICAL CHARACTERISTICS
Installation Characteristics
Functional Characteristics
Interface Characteristics
VT55 OPTIONS . . . . . . . .

CHAPTER 2

SITE PREPARATION AND PLANNING

2.1
2.2
2.3
2.3.1
2.3.2
2.3.3
2.3.4
2.4
2.5

PURPOSE AND IMPORTANCE
INSTALLATION SPACE
OPERATING ENVIRONMENT
Operating Temperature and Humidity
Device Dissipation . . . . . . .
Electrical Noise Tolerance
Vibration, Atmosphere, Altitude
POWER REQUIREMENTS
SIGNAL CABLES . . . . . . . . . .

CHAPTER 3

INSTALLATION

3.1
3.2
3.3
3.3.1
3.3.2
3.3.3
3.4
3.4.1
3.4.2
3.4.3
3.4.4
3.4.5
3.4.6
3.5
3.5.1
3.5.2
3.5.3
3.5.4
3.6
3.7

UNPACKING
............... .
PRELIMINARY AND ACCEPTANCE TESTING
MANUAL TESTING
....... .
Miscellaneous Functions . . . .
Graph Drawing Mode Functions
Hold Display Mode Functions
CONNECTING THE VT55 TO AN INTERFACE
20 rnA Interface Cable with Mate-N-Lok Connector
20 rnA Interface Cable with 283B Connector
EIA Interface Cable . . . . . .
Changing VT55 Interface Cables
Setting the Baud Rate Switches
Setting the Parity Switches ..
SERIAL LINE SIGNAL FORMAT
Word Length and Parity
Baud Rate
Levels
Synchronization
ADJUSTING VT55 NOMINAL LINE VOLTAGE
CHANGING VT55 LINE FREQUENCY - NON-COPIER MODELS ONLY

iii

1-1
1-3
1-4
1-4
1-4
1-6
1-6
1-7
1-8
1-8

2-1
2-1
2-2
2-2
2-2
2-2
2-2
2-2
2-3

3-1
3-1
3-2
3-3
3-5
3-6
3-6
3-7
3-8
3-9

3-10
3-10
3-10
3-10
3-13
3-14
3-14
3-14
3-16
3-18

CONTENTS (Cont)
Page
CHAPTER 4

TECHNICAL DESCRIPTION

4.1

4.3.2
4.3.3
4.3.4
4.3.5
4.4

INTRODUCTION . . . . . .
RASTER SCAN TECHNIQUE
COMMUNICATING WITH THE VT55
Alphanumeric Mode
Graphics Mode . . . . . . . . .
Graph Drawing . . . . . . . . .
Escape Mode and Special Terminal Commands
VT55 Keyboard
................. .
VT55 COPIER

CHAPTERS

OPERATION AND PROGRAMMING

5.1
5.2
5.2.1
5.2.2
5.3
5.3.1
5.3.1.1
5.3.1.2
5.3.1.3
5.3.1.4
5.3.1.5
5.3.1.6
5.3.1.7
5.4
5.4.1
5.5
5.6
5.6.1
5.6.2

INTRODUCTION . . . . .
OPERATION . . . . . . .
Powering on the VT55
VT55 Models with Internal Copier
THE KEYBOARD . . .
Using the Keyboard
ASCII Codes
Shift Key ..
CAPS LOCK Key
REPEAT Key ..
SCROLL and COPY Keys
BREAK Key
Auxiliary Keypad
ALPHANUMERIC MODE
Alphanumeric Cursor
GRAPHIC MODE . . . .
DISPLAY CALIBRATION SPECIFICATION
Calibration of the Graph Drawing Field
Alignment of Alphanumerics on the Graph
.......... .
Drawing Data Field
Definition of Graph Marker on the Graph
....... .
Drawing Data Field
GRAPH DRAWING MODE . . . . . . .
Graph Drawing Command Characters
Graph Drawing Data Characters
Other Characters
ESCAPE MODE . . . .
Cursor Down . . .
Reverse Line Feed
Cursor Up
Cursor Right
Cursor Home
Direct Cursor Addressing

4.2

4.3
4.3.1

5.6.3
5.7
5.7.1
5.7.2
5.7.3
5.8
5.8.1
5.8.2
5.8.3
5.8.4
5.8.5
5.8.6

4-1

4-1
4-1
4-2

4-3
4-3
4-4

4-4
4-5

5-1
5-1
5-1
5-1
5-4
5-4
5-5
5-5
5-5
5-7
5-7
5-7
5-7
5-8
5-8
5-10

5-11
5-11
5-12
5-13
5-14
5-14
5-14
5-17
5-17
5-17
5-17
5-18
5-18
5-18
5-18

CONTENTS (Cont)
Page
5.8.7
5.8.8
5.8.9
5.8.10
5.8.11
5.8.12
5.8.13
5.8.14
5.8.15
5.8.16
5.8.17
5.8.18
5.8.19
5.8.20
5.9
5.10
5.11
5.11.1
5.11.2
5.11.3
5.11.4
5.11.5
5.11.6
5.11.7
5.11.8
5.11.9
5.11.10
5.11.11
5.11.11.1
5.11.11.2
5.11.11.3
5.11.11.4
5.11.11.5
5.11.11.6
5.11.11.7
5.11.11.8
5.11.11.9
5.11.11.10
5.11.11.11

Erase to End of Line .
Erase to End of Screen
Identify Terminal Type
Enter Hold Screen Mode
Exit Hold Screen Mode
Enter Alternate Keypad Mode
Exit Alternate Keypad Mode
Copy Screen . . . . . .
Enable Auto-Print Mode
Disable Auto-Print Mode
Enter Graphic Mode
Exit Graphic Mode '"
Enter Graph Drawing Mode
Exit Graph Drawing Mode
HOLD SCREEN COMMAND
CONTROL CODES . . . . . .
GRAPHING COMMANDS AND EXAMPLES
NOOP . . . . . . . .
Load Enable Register 0
Load Enable Register 1
Load Graph 0
Load Graph 1
Load Graph Marker Memory for Graph 0
Load Graph Marker Memory for Graph 1
Load Horizontal Line Coordinate
Load Vertical Line Coordinate
Load Starting X Coordinate
Example Sequence . . . . . .
Reset Logic . . . . . . .
Enter Graph Drawing Mode
Enable Graphs
Load Graph 0 . . .
Load X Coordinate
Load Graph 1 . . .
Load Graph Marker Memory for Graph 0
Load Graph Marker Memory for Graph 1
Load Horizontal Line Coordinate
Load Vertical Line Coordinate
Controlling Display Status

5-19
5-19
5-19
5-19
5-19
5-20
5-20
5-20
5-20
5-20
5-20
5-20
5-20
5-20
5-21
5-22
5-22
5-24
5-24
5-25
5-25
5-26
5-26
5-27
5-27
5-27
5-28
5-29
5-29
5-29
5-29
5-29
5-29
5-29
5-29
5-29
5-29
5-29
5-29

CONTENTS (Cont)
Page
CHAPTER 6
6.1
6.2
6.2.1
6.2.2
6.2.3
6.2.4
6.2.5
6.3

MAINTENANCE
PHILOSOPHY . . . . . . . . . . . . . .
CORRECTIVE MAINTENANCE
Recommended Spare Parts and Tools
Basic Description of Internal Signal Flow
Troubleshooting Procedure
Copier Replacement Procedure
Video Adjustment Procedure
PACKING OF DECscope . . . . .

6-1
6-1
6-1
6-3

6-4
6-13
6-14
6-15

ILLUSTRATIONS
Figure No.
1-1
1-2
1-3
2-1
2-2
3-1
3-2

3-3
3-4
3-5

3-6
3-7
3-8

3-9
4-1
4-2
4-3
5-1
5-2
5-3

5-4
5-5

5-6
6-1
6-2

6-3
6-4
6-5
6-6
6-7

Title

Page

VT55 with Typical Display
. . . . . . . . . . . . . . .
Typical VT55 Displays
.............. .
VT55/Computer Interconnection and Simplified Block Diagram
VT55 Installation Dimensions . . . . . . . . . . .
VT55 Power Connectors
............ .
20 rnA Interface Cable with Mate-N-Lok Connector
20 rnA Interface Cable with 283B Connector .. .
EIA Interface Cable . . . . . . . . . . . . . . . .
Location of Underside Switches and Jumpers (Bottom View)
Transmission (or Reception) of ASCII "B" with Parity Switch
Set to NONE . . . . . . . . . . .
20 rnA Current Loop Connection .
Access to AC Power Terminal Strip
10-Screw Terminal Strip . . . . . .
Location of WI and W2 on the M7024 Graphing Module
Interface Modes .
VT55 Keyboard .
VT55 with Copier
VT55 Controls
State Diagram Showing Change Between Alphanumeric, Graphic,
Graph Drawing, and Escape Modes
VT55 Keyboard . . . . . . . . . . . . . . .
Graph Drawing Data Field Calibration
X and Y Position Specification for Letter "E"
Graph Marker for Graph Point Y = 3228
VT55 Simplified Signal Flow Diagram
ROM, UART, and Timing Module .
VT55 DECscope Assembly Drawing .
Removal of Copier Cover . . . . . .
Location of Controls on Power Supply Monitor Module
Alignment of Alphanumerics and Graphics
VT55 Packing . . . . . . .
. . . . . . . . . . .

1-1
1-2
1-3
2-1
2-3
3-8

3-9
3-10
3-11

3-13
3-14
3-17
3-17

3-19
4-1
4-5

4-6
5-2
5-3
5-6
5-12

5-12
5-13

6-3
6-4
6-5
6-13
6-14
6-15
6-16

TABLES
Table No.

I-I
1-2
3-1
3-2
3-3

3-4
3-5

3-6
3-7
3-8
5-1
5-2
5-3
5-4

5-5
5-6
5-7
5-8

5-9
6-1
6-2

6-3
6-4
6-5

6-6

Title
Summary of VT55 Keyboard and Displayable Features
VT55 Models . . . . . . . . . . . . . . . . . . .
20 rnA Interface Cable Wire Color-Pin Assignments
(Mate-N-Lok) . . . . . . . . . . . . . . . . . . .
20 rnA Interface Cable (Mate-N-Lok) Part Numbers
20 rnA Interface Cable Wire Color-Pin Assignments (283B)
20 rnA Interface Cable (283B) Part Numbers
EIA Interface Cable Wire Color-Pin Assignments
..... .
EIA Interface Cable Part Numbers
Baud Rate and Interface Mode Selection Switch Positions
AC Terminal Strip Jumpers for Different
................ .
Nominal Line Voltages
Code Ensemble of ASCII Keyboard - Transmitted Codes
Codes Transmitted to Host by VT55
......... .
Code Ensemble in Alphanumerics Mode - Received Codes
Alphanumeric Cursor Control .
Display of the Codes 136-176
Graph Drawing Data Characters
.....
Character Values
Escape Mode Characters . . .
Control Codes - All Modes
Recommended Spare Parts Kit
Recommended Tool Kit .. .
.... .
Capacitor Voltages
Transformer Secondary Voltages
CRT Base Voltages . . .
VT55 Shipping Package

vii

Page
1-3
1-5
3-7
3-7
3-8
3-8
3-9
3-9
3-12
3-18
5-5
5-8

5-9
5-9
5-10
5-15
5-16
5-18
5-23
6-2
6-2
6-11
6-12
6-12
6-15

CHAPTER 1
INTRODUCTION

1.1 GENERAL DESCRIPTION
The VT55 DECscope is a serial line-interfaced computer terminal that features alphanumeric and
graphical display in a single unit with upper- and lowercase characters. When interfaced to a host
computer, the VT55 can display as many as two graphs or histograms supported by programmable
grid and programmable graph markers. When alphanumeric information is added to this picture, a
completely defined graph is displayed. As many as 24 lines of 80-character alphanumeric information
may be displayed on the VT55. Control of the VT55 is via the console keyboard and keypad, which is
included with the unit. By using only the alphanumeric capability of this DECscope, with the keyboard
and keypad, the VT55 may be utilized as a 24-line programmers' terminal. Some models of the VT55
include a copier unit that generates a paper copy of the screen's contents, alphanumeric and graphic,
for the user's convenience. Figure 1-1 shows a VT55 with a typical graph displayed; Figure 1-2 shows a
typical line graph, histogram, grid, and the superposition of the three. Table 1-1 summarizes the
important keyboard and displayable VT55 features.

8009-4

Figure 1-1

VT55 with Typical Display

1-1

7717 1

7717-2

Graph 0 with Graphing
Markers (Line Graph)

Graph 1 (Histogram)

77173

7717-4

Labeled Grid

Superposition of a, b, and c

Figure 1-2 Typical VT55 Displays

The VT55 is packaged in a compact, attractive, desktop mounting cabinet. Information is displayed on
a 12-inch (diagonal measurement) cathode ray tube which faces the front of the unit along with the
console keyboard. All electronics are housed within the VT55 cabinet. Models with copiers have the
copier mechanics and the hard copy output located at the top right side of the cabinet.
Typical applications for the VT55 are:
•

A business where sales, profit, or inventory is computerized, and changes due to varying
parameters are being studied.

•

A medical environment where transducers monitor vital body functions; data is computerized and displayed as graphical output.

•

An educational environment where displayed graphs may be altered by varying the parameters of the representative equations.
1-2

Table 1-1

Summary of Vf55 Keyboard and Displayable Features·

Item

Description

Keyboard

ANSI X 4.14 with control keys similar to office typewriters.
Three key rollover and audible feedback is provided.

Keypad

Useful for direct cursor addressing and other number entry
applications.

Displayable Alphanumerics

24 lines, 80 characters per line. 96-character ASCII including
upper- and lowercase letters, numerals, and special characters
that may be displayed on the CRT and printed on the copier
output. Direct cursor addressing in the alphanumerics mode.

Displayable Graphs

Two individually chosen graphs or histograms; 512 points along
the horizontal axis. Each graph must be single-valued along the
vertical axis. Two points cannot be programmed in the same
vertical column on the same graph.
Graph grid composed of as many as 512 vertical lines and 236
horizontal lines, all individually selectable.
Graph markers that may be selected to straddle as many of the
512 points on each of the two graphs as desired.

Display Area

Approximately 203 X 127 mm (horizontal X vertical) - 8 in. X 5
in.
7- X 7-point character font.

*Refer to Paragraph 1.5 for complete specifications.

1.2 HARDWARE DESCRIPTION
The VT55 is normally used in conjunction with a host computer. A serial line cable makes the connection between the terminal and the computer. To transmit information to the computer, the operator presses a key on the terminal keyboard. This transmits a code, representative of the key that was
pressed, to the terminal's serial line output. The computer, at the other end of the serial line cable,
responds to the key in accordance with the computer's program (Figure 1-3).

I VT55 - I KEYBOARD

- - - - 1-------*1

g
N
T

-, SERIAL

SERIAL
LINEIN

LINE OUT

I
I

~ \V

CABLE

SERIAL

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

SERIAL
LINE OUTI

\VI

COMPUTER

8
INTERNAL PROGRAM

I
I
I

L ______ --1
CP-2145

Figure 1-3

VT55jComputer Interconnection and Simplified Block Diagram

1-3

When information is transmitted from the computer's serial line output to the DECscope serial line
input, it is examined by the VT55 control electronics to determine whether the information is alphanumeric data or terminal control information. If the information is alphanumeric or graph data, it will be
stored in the VT55's internal memory. The contents of this memory are constantly displayed on the
cathode ray tube as alphanumeric characters and graph curves and lines.
When the information received via the serial line is terminal control information such as "Erase Lines
of Characters," "Switch to Graph Drawing Mode," or "Generate Hard Copy," the operation of the
terminal is modified accordingly. The terminal then waits for new information. As shown in Figure 13, the copier is connected at the interface of the memory and display. When a Generate Hard Copy
command is issued, the current displayable contents of the memory, alphanumeric and graphic, will be
duplicated onto the copier's output. This copier feature is available on some models of the VT55.
1.2.1 VT55 Technology
The VT55 uses a rather unique combination of technologies to deliver a product with maximum
performance at a minimum price. For example, the cabinet is a modern, high~impact, injection-molded
design yielding strength, good design, and light weight. The internal electronics is a microprocessor
design. A specialized computer program is actually constantly running within the terminal. It is always
waiting to service a request to transmit data from the keyboard to the serial line output, enter data into
the internal semiconductor memories, service a copier request, etc. A separate memory is allotted to
alphanumeric data, graph data, and terminal control information.
The display of the alphanumeric and graphic data is performed by the raster scan method. This is
basically the same method utilized by consumer television sets to produce their displays. The method is
reliable and produces bright, flicker-free information output. The generation of the raster scans and
intensification information are synchronized to the microprocessor, thus eliminating the need for vertical and horizontal hold controls.
The optional copier uses a scanning technique similar to methods utilized in facsimile picture transmission systems of the 1940's. This technique, in conjunction with the use of modern recording paper,
generates high-quality copy in a minimum-size package with a minimum number of moving parts.
1.3 VT55 MODELS
There are 15 models of the VT55 DECscope. They are listed in Table 1-2 and differ in their line
voltage/line frequency and interface requirements. Some models include the copier.
The VT55-EB, EF, HB, FB, FF, and JB models have internal jumpers that allow altering the nominal
input line voltage from 230 Vac to 240 or 220 Vac. Models VT55-EC, EH, and HC allow the nominal
line voltage to be increased from 100 Vac to 110 or 127 Vac.
The capability to vary the nominal input line voltage allows the unit to be tailored to the standard of
the country in which the VT55 is installed. Models with 50/60 Hz capability are always shipped
configured for 50 Hz operation.
Units with Mate-N-Lok connectors are primarily intended for interfacing with DIGITAL serial line
communications interfaces. Units with RS232C connectors are compatible with RS232C Standard
Dataphone and Modem applications. Connector 283B is a telephone-type plug compatible with DECsystem -1 0 applications.
1.4 RELA TED DOCUMENTS
VT55 models FA, FB, FE, FF, JA, and JB are equipped with the VT50 copier. A VT50 Series Copier
User's Manual (EK-VT5C-OP-001) is shipped with these models. Refer to the copier manual when
performing maintenance tasks on the copier.

1-4

Table 1-2 VTSS Models
Model

Line Voltage
(Vac)

Line Frequency
(Hz)

Copier

Interface

VT55-EA

115

20 rnA with Mate-N-Lok
connector

VT55-EB

220/240

50/60

20 rnA with Mate-N-Lok
connector

VT55-EC

100/127

50/60

20 rnA with Mate-N-Lok
connector

VT55-EE

115

EIA with RS232C
connector

VT55-EF

220/240

50/60

EIA with RS232C
connector

VT55-EH

100/127

50/60

EIA with RS232C
connector

VT55-HA

115

20 rnA with 283B
connector

VT55-HB

220/240

50/60

20 rnA with 283B
connector

VT55-HC

100/127

50/60

20 rnA with 283B
connector

VT55-FA

115

Yes

20 rnA with Mate-N-Lok
connector

VT55-FB

220/240

Yes

20 rnA with Mate-N-Lok
connector

VT55-FE

115

Yes

EIA with RS232C
connector

VT55-FF

220/240

Yes

EIA with RS232C
connector

VT55-JA

115

Yes

20 rnA with 283B
connector

VT55-JB

220/240

Yes

20 rnA with 283 B
connector

1-5

1.5 TECHNICAL CHARACTERISTICS
The VT55 DECscope is designed to operate in a typical office or laboratory environment. It will
operate over a wide temperature and humidity range and is insensitive to most power "brownout"
conditions. The following paragraphs itemize the installation, functional, and interface characteristics
of the VT55.
1.5.1 Installation Characteristics
Cabinet Dimensions
Height
Width
Depth
Minimum Table Depth

360 mm (14.1 in)
530 mm (20.9 in)
690 mm (27.2 in)
450 mm (17.7 in)

System Weight
With Copier
Without Copier

25.8 kg (57 lb)
22.2 kg (49 lb)

Temperature
Operating Environment
With Copier
Without Copier
Nonoperating Environment

15° to 32° C (58° to 90° F)
10° to 40° C (50° to 104° F)
-40° to 66° C (-40° to 151° F)

Humidity (N oncondensing)
Operating Environment
With Copier
Without Copier
Nonoperating Environment

20 to 80%
10 to 90%
o to 95%

Power

Model

Nominal Line
Voltage (Vac)

VT55-EA, EE, HA
VT5S-EB, EF, HB*
VTSS-EC, EH, HCt
VTSS-FA,FE,JA
VTSS-FB, FF, JB*

115
240
100
115
240

Voltage Range (Vac)
-13%
+9%
100
209
87
100
209

Maximum
Line Current (Amps)

Line
Freq (Hz)

1.5
0.7
1.7
2.4
1.2

60 ± 1
50/60 ± 1
50/60 ± 1
60 ± 1
50 ± 1

126
262
109
126
262

• Jumper selectable for 230 or 220 Vac, -13%, +9%

t Jumper selectable for 110 or 127 Vac, -13%, +9%

Power Line Voltage Transients

Voltage transients must be less than 300 V peak-topeak to ensure no data errors. Voltage transients
on the power line greater than 1000 V peak-to-peak
may cause damage to the terminal.

Safety and Ground Current Leakage

3.5 rnA max

Fault Protection

Circuit breaker

Power Cord Length

2.74 m (9 ft)
1-6

I.S.2 Functional Characteristics
Video Display
Size
Active Screen Size
Method
Phosphor
Linearity
Alphanumerics
Character Lines
Character Columns
Control
Character Set
Special Features
Character Matrix
Graphics
Resolution
Graphs or Histograms
Grid
Graph Markers
Special Features
Special Features
Hold Screen Mode
Terminal Identifier
Keyboard
Format
Keyclick
Error Correction
Special Keys

Auxiliary Keypad
User Controls
Intensity
Parity
Power /Logic Reset
Baud Rate/Interface Mode

305 mm (12 in) diagonal
203 X 127 mm (8 X 5.0 in)
Raster scan, roll-free
P4
± 1/2 point
24
80
Blinking alpha cursor
96-character, upper- and lowercase ASCII subset,
32 control characters, and graphic characters.
Upward and downward scroll, bell, erase, tabulate,
cursor control
7X 7
512 horizontal X 236 vertical points
Two single-valued functions of x, each individually
controlled
512 vertical lines and 236 horizontal lines, each
individually controlled
512/graph, total of 1024. Each graph marker is
individually controlled
Individual blanking and un blanking of all graph
features, clear all graphs.
Allows interruption of data transmission for
extended display viewing.
Terminal will respond with ESC/E when receiving
ESCZ.
ANSI X 4.14-1971 standard typewriter keyboard
Audible feedback on each keystroke
Three-key rollover to reduce errors caused by fast
typing
Extra control keys for special and commonly used
control functions include ESC, TAB, SCROLL,
BACKSPACE, BREAK, LINE FEED,
RETURN,COPY,REPEAT
Extra control keys for transmitting 2-character
escape sequences and special character codes
Variable to adjust character and graph brightness
Even or no parity, switch selectable; odd parity,
jumper-selectable
Turns line voltage on and off and resets unit to
Alphanumeric mode.
Allows choice of baud rate, full duplex, full duplex
with local copy, and local mode with rotary
switches.
1-7

Copier
Image Copied
Time/Print
Copy Size
Paper Roll Size
Character Format
Graph Field
User Controls

Display on screen less the alphanumeric cursor
Approximately 25 seconds/copy
Approximately 76.2 mm (3 in) high X 177.8 mm (7
in) wide
36.5 m (120 ft) long X 216 mm (8-1/2 in) wide
7 X 7 dot matrix
512 points horizontal X 236 points vertical
Variable character width adjustment

1.5.3 Interface Characteristics
Type

20 rnA current loop or EIA, depending on model

Speed
Full Duplex
Full Duplex with Local Copy
Full Duplex, Split Speeds

Transmission
1O-Bit Length
ll-Bit Length
Parity

75, 110, 150, 300, 600, 1200, 2400, 4800, and 9600
baud 110, 150,300,600, 1200,2400,4800, and 9600 baud
Transmission at 75, 150, 300, or 4800 baud with
reception at 110, 600, 1200, 2400, 4800, or 9600
baud
75, 150,300,600, 1200,2400,4800, and 9600 baud
110 baud only
Generated on transmission as odd or even parity or
a mark. (No parity is switch-selectable.) Parity is
not checked on reception.

Supplied Cable Length
20 rnA models
EIA models

7.6 m (25 ft)
7.6 m (25 ft)

Maximum Cable Length
20 rnA models
EIA models

304.8 m (1000 ft)
15.2 m (50 ft)

1.6 VT55 OPTIONS
There are three cable options available for the VT55. These are:
1.

BN 52C-7F 7.6-m (24.9-ft), current loop cable with a 283B telephone type plug

BN52B-7F 7.6-m (24.9-ft), 20-mA adapter cable with an 8-pin Mate-N-Lok connector

BN52A-7F 7.6-m (24.9-ft), EIA cable with the standard 25-pin EIA connector.

These optional cables allow the user to convert his VT55 to either a 20-mA current loop interface or an
EIA interface in the event his interface requirements change after purchasing the VT55. Optional
interface cables may be purchased from the nearest Digital Equipment Corporation Sales Office.

1-8

CHAPTER 2
SITE PREPARATION AND PLANNING

2.1 PURPOSE AND IMPORTANCE
In order to facilitate installation of the VT55 DECscope upon receipt from the factory, the user should
ensure that site installation requirements are met. As the VT55 is designed to operate in most office
and laboratory environments, special alterations to the installation site will not normally be required.
The paragraphs which follow detail the user's responsibility prior to installation. These concern installation, space, operating environment, power, and signal connection requirements. If any questions
arise concerning site preparation, please call the local DIGITAL Field Service Representative.
2.2 INSTALLATION SPACE
The VT55 is a tabletop mounting device. The cabinet is 360 mm (14.1 in) high, 530 mm (20.9 in) wide,
and 690 mm (27.2 in) deep. A table depth of 450 mm (17.7 in) is required for unit placement. A space of
640 mm (25 in) should be allowed in vertical height above the table to tip the unit into a position that
allows access to switches and data cables. These dimensions are diagrammed in Figure 2-1.

T
14.1"

25"

8009-5

Figure 2-1

VT55 Installation Dimensions
2-1

8009-3

The VT55 must be kept at least 50 mm (2 in) from the nearest obstruction in the rear. This permits
adequate air flow space for internal convection cooling. VT55 models FA, FB, FE, FF, JA, and JB
with internal copiers require clearance on the top and right side of the unit for access to the copier
paper housing and copier paper output.
The VT55 should be positioned so that access to the keyboard and work material is comfortable. A
page holder is mounted just to the right of the video screen to aid in transcribing information from
paper to the VT55 keyboard.

2.3

OPERATING ENVIRONMENT

2.3.1 Operating Temperature and Humidity
The VT55 is designed to operate in a typical office or laboratory environment. Acceptable limits of
ambient temperature are 10 0 to 40 0 C (50 0 to 104 0 F). Relative humidity must be between 10 and 90
percent with a maximum wet bulb temperature of 82 0 F and a minimum dew point temperature of 2 0
C (36 0 F). These parameters ensure that there is no condensation on the unit.
2.3.2 Device Dissipation
Nominal device dissipation for the VT55 DECscope is approximately 500 Btu/hr. No special cooling
consideration is needed for the terminal. However, in worst case use where the VT55 has a copier
installed that is running 100 percent of the time with the line voltage at maximum, the thermal load
could increase to 1000 Btu/hr.
2.3.3 Electrical Noise Tolerance
Electromagnetic fields in the vicinity of the VT55 can cause distortion or jittering of the video display.
This is caused by distortion in the travel of the cathode ray tube electron beam due to the fields. Items
that generate electromagnetic fields such as power transformers, fluorescent lights, and transmitting
antennas should be kept away from the VT55. An acceptable electromagnetic field intensity is less than
1 volt/meter, at any frequency, measured in the vicinity of the VT55.
2.3.4 Vibration, Atmosphere, Altitude
There are no special considerations that must be made for the VT55 with regard to these parameters at
most installation sites. The installation site must be free of excessive vibration and be free of excessive
dust, metal particles, and corrosive fumes in the air. The product can operate in altitudes up to 2730 m
(8000 ft) above sea level.
These requirements are easily met in most installation cases. When it is questionable that a given site
meets these requirements, the local DIGITAL Sales or Service Representative should be consulted.

2.4 POWER REQUIREMENTS
It is the user's responsibility to provide an adequate source of ac power to the VT55. The power source
must deliver voltages that are suitable to operate the unit and must be capable of providing the
machine with adequate current. The power lines must also be relatively free of voltage transients.
Paragraph 1.5.1 defines line voltage, transients, frequency, and maximum current requirements for the
various models of the VT55. Maximum current values for the copier versions assume that the copier is
running 100 percent of the time.
The 240 Vac models of the VT55 may be easily modified upon arrival at the site for 220 Vac or 230 Vac
nominal line voltage. The 100 Vac version may be converted for 110 Vac or 127 Vac nominal line
voltages. The maximum deviations from nominal of +9% and -13% apply in these cases as well. In
addition, 100 Vac and 240 Vac machines are shipped from the factory wired for 50 Hz operation. If
required, non-copier units may be easily altered to operate at 60 Hz. Chapter 3 of this manual explains
these procedures. VT55 units with copier operate at only 50 Hz or 60 Hz, depending on the model
(Table 1-2).
2-2

VT55-EA, EB, EE, EF, HA, and HC require 185 W of power while the copier versions, VT55-FA, FB,
FE, FF, lA, and lB, require 300 W maximum. The local DIGITAL Field Service Representative
should be consulted if the integrity of the power source is questionable.
Each unit is shipped with a 2.75-m (9-ft) power cord. A NEMA (National Electronic Manufacturer's
Association) 5-15P male plug is provided on VT55-EA, EC, EE, EH, FA, FE, HA, HC, and lA, while
NEMA 6-15P is installed at the end of VT55-EB, EF, FB, FF, HB, and lB power cords. Figure 2-2
shows mating plugs and receptacles.

PLUG

RECEPTACLE

PLUG

® ®

NEMA# 5-15P
DEC # 90-08938

RECEPTACLE

6-15P
90-08853

5-15R
12 - 05351

6-15R
12-11204

VT55 EB,EF,HB,FB,FF,JB

VT55 EA,EE.HA,EC,EH,HC,FA,FE,JA

cp- 2522

Figure 2-2 VT55 Power Connectors

When the VT55 is interfaced to a computer with switched convenience outlets, it is convenient to allow
the terminal to be powered by the master console power key by utilizing these outlets.
2.5 SIGNAL CABLES
VT55 models EA, EB, EC, FA, FB are shipped with a 7.6-m (24.9-ft), 20-mA signal cable. The signal
cable is terminated with an 8-pin, male Mate-N-Lok connector suitable for connection to most
DIGITAL serial line interface options. This connector may be removed and replaced as desired.
Models EE, EF, EH, FE, FF are shipped with a 7.6-m (24.9-ft), EIA signal cable. This cable is terminated with a male 25-pin connector which is compatible with EIA Standard RS232C.
Models HA, HB, HC, lA, lB are shipped with a 7.6-m (24.9-ft), 20 rnA signal cable. This cable is
terminated with a telephone-type 283 plug compatible with DECsystem-lO applications.
Refer to Chapter 3 for details on part numbers and pin assignments if this information is required at
site planning.

2-3

8. Test the unit by pressing keys and checking for corresponding characters on the screen.
Press COpy if applicable. This ensures that the video, microprocessor, keyboard, and copier are functional in the local mode. The hard copy output received at the copier after 25
seconds should be an accurate representation of the video screen display. The blinking
alphanumeric cursor will not appear on the copy. Use the pinch roller release lever to release
the paper to examine the copy. The copy may be torn off on the serrated edge of the cover.
9. Connect the VT55 to the computer by following the procedure outlined in Paragraph 3.4.
10. To ensure that all of the VT 55 logic functions properly at operating speeds through a remote
interface, acceptance testing is to be performed with a host computer using the MAINDEC
programs listed below. However, before performing the acceptance test, tilt the VT55 on its
back and set SI and S2 to the desired baud rate and full duplex mode. If the VT55 is to be
used with a PDP-8 or a PDP-II host computer, the following DIGITAL-supplied MAINDEC programs must be used:

PDP-8

PDP-ll

Title

MAINDEC-08-DHVT A
MAINDEC-08-DIVTC

MAINDEC-II-DZVTC
MAINDEC-II-DZVTD

VT50-A, B, H Acceptance Test
VT55 Acceptance Test

The VT50-A, B, H Acceptance Test tests all alphanumeric functions and the copier. The VT55 graphing capability is tested by the VT55 Acceptance Test. At least one pass of each diagnostic should be run
in accordance with the supplied diagnostic writeup. The writeup should be read thoroughly to ensure
full understanding of what constitutes proper operation of the terminal. Please note that any terminal
errors will be found by careful examination of the video screen output, not by error reporting on the
console device. Users who have units with copier capability should ensure full copier operation by
pressing COPY after a graphic picture is displayed when running the VT55 Acceptance Test.
If the VT55 is to be used on a processor other than a PDP-8, PDP-II, or other DIGITAL processor,
adjust the baud rate and interface mode switches to the desired setting. Any piece of software known to
operate the VT55 may be utilized for final test of the unit. However, it is not the responsibility of the
DIG ITAL Field Service Representative to ensure the integrity of user software.
If running the acceptance tests uncovers faults in the VT55, the interface, or the computer, the manual
test in Paragraph 3.3 can be used to isolate the fault with the VT55 off line.

3.3

MANUAL TESTING
1. The VT55 must be connected to the computer interface as outlined in Paragraph 3.4 and the
mode switch (SI) must be set for full duplex with local copy (half duplex) operation.

2. The computer does not require any program for the following test since the data transmitted
by the VT55 is looped back to the receiver via internal VT55 connections. The interface
cable must be connected to the computer interface to assert proper logic conditions at the
optical couplers. Therefore, the interface must be powered up. If no computer is connected
to the VT55, set the VT55 in the local mode as described in Paragraph 3.2, step 4. In this
condition, the interface will be bypassed.

3-2

3.3.1

Miscellaneous Functions

1. Test the VT55 by pressing keys and checking for corresponding characters on the screen. All
characters should be exercised in both upper- and lowercase. As characters are typed, the
cursor should move to the right one character position. Each time a character is typed, the
VT55 will make a clicking sound. When 80 characters have been typed on one line, the
cursor should be positioned under the 80th character.
2. With 80 characters displayed on one line, and the cursor positioned under the 80th character, type an 81st character. Since no more space is available on the line, the 80th character
will be replaced by the 81st character and the cursor will remain under the last character
(80th) on the line.
3. While displaying 80 characters, press the BACKSPACE key. Note that the cursor moves
toward the left of the display one character position for each time the BACKSPACE key is
depressed.
4. Using the BACKSPACE key, position the cursor toward the middle of the display and then
type additional characters. Note that the characters previously displayed are replaced with
the characters just typed.
5. Press the space bar and note that the cursor moves one character position to the right and
replaces a character with a space.
6. Press the RETURN key. The cursor should move to the left margin of the line it is on.
7. Press the LINE FEED key. The cursor should move down one line.
8. Type another line of characters. This line of characters should appear under the line previously typed.
9. Every eight columns (character positions) there is a tab stop. Press the TAB key and note
that the cursor moves to the right until it reaches a tab stop. Press the TAB a second or third
time and note that the cursor is reaching the tab stops. Also note that when the TAB key is
pressed, characters are not erased from the display.
10. Use the RETURN and LINE FEED keys to position the cursor at the beginning ofthe next
line. Press the TAB key and verify that the cursor moves eight columns to the right. Type an
"X" to mark the spot where the cursor stopped. Continue to press the TAB key and then
type an "X" to see where all the tab stops (eight of them) are. Notice that when the cursor
reaches column 72, the TAB key will move the cursor only one column to the right. If the
cursor is at the end of a line, the TAB key will not move the cursor.
11. Using the RETURN and LINE FEED keys, move the cursor to the start of the next line.
Type two or three characters, then press the TAB key. The cursor should travel to the TAB
stop. Type characters, press the BACKSPACE key, then press the TAB key. Note that the
TAB key does not always move the cursor eight columns to the right, but only the number of
columns necessary to reach the next TAB stop.

3-3

12.

Press the DELETE key and note that nothing happens to the display. The VT55 ignores this
key command. However, when a computer receives the DELETE command, special action
may be taken.

13.

Hold the SHIFT key down and, at the same time, type some characters. Note that the
characters displayed are uppercase. Release the SHIFT key.

14.

Press the CAPS LOCK key. Note that this key clicks and locks in the down position. Type
some characters and note that they are displayed in uppercase. Press the CAPS LOCK key
again and type more characters. These characters are displayed in lowercase.

15. Type all the numeral and symbol keys, except the ones with special words labeled on them.
Use the SHIFT key to display the symbol which is on the top half of the key. (Note that the
CAPS LOCK key will not perform the same function as the SHIFT key on keys that are not
letter keys. That is, to display a "G" rather than a "g," press either the SHIFT key or the
CAPS LOCK key. However, to display an "@" instead of a "2," the SHIFT key must be
used.)
16.

Press the CAPS LOCK key and type "ONE-TWa-THREE." Note that the CAPS LOCK
key does not have to be unlocked to type the dashes.

17. Type several short lines, following each line with a RETURN and LINE FEED until the
bottom line (24th line) of the display is completed. Now press LINE FEED. Since the VT55
must provide a new line, it moves the information on the display up one line to make room.
Notice that the information that was displayed on the top line has gone off of the screen.

18. Type several more lines of characters. These lines are placed at the bottom of the display and
displace the same number of lines from the top of the display.
19.

Press the BREAK key and note that nothing happens to the display. The VT55 ignores this
key command. However, when a computer receives the BREAK command, special action
may be taken.

20. The red CTRL (control) key can be used with alphabetic keys to produce commands which
are neither upper- nor lowercase letters; i.e., CTRL H = BACKSPACE, CTRL M =
RETURN, CTRL J = LINE FEED. Hold down the CTRL key and type an H, then an M,
then a J. The cursor should backspace one character position, move to the left display
margin, and then move down one line.
21.

Press CTRL G. The VT55 internal buzzer should sound.

22. The red ESC (SEL) key is used to give the VT55 special commands. Press ESC and then type
H. Note that the cursor moves to the upper left corner of the display.
23.

Press ESC and then type K. The top line of the display will be erased.

24.

Press ESC and then type J. The eritire display will be erased.

25. Type numerals 0 through 9 on the auxiliary keypad. Numerals 0 through 9 will be displayed.
The auxiliary keypad numeral keys perform the same functions as the numeral keys on the
main keyboard.
26.

Press the period (.) key on the auxiliary keypad. A period (.) will be displayed. The period
key on the auxiliary keypad performs the same function as the period on the main keyboard.
3-4

27. Press the ENTER key on the auxiliary keypad. The cursor should move to the left margin of
the display. The ENTER key on the auxiliary keypad performs the same function as the
RETURN key on the main keyboard.
28. Press the arrow keys on the auxiliary keypad one at a time. The cursor should move in the
direction of the arrows on the keys, i.e., down for the down arrow, to the right for the right
arrow, left for the left arrow, and up for the up arrow.
29. For VT55s with a copier, press the COpy key. The information displayed will be printed on
the copier output.
30. Hold down the REPEAT key and then press and release any character key. The character
will be repeated on the display as long as the REPEAT key is held down.
31. Press ESC Z. The character E will be displayed. The VT55 will always respond with the
terminal identifier sequence ESC IE to ESC Z being received. ESC I is not displayed in this
case; it is part of the "escape sequence" described in Chapter 5.
3.3.2 Graph Drawing Mode Functions
1. Press ESC 1; the VT55 is in Graph Drawing mode. (The VT55 will no longer recognize keys
pressed as letters and numbers.)
2. Press I, then 0 to clear graph drawing memory. Be sure to depress SHIFT or CAPS LOCK
to obtain uppercase letters.
3. Press A, then #; graph 0 is now enabled and a horizontal line should appear near the bottom
of the display.
4.

Press B, then 6 and 6 again; a single dot should be displayed near the upper-left corner of the
display.

5. Press 6 twice, nine more times; a to-dot horizontal line should be in the upper-left corner of
the display. (
6.

Press A, then ); the to-dot horizontal line should become a solid histogram (solid intensity
bar from 10-dot horizontal line to baseline).

7. Press A, then %; the solid histogram should disappear and the horizontal baseline of graph I
should appear on the bottom of the display.

8. Press H, then 5, followed by 5; this sets the next image near the center of the display.
9.

Press J, then 6, followed by 6; a single dot should appear near the top center of the display.

to. Press 6 twice, nine more times; a to-point horizontal bar should appear near the top center
of the display.
11. Press A, then 9; the to-point horizontal line of graph I should become a histogram and the
histogram of graph 0 should reappear.
12. Press I, then I; this enables generation of grid lines and graph markers in the following steps.
13. Press L, then 0 twice; a single, long vertical line should appear near the left side of the
display.
3-5

14. Press C, then 1 twice; a 16-point vertical cursor should appear about 2.54 cm (1 inch) to the
right of the long vertical line. This is a cursor 0 graph marker.
15. Press K, then 2 twice; a second 16-point vertical cursor should appear on the display. This is
a cursor 1 graph marker.
16. Press D, then 0 twice; a long horizontal line should appear just above the lower graph 0 and
graph 1 baselines.
17.

Press A, then SPACE; all graph drawing information should be blanked from the display.
Press 9 to restore.

18. At this point, press the COpy key if the VT55 has a copier installed. An exact reproduction
of the display should appear at the copier paper output after 25 seconds. It is normal for the
graph to disappear from the display for this short period of time.
19. Press I, then 0; this is the Graph Clear command. All graph drawings should clear off the
display, leaving only the horizontal baselines of graph 0 and graph 1.
20.

3.3.3

Press ESC, then 2; the VT55 is now restored to the Alphanumeric mode. Typing on the
keyboard should again display alphanumeric characters on the display. The horizontal baselines of graph 0 and graph 1 will remain displayed on the screen.

Hold Display Mode Functions
1.

Place the alphanumeric cursor on the bottom line.

Press ESC [ to enter Hold Display mode.

Press the LINE FEED key.

4. Type VT55; characters do not appear on the display.
5.

Press the SCROLL key; the message "VT55" should now appear on the display.

Press ESC \ to exit Hold Display mode.

Press the LINE FEED key; check for the message to scroll up.

3.4 CONNECTING THE VT55 TO AN INTERFACE
Models of the VT55 are supplied with interface cables for a 20 rnA current loop with a Mate-N-Lok
connector, a 20 rnA current loop with a 283B connector, or an EIA interface with an RS232C connector. The various VT55 models and their associated interface cables are as follows:
Model

Interface Cable

VT55-EA, EB, EC, FA, FB
VT55-HA, HB, HC, JA, JB
VT55-EE, EF, EH, FE, FF

20 rnA with 8-pin Mate-N-Lok connector.
20 rnA with a 283B telephone type connector.
EIA interface with RS232C connector.

All interface cables are connected to the VT55 by means of a plug-in card and are 7.6 m (24.9 ft) in
length. Optional interface cables (Paragraph 1.6) may be purchased from DIGITAL to allow the VT55
user to change the interface as requirements change. After connecting the VT55 to the interface, the
baud rate and mode switches must be set per the requirements of Paragraph 3.4.5 or of the user.

3-6

3.4.1 20 mA Interface Cable with Mate-N-Lok Connector
VT55 models EA, EB, EC, FA, and FB are supplied with the standard 20 rnA interface cable terminated with an 8-pin Mate-N-Lok connector. The connector may be removed and replaced if desired to
suit customer requirements. Table 3-1 and Figure 3-1 provide signal and pin information for the cable
and connectors. Table 3-2 provides part numbers for users who want to order parts from DIGITAL to
construct longer cables.
NOTE
For DLll-As and DLll-Cs that operate with 20 mA
current loops, capacitors are used to filter the receive
line and slow the switching time of the transmit line.
To avoid excessive distortion above ISO baud, the
capacitance in each of these two circuits must be
reduced. This is accomplished by clipping C29 (0.47
mF) and C31 (1000 pF), both shown on drawing DL3. Similar attention must be paid to other interfaces
requiring such modifications. Refer to the specifications for these devices at installation time.

Table 3-1

20 mA Interface Cable Wire Color-Pin Assignments (Mate-N-Lok)

Interface Cable
Wire Color

Adapter Card
Pin Number

Signal
Assignment

Mate-N-Lok
Pin Number

White
Black
Green
Red

HWI
HW2
HW3
HW4

Receive Positive
Receive Negative
Transmit Positive
Transmit Negative

5
2
7
3

Table 3-2 20 mA Interface Cable (Mate-N-Lok) Part Numbers
Description*

Part Number

Cable Type, 4-wire No. 22 stranded

DEC 91-07706

Connector Type, 8-pin Mate-N-Lok (Male)

DEC 12-09340-01 or
AMP 1-480460 or equivalent

Connector Male Pins, 4 required

DEC 12-09378-03 or
AMP 350079-4 or equivalent

Connector Type, 8-pin Mate-N-Lok
(Female)

DEC 12-09340-00 or
AMP 1-480459 or equivalent

Connector Female pins, 4 required

DEC 12-09379-03 or
AMP 350078-4 or equivalent

VT52 20 rnA Adapter Card

DEC 5411759

*Standard cable length is 7.6 m (24.9 ft) - BN52B-7F; maximum cable length is 304.8 m (1000 ft).

3-7

MATE -N-LOK
COMPUTER
CONNECTOR

VT52 ADAPTER CARD
7

e7
0

e5
04

e2
01

·1

24.9 It (7.6 m)

CP -2515

Figure 3-1

20 rnA Interface Cable with Mate-N-Lok Connector

3.4.2 20 mA Interface Cable with 283B Connector
VT55 models HA, HB, HC, JA, and JB are supplied with a 20 rnA interface cable terminated with a 4prong, telephone-type plug which can be used for DECsystem-1O applications. The 4-prong plug can
be removed and replaced with a connector more suitable for customer requirements if desired. Table 33 and Figure 3-2 provide signal and pin information for the cable and connectors. Table 3-4 provides
part numbers for users desiring to order parts from DIGITAL to construct longer cables.

Table 3-3 20 mA Interface Cable Wire Color-Pin Assignments (283B)
Interface Cable
Wire Color

Adapter Card
Pin Number

Signal
Assignment

283B
Pin Color

White
Black
Green
Red

HWI
HW2
HW3
HW4

Receive Positive
Receive Negative
Transmit Positive
Transmit Negative

Black
Yellow
Green
Red

Table 3-4 20 mA Interface Cable (283B) Part Numbers
Description*

Part Number

Cable Type, 4-wire, No. 22 stranded
Connector Type, 4-prong telephone (Male)
Connector Type, 4-socket telephone (Female)
VT52 20 rnA Adapter Card

DEC 91-07706
DEC 12-05857-01
DEC 12-05857-02
DEC 5411759

*Standard cable length is 7.6 m (24.9 ft) - BN52C-7F; maximum cable length is 304.8 m (1000 ft)

3-8

SCREW SIDE
BLACK

~® GREEN
® 0 ® RED

COVER HELD IN
YELLOW
PLACE WITH SCREW

VT52 ADAPTER CARD

BN52C -7F INTERFACE CABLE

@O'~ SCREW HOLE
@ '@ FOR COVER

TELEPHONE
TYPE PLUG

14-1----------

24.911 (7.6 m)

PIN SIDE

----------+1-1
CP - 2516

Figure 3-2

20 rnA Interface Cable with 283B Connector

3.4.3 EIA Interface Cable
VT55 models EE, EF, EH, FE, and FF are supplied with an EIA cable terminated with the standard
25-pin EIA type connector. The connector may be removed and replaced if desired to suit customer
requirements. Table 3-5 and Figure 3-3 provide signal and pin information for the cable and connectors. Table 3-6 provides part numbers for users who want to order parts from DIGITAL to construct longer cables.
Table 3-5

EIA Interface Cable Wire Color-Pin Assignments

Interface Cable
Wire Color

Adapter Card
Pin Number

Signal
Assignment

EIA
Pin Number

Green
Blue
Red
White

HI
H2
H3
H4

1
2
3
4

Black
Orange

H5
H6

Protective Ground
Transmitted Data
Received Data
Request to Send
(Wired true = + 10 V)
Signal Ground
Data Terminal Ready
(Wired true = + 10 V)

Table 3-6

7
20

EIA Interface Cable Part Numbers

Description*

Part Number

Cable Type, 6-wire, No. 20 stranded
Connector Type, 25-Pin (Male)
Connector Male Pins, 6 required
Strain Relief, 2 required
VT52 EIA Adapter Card

DEC 1700012
DEC 12-10493-31
DEC 12-10493-39
DEC 12-10493-50
DEC 54-11448-1-0

*Standard cable length is 7.6 m (24.9 ft); maximum cable length is 15.25 m (50 ft).

3-9

EIA
CONNECTOR

VT52 ADAPTER CARD
BN52A -7F INTERFACE CABLE

H5
Ht--c----

PINS
14

o
0

25:

PINS
1

114-11----- 24.9ft (7. 6m) ---------1-1
CP-2517

Figure 3-3

EIA Interface Cable

3.4.4 Changing VT55 Interface Cables
The following procedure should be followed when changing interface cables.
I.

Set the VT55 on its top side and remove the base assembly from the bottom of the unit.
(Refer to Chapter 6.)

Carefully remove the interface adapter card (EIA or either one of the two 20 rnA adapter
cards) from the ROM/UART module. (Refer to Figure 6-3, items 41, 42, aQd 43.)

Carefully install the new interface adapter card on the ROM/UART module. Orient the
adapter card as shown in Figure 6-3. The components of the adapter card face down, in the
direction of the microprocessor module on the underside of the unit. Ensure that the set of
pins on the adapter card and the female socket connectors on the ROM/UART module are
properly aligned before pressing the adapter card onto the ROM/UART module.

Route the interface cable through the hole in VT55 base and replace the base assembly.

3.4.5 Setting the Baud Rate Switches
When the VT55 is installed, an understanding of the baud rate and interconnection requirement of the
host device is imperative. Paragraphs 4.3 and 4.4 discuss the meaning of full duplex, local mode, baud
rate, EIA, parity, etc.
To set baud rate and interface mode switches SI and S2 (Figure 3-4):
I.

Tilt the unit on its back as shown in Figure 2-1.

With a coin or screwdriver, turn switches Sl and S2 fully counterclockwise. The switches are
now in position I-A (Figure 3-4).

Refer to Table 3-7 to adjust switches SI and S2 to the desired interface mode and baud rate.

3.4.6 Setting the Parity Switches
Parity for the VT55 is selected by means of the PARITY NONE/EVEN switch and jumpers W5 and
W6 on the underside of the unit. Figure 3-4 shows the locations of the switch and jumpers. To gain
access to the parity switch and jumpers, lay the VT55 on its top side and remove the metal screen.
3.5 SERIAL LINE SIGNAL FORMAT
When data is transmitted from the VT55 to the host computer or received from the host computer by
the VT55, an agreed-upon protocol must be utilized in order for the two units to recognize each other's
transmission. This protocol is specified by EIA (Electronic Industry Association) Standard RS232-C.

3-10

Access to these switches
is gained by removing
the metal screen.
PARITY
(RESERVED)
NONE

C!p

E
G F

1 -.€l- 5

7 6

W700
1 §
o ;:!

Switch S2
Switch S1
at position G
at position 1
Shown set for 9600 Baud, off-line use
JUMPER

SHOULD BE INSTALLED FOR

W7
W6

W6 W5

50 Hz operation
Odd parity insteady of even (if
parity switch is in "Even"position)
Space parity instead of mark (if
parity switch is in "None" position)

SWITCH S2 generally dictates receiving speed. In the "match" positions,
indicated with an asterisk, the receiving speed will be the same as the
transmitting speed, which must be indicated by the setting of switch S1.
Position A is the most counterclockwise position.

SWITCH S1 generally dictates transmitting speed. In the "match" positions, indicated with an asterisk, the
transmitting speed will be the same as
the receiving speed, which must be indicated by the setting of switch S2.
Position 1 is the most counterclockwise position.

A - Match (Bell 103) - Local Copy*
B - 110 Baud
C - Match (Bell 103)*
D - 600 Baud
E -1200Baud
F - 2400 Baud
G - 9600 Baud

1 2 3 4 5 6 7 -

EVEN

o 0
t

Off-Line*
Full Duplex with Local Copy*
Full Duplex*
300 Baud
150 Baud
75 Baud
4800 Baud

The terminal will not operate with both switches in the "match" position. See
the Users' Manual for a complete list of switch settings. For Teletype (Model 33)
compatibility, set S1 to 3 and S2 to B (Full Duplex 110 Baud).

CP-252B

Figure 3-4

Location of Underside Switches and Jumpers
(Bottom View)

3-11

Table 3-7 Baud Rate and Interface Mode Selection Switch Positions

Environment

Desired Baud Rate
Transmit
Receive

Sl-S2
Switch Positions

Maximum Delay Time*

Off-Line

9600
4800
2400
1200
600
110

l-G
N/A
l-F
I-E
I-D
I-B

These first five settings are
functionally equivalent. (Since
no interdevice communication
occurs, the baud rate is irrelevant.)
If the tenninal is set for Off-Line
use at 110 baud, the rate of repetition achieved with the REPEAT
key will be noticeably slower.

Full Duplex
With Local
Copy

9600
4800
2400
1200
600
300
150
110

9600
4800
2400
1200
600
300
150
110t

2-G
7-A
2-F
2-E
2-D
4-A
5-A
2-B

8.3 IDS
16.7 IDS
33.3 IDS
67 IDS
133 IDS
267 IDS
533 IDS
800 IDS

Full Duplex

9600
4800
2400
1200
600
300
150
110
75

9600
4800
2400
1200
600
300
150
IlOt
75

3-G
7-C
3-F
3-E
3-D
4-C
5-C
3-B
6-C

9.4 IDS
18.8 IDS
37.5 IDS
75 IDS
150 IDS
300 IDS
600 IDS
900 IDS
1200 IDS

4800
4800
4800
4800
4800

9600
2400
1200
600
110t

7-G
7-F
7-E
7-D
7-B

7.3 IDS
41.7 IDS
88 IDS
179 IDS
1100 IDS

300
300
300
300
300

9600
2400
1200
600
IlOt

4-G
4-F
4-E
4-D
4-B

Negative
Negative
25 IDS
117 IDS
1033 IDS

150
150
150
150
150

9600
2400
1200
600
110t

5-G
5-F
5-E
5-D
5-B

Negative
Negative
Negative
50 IDS
967 IDS

3-12

Table 3-7 Baud Rate and Interface Mode Selection Switch Positions (Cont)

Environment

SI-S2
Switch Positions

Desired Baud Rate
Transmit
Receive

75
75
75
75
75
75

9600
4800
2400
1200
600
110t

Maximum Delay Time*

6-G
N/A
6-F
6-E
6-D
6-B

Negative
Negative
Negative
Negative
Negative
833 ms

*The host must receive and respond to an XOFF signal from the terminal within this interval from the time the VTSS
transmitted it, to ensure that the Silo does not overflow. The indicated times include processing time as well as
transmission delays both ways. "Negative" indicates that Silo overflow may occur regardless of delay and processing
time. See Paragraph 3.5.4 of this manual for a complete description of the XOFF signal.

t At these settings, the terminal will supply an extra stop bit on each character it transmits, and wait for one on each
character it receives. A total of 11 bits per character, rather than 10, will be used at these settings.

3.5.1 Word Length and Parity
Data words transmitted and received are 7-bit ASCII for Alphanumeric and Graphic modes, or 7-bit
binary for Graph Drawing mode. Chapter 5 explains decoding of these seven bits. When these seven
bits are interchanged between host and terminal, they are transmitted serially at the baud rate. The
seven bits are contained within start and stop and parity bits to synchronize the interface when reading
the seven bits of data.
At all baud rates except 110 baud, the transmitted data from the VT55 is in a 10-bit format. Data
received by the VT55 is in a to-bit format as well. With the parity switch set to NONE, a transmitted
alphanumeric character "B" (ASCII 102) will appear as shown in Figure 3-5.
If the parity switch is set to EVEN, the stop bit in the ninth state is affected. This bit is no longer

termed the "stop" bit but is called the "parity" bit. The parity bit is adjusted by hardware to ensure
transmission of an even number of marks during any lO-bit word transmission. The fact that the
receiver (the host computer) knows that an even number of marks is being transmitted allows it to
detect errors that might occur due to poor communication links. In the example shown in Figure 3-5,
state 9 would change from a mark to a space if the switch was set to EVEN. The even parity feature
should only be used with a computer that can check incoming data for even parity. The VT55 does not
check received data for parity; no error detection is done within the terminal.
TIME
STATE

10
MARK

S
T
A
R
T

","

"0"

"," "0"

SPACE
S
T

S
T

P
CP-2140

Figure 3-5 Transmission (or Reception) of ASCII "B" with Parity Switch Set to NONE

3-13

The transmitted and received word lengths are changed to 11 bits when 110 baud is chosen. This allows
the VT55 to connect with II-bit interfaces originally designed for 110 baud Teletype® applications.
The effect of adding the 11 th bit is the same as adding 2008 to any 7-bit data word. In Figure 3-5, code
1028 (B) would become code 3028 • An extra mark would be inserted between time states 1 and 2,
making an II-bit word. The parity switch should be set to NONE to conform with a standard II-bit
Teletype format.

3.5.2 Baud Rate
'"
The term baud means bits per second. A baud rate of 9600 means that 9600 bits are being transmitted
over the serial line in 1 second. Since each word is 10 bits long, 960 words per second will be transmitted at this baud rate setting. Similarly, since 110 baud corresponds to transmitting 110 bits per
second where each word is 11 bits long, 10 words per second are being transmitted. Baud rate is
normally determined by the host computer and any restrictions imposed by the interconnecting transmission media.
3.5.3 Levels
When the VT55 is utilized with a 20 rnA interface, a mark corresponds to the presence of a 20 rnA
current in the transmitter or receiver current loop, while a space corresponds to an absence of this
current. (Refer to Figure 3-6.)
------,,

r---

I
I

RCVR

XMTR

RCVR

L ___

HOST
COMPUTER

VT55

CP-Z139

NOTE
The tolerance on the 20 rnA current is ± 10%.

Figure 3-6 20 rnA Current Loop Connection
With the EIA interface, these currents are replaced by voltage levels. A transmitted mark will be
designated by a + 10 V signal level and a space by a-I 0 V signal level. Received data will be interpreted
as a logical 1 or mark if between + 5 V and +25 V, and a space if between -5 V and -25 V. Both
transmitted and received signals comply with EIA Standard RS232C.

3.5.4 Synchronization
Some terminals cannot process characters and commands as fast as the codes come in from the host.
These terminals require the host to send several meaningless characters, called fillers, to the terminal in
order to "pass the time" until the terminal is ready to process more data. The control codes 000 (NUL)
and 177 (DEL) are most often used as fillers. Although no DECscope requires the host to send fillers at
any time, the DECscope will ignore 000 and 177 when it receives them, so that software which uses
these characters as fillers will not have to be rewritten to eliminate them.

®Teletype is a registered trademark of Teletype Corporation.

3-14

The only time the terminal fails to keep up with the data coming in from the host is when it is passing
data to the electrolytic copier. In this case, rather than require the host to compute a complicated
formula for how many fillers to send out and when, the terminal calculates on its own when it is getting
behind, and sends a signal to the host to request it to stop transmitting. This signal is the control code
XOFF (023). When the terminal has reduced its backlog, it sends another signal to the host to tell it to
resume. This signal is XON (021). The terminal depends on the host to suspend its transmission when
the host receives XOFF, and to resume the transmission from the point at which it left off when it
receives XON.
The host's response to XOFF, however, will not be immediate, simply because it will take time for the
XOFF code to travel down the wire to the host. After the terminal transmits XOFF to signify that it
cannot process more data, it may receive several more codes before the host actually stops
transmitting.
Therefore, the terminal places these codes in an internal buffer called the Silo, to be processed at a later
time. When the terminal is ready to process more codes, it will take the codes out of the Silo in the
order in which they were received, and process them before it sends XON to the host to request more
data.
Effectively, there is room for 13 codes in the Silo. For most applications, this is sufficient to allow for
transmission delays. It is not meant to be sufficient to let the host ignore XOFF. If the Silo fills up and
the host keeps sending, the Silo "overflows." The codes in the Silo are removed and processed by the
terminal in the order in which they were received, regardless of the situation at the terminal which had
caused the delay.
One cause of the Silo overflowing is excessive processing time and transmission line delay. To illustrate, if the DECscope were connected to the host so that messages from the terminal reached the host
10 seconds after leaving the terminal, an XOFF sent by the terminal might not reach the host in time to
halt the host's transmission.
Another cause for Silo overflow is the use of a receiving speed which is much more rapid than the
transmitting speed. If the DECscope were set to receive at 9600 baud while transmitting at 300 baud,
9600/300 = 32 characters which could be received in the time it took the terminal to send XOFF. This
problem would not exist if the transmission speed were also 9600 baud. Thus, the use of high baud
rates is not by itself a cause of synchronization failure, although it does magnify the effect of long delay
times.
The sufficiency of the DECscope's Silo can be calculated using the formulas:
(D + d + P) Y + (2Y IX) + 2
to yield the number of characters that the VT55 would have to buffer at worst case. If this equation
produces a number 13 or less, the DECscope will fit the application since the effective length of the Silo
is 13 characters. In the equation above:
D

= Transmission delay from terminal to host (in seconds)

= Transmission delay from host to terminal (in seconds)

Processing delay (in seconds). (P is the interval from the time the host physically receives XOFF and the time software transmits its last character before stopping in response
to the XOFF.)

3-15

= Receiving speed of the terminal (in char/second·)

= Speed at which the terminal transmits (in char/second·)

The 2s in the formula reflect the fact that the host's interface hardware and that of the DECscope (that
is, the U AR Ts) will buffer two characters in each direction.
As an example, consider a VT55 communicating to a host at 300 baud (transmitting and receiving)
over a telephone line. The telephone company specifies that the worst-case transmission delay is not
greater than 50 ms. In this example, the processing delay was zero since, when the host received XOFF,
it trapped to a routine which disabled output. So the last character the host transmitted to the terminal
would occur before receiving XOFF. Since transmitting and receiving speeds are both 30 char/second,
the equation is:
(0.050 + 0.050 + 0) 30 + 2 (30/30) + 2 = 7
A terminal with a 7-character input buffer is required. The DECscope, with its 13-character Silo, is
adequate for the application.
When the DECscope is set for local copy, the effective length of the Silo is reduced to 12 characters,
since the XOFF transmitted by the terminal goes into its own Silo. Note that in this situation the
operator could cause the Silo to overflow by continuing to type, since the characters typed will likewise
go into the Silo.
Table 3-7 lists each combination of receiving and transmitting baud rates, the time (D + d + P) by
which the host must be able to respond to XOFF in order to prevent Silo overflow.
Software must respond to XONj XOFF if the copier is being used and there is a possibility that the host
will be sending data to the terminal while the copier is running.

In Hold Screen mode, the terminal uses these same XOFF and XON signals again, not because the
terminal cannot keep up with the data from the host, but simply to hold the present data on the screen
until the operator gives the terminal a request for new data. Software must respond to XONj XOFF if
Hold Screen mode is being used.
3.6 ADJUSTING VT55 NOMINAL LINE VOLTAGE
Adjusting the nominal operating line voltage of the VT55 requires a simple tap change of the power
transformer utilized on VT55 models EB, EC, EF, EH, HB, HC, FB, FF, and JB. The procedure for
altering the nominal line voltage is as follows:
I.

Remove the ac power cord. You are altering the wiring on the primary side of the transformer. If the VT55 does not have a copier, then remove the copier cover (Figure 6-4) and
proceed to step 6.

Place the VT55 on its top side and remove the base assembly.

3. Check to see that the ac power cord is removed.
4.

Remove the ROM/UART module. (Refer to Paragraph 6.2.3.)

Remove the front panel on the right of the CRT (Figure 3-7) by applying pressure to the
plastic tab which is accessible through the bottom on the VT55.

*9600 baud = 960 char j second, 1200 baud = 120 char j second, etc. but 110 baud = 10 char I second.

3-16

·,

REMOVE THIS PANEL
TO GAIN ACCESS TO
TERMINAL STRIP

8009-5

Figure 3-7

Access to AC Power Terminal Strip

6. Locate the 10-screw terminal strip mounted behind the front panel (Figure 3-8).

JUMPER

WHT

TO TRANSFORMER
CP-2134

NOTE
Terminal assignments shown wired for 240 Vac.

Figure 3-8

lO-Screw Terminal Strip

3-17

Remove existing jumpers on the top screws of the lO-screw terminal strip. Add new jumpers
to these in accordance with Table 3-8. Also move the black and white wires from the POWER/LoGIc RESET switch to the new terminals as indicated.

Double check jumpers added in step 7. An error will damage the terminal.

9. Replace the front panel.
10. Replace the ROM/UART module.
Table 3-8
VT55 Model No.
EB,EF, FB,
FF, HB,lB

EC,EH,HC

11.

AC Terminal Strip Jumpers for Different Nominal Line Voltages

Optional Nominal
Line Voltage (V ac)

{
{

Black Wire
from Switch

White Wire
from Switch

Add
Jumpers

220
230
240

3
4
4

5
5
5

1-7
1-6
1-7

100
110
127

2
3
4

1
1
1

2-6, 1-5
3-7, 1-5
4-8, 1-5

Replace the base assembly and write the new nominal line voltage on the base decal.

12. Set the VT55 on its base.
13. The nominal line voltage alteration is now complete. Connect the ac power cord to a suitable ac source.
3.7 CHANGING VT55 LINE FREQUENCY - NON-COPIER MODELS ONLY*
VT55 DECscope models EB, EC, EF, EH, HB, and HC are shipped configured for 50 Hz power
source operation. In some circumstances, 60 Hz power source operation may be required. This is easily
accomplished by clipping three jumpers within the VT55. These three jumpers control whether the
information display is refreshed 50 or 60 times per second. Annoying screen flicker is eliminated by
refreshing the VT55 screen at the same frequency as overhead lighting fixtures and predominant electromagnetic fields in the room. The VT55 may be changed to operate from a 60 Hz power source as
follows:
1.

Turn the VT55 on its back and remove the VT55 base assembly. (Refer to Figure 6-3.)
Ensure that the power cord is unplugged.

Locate the ROM/UART module in Figure 6-3. Clip jumper W7 adjacent to the baud rate
switch as shown in Figure 3-4.

Locate jumpers WI and W2 on the M7024 Graphing module. Locate P4 on Figure 6-3 and
then use Figure 3-9 to find WI and W2. Clip out jumpers WI and W2 for 60 Hz operation.

Reassemble the VT55. The screen will now refresh at a rate of 60 times per second.

*Units with copiers cannot be altered because copier motors are fixed at 50 Hz or 60 Hz.

3-18

......... C........,.,JUII~U

~UL~

do oO[JlOoOo 00 [JlOoooOO
~Wl
0
OOIJlO~OO

ODD

00 aOODOD do

00 00

=--W2

CP-2138

Figure 3-9

Location of WI and W2 on the M7024 Graphing Module

3-19

CHAPTER 4
TECHNICAL DESCRIPTION

4.1 INTRODUCTION
This chapter presents a technical description of major features and operating modes of the VT55
DECscope. A brief discussion of the raster scan technique, communicating with the VT55, the
Alphanumeric and Graph Drawing modes, the Escape mode and special terminal commands, and the
copier are presented.
4.2 RASTER SCAN TECHNIQUE
As mentioned in Chapter 1, the display of alphanumeric and graphic data is performed by the raster
scan method. Raster scan infers that the beam of the cathode ray tube is swept completely across the
phosphor surface of the tube 50 or 60 times (depending on the power line frequency) in a second.
Information, generated via the memory, turns the beam on and off, intensifying dots on the phosphor
at the proper time to produce characters and graphics on the video screen. This is basically the same
method used by consumer television sets. The method is reliable and produces flicker-free display
information. No vertical or horizontal hold controls are used by the VT55. These controls are eliminated by synchronizing the raster scan and intensification information to the VT55 microprocessor.
4.3 COMMUNICATING WITH THE VT55
For most applications, the VT55 will be interfaced to a host computer and operated in the full duplex
communication mode. Figure 1-3 shows a simplified full duplex interface. Data transmitted by the
VT55 is received by the host computer. Data received by the VT55 is determined solely by the host
computer. This is the typical interface that is desired in an interactive man-machine environment.
However, two other terminal interface modes are available. These modes are shown in Figure 4-1. The
terminal mode switch is set to the full duplex mode. When the local copy switch is closed, data received
from the host computer or transmitted by the VT55 appears on the video screen.

REMOTE

FROM HOST
COMPUTER

_o---------<~

SCREEN

MEMORY

RECEIVER
(
~

KEYBOARD

____________

FULL DUPLEX
SWITCH
______
.TO HOST
COMPUTER

TRANSMITTER

FULL DUPLEX
WITH LOCAL COpy
CP-2144

Figure 4-1

Interface Modes

4-1

This full duplex with local copy mode of operation is useful when the host computer does not generate
an immediate echo or response to a keyboard transmission, and a visual verification of the information
being sent is desired. Making this local copy connection in no way affects the way in which the host
computer interfaces with the VT55. When no connection with the host computer is desired, the serial
line input/serial line output may be disconnected by placing the VT55 in the local mode. Local mode is
normally used for off-line hardware maintenance, training, or use of the copier. Selection of the mode
of the terminal interface is made via switches under the VT55. These switches also determine the
transmit and receive baud rates of the unit.
The VT55 is capable of sending and receiving at baud rates from 75 to 9600 bits per second. Data is
sent in serial digital format for the 20 rnA interface models. A logical 1 is represented by the presence
of a 20 rnA current, a logical 0 by the absence of current. The 20 rnA current is referred to as a 20 rnA
current loop. The EIA interface models allow the use of the VT55 with RS232-C signals. EIA specifications define a logical 1 as a negative voltage and a logical 0 as a positive voltage.
When a digital word is transmitted to the host computer from the VT55 keyboard, it is transmitted as
the ASCII representation of the depressed key. The VT55 will transmit the 96-character ASCII set
which includes all upper- and lowercase letters plus numbers and special symbols. In addition, the set
of 32 control codes may be transmitted.
When a digital word is received by the VT55, it may be interpreted in one of three ways: as alphanumeric/graphic information, as terminal control information, or as graph drawing information. When
the terminal is powered on, it assumes the Alphanumeric mode and translates, in accordance with
ASCII, incoming displayable information as the 96-character set mentioned above. The letter or
numeral representation of the transmitted code is stored in the alphanumeric memory and is displayed
on the screen. Certain codes in the 32-character control code set of ASCII, when received, affect VT55
operation. These control codes, sometimes in combination with alphanumeric characters, cause the
screen to erase, the VT55 internal bell to sound, a hard copy to be generated, etc., when received. One
sequence switches the VT55 to the Graph Drawing mode.
When the VT55 is in the Graph Drawing mode, the receipt of the 96-code set that represented numbers
and letters in the Alphanumeric mode is now interpreted in accordance with a unique code that treats
the digital information as graph instructions and data. These place the graph points, markers, and
grids on the display, superimposed on any alphanumeric information. A different control code
sequence reverts the VT55 back to the Alphanumeric mode.
4.3.1 Alphanumeric Mode
When the VT55 is powered on, the terminal is in the Alphanumeric mode. Data received via the serial
line input is stored in the alphanumeric memory and printed as characters, numerals, and symbols on
the screen if the code represents a printable character.

Received printable symbols are displayed on the video screen in a 24-horizontal line by 80-vertical
column format. Each character is displayed as a 7 X 7 dot matrix; the matrix size is approximately 4.0
mm X 4.0 mm. Character placement on the screen is controlled by the alphanumeric cursor. The
cursor is a blinking, underlining line that indicates the screen position in which the next received
character is to be placed. When receiving a new character, the cursor automatically increments to the
right. The cursor may also be shifted around the screen by receipt of alphanumeric cursor control
commands or the auxiliary keypad. When the VT55 receives a line feed code, the alphanumeric cursor
moves down one line. Upon receipt of the alphanumeric carriage return, it moves to column one of the
current line. By the use of other control sequences, the cursor may be moved up one line, back one
space (backspace), right one space, homed (line one-column one), and tabulated (tab) eight spaces to
the right. The alphanumeric cursor may also be used to erase alphanumeric text from the cursor
position to the end of the current line or to the end of the entire screen. These features of the alphanumeric cursor greatly facilitate text handling on the VT55.
4-2

In order to plot graph data on the VT55 screen, it is necessary to take the terminal out of the Alphanumeric mode. This is accomplished by receipt of a special control sequence that causes the VT55 to treat
receipt of noncontrol ASCII as graph plotting information rather than alphanumeric information. The
VT55 may be returned to the Alphanumeric mode by the receipt of a second unique control sequence.

4.3.2 Graphics Mode
Graphics mode performs in essentially the same way as Alphanumeric mode with regard to character
font, control codes, etc. The only difference is that the subset of 32 lowercase characters is replaced by
a set of 30 special characters to extend the capability of the terminal in printing equations and fractions
and plotting line graphs.
4.3.3 Graph Drawing
When the VT55 is in the Graph Drawing mode, incoming digital data, aside from the' control codes, is
treated as graph plotting information. This data represents Graph Drawing mode instructions and
data points as defined in Chapter 5. The VT55 can plot two 512-point graphs, graph 0 and graph 1,
each represented as a line graph or histogram. Vertical and horizontal lines that span the viewable
screen area may be displayed anywhere in the viewable area of the screen. These lines can be combined
to form grids to aid in labeling values on the graphs. The VT55 has graph markers as an additional
graph feature. These are short vertical markers useful in defining important features on the graph. All
graph features of the VT55 are individually controllable by the programmer. All data is stored in a
separate graph memory. Keeping graph information separate from the alphanumeric information
storage allows superposition of the alphanumeric and graph information and allows the removal of
one type of information without affecting the other. For example, a label could be printed over a graph
and then removed without affecting the graph.
Figure 1-2 shows the individual types of data displayable on the VT55, and how they are superimposed
to form a complete representation. Each graph image on the screen is formed by storing X-Y coordinate points in memory and then point-plotting the image on the screen.
Figure 1-2a shows graph 0 as a typical line graph with the graph markers that may be plotted on the
VT55. The graph may have a horizontal resolution of 512 points and a vertical resolution of 236
points. The graph must be a single-valued function of X. Two points may not be stored on a single X
coordinate on a given graph. A graph marker may be individually turned on or off at any X coordinate
on either graph 0 or graph 1. The graph marker will automatically be attached to the graph at the
corresponding Y position.
Figure 1-2b shows a histogram display on graph 1. When a histogram is desired, the VT55 simply
intensifies all points below the line graph envelope of the desired histogram; that is, if graph 0 is desired
as a histogram, the VT55 intensifies all points under the outline as shown in Figure 1-2a. Of course, the
histogram intensification could be removed, leaving just a line graph. Graph 1 has the same specification, with regard to resolution and graph markers, as graph O.
Figure 1-2c shows a typical grid formed on the screen of the VT55 by combining horizontal and
vertical lines. As many as 512 horizontal and 236 vertical lines may be individually displayed and
cleared from the screen. With this flexibility, almost any other type of coordinate scheme, such as
logarithmic, may be plotted.
Figure 1-2d shows the three previous plots superimposed. A well-defined graph drawing display has
been formed. Note how graph 0 is clearly visible through the histogram of graph 1. Alphanumerics are
viewable through the histogram as well. However, alphanumeric characters would not be readable if
both graphs were displayed as histograms and alphanumeric characters were plotted in an overlapping
area of the two graphs. Also note that column I and line 24 of the alphanumeric display are outside of
the graph field for easy readability of coordinate labels.

4-3

The VT55 has an ensemble of nine graph commands which permit loading of individual points on the
two graphs and loading and clearing of individual markers, vertical lines, and horizontal lines. The
user may choose to blank this data from the screen, but hold the data in the internal graph memory for
later viewing. All graph memory may be cleared with one instruction when desired.
4.3.4 Escape Mode and Special Terminal Commands
There are certain control commands which are not unique to the Alphanumeric or Graph Drawing
operation. These commands are utilized to facilitate interaction with the host computer or the VT55
copier. Important to these control commands is the concept of the Escape mode sequence. When in
Escape mode, the VT55 microprocessor examines the next incoming character to see if this 2-character
escape sequence has any special meaning. If the 2-character escape sequence has a meaning to the
VT55, the operation of the VT55 will be modified. If the sequence has no meaning, the VT55 returns to
the Alphanumeric or Graph Drawing mode. Escape mode is instrumental in alphanumeric cursor
control and switching from Alphanumeric to Graph Drawing mode and back again.
The codes ESC [ put the VT55 into a Hold Screen mode. Hold Screen mode is used normally in
Alphanumeric mode to halt the transmission of data to the serial line input. It is sometimes desired to
hold a screen full of data for examination before allowing more data to be displayed. When a 25th line
of data is received, the VT55 normally scrolls the first line off the screen. When Hold Screen mode is
invoked, a control signal termed XOFF is transmitted to the host computer to request the computer to
stop transmission. If the computer is programmed to respond to this code, the user can examine the
display at his leisure. Using the special key labeled SCROLL in conjunction with the SHIFT key, the
VT 55 can be made to display one or more lines of text by scrolling once, or a whole new display of text
by scrolling 24 times. This is accomplished by sending an XON control character to the computer to
request it to resume sending.
Hold Screen mode is terminated by receipt of the 2-character escape sequence ESC \. Escape
sequences are also used to allow the VT55 to identify itself. Anytime the VT55 receives the 2-character
escape sequence ESC Z, it responds by transmitting the 3-character sequence ESC / E, which identifies
the terminal as a VT55, a unit with graphic capability. A VT55 or similar DIGITAL product will
always respond with this unique code.
Lastly, escape sequences are utilized for the VT55 copier operation. When a VT55 with copier receives
these defined 2-character escape sequences, it can be made to perform the following three things:

Copy all lines from the top of the screen to and including the marker line. Any graph
drawing information above the alphanumeric cursor line will be copied as well.

Copy all lines up to but not including the alphanumeric cursor line; then copy a new line
after each new line feed is received (called auto-copy mode).

Disable the auto-copy mode.

In addition, a special key on the VT55 keyboard labeled COPY can either copy the entire screen or
invoke the auto-copy mode (with use of the SHIFT key). Auto-copy mode is designed to furnish a line
printer type listing when dealing strictly with alphanumeric text. It should not be used when graph
drawing data is displayed on the screen.
4.3.5 VT55 Keyboard
The VT55 keyboard is compatible with ANSI Standard X 4.14 standard typewriter keyboard. It is also
the standard keyboard layout incorporated on other terminals manufactured by Digital Equipment
Corporation. This keyboard layout, shown in Figure 4-2, is similar to the layout found on an office
typewriter, rather than on a keypunch machine. It contains the keys that generate the 96-character set
of ASCII and a control key for the generation of the 32 control codes. Unique VT55 function keys are
on the keyboard as well.
4-4

8009-2

Figure 4-2

VT55 Keyboard

The unique control keys on the keyboard are duplicates of control codes that may be generated from
combinations of other keys on the keyboard. However, since these keys tend to be utilized quite often
by the operator, they are furnished as separate keys. These keys include ESC, TAB, BACKSPACE,
LINE FEED, RETURN, and REPEAT.
Two keys which do not transmit data via the serial line port of the VT55 are also contained on the
keyboard. These keys are SCROLL and COPY. The actual functions of these keys were explained in
Paragraph 4.3.3.
In addition to the above-mentioned keys, the VT55 contains a BREAK key. The break function is
included to allow the interruption of incoming data when utilizing the VT55 in a half duplex system.
Chapter 5 covers this subject in more detail.
The VT55 keyboard electronics has a feature called 3-key rollover. This feature allows error-free data
transmission from the VT55 even if two keys are held down while a third key is depressed, as long as
one of the first two keys is released before the third key is released. Three-key rollover is a feature
meant to reduce missing character errors due to very fast typing.
The keyboard also produces an audible keyclick sound that gives the operator feedback telling him
that the key was properly depressed. This feature may be disabled for quiet environments. Additionally, the 16-key auxiliary keypad provides for special user characters as well as marker position keys.
4.4 VT55 COPIER
VT55 models FA, FB, FE, FF, lA, and lB are supplied with an internal copier. The copier allows the
user to make a hard copy duplicate of the video screen. The paper copy includes the displayed graphs
as well as the alphanumeric characters. The copier is contained within the VT55 cabinet and the paper
output emerges from the right side of the terminal. (Refer to Figure 4-3.) Upon command, the VT55
generates a hard copy of the display on the screen. Using a scanning technique, the terminal generates
the image of the display in about 25 seconds. The display image is approximately 3 inches high. As the
copier uses rolled paper, many lines of text and graphs may be formatted on a standard letter-size sheet
of paper. The roll is 8-1/2 inches wide by 120 feet long. Hence, approximately 480 display images may
be copied onto a roll of paper.
4-5

7717-5

Figure 4-3

VT55 with Copier

Each character of the screen's alphanumeric content is reproduced as a 7 X 7 dot matrix on the paper.
Graphs are represented in a 512 X 236 point matrix as on the VT55 screen. Characters, graphs,
histograms, grids, and graph markers may be copied by the copier.
Superimposed information on the screen appears as one shade on the hard copy output. The hard copy
unit is capable of producing one shade of intensity while the video screen can simulate two. Therefore,
it might be necessary to manually fill in lines from the hard copy output due to superposition. Superposition usually occurs when a histogram is overprinted on other data.
As mentioned previously, the VT55 copier uses a scanning technique similar to the method employed
in facsimile picture transmission. The technique consists of scanning an electrode across a surface of
special recording paper in exactly the same pattern as the raster scan on the VT55 screen. An electrode
on the opposite face of the paper receives a voltage from the VT55 microprocessor in conjunction with
the image to be recorded. The potential across the two electrodes causes a current to pass through the
recording paper and actually plates metal from one of these electrodes onto the paper to form a dark
point.
The VT55 copier is of simple design and thus is extremely reliable and requires little maintenance. All
that is required of the user is the changing of the metal anode electrode, the removal of excess paper
build-up from the cathode electrode when changing to a new roll of paper, and the moistening of the
paper wet pad. There are no adjustments to be concerned with. The copier is controlled manually from
the keyboard or by special control codes as mentioned in Paragraph 4.3.4.

4-6

CHAPTER 5
OPERATION AND PROGRAMMING

5.1 INTRODUCTION
This chapter first discusses the normal, everyday procedures involved in utilizing the VT55 with a host
computer, data set, or other communications device. It is assumed that the terminal was installed in
accordance with the installation procedure of Chapter 3. Any changes of the baud rate/mode switches,
serial line connections, or parity switch are discussed in that chapter. In addition, an off-line procedure
is included in Chapter 3 to familiarize you with some of the terminal's functions.
The second part of this chapter discusses the terminal's capabilities in Alphanumeric, Graphic, Graph
Drawing, and Escape modes. An explanation of how these modes work together and concise charts
containing programming data are included. The end of this chapter contains sample programs that
exercise many of the functions discussed.

5.2 OPERATION
5.2.1

Powering on the VT55
1.

Apply power to all components of the terminal by setting the POWER/LOGIC RESET
switch to ON.

Wait about 1 minute. The blinking alphanumeric cursor (Figure 5-1) will appear in the top
left corner of the screen; this is called the home position.

If the blinking cursor fails to appear after 1 minute, the intensity control may be set too low.

Similar to the brightness on a standard television set, the intensity control should be adjusted to achieve maximum image clarity on the screen.
4.

The terminal is now ready to operate with a host device or, if in local mode, may be exercised off line.

5.2.2 VT55 Models with Internal Copier
Operation of the VT55 is covered in the VT Series Copier User's Manual, EK-VT5C-OP-OOl. It is
included with each VT55 DECscope that contains a copier.

5-1

ESC F puts the terminal in Graphic mode. ASCII codes which usually stand for the lowercase letters
and some symbols will cause 1 of the VT55's 30 special symbols (Table 5-4) to be displayed on the
screen.
Control codes, when received in Graphic mode, are performed and the unit remains in Graphic mode.
Escape sequences put the terminal into Escape mode, and after the function is performed, the terminal
reverts back to the Graphic mode. Again there are two exceptions, ESC G and ESC 1. ESC G switches
the terminal back to Alphanumeric mode.
ESC I, whether the terminal is in Graphic or Alphanumeric mode, switches the terminal to Graph
Drawing mode. Incoming data that is normally a printable ASCII character becomes information for
generating graphs on the screen. When control codes are received, they are performed and the unit
remains in Graph Drawing mode. Once again,.escape sequences put the terminal in Escape mode, and
after the function is performed, the unit returns to Graph Drawing mode. Here again, there are exceptions. The sequence ESC 2 causes the terminal to leave Graph Drawing mode and return to whatever
mode, Graphic or Alphanumeric, the terminal was in before it entered Graph Drawing mode. This is
the only sequence that will bring the terminal out of Graph Drawing mode. ESC F and ESC G will not
cause the terminal to leave Graph Drawing mode, but they will control the mode that the terminal will
enter when ESC 2 is received. In Figure 5-2, the only difference between the two Graph Drawing
modes is the mode that the terminal will go to if ESC 2 is received. Otherwise, they are identical.
Hence, the terminal can be looked at as having three permanent states - Alphanumeric mode, Graphic
mode, and Graph Drawing mode. These three modes are switched by escape sequences. A temporary
mode, Escape mode, is used for exercising special control functions. In addition, standard ASCII
control functions, such as line feed, may be exercised at any time without disturbing the mode of the
terminal.
In some cases it is possible for a program that is under development to erroneously leave the terminal
in Escape, Graph Drawing, or Graphic mode when control of a monitor program on the host computer is desired. The POWER/LOGIC RESET switch will always return control to the Alphanumeric
mode when this is required. The keyboard will always transmit appropriate ASCII codes when a key is
pressed, but the terminal will misinterpret alphanumeric input as graphic, graph dTawing, or escape
code data if the terminal is not in the Alphanumeric mode.
5.3 THE KEYBOARD
5.3.1 U sing the Keyboard
The operator uses the keyboard to transmit codes to the host. Using the keyboard is the only way the
operator can transmit codes to the host. In general, transmitting codes to the host is the only function
the keyboard performs.
Some keys transmit one or more codes to the host immediately when typed. Other keys, such as the
SHIFT keys, do not transmit when typed. Instead, when held down, they modify the codes transmitted
by other keys. The code-transmitting keys cause the terminal to make a clicking sound to verify to the
operator that the keystroke has been processed by the terminal. If two code-transmitting keys are held
down together, two codes will be transmitted according to the order in which the keys were typed. The
terminal will not wait for the keys to be lifted, but will transmit both codes as soon as possible after the
keys are first typed. If three such keys are held down simultaneously, the codes for the first two keys
are transmitted immediately; the code for the third will be transmitted if and when one of the first two
keys is lifted.
Table 5-1 shows the standard ASCII code that may be generated by the keys of the VT55 keyboard.
Columns 0 and 1 are termed control codes. They are the set of 32 nonprintable characters normally
used for terminal control. Columns 2-7 are called printable characters. They contain upper- and
lowercase letters, numbers, and symbols.
5-4

Table 5-1 Code Ensemble of ASCII Keyboard - Transmitted Codes
b7

0
0

b3
.j,

b2
.j,

bl
.j,

1
0

0
1
0

1
1

1
0

1
1

NUL

DLE

SOH

DCI

STX

DC2

ETX

DC3

EOT

DC4

ENQ

NAK

ACK

SYN

Row.j,

7
P

BEL

ETB

CAN

(

)

SUB

ESC

[

+
,

'1"

<
=

I
I

DEL

When a key is pressed, the ASCII code is transmitted to the host device when the terminal interface is
set to a full duplex mode. Any effect on the VT55 will only occur when the computer transmits a
response to the VT55. When in local or full duplex with local copy mode, the terminal will be directly
affected by this code. This is discussed in subsequent paragraphs.

5.3.1.1 ASCII Codes - Table 5-1 shows the ASCII code that each key on the keyboard generates. The
binary representation of the ASCII character is found by taking the three binary bits of the column, b 7
b6 b 5, and merging these with the b4 b 3 b 2 b l of the row to form b 7 b6 b 5 b4 b 3 b 2 b l • For example, the
binary representation of letter H is b 7 b6 b 5 b4 b 3 b 2 b l = 1001000. As ASCII characters are normally
referred to in octal, H would correspond to 1108 •
5.3.1.2 Shift Key - Lowercase letters, numerals, and some symbols are generated directly by pressing
the corresponding key on the keyboard. Uppercase letters and symbols - !@#$%&*O+[:"<> and?are generated by pressing SHIFT in conjunction with the correctly labeled key (Figure 5-3).
5.3.1.3 CAPS LOCK Key - When this key is in the pressed position, it causes the terminal to generate
uppercase letters instead of lowercase letters. This is its only function. None of the other keys are
affected.

5-5

However, if software must be able to distinguish between, for instance, the pressing of the 5 key on the
auxiliary keypad and the pressing of the 5 key on the main keyboard, the host can give the terminal a
command to place it in Alternate Keypad mode. In Alternate Keypad mode, the functions of the blank
keys and the keys labeled with arrows do not change, but the numeral, decimal point, and ENTER
keys all transmit unique escape sequences. Now all the keys on the auxiliary keypad are user-definable.
Table 5-2 lists the codes that may be transmitted with these keys.
Table S-2
Key

o
1

2
3
4

5
6

7
8
9

ENTER

Codes Transmitted to Host by VTSS

VTSSNotln
Alternate Keypad Mode

VTSSln
Alternate Keypad Mode

060
061
062
063
064
065
066
067
070
071

033077 160 (ESC? p)
033077 161 (ESC? q)
033077 162 (ESC? r)
033077 163 (ESC? s)
033077 164 (ESC? t)
033077 165 (ESC? u)
033077 166 (ESC? v)
033077 167 (ESC? w)
033 077 170 (ES C ? x)
033077 171 (ESC? y)

056

033077 156 (ESC? n)

015

033077 115 (ESC? M)

NOTE
None of the keys on the auxiliary keypad are affected by depressing the SHIFT, CAPS LOCK, or
CTRL keys.

S.4 ALPHANUMERIC MODE
When the terminal is in Alphanumeric mode, information received by the terminal is interpreted as
letters, numerals, and terminal control information. The data may be generated by the host computer
or the keyboard, depending on the interface mode of the machine. Where data is placed on the screen
is determined by the position of the alphanumeric cursor.
S.4.1 Alphanumeric Cursor
The alphanumeric cursor is controlled by either control codes or escape sequences. This cursor may be
moved whether the terminal is in Alphanumeric, Graph Drawing, Graphic, or Escape mode. Table 5-3
lists the codes that the VT55 responds to when it is in Alphanumeric mode. Although the function of
control codes and escape sequences will be discussed in later paragraphs, as they are common to all
three modes of terminal operation, it is worthwhile to mention codes BS, HT, LF, and CR now.
Backspace, Horizontal Tabulate, Line Feed, and Carriage Return are utilized to move the alphanumeric cursor around the screen. The placement of this alphanumeric cursor determines where the next
character will be placed on the screen. All cursor control commands are listed in Table 5-4.

5-8

Table 5-3 Code Ensemble in Alphanumerics Mode - Received Codes

0
0

,:
bs

0
1

0
b3

0
0

1
1

BEL

&
,

(

)

•

[

I
I

Row~

ESC

DEL

Table 5-4 Alphanumeric Cursor Control
Code (ASClI s )

Type Command

Backspace (010)

Control Code

Return (015)
Line Feed (012)

Action
Moves cursor left one position.
Moves cursor to left margin of current line.
Moves cursor down one line.

Tabulate (011)

Control Code

Moves cursor to next tab stop. Tab stops are set every
8 spaces to the nnd character position. After the
nnd position, Tab moves the cursor one space to the
right.

ESC A (033,101)

Escape Sequence

Moves cursor up one line.

ESC C (033,103)

Moves cursor right one position.

ESC H (033, 110)

Moves cursor to upper·left corner of the screen
(Home Position).

ESC J (033, 112)

Erases screen from cursor to bottom of screen.

ESC K (033, 113)

Escape Sequence

Erases current line from cursor to right margin.

5-9

5.5 GRAPHIC MODE
There are 30 special symbols which can be displayed on the screen of the VT55. These special symbols
can be entered in the screen only by placing the terminal in Graphic mode. Normally, the codes
136-176 stand for the lowercase letters and some symbols. In Graphic mode, each code in this range
will call out one of the special symbols to be placed on the screen.
Codes 040-135 are unaffected. The symbols they represent can be placed on the screen whether or not
the terminal is in Graphic mode.
The VT55 uses the control codes to mark the position of the special symbols in its internal memory.
Therefore, the special symbols and the lowercase letters can coexist on the screen. The special symbols
will remain on the screen where they were entered even if the terminal is subsequently taken out of
Graphic mode.
Table 5-5 shows all the special symbols generated in Graphic mode.
Table 5-5

Display of the Codes 136-176

NOTE
This table describes the appearance of the VT55's
special symbols, which are entered into the screen
using the codes 136-176 and with the terminal in
Graphic mode.
Code Received
by Terminal
136
137
140
141
142
143
144
145
146
147
150
151
152
153
154
155
156
157
160
161
162
163
164
165

Display on VT55 Screen
In Graphic Mode
Not in Graphic Mode
Reserved
Reserved
Reserved
Solid rectangle

1/
3/
5/
7/
degrees
plus or minus
right arrow
ellipsis
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

5-10

a
b
c
d
e
f
g
h

J
k
1
m
n
0

p
q
r
s
t
u

Table 5-5

Display ofthe Codes 136-176 (Cont)

Code Received
by Terminal
166
167
170
171
172
173
174
175
176

Display on VT55 Screen
In Graphic Mode
Not ID Graphic Mode
subscript 2
subscript 3
subscript 4
subscript 5
subscript 6
subscript 7
subscript 8
subscript 9
paragraph

v
w
x
y
z
(

)

USES OF THE SPECIAL SYMBOLS
1.

The codes 154-163 cause eight horizontal lines at various scans within the character position
to be displayed. These bars can be used to paint a line on the screen with more accuracy than
would be possible using only minus signs and underlines.

The codes 142-145 (1/,3/,5/, and 7/) are used before the subscripts to form fractions. For
example, the fractions 1/8,1/4,3/8,1/2,5/8,3/4, and 7/8 can be formed using these four
symbols and the subscripts.

The ellipsis (151) appears as three dots in the character position ( ... ). The spacing of these
three dots is such that several of these symbols placed adjacent to one another in the screen
will produce a smooth line of dots.

The symbols produced by the codes 136-140 are reserved for possible future use.

5.6 DISPLA Y CALIBRATION SPECIFICATION
VT55 video screen data is actually composed of a graph drawing data field with overlaid alphanumeric
information. The graph drawing data field consists of 236 points along the Y axis (vertical) and 512
points along the X axis (horizontal). Superimposed on this graph drawing data field is 79 columns by
23 lines of alphanumeric information. The remaining row and column of alphanumeric data is outside
the graph drawing field. This aids in clear labeling of full screen graphs.
The following paragraphs define the graph drawing data field calibration and the alignment of
alphanumeric data on this field. With an understanding of this, the programmer can plot a graph and
then label it.
5.6.1 Calibration of the Graph Drawing Field
The graph drawing data field is addressed as a 3548 X 10008 matrix on the video screen as shown in
Figure 5-4.
Two "data characters" are transmitted to uniquely define either an X or Y point on the graph drawing
data field. The use of data characters and graphic drawing command characters is discussed in Paragraph 5.7.

5-11

~----------------------------~ __ x
(777.,0.'

CP-2135

Figure 5-4 Graph Drawing Data Field Calibration

5.6.2 Alignment of Alphanumerics on the Graph Drawing Data Field
VT55 alphanumeric characters are point-plotted on the video display in a 7 X 7 (horizontal by vertical)
dot matrix. Alignment of the alphanumeric character on the graph drawing data field is specified by
where the bottom dots of the 7 X 7 matrix fall in the Y direction and by where the left-hand edge of
this matrix is aligned in the X direction. Figure 5-5 shows this in detail for the character "E."

• ••••••
I

•I 0 0 0 0 0 0
.000000

•••••••
I
I

•I 0 0 0 0 0 0
.00 0 000
I
Y POSITION---------.-.-.-.-.-.-.
I
I
I

X POSITION
CP-2520

Figure 5-5

X and Y Position Specification for Letter "E"

In the Y direction, an alphanumeric character may be placed at 10-point (128) increments, beginning
with line 48, on the graph drawing on the data field. A total of23lines of characters reside on the graph
drawing data field. The 24th line is below this field at Y = -6 8 • As the Y position of an alphanumeric
character is usually determined by the number of "line feed" characters that the terminal receives after
the alphanumeric cursor is homed, the Y position on the graph drawing field is defined by:
Y = 224 - IOL

o ~ L ~ 23

All values in decimal.

where:
Y
L

= Y position on graph drawing data field
= Integer number of line feed characters from top line on screen.

5-12

In the X direction, characters are plotted on 6-1/2 dot boundaries beginning with the second column
of characters. Hence, 79 columns of characters reside in the graph drawing data field while the first
column is displayed to the left of this field. This first column begins at X = 6.5 s. Mathematically
expressed in terms of the number of "space" characters from the left column of the screen:
X = 6.5S - 6.5

0..;; S";; 79

All values in decimal.

where:

X = X position on graph drawing data field
S = Integer number of space characters from left column on the screen.
Examples:
Suppose that 14 line feed characters, followed by 31 space characters, followed by the letter E are
transmitted to the terminal while in Alphanumeric mode. The position of "E" will be:

Y = 224 - 10 (14) = 84

X = 6.5 (31) - 6.5 = 195
The decimal X, Y coordinate of the E is (195, 84), which corresponds to (303s, 124s) in octal.
Plotting a vertical graph line at X = 303 s and plotting a horizontal graph line at Y = 124s, in
Graph Drawing mode, will intersect the letter E in its lower left corner as shown in Figure 5-5.
If the example were changed so that 23 line feeds were transmitted, the new Y coordinate would
be:

Y = 224 - 10 (23) = -6
The letter E is now 6 dots below the line Y = 0 and outside the graph drawing field.
5.6.3 Definition of Graph Marker on the Graph Drawing Data Field
Graph markers may be displayed on any or all of the 512 points on each of the two VT55 graphs. The
marker will be displayed as a vertical 20s-point line, except between lines Y = 340s and 353s, where it is
only 14s points high.
The graph marker always starts at 20s increments (e.g., Os, 20s, 60s, l00s, 340s) along the Y axis. For
example, a graph marker desired over a point at Y = 322s will begin at Y = 320s and end at 337 s. A
graph marker placed on graph point Y = 350s will be plotted from Y = 340s to Y = 353 s (top of graph
drawing data field). Figure 5-6 shows the example of a graph marker plotted over graph point Y =
322s.

PART OF
GRAPH PLOT
Y =3378 -

y =32~ Y =3208 -

CP-2149

Figure 5-6 Graph Marker for Graph Point Y = 322s

5-13

5.7 GRAPH DRAWING MODE
This section describes the operation of the VT55 while in the Graph Drawing mode, plus the ways of
entering and exiting this mode.
Basically, the VT55 uses the same character set in Graph Drawing mode as the other two modes. To
enter the Graph Drawing mode, an escape sequence is necessary, specifically ESC (033 8) followed by
the numeral 1 (061s). To return to the previous mode, the sequence is ESC followed by the numeral 2
(033 8, 0628)'
5.7.1 Graph Drawing Command Characters
All graph drawing commands begin with a character from the normal character set. Specifically, these
are @, A, B, C, D, H, I, J, K, L.
1.

The @ is the "no operation" command.

Band J cause graphs 0 and 1 to be loaded.

A and I control the status of the display.

C and K cause markers to be loaded or erased.

D and L cause horizontal and vertical lines to be loaded or erased.

H is the command to set up an X coordinate.

These commands are followed (with the exception of the @) by either one or two characters. These
characters act as data for the command characters and as a result they will vary. (The command
characters are fixed.)
5.7.2 Graph Drawing Data Characters
The data characters all have a particular format in which the most significant bit is always 0, and the
next most significant bit is always 1. This format results in the use of a subset of the total 96-character
set. That subset is the group of characters with an octal representation of OXY, where X and Y vary as
follows:
The value of X ranges from 4 to 7.
The value of Y ranges from 0 to 7.
Paragraph 5.10 details how these data characters are used in conjunction with the graph drawing
command characters of Paragraph 5.7.1. Each graph drawing data character is contained within the
OXY subset and one or two OXY characters need to be transmitted to completely define the graph
drawing command character operation. When two characters define the operation, the first character
is taken as the least significant part of a IO-bit data word while the second character defines the most
significant part of the 10-bit data word. Table 5-6 lists the interpretation of ASCII characters that are
used as the least significant part of a IO-bit data word. Table 5-7 lists the interpretation of these
characters when used as the most significant part of the IO-bit word. When a least significant word and
a most significant word are received by the VT55, their data values are simply added to form the IO-bit
data value.
When only a single graph drawing data character is used in conjunction with a graph drawing command character, the low-order data values of Table 5-6 are applicable.

5-14

Table 5-6 Graph Drawing Data Characters
(When Used as Low-Order Part of Data Word)
Character
SPACE
!

Octal Value

Data Value*

40
41
42
43

0
1
2
3
4
5
6
7
10

45
46
47
50
51
52
53
54
55
56
57
60
61
62
63

&
, (apostrophe)
(
)
*

+
, (comma)
- (minus)

0
1
2
3
4
5
6
7
8
9

65
66
67
70
71
72

=
>

73
74
75
76
77

*Used as bottom five bits of a data word.

5-15

12
13

14
15
16
17
20
21
22
23
24
25
26
27
30
31
32
33
34
35
36
37

Table 5-7 Character Values
(When Used as High-Order Part of Data Word)
Character

Octal Value

Data Value*

SPACE

40
41
42
43
44
45
46
47
50
51
52
53
54
55
56
57
60
61
62
63
64
65
66
67
70
71
72
73
74
75
76
77

0
40
100
140
200
240
300
340
400
440
500
540

!
"

$
%
&
• (apostrophe)
(
)
*

+
, (comma)
- (minus)

0
1
2
3
4
5
6
7

8
9

600
640
700
740
1000
1040
1100
1140
1200
1240
1300
1340
1400
1440
1500
1540
1600
1640
1700
1740

·Used as top five bits of a data word.

When two characters must be transmitted to form a lO-bit data value, the low-order part of the data
word must always be transmitted first. The high-order part of the data word must be transmitted next.
The VT55 will misinterpret the use of received graph drawing data characters if they are not received in
the proper order.

5-16

Examples
The sequence & # used as data in a LOAD X COORDINATE command has the following
characteristics:
1.

Octal values 046 and 043 respectively.

Data values as follows: Since the # is to be used to form the most significant five bits of
the data word, its data value (from Table 5-7) will be 140. The addition of this value to
that of the & character (6 from Table 5-6) results in an address of 146 on the horizontal
axis.

Reversing the situation to send the data word 146 you would take the least significant five bits
(00110) and add the standard bit pattern (01) to form the total pattern 0 100 110 or 46 (the
character &). The same process would hold true for the most significant five bits, 00011, with the
standard pattern 01 being added. The result, 0 100 011 or 43 (the character #), would be sent to
the VT55 as the second character.
Viewed this way, it can be seen that characters can be formed and sent to the VT55 by a program which
need not keep track of what the actual characters were. As long as the bit pattern is correct, the proper
action will result.

5.7.3 Other Characters
If while in Graph Drawing mode a character other than a graph drawing command or graph drawing
data character is received, it is examined to see if it is either an escape character or a control code. If
neither is true, then the character is ignored, and the VT55 remains in Graph Drawing mode.
5.8 ESCAPE MODE
Although there are three main modes of operation, Alphanumeric, Graphic, and Graph Drawing,
there is an intermediate mode called Escape mode. This mode is entered at each occurrence of the ESC
(escape) character. One example is the switch from Alphanumeric to Graph Drawing mode. The
sequence is ESC 1 (033 8 followed by 0618). Upon receiving the ESC character, the VT55 enters Escape
mode. In this mode, it is capable of acting upon a specific set of characters to perform functions such
as homing the cursor or erasing portions of the alphanumeric display. Table 5-8 gives the characters
which the VT55 can act upon while in Escape mode and the resulting action. All references to "cursor"
refer to the alphanumeric cursor. The following paragraphs detail the action of each escape sequence.
5.8.1 Cursor Down
Cursor Down is invoked by ESC B (033 102). The cursor is moved down one character position to the
same column of the line below where it was. If the cursor was on the bottom line of the screen to begin
with, it stays where it was, and no scroll occurs.
5.8.2 Reverse Line Feed
Reverse Line Feed is invoked by ESC I (033 Ill). The cursor is moved up one character position to the
same column of the line above the one it was on. If the cursor was on the top line to begin with, it stays
where it was, but all the information on the screen moves down one line. The information that was on
the bottom line of the screen is lost; a new blank line appears at the top line. This process is also called
"downward scroll."

5-17

Table 5-S
Character

Octal Code

101
102
103
106
107
110
111
112

B
C
F
G
H
I
J

113

y
Z

131
132

[

133
134

]

135

136
137

>
I
2

075
076

061
062

Escape Mode Characters
Action
Moves cursor up one line.
Moves the cursor down one line.
Moves cursor right one position.
Enter Graphic mode.
Exit Graphic mode.
Moves cursor to the home position.
Scroll screen down one line.
Erases text from cursor position to bottom of
screen.
Erases text from cursor position to right margin.
Direct cursor address command.
VT55 transmits its identifying sequence ESC / E
(033,057,105).
Enables Hold Screen mode.
Disables Hold Screen mode.
Copies all lines of text from top of screen to line
containing cursor.
Enables auto-print mode.
Disables auto-print mode.
Enter Alternate Keypad mode.
Exit Alternate Keypad mode.
Enter Graph Drawing mode.
Exit Graph Drawing mode.

5.S.3 Cursor Up
Cursor Up is invoked by ESC A (033 101). The cursor is moved up one character position to the same
column of the line above the one it was on. If the cursor was on the top line to begin with, it stays
where it was, and no scroll occurs.
5.S.4 Cursor Right
Cursor Right is invoked by ESC C (033 103). The cursor is movved one column to the right. If the
cursor was at the end of the line to begin with, it does not move. No character on the screen is erased.
5.S.5 Cursor Home
Cursor Home is invoked by ESC H (033 110). The cursor is moved to the home position - the character position at the upper left corner of the screen. If it was there to begin with, it stays there.
5.S.6 Direct Cursor Addressing
Direct Cursor Addressing is invoked by ESC Y (033 131). The next code after ESC Y that the host
sends to the terminal will not be displayed but will be interpreted as specifying one of the lines on the
screen. The character the terminal receives after that will not be displayed but will be interpreted as
specifying one of the columns on the screen. The cursor will be moved to the character position at the
specified line and column. The complete Direct Cursor Addressing command has this form:
ESC Y line # column #
and consists of four characters from the host. Control codes or other escape sequences should not be
embedded in this string of four characters (doing so will produce unspecified results).
5-18

For line number, the host sends the octal code 040 to specify the top line of the screen, 041 to specify
the line below the top line, and so forth. The bottom line is specified by 067. The VT55 will not move
the cursor vertically if the vertical parameter is out of bounds. A Direct Cursor Addressing command
with the first parameter greater than 067 can be issued to the VT55 to move the cursor arbitrarily in the
horizontal direction without the flickering of the video that the full Direct Cursor Addressing command can cause.
For column number, the host sends the octal code 040 to specify the leftmost column in a line, and 157
to specify the rightmost column. If the column number specified is greater than 157 and, therefore,
does not specify a column that exists on the screen, the cursor is moved to the rightmost column on a
line.

5.8.7 Erase to End of Line
Erase to End of Line is invoked by ESC K (033 113). All the information at the cursor position and to
the right to the end of the line is erased. (Spaces are deposited at those character positions.)
If the cursor is at the rightmost column on a line, the character at the cursor position will be the only
character to be erased. If the cursor is at the leftmost column on a line, the entire line will be erased.

5.8.8 Erase to End of Screen
Erase to End of Screen is invoked by ESC J (033 112). All the information from the cursor position to
the end of the screen is erased. This function does what Erase to End of Line does and, in addition,
erases the information in every line below the line the cursor is on.
If the cursor is at the lower right corner of the screen, one character will be erased. If the cursor is at the
home position of the screen, all the information on the screen will be erased.

5.8.9 Identify Terminal Type
Identify Terminal Type is invoked by ESC Z (033 132). When the terminal receives ESC Z, it transmits
a 3-character escape sequence to the host. This escape sequence tells the host two things:
1.

The terminal is switched on, connected to the host, and responding to commands.

The terminal is a VT55, with graph drawing capacity.

During the time it is transmitting the 3-character escape sequence, the terminal will disable the keyboard so that characters typed will not be embedded in the escape sequence. Under no circumstances
will the keyboard be locked for more than half a second. Because the transmitting baud rate is set
higher so that it takes less time to transmit the three characters, the time for this command will drop to
under 1/15 second.
On receiving this command, the VT55 will respond with ESC / E (033, 057, 103).

5.8.10 Enter Hold Screen Mode
Enter Hold Screen Mode is invoked by ESC [ (033 133). The terminal enters Hold Screen mode as
described in Paragraph 5.9. Data will not be allowed to scroll off the screen without permission from
the operator via the SCROLL key. After entering Hold Screen mode, the first command that would
cause a scroll to occur will not be immediately processed, and the terminal will send XOFF to the host.
Hold Screen mode remains in effect until the Exit Hold Screen Mode command disables that feature.

5.8.11 Exit Hold Screen Mode
Exit Hold Screen Mode is invoked by ESC \ (033 134). The terminal exits Hold Screen mode. Data will
be allowed to scroll off the screen if it has to in order to make room for new data coming from the host.
5-19

S.S.12 Enter Alternate, Keypad Mode
Enter Alternate Keypad Mode is invoked by ESC = (033 075). The terminal enters Alternate Keypad
mode, in which the numeral keys, decimal point key, and ENTER key transmit unique escape
sequences, allowing the software to distinguish between them and keys on the main keyboard, and to
assign its own meaning to each key.
Alternate Keypad mode will not be in effect until the host issues this command and, once enabled, will
remain in effect until the host uses the Exit Alternate Keypad Mode command to disable that feature.
S.S.13 Exit Alternate Keypad Mode
Exit Alternate Keypad Mode is invoked by ESC > (033 076). The terminal exits Alternate Keypad
mode. Now the numeral, decimal point, and ENTER keys transmit codes that are indistinguishable
from the codes transmitted by the numeral, decimal point, and RETURN keys on the main keyboard.
Applications which do not need to redefine the meanings of these 12 keys will work correctly with the
operator using the keypad for entry of numeric data.
S.S.14 Copy Screen
Copy Screen is invoked by ESC [ (033 135). The contents of the CRT screen will be copied. See
Paragraph 5.2.2 for detailed operation of the copier.
Note that the graph drawing information will temporarily blank from the viewing screen during the
operation. The alphanumeric cursor will not be copied on the hard copy output.
S.S.IS Enable Auto-Print Mode
Enable Auto-Print Mode is invoked by ESC A. (033 126). The terminal enters Auto-Print mode, in
which the copier will copy each line of print as the cursor is moved to the line below by a line feed or
ESCB.
S.S.16 Disable Auto-Print Mode
Disable Auto-Print Mode is invoked by ESC - (033 137). The terminal leaves Auto-Print mode and
returns to normal operation.
S.S.17 Enter Graphic Mode
Enter Graphic Mode is invoked by ESC F (033 106). When codes in the range 136-176 are received,
they will be converted to the special symbols before being placed on the screen. This remains true until
the terminal receives the Exit Graphic Mode command.
S.S.IS Exit Graphic Mode
Exit Graphic Mode is invoked by ESC G (033 107). The codes 136-176 resume their standard (ASCII)
meanings.
See Paragraph 5.5 for additional Graphic mode information.
S.S.19 Enter Graph Drawing Mode
Enter Graph Drawing Mode is invoked by ESC 1 (033 061). The terminal will interpret standard
ASCII codes as graph drawing data and commands. This remains true until the terminal receives the
following command.
S.S.20 Exit Graph Drawing Mode
Exit Graph Drawing Mode is invoked by ESC 2 (033 062).- The terminal will revert to the mode of
operation it was in before entering Graph Drawing mode. Graph Drawing mode is explained in Paragraph 5.7.

5-20

5.9 HOLD SCREEN COMMAND
The Hold Screen command allows the operator to control the rate at which data enters and leaves the
screen. This is important because the DECscope can operate at such fast speeds that data from the host
might remain on the screen for only a few seconds before it scrolls up and off the top of the screen to
make way for new data. Other terminals simply allow the data to leave the screen and be lost, regardless of whether the operator has had time to read it. The DECscope does this, too, when it is not placed
in Hold Screen mode.
The DECscope has a synchronization scheme with the host, which was described in Paragraph 3.5.4.
Whenever, for any reason, it cannot process data from the host, it automatically transmits the control
code XOFF (023). When it is ready again, it transmits XON (021). The terminal depends on the host to
suspend its transmission promptly when the host receives XOFF from the terminal, and resume the
transmission where it left off upon receiving the XON. When software places the terminal in Hold
Screen, the terminal refuses to perform scrolls. If the host commands it to scroll by sending the terminal a LF (012) when the cursor is on the bottom line, the terminal will place the LF in the Silo to be
executed later, and send XOFF to the host. The XOFF means that the terminal is not ready for more
data from the host, because the terminal assumes that the operator is not ready for more.
The operator tells the terminal that he is ready to see more data - specifically, one more line of databy pressing the SCROLL key. When he does so, the terminal processes the LF character out of the
Silo. When it does this, a scroll occurs. Then the terminal takes from the Silo any other characters that
may have arrived from the host before the host responded to the XOFF and suspended its output.
Each character in the Silo is displayed on the screen or, in the case of commands, executed exactly as if
it had just been received - unless it is another LF causing another scroll. If the terminal encounters a
LF in the Silo, it stops processing characters out of the Silo until the operator presses the SCROLL key
again.
If the terminal processes all the characters in the Silo without finding a LF, it transmits XON to the
host to notify it that the terminal is again ready to receive characters. It will display all the characters
and execute all the commands until it is again ordered to perform a scroll. Then it will again send
XOFF, store the LF in the Silo, and wait for the operator to press the SCROLL key.
If, after the terminal transmits XOFF, the host keeps transmitting to such an extent that the Silo fills

up completely, then, rather than allow data to be lost, the terminal will perform the scroll it was
commanded to perform despite Hold Screen and remove the characters from the Silo and interpret
them, reducing the backlog. However, the terminal does not exit Hold Screen; if it encounters another
command to scroll the display, either within the Silo or directly from the host, it will not scroll, and it
will begin to buffer incoming characters once again.
The operator types the SCROLL key to request that another line be admitted to the screen. The
terminal translates this request into "start" and "stop" commands - XON and XOFF - and sends
them to the host in such a way that just enough data comes to the terminal to satisfy the operator's
request for one more line.
The operator can type the SCROLL key with the SHIFT key down to make a request for a new
screenful of data. As with the unshifted SCROLL request, the terminal begins to process characters
again and sends XON to the host when the characters that accumulated in the Silo have all been
processed. But the shifted SCROLL request tells the terminal to allow an entire screenful of new data
to enter the screen before shutting off the transmission from the host.

5-21

Therefore, by using Hold Screen, the software can control the page-by-page output to the terminal.
The software need not maintain counters of how many lines have been output to the terminal since the
last request. Since the terminal calculates when its own screen has been filled with new data and takes
the initiative to notify the host, the software does not need to know the capacity of lines of the terminal
it is outputting to. Current DIGITAL software operating systems such as RSTS-ll, RSX-ll, RT-ll,
TOPS-lO, and TOPS-20 support XON/XOFF control.
5.10 CONTROL CODES
Of the 128 codes in the ASCII set, 32 are considered to be control codes. This means that they do not
represent characters that can be displayed on the screen. Some are meant to be interpreted as commands. To generate a control code, the CTRL key must be depressed simultaneously with another key.
CTRL has the effect of forcing b 7 and b s to 00. Therefore, control codes can be generated by depressing CTRL and a key listed in columns 4 or 5 to generate the codes of columns 0 and 1, respectively. For
example, CTRL G would generate binary code b 7 b6 b s b4 b 3 b 2 b l = 0000111 or 0078 (BEL). Careful
examination shows that CTRL ' would generate BEL as well, but since the keys of columns 2 and 3
would generate codes equivalent to columns 4 and 5, control codes are normally referred to the keys in
columns 4 and 5.
The execution of control codes is possible in all four (Escape, Graph Drawing, and Alphanumeric)
modes. After execution of the control code, the mode remains the same as before. (Refer to Figure 5-2
for flow diagram.)
The possible control codes and their operations are detailed in Table 5-9. Some control codes may be
transmitted by typing a single key. These are designated by an asterisk.
Eventually, however, with only 32 control codes, terminals are going to run out of codes to stand for
all their various functions. Therefore, one of the control codes, called ESC (escape), has been reserved
for use to declare that the code that follows it, though it represents a character that could be displayed
on the screen, must instead be interpreted as a command. For example, if the terminal receives the
code 102, it will display a "B" on the screen. But if it receives 033, the code for ESC, and then 102, it
will not display a "B" but instead will perform a special command.
Escape sequences exist to position the cursor, erase part or all of the information on the screen, place
the terminal in special modes in which it behaves differently, and force the terminal to identify itself to
the host.
If a control code is sent to the terminal between the ESC and the final character, the function specified
by the control code is performed when the control code is received, and the function specified by the
escape sequence is performed when the final character is received. If the VT55 receives ESC ESC, it
will remain in Escape mode and wait to interpret the next received character as part of the escape
sequence.

5.11 GRAPHING COMMANDS AND EXAMPLES
This paragraph describes the character sequences needed to initiate the graphing functions of the
VT55. The VT55 is completely controlled by the characters sent to it over the asynchronous ASCII
line, and a command can be constructed out of simple 2- and 3-character sequences. Typically, the first
character in any string is fixed while subsequent characters will vary. This is because the bits in the
second (and third) character are assigned specific functions. Since these functions are optionally
selected by the user's program, the bit pattern and thus the character will vary.

5-22

Table 5-9

Control Codes - All Modes

Octal
Code

Key

000
001
002
003
004
005
006
007
010*
011 *

NUL (Null)
SOH (Start of Heading)
STX (Start of Text)
ETX (End of Text)
EOT (End of Transmission)
ENQ (Enquiry)
ACK (Acknowledge)
BEL (Bell)
BACKSPACE BS (Backspace)
TAB
HT (Horizontal Tab)

012*
013
014
015*

LINE FEED
RETURN

016
017
020
021t
022
023t
024
025
026
027
030
031
032
033*
034
035
036
037

ESC

Terminal Action

Character

LF (Line Feed)
VT (Vertical Tab)
FF (Form Feed)
CR (Carriage Return)
SO (Shift Out)
SI (Shift In)
DLE (Data Link Escape)
DC 1 (Device Control 1)
DC2 (Device Control 2)
DC3 (Device Control 3)
DC4 (Device Control 4)
NAK (Negative
Acknowledge)
SYN (Synchronous Idle)
ETB (End of Transmission
Block)
CAN (Cancel)
EM (End of Medium)
SUB (Substitute)
ESC (Escape)
FS (File Separator)
GS (Group Separator)
RS (Record Separator)
US (U nit Separator)

None
None
None
None
None
None
None
Rings buzzer.
Moves cursor one space left.
Tabs eight spaces right (one space right after
column 72).
Moves cursor down one line.
None
None
Moves cursor to far left and down one line
(scrolls if required).
None
None
None
None
None
None
None
None
None
None
None
None
None
Switches the unit to escape mode.
None
None
None
None

*These codes may be entered by special keys on the keyboard.
tOC! is also referred to as XON; OC3 is also referred to as XOFF.

5-23

In the repertoire of the VT55, there are a total of nine instructions. One of these (the NO OP instruction) is a single-character command, two are 2-character commands, and six are 3-character sequences.
It should be noted that the first "command character" transmission implies that only the last characters need be sent since the first character sets up the type of operation (Load Graph 0, etc.), and the
VT55 will continue to operate in this way until a new command is issued. Many data characters may
now be sent to the terminal, thus increasing the plotting rates.
Prior to sending any of these character strings (Graph Drawing commands), the VT55 must be in
Graph Drawing mode. This is accomplished by sending an escape character (033 8) followed by the
numeral 1 (06ls).
5.11.1

NO OP

1st Character: @ (1008 )
2nd Character: None
3rd Character: None
5.11.2

Load Enable Register 0

1st Character: A (101 8
2nd Character: Variable (see below)
3rd Character: None
Explanation
The second character is formed by setting bits where the bits have the following functions:
01XXXXX

The most significant bit is always O. _ _ _ _ _ _ _ _ _ _---'t

Always 1. - - - - - - - - - - - - - - - - - - - - - - - '

If 1, displays graph 1 as a histogram. - - - - - - - - - - - - - - - '

If 1, displays graph 0 as a histogram. - - - - - - - - - - - - - - - - - '

If 1, allows graph 1 to be d i s p l a y e d . - - - - - - - - - - - - - - - - - J

If 1, allows graph 0 to be displayed. - - - - - - - - - - - - - - - - - - '

If 1, allows all enabled graphic information to be displayed. (This is the least significant bit in the data
character. )
NOTE
Do not enable graph 0 and histogram 0 or graph 1
and histogram 1 at the same time. This intensifies the
graph envelope, which is undesirable for most
applications.

5-24

5.11.3 Load Enable Register 1
1st Character: I (1118)
2nd Character: Variable (see below)
3rd Character: None
Explanation
The second character is formed by setting bits where the bits have the following functions:

OIXXXXX

_______~t- ,I ~ • .~ ~ ,
1. The most significant bit is always O.
2.

Always I. --------------------~

If 1, clears any vertical lines, horizontal lines, graph 0,graph 1, and any graph markers. ---------------1

If I, allows graph markers to be displayed for graph
1.
------------------------~

If
for graph
O. 1, allows graph markers to be displayed
__________________________
--J

If 1, allows display of vertical lines. - - - - - - - - - - - - - - - - - - - '

If 1, allows display of horizontal lines. (This is the
least significant bit in the character.) - - - - - - - - - - - - - - - - - - - - '
NOTE
It is good programming practice to initialize the
graph drawing memories with the sequence I 0
(clear) when initializing a graph drawing program.

5.11.4

Load Graph 0

1st Character: B (102 8)
2nd Character: Variable (see below)
3rd Character: Variable (see below)
Explanation of Second Character

1 X
\

X X X X
I
v

The most significant bit is always O.

Always I.

Remaining bits form the low-order five bits of an 8bit Y value. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _---.J

5-25

Explanation of Third ataracter
1 X

•!

The most significant bit is always O.

Always 1.

Unused

Remaining bits form the high-order three bits of the
8-bit Y value.

X
T

5.11.5 Load Graph 1
1st Character: J (1128)
2nd Character: Variable (same as in Paragraph 5.11.4).
3rd Character: Variable (same as in Paragraph 5.11.4).

5.11.6 Load Graph Marker Memory for Graph 0
1st Character: C (103 8)
2nd Character: Variable (see below)
3rd Character: Variable (see below)
Explanation of Second Character

The most significant bit is always O.

Always 1.

Remaining bits form the low-order five bits of a 9-bit
X address.

Explanation of Third Character

,
0

The most significant bit is always O.

Always 1.

If 1, causes marker to be loaded; if 0, causes marker
to be erased.

Remaining bits form high-order four bits of the 9-bit
X address.
NOTE
Graph 0 or histogram 0 must be enabled for graph
markers to be displayed.
5-26

1 X

X X

5.11.7 Load Graph Marker Memory for Graph 1
1st Character: K (113 8)
2nd Character: Variable (same as in Paragraph 5.11.6)
3rd Character: Variable (same as in Paragraph 5.11.6)
NOTE
Graph 1 or histogram 1 must be enabled for graph
markers to be displayed.
5.11.8 Load Horizontal Line Coordinate
1st Character: 0 (1048)
2nd Character: Variable (see below)
3rd Character: Variable (see below)
Explanation of Second Character
0

1 X
\

X X

The most significant bit is always O.

Always 1.

Remaining bits form the low-order five bits of an 8bit Y position. ------------------------------------------------'

X X
I

Explanation of Third Character

_ _

01XXXXX

----ot

'T'

The most significant bit is always O.

Always 1.----------------------------------------~

If 1, causes horizontal line to be loaded; if 0, causes
horizontal line to be erased. ---------------------------------1

Unused --------------------------------------------------.1

Remaining bits form the three high-order bits of the
8-bit Y position. --------------________________________________---'

5.11.9 Load Vertical Line Coordinate
1st Character: L (1148)
2nd Character: Variable (see below)
3rd Character: Variable (see below)

5-27

Explanation of Second Character

1 X

•J

The most significant bit is always O.

Always 1.

Remaining bits form low-order five bits of a 9-bit X
position.

Explanation of Third Character

The most significant bit is always O.

Always 1.

If 1, causes vertical line to be loaded; if 0, causes vertical line to be erased.

Remaining bits form the high-order four bits of a 9bit X position.

1 X

5.11.10

Load Starting X Coordinate

1st Character: H (1108 )
2nd Character: Variable (see below)
3rd Character: Variable (see below)

Explanation of Second Character
0

The most significant bit is always O.

Always 1.

Remaining bits form the low-order five bits of a 9-bit
X position.

Explanation of Third Character

,
0

1 X

X
T

The most significant bit is always O.

Always 1.

Unused

Remaining bits form the high-order four bits of a 9bit X position. - - - - - - - - - - - - - - - - - - - - - - - - - - - - '

5-28

5.11.11 Example Sequence
This example uses all of the Graphic Drawing control commands except @ (100g) - no operation. The
terminal should be in local mode or attached to a host computer that will echo data typed on the
keyboard.
5.11.11.1

Reset Logic - Flip the POWER/LOGIC RESET switch to reset all circuits.

5.11.11.2

Enter Graph Drawing Mode - Type ESC 1 (033 g, 06ls).

5.11.11.3 Enable Graphs - Type A, '(apostrophe), I, ? (lOIs, 047 s, His, 077 s). This enables all graphs,
lines, and cursors. When the apostrophe is pressed, a horizontal line will appear at the bottom of the
screen to show that the graphs are enabled. Press the SHIFT key for an uppercase character.
5.11.11.4 Load Graph 0 - Type B, 1, 1 (102s, 061 g, 061 s). A single point will appear near the lower left
of the screen.
Type 1 twice, eight more times. This will form a 9-point horizontal line.
Notice that B need not be pressed again as the terminal will continue loading graph 0 until it receives
another command.
5.11.11.5 Load X Coordinate - Type H, SPACE, (
to 400g, or the middle of the screen.

(110s, 040s, 050g). This sets the next X coordinate

5.11.11.6 Load Graph 1 - Type J, 6, 6 (112s, 066 s, 066g). A single point will appear near the top center
of the screen.
Type 6 twice, eight more times. There will now be a 9-point horizontal line near the top center of the
screen.
5.11.11.7 Load Graph Marker Memory for Graph 0 - Type C, $,0 (103g, 044g, 060g). This will generate
a 16-point vertical cursor that will intersect the graph 0 horizontal line.
5.11.11.8 Load Graph Marker Memory for Graph 1 - Type K, $, 8 (113 s, 044 s, 068g). This will generate a I6-point vertical cursor that will intersect the graph 1 horizontal line.
5.11.11.9 Load Horizontal Line Coordinate - Type D, +, 7, 2, 3 (104g, 053 8, 067g, 062g, 063 8), This
generates horizontal lines at the top and the center of the screen.
5.11.11.10 Load Vertical Line Coordinate - Type L, SPACE, 0, ?, ?, SPACE, 8 (1148, 040g, 060g, 077 8 ,
040g, 068 8), This generates vertical lines at the right and left extremes of the graph drawing field, and
through the center. The left line should touch the left side of graph 0, and the center line should touch
the left side of graph 1.
5.11.11.11 Controlling Display Status - All information is now loaded. The display status can be
controlled with enable registers 0 and 1.
Type A, 9 (lOI g, 069 8), Graphs 0 and 1 will both become histograms.
Type 7 (067 g). Graph I will show as a histogram; graph 0 will show as a graph.
Type 5 (065 8 ). Graph I will show as a histogram; graph 0 will not show at all.
Type - (minus) (055 8 ). Graph 0 will show as a histogram; graph 1 will show as a graph.
5-29

Type 0 (0608). The entire screen will go blank.
Type' (apostrophe) (0478). Everything will reappear; graphs 0 and 1 will both show as graphs.
Type I, . (period) (111 8, 056 8). All horizontal lines will disappear. (This does not include the baseline
which is part of the graphs.)
Type, (comma) (0548). All vertical lines will diaappear.
Type ( (0508). Graph 0 graph markers will disappear.
Type SPACE (0408). Graph 1 graph markers will disappear.
Type /. All lines and graph markers will reappear.
Type ESC 2 (033 8, 0628). Typing characters on the keyboard will place characters on the screen as the
VT55 should now be in Alphanumeric mode. The graph drawing will remain on the screen.

5-30

CHAPTER 6
MAINTENANCE

6.1 PHILOSOPHY
Major assemblies used within the VT55 are designed to be repaired by subassembly replacement. This
is in contrast to field replacement of defective components. There are a number of reasons for utilizing
this service technique.
The first is the fact that the two microprocessor modules that form the basis for control of the VT55
perform closely interrelated functions via circuits scattered across the two boards. The interaction of
these two modules is very complex. Without the use of specially designed and programmed Exclusive
OR (XOR) testers, a very detailed knowledge of the microinstruction set is needed to pinpoint a
problem. Three other major assemblies are the power supply/monitor module, graphing control module, and keyboard. As these modules all depend on the microprocessor for proper timing and data
information, it is difficult to effectively troubleshoot them. Special testers exist to functionally test
these modules when detached from the microprocessor.
For these reasons, swapping major assemblies is a far more efficient means of repairing VT55 malfunctions. All spare VT55 modules stocked in DIGITAL Field Service Offices have been tested on a special
tester and then are tested under actual operating conditions within a VT55. Special module-testing
VT55 units are exposed to a thermal cycle test where each subassembly is made to operate over the
complete temperature range of the VT55. These precautions ensure that highest quality spare parts are
stocked at DIGITAL Field Service Offices throughout the world. These same parts are available to the
VT55 user who wishes to maintain his own spare parts kit for performing his own terminal service.

6.2 CORRECTIVE MAINTENANCE
6.2.1 Recommended Spare Parts and Tools
This paragraph describes procedures for tracing a problem to one of the replaceable subassemblies
within the VT55. When attempting to repair a VT55, it is imperative to bring a VT55 spare parts kit,
plus a few simple tools for disassembly and some minor testing. Recommended spare parts and a
recommended tool kit are listed in Tables 6-1 and 6-2.
The recommended spares kit can be used to repair 95 percent of machine failures. Figure 6-1 shows the
relationship of these parts and the other parts that constitute the VT55.
The two functional parts that are not included in the spares kit are the transformer mounting plate
assembly and the CRT assembly. Both items demonstrate a very low failure rate. The transformer
mounting plate may be ordered, when required, through the local DIGITAL Field Service Office. The
CRT assembly contains the 12-inch cathode ray tube. Due to the safety hazards involved in handling
this part, the entire VT55 must be returned to a DIGITAL Repair Depot when a CRT repair is
required. Depots have the necessary protective gloves and goggles required for this operation.

6-1

Table 6-1

Recommended Spare Parts Kit

DEC Part No.

Description

VT55Modeis

5411450
5411745
5411815
5411743-4t
5411743-2*
23-128A9-oo
23-129 A9-oo
23-130A9-oo
23-131 A9-oo
M7024
5411170-4
BN52A-7F
BN52B-7F
BN52C-7F
7010068-0
7010068-2

Monitor, Power Supply, VT55
Data Path, Memory, Decoders
8K Character Generator
ROM, UART, and Timing
ROM, UART, and Timing
512 X 4 ROM
512X4ROM
512 X 4 ROM
512 X 4 ROM
VT55 Graphing Control Module
Keyboard VT51/VT50H
VT52 EIA Cable
20 rnA Adapter Assembly
Current Loop to 283B Plug Assembly
Copier Assembly 115 V /60 Hz
Copier Assembly 220 V/240 V, 50Hz

All models·
All models·
All models·
All models·
All models·
All models·
All models·
All models·
All models·
All models·
All models·
EE, EF, EH, FE, FF
EA, EB, EC, FA, FB
HA, HB, HC, JA, JB
FA, FE,JA
FB, FF,JB

*This includes VT55 models EA, ED, EC, EE, EF, EH, HA, HD, HC, FA, FD, FE, FF, JA, JC only.
tFor DIGITAL Field Service only; VT52-variation module requires the substitution of four pluggable
ROMs (128A9, 129A9, 130A9, 13IA9) before it may be used in the VT55.
:j:For OEM users; this module already has the four ROMs incorporated on the module.

Table 6-2

Recommended Tool Kit

Quantity

Description

1
1

Phillips head screwdriver
Slotted head screwdriver
#8 nutdriver, #10 nutdriver
Volt-ohmmeter or digital voltmeter
Wire stripper
Assorted pieces of insulated wire, #22 gauge
Jar of Thermal Joint Compound (DEC Part No. 9008268 or equivalent)
Transformer, 0.075-inch slug alignment tool
General Cement No. 9300 or equivalent

1
1
1

6-2

6.2.2 Basic Description of Internal Signal Flow
Figure 6-1 is a simplified signal flow diagram to aid in understanding the function of each subassembly. The microprocessor modules and the graphing module comprise the heart of the VT55.
When data is received via the keyboard or serial line, it is fed into the ROM, UART, and timing
module (Figure 6-2). If the data is alphanumeric or graphic, it will be stored in the data path, memory,
and decoder module. If the data is meant for graph display, it is stored in the graphing module.
Control of terminal operation is within the microprocessor module pair. Unique graph drawing information control is contained within the graphing module. In conjunction with timing circuits contained
within all three of these modules, alphanumeric, graphic, and graph drawing video data are logically
ORed into the monitor section of the power supply/monitor module. In fact, it is possible to remove
the graphing module from the terminal and still retain full alphanumeric and graphic capability. The
monitor controls the sweeping of the raster scan over the face of the CRT and unblanking of the beam
when a point is to be intensified on the screen. The monitor also generates all the high voltages
necessary for CRT operation. Timing information from the data path, memory, and decoder module is
critical to properly operate the monitor. Voltage from the power supply section of the power supply/monitor module is converted into the voltages required to operate the tube. Of course, the power
supply delivers power to all circuits in the VT55 logic as well.
Beam position information is deflected on the cathode ray tube by means of a yoke placed around the
glass neck of the tube. The intensity for the beam is applied to the tube's cathode. Other electrodes of
the tube have signals applied to them that control focus and intensity of the screen image. In addition,
heater voltage and anode voltage are supplied to the CRT.
WARNING
The VTSS anode connection is made via a high-voltage diode on the power supply/monitor module,
through high-voltage wire, to the glass envelope of
the CRT. This circuit develops 11,000 V, so extreme
caution must be used when dealing with it.

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

r--

•••••••••

t
-r

XFMR.
POWER
MTG ........... SPLY.

PLATE

i--...J

CRT
ASSY.

L--'1

-----,

L __ -,

I
I

I fMiCRoPRO'CESSOR KEYBOARD

~--~

-1-t

,......L...J..I..J....I--J._ _..,

ROM,UART

TIMING

L
\.!..,....!J

-SIGNAL FLOW I
SCI~ltL
----D.C. VOLTAGE L ______ - - - - ...... A.C. VOLTAGE
CP-2136

Figure 6-1

VT55 Simplified Signal Flow Diagram

6-3

128A9

W6{

IN'EVEN PARITY
OUT' ODD PARITY

129A9

130A9

131A9

W5{ IN'BIT 7 SPACE
OUT BIT 7 MARK
JUMPER WIRE TABLE
VT52
WI OUT
W2 OUT
W3
IN
W4 IN

Figure 6-2

VT51 VT55
IN
OUT
IN
OUT
IN
OUT
IN
OUT

CP-2675

ROM, UART, and Timing Module

The purpose of the transformer mounting plate assembly is to convert the incoming ac voltage to lower
ac voltages that may efficiently be used by the power supply circuit. It also contains a primary circuit
breaker and the POWER/LOGIC RESET switch.
When a copier is connected on the VT55, ac wiring for the copier motor is derived from the transformer mounting plate assembly. All other signals, including dc voltages, come from the data path,
memory, and decoder module.
6.2.3 Troubleshooting Procedure

1. If the VT55 does not function properly, check the setting of the transmission speed and
interface mode switches, as well as the placement of the parity switch. Paragraph 3.4.6
discusses these settings.
2. If a problem still exists, turn power off and tip the unit on its top. Be sure to rest it on a
clean, smooth surface to avoid scratching the top surface.
3. Remove the base (Figure 6-3). Locate the graphing module (item 82).
6-4

NorES:
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( 176",,,.,5).
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EXACTl,..Y AS S~OWN.

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S. REMOvE. ONE. (I) PLASTIc.. rAe ON
VT55 EB,EC.EF,~H,H8.ANO He
MODELS AS SHOWN.
G. rrE:M72 MAY BE. REPLftCEO 'N!TH

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Figure 6-3 VT55 DECscope Assembly
Drawing (Sheet 1 of 4)
6-5

(15(QTY51

32
33
31 (QTY2l

~l
F

72oR74OR75

OR
35

36
OR

GND LUG

sJE.

NOTE'S

~
I "

/
c

"-,,- I REF
OR

73 REF

A
D

Figure 6-3

VT55 DECscope Assembly Drawing (Sheet 2 of 4)
6-6

THIS DRAWING AND SPECIFICATIONS. HEREIN ARE THE PROPERTY OF DIGITAL EQUIPMENT CORPORATION AND
SHALL NOT BE REPRODUCED OR COPIED OR USED IN WHOLE OR IN PART AS THE BASIS FDA THE MANUFACTURE
OR SALE OF ITEMS WITHOUT WRITTEN PERMISSION. COPYRIGHT C 1976 • DIGITAL EQUIPMENT CORPORATION

~~r------------------'--------------------------------------11

I,~O~.

DESCRIPTION

~~~~~~t

..,'" ..,U

~I~~I~~
E; IE; E; IE;
E;

~ ~I~ ~ ~ ~ ~

~ ~

ra:I

raJ

ra:I
ra:I

a..

E-MD-7411540-0-0

9008141-04

LABEL, ALUMINUM, ADHESIVE

9009672 00

SPACER, PLASTIC,

STANDOFF PLASTIC SNAP-IN. 250 LG
SHWH, TH'D ROLLING SCREW #6. X .31

9009670 00
9009680-01

9009680-04

SHWH, TH'D ROLLING SCREW #6 X .50

222

PART NO.

ral

II(

2
I

SHELL 1IT5/6

.75 LD. X .31 LG

PLASTIC, SNAP-IN

.62~

2
2

333

9009670-01

STANDOFF,

222

D-CS-5411450-0-1

MONITOR, POWER SUPPLY 1IT55

11111

D-IA-7412866-0-0

HEAT SINK

1
1

11111111

SHWH, TH'D ROLLING SCREW #8 X .75

1
1

9009679-02

1
1

11
12

9009683-01
1012412-0-0

!MI

1012065-0-0

CAP. 23,000 UF 20v +751

2-~\D-7414797-0-0

CLAMP

9009681-01

SHWH, TH'D ROLLING SCREW #10 X .62

9009682-01

SHWH, TH'D #8-32 X .38

.3-AD-7010858-01

BASE ASSEMBLY

1111111

A-SP-3700184-0-0

PACKAGING Il'STRUCTIONS w/o COPIER

BLACK PASSIVATED SCREW "8 X .38
CAP. 6000 MFD. 40 VDC G % AL EL

11111111

10 10 10 10 10 10

10 10 10 10 10 10 10 10

10 10 10

-lOr, AL EL

CAPACITOR

22222222

1
1

1
1 1
III

11111111
11111111

1
1

484848484848
555

484851 51 51 51 51 51

555

51 51 51

11111111
1
1
1--------

n-CS-5411745-0-1

DATA PATH, 1·jE!'10RY A..""D DECODERS

D-CS-5411743-2-1

ROM UART AND TIMING

111111111

0-AD-7010859-0-0

CRT ASSEMBLY

111111111

2-~\D-7413327-0-0

'ITG BRACKET KYBD RIGHT

111111111

D-CS-5411170-4-1

KEYBOARD VT51/1IT5/'1 H

111111111

C-IA-7413326-0-0

BRKT PC BD TO SHELL

D-~\D-7413103-0-0

COVER, CASSETTE OPENING

l-IA-7413432-0-0

BEZEL, KEYBOARD

/'I-UA-7010068-0-0

COPIER ASSEMBLY 115V/60HZ

I--

111111111
1111111

11111111

-'~~28~~/'I~-~U~A~-~7~0~1~0~0~6~8-~1~-~0~__~C~O~P~I~E~R~A~S~S~E~~~ffi~L~Y~1~¢~/'I~V/~1~27~V~,~5~3~H~Z~______-#~-~~-+-=-+--~~-~~-~~-~-=-+--~~-~~-~~l1--~~-~~I~~-+-=t-ll+-~--t--t--t-1--1--+--+--+--+-1~-t--t-~--t--t--r-i--+--+--+--+--t--r--r--t-~

/6-UA-7010068-2-0

COPIER ASSEMBLY 22/'1V/24/'1V, 50HZ

9006038-01

S.P.P.H. MACHINE SCREW #8-32 X .44

B 8

9009681-00

SHWH HI-LO SCREW #10 X .50

C-IA 7413332-0-0

BLANK COVER

HOLDDOWN BAR

l-AD-7010846 0-0

COVER BLA.''K FIXED ASSEM8LY

I-MD-7412574-0-0

COPIER COVER (1IT5/'1)

zl[';

I-MD-7412574-1-0

COPIER COVER (1IT53)

E-AD-7010773-1-0

TRA:'SFO~'lER ASSY

(1l5V,6/'1HZ)

E-AD-701077 3-2-0

7RAl'5FORl'1£R ASSY (2 20v/240v, 50/6/'1HZ)

E-AD-7010773-3-0

TRARSFOP:·lER ASSY (1/'1/'1V/127V)

9006563-00

KEPNUT #8-32

~ 1

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41

l-IA-BI;52A-7F

VT52 EIA CARn ~-"D CABLE ASSEMBLY

l-IA-BN52B-7F

2/'1 ~lA. ADAPTER ASSY

l-IA-BN52C-7F

CURREllT LOOP TO 283B PLUG

g ~~ ~ ~ ~~~~N..o
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TITLE

UN IT ASSY
VT55

:J.t! /ff.~ '/~

NEXT HIGHER ASSV.

10-~';-VT 5::, -0-7:
SCALE

SHEET
DEC FORM NO.
ORe 110 A

SIZE CODE[

NUMBER

REV.

C PLI VTS::':-o-0
DIST.

I I I I

1
Figure 6-3 VT55 DECscope Assembly
Drawing (Sheet 3 of 4)

6-7

4. Note the position of the graphing module plugs. Disconnect plugs PI, P2, P3, and P4. The
graphing module is now disconnected from the VT55 circuit.
5. Turn power on and check to see if the problem is gone. The terminal should be able to
display and transmiit alphanumeric data.
6. Replace the graphing module if necessary.
7. Remove jumpers WI and W2 when line frequency is 60 Hz (Figure 3-9).
8. Adjust the graphic and alphanumeric alignment by following the procedure in Paragraph
6.2.5, steps 9-12.
9. If the problem remains, disconnect the keyboard connector, PI, from the ROM, UART,
and timing module (item 20).
10.

Place the good, spare keyboard on a piece of cardboard on top of the ROM, UART, and
timing module.

11. Connect the spare keyboard in place of the internal keyboard. Always be careful to note
proper orientation of module interconnection plugs. Figure 6-3 accurately depicts routing of
interconnections. A twisted or misplaced cable can cause a module failure.
12. If keyboard replacement does not solve the problem, totally remove the graphing module
per Figure 6-3 by removing the four stand-offs that mount this module. Do not misplace any
of the hardware associated with these stand-offs. Detail A in Figure 6-3 shows the assembly
of the four stand-offs.
13. If the VT55 terminal has a copier, disconnect copier signal connections by first removing all
screws that fasten the data path, memory, and decoder module and ROM, UART, and
timing module to the shell. Proceed to step 20 if the machine has no copier.
14. Disconnect Pion the keyboard, and P3 and P4 on the data path module (item 19).
15. Gently lift the module pair to expose the connector from the copier.
16. Remove 11 on the copier by gently pulling at either side of the connector or by gently prying
with a screwdriver. Note the orientation before removing.
17. Replace PIon the keyboard, and P3 and P4 on the data path module.
18. Replace a few screws to properly center the modules in the shell. Be sure to include some
screws in the 11-11 area, the interconnection of item 20 (ROM, U AR T, and timing module)
and item 19 (data path module). A misalignment with the metal CRT bracket below may
cause a short circuit.
19. If the unit functions now, the copier must be replaced. Refer to Paragraph 6.2.4 for the
copier replacement procedure.
20. If the VT55 is still faulty, the microprocessor module pair should be replaced next. With the
copier still detached, remove the ROM, UART, and timing module and the data path module as a pair, as before.

6-9

21. Remove the replacement modules from their packing material and place them on a flat
surface. Insert the 8K character generator module into the data path module. Orientate the
module so that the part designated "rear" on the module points to the rear of the machine.
Insert ROMs 23-128A9, 23-129A9, 23-130A9, and 23-13IA9, into the ROM, UART, and
timing module as shown in Figure 6-2. Check jumpers W5, W6, and W7 for the desired
configuration
22. Carefully attach 11 to 11 by pressing the two boards together. Ensure that all 77 pins are
properly aligned or damage to the modules will occur. Alternating slight pressure on either
side until the connectors are firmly seated is the best way to make this attachment.
23. Retest the VT55 by installing the replacement module pair into the VT55 shell.
24. Replace copier plug 11.
25. Replace PIon the keyboard, and P3 and P4 on the data path module.
26. Replace a few screws to properly center the modules in the shell. Be sure to include some
screws in the 11-11 area between the ROM, UART, and timing module and the data path
module.
27. If the machine functions now, the module pair should be left in the machine. Replace all
screws that fasten these modules and add the graphing control module (per detail A on
Figure 6-3).
28. Remove jumper W7 on the ROM, UART, and timing module if the machine is to be run on
a 60 Hz power line. Leave this jumper installed if this is a 50 Hz machine. W7 adjusts the
screen frame rate to that of the line frequency to eliminate annoying flicker. The location of
W7 is shown in Figure 6-2.
29. The shell may now be reassembled. Detach the original ROM, UART, and timing module
from the data path module and return these to the Field Service Repair Depot. Use the
packing material that came with the new board set. Detachment is facilitated by gently
prying with a screwdriver on alternate sides of the 11-11 interconnection.
30. If the new microprocessor module pair did not correct the defect, the power supply/monitor
module must be replaced next. Remove the new microprocessor module set, detach as
above, and put them back into their packing material.
31. Remove the heat sink and heat shield, items 9 and 77 in Figure 6-3. These are the pieces of
metal that form the back of the machine.
32. Remove the high-voltage diode cap from diode D 1 on the power supply/monitor module
(item 8).
WARNING
High voltage may be present on the Dl terminal;
therefore, handle only insulated material when
discharging.

6-10

33. Connect a #22 insulated wire from the exposed terminal of D 1 to the bare area of metal on
the large power transistor heat spreader that is mounted on the power supply/monitor module. This will discharge any high voltage that might appear on Dl due to a charged CRT.
After 15 seconds remove the wire. Avoid getting white thermal compound on clothing when
working on the power supply/monitor module.
34. Remove the connectors J4, PI, P2, and J3 from the power supply/monitor module as shown
in Figure 6-3.
35. Detach the six connectors (P3, P4, P5, P6, P7, and P8) from the three electrolytic capacitors
(Cl, C2, and C3). Note the wire table indicating these connections in Figure 6-3.
36. The module may now be replaced by removing one screw from the heat spreader bracket
(item 10).
37. Release the board by gently pinching the four plastic studs that hold it to the frame, one at a
time.
38. Reassemble the VT55 with the new power supply/monitor module and retest.
WARNING
Do not attempt to replace high-voltage diode D1.
Use the diode already in the shell. Replacement of
D 1 or the CRT assembly must be performed at a
DIGITAL Field Service Depot due to the hazards of
high voltage and glass tube implosion. The new diode
D 1 should be placed in the module carton with
returned module.

39. If the terminal is now functional, refer to Paragraph 6.2.5 for the video adjustment
procedure.
40. If the unit still does not function, the transformer mounting plate assembly, Cl, C2, C3, and
the CRT assembly may be at fault.
41.

Remove the microprocessor module set and measure the dc voltage across Cl, C2, and C3
with a voltmeter. Then measure the ac voltage. Voltages should be within 20 percent of the
values given in Table 6-3. The reading is taken with the negative meter lead on the negative
capacitor terminal.
Table 6-3 Capacitor Voltages
(All Logic Modules Disconnected)
Capacitor

DC Voltage (V)

Vdc Plus AC Ripple (Vrms)

Cl
C2
C3

26
13
25

28
14
27

6-11

42. If the capacitors test satisfactorily, test the secondary voltages of the transformer by measuring voltage at J1. Remove J1 from 13 on the power supply module to make this measurement (Table 6-4).
43. If the unit is still not functional, only the CRT assembly can be at fault at this point.
Although the CRT assembly must be replaced at a Field Service Depot, voltages being
supplied to the CRT neck socket may be verified with Table 6-5 if desired. Measure at the
power supply end of the CRT cable with respect to ground on the heat spreader bracket.

High voltage applied to the anode of the CRT may be verified by listening for a faint, high-pitched,
high-voltage power supply noise, characteristic of a functional high-voltage circuit.

If these procedures do not repair the VT55, reassemble the entire unit. Repack the unit as described in
Paragraph 6.3 and return it to a DIGITAL Field Service Depot Repair Facility.
Table 6-4 Transformer Secondary Voltages
(Items 36, 37, 38, 72, 74, or 75 on Figure 6-3)
Colors

Jl Terminals

VIO-VIOjBLK
VIO-VIOjBLK
BLU-BLUjBLK
BLU-BLUjBLK

J1-4 to J1-6
J1-5 to J1-6
J1-7 to J1-9
J1-8 to J1-9

Voltage (Vac rms ± 20%)

13
13

27
27

Table 6-5 CRT Base Voltages
Color

Pin No.

Voltage (Vdc ±20%)

Green

0-+ -80 (Intensity Potentiometer)

Grid 1 (Intensity)

Yellow

40 V (with negative logic pulses
for video data generation)

Cathode

Brown
Black

3
4

-12
Ground

Heater - a visible glow
should be apparent in
neck of tube.

Description

Not used

5
Red

+360

Screen Grid

Blue

-80 -+ +450 (Focus Potentiometer)

Focus

6-12

6.2.4

Copier Replacement Procedure

Remove the ac power cord. The copier is installed in an area where dangerous voltages are
present when the power cord is installed.

Remove the copier cover (item 35 in Figure 6-3). It may be easily released by locating one of
the two spring clips which hold it to the shell and pressing with a screwdriver or similar tool
(Figure 6-4).

7717-11

Figure 6-4

Removal of Copier Cover

Remove the copier from the shell. Figure 6-3 shows the five screws that hold the copier to
the shell. Gently lay the copier on a table or shell to facilitate detaching connecting cables.

Detach the Mate-N-Lok connector that connects the copier to the ac line voltage and transformer secondary. It has four wires - red, red/yellow, black, and white.

Gently remove copier signal connector 11. A slight rocking motion may be used to accomplish this. A void breaking or bending the connector pins. Holding the data path module
from the bottom willl keep the mating socket steady while 11 is being retracted. Note positioning of J 1 before removing.

6-13

Remove the green copier ground wire from the ground lug on the inside of the transformer
mounting plate assembly. A nutdriver should be utilized to remove the fastening nut.

Install the new copier by reversing the above steps. Pack the defective copier in the supplied
packing material and return to a DIGITAL Field Service Depot for repair.

6.2.5 Video Adjustment Procedure
With the heat shield and heat sink removed from the power supply/monitor module, the image on the
VT55 may be checked for accurate alignment. Normally, slight adjustment of the monitor will be
required to make the display meet specifications. Refer to Figure 6-5 when making these adjustments.

~ILJlJllll

@ HaRtz. SIZE

VERTICAL

SIZE

LIN

@
FOCUS
INTENSITY

"----1
CP-2521

Figure 6-5

Location of Controls on Power Supply Monitor Module

1. With the terminal in local mode, type the following sequencc~ of characters on the keyboard:
ESC, 1, A, ?, I, ?, L, SPACE, 0, ?, ?, D, +,7, ESC, and 2. A rectangle should appear on the
screen. This part of the procedure is for adjusting the box to 203 X 127 mm (S.O X 5 inches)
in size to within ±6.5 mm (1/4 inches).
2. If the rectangle is not parallel with the four edges of the plastic frame around the glass face
of the tube, the yoke must be adjusted. The yoke is accessed through a hole under the two
top covers on the VT55. Remove the blank cover and copier cover, items 35 and 36 in Figure
6-3.
3.

Loosen the yoke clamp screw and ensure that the yoke is as far forward on the CRT neck as
possible. Turn the yoke to make the box parallel with the four sides of the VT55's front.

Center the pattern on the screen by adjusting the tabs on the yoke.

Adjust the horizontal size control for the 203-mm (S.O-inch) dimension of the box.

6. Adjust the vertical size control for the 127-mm (5-inch) dimension of the box. Flip the
POWER/LOGIC RESET switch once to clear the screen.

6-14

7. Type some characters on the top line, bottom line, and in the center of the VT55 screen.
Adjust the vertical linearity control so that character heights are nearly equal on all three
lines.
8. When this procedure is complete, all box adjustments, including centering of the box, must
be ± 6.5 mm (1 14 inch). All characters must look equal in size, on all 24 lines of text, when
viewed from a normal operator's position in front of the terminal.
9. Type two vertical columns of the letter H in the first two columns of the video screen. Type
one vertical column of the letter H in column 80.
10.

Replace the rectangular box on the screen as in step 1 of this procedure.

II. Ensure that the left side of the rectangular box passes through the left side of the second

column of the letter H. The right side of the rectangular box should pass through the right
side of the column of Hs in column 80 (Figure 6-6).
12. If the alignment of the alphanumeric and graphic data is not as shown in Figure 6-6, turn the
potentiometer on the M7024 graphing module until the display agrees with Figure 6-6.

H~--------------------------~
CP-Z1S7

Figure 6-6 Alignment of Alphanumerics and Graphics

6.3 PACKING OF DECscope
The VT55 shipping package is described in Table 6-6 and packing instructions follow.
Table 6-6 VT55 Shipping Package
Quantity

Part No.

Description

9905698
9905699
9905639
9905729
9009634
9905935
9905937
9007032
9905936
9905938

Regular slotted carton
Foam pieces (1 set equals 6 pieces)
Laminated buildup
Carton sealing tape
Tape (Scotch, Y-8921)
318 square wood
Taped tube
Tie wrap 6-3/4 long
Convoluted polyurethane foam pad
Paper tube

1 (set)
1

14 ft
2 ft
1
1

2
1

6-15

1. Refer to Figure 6-7. Set up the regular slotted carton (9905698). Tape with one strip of
carton sealing tape (9905729) across the middle and one strip across each end.
3/8" SQ. WOOD DOWEL

9905935
PAPER ROLL INSIDE
TAPED TUBE

9905937

cp- 2529

Figure 6-7 VT55 Packing

2. Place the laminated buildup (9905639) on top of the keyboard. Place one strip of tape
(9009634) on each end of the buildup. Pull the tape down and secure it to the base of the
VT55.

3. Remove the terminal cover to expose the copier.
4. Tie the anode holder to the copier chassis using two tie wraps (9007032)

6-16

5. Pick up the anode t.older, place the grey convoluted foam pad (9905936) between the rollers,
replace the anode holder, and tuck the foam snugly into each end.
6. Replace the terminal cover.
7. Place the VT55 terminal into the front and rear foam pieces (9905699). Place the assembly
into the regular slotted carton (9905698).
8. Place the right and left rear vertical foam pieces (9905699) into position on the VT55
terminal.
9. Place the top right and top left foam pieces (9905699) into position on the VT55.
10. Place the power cord into the void beside the VT55 terminal.
11. Place the user's manual on top of the VT55 terminal.
12. Slide a square wooden rod into the slots in the top foam pieces. Tape the rod into the slots
using two strips of tape (9009634) on each foam piece.
13. Close and seal the regular slotted carton using one strip of carton sealing tape (9905729)
across the top and one strip across each end panel.

6-17

Reader's Comments
VT55-E, F, H, J DECgraphic SCOPE
USERS' MANUAL
EK-VT55E-TM-OOI

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