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DEC-15-H2DC-D

September 1971

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Document:	PDP-15 User's Handbook Vol. Peripherals
Order Number:	DEC-15-H2DC-D
Revision:
Pages:	99
Original Filename:	http://bitsavers.org/pdf/dec/pdp15/DEC-15-H2DC-D_UsersHbkVol2.pdf

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Digital Equipment Corporation
Maynard, Massachusetts

PDP-15 Systems

User's Handbook
Vol.2 Peri herals

DEC-lS-H2DC-D

PDP-15 SYSTEMS
USER'S HANDBOOK
VOLUME 2 PERIPHERALS

DIGITAL

EQUIPMENT

CORPORATION • MAYNARD, MASSACHUSETTS

1st Edi tion September 1970
2nd Printing (Rev) November 1970
3rd Printing (Rev) May 1971

The material in this manual is for information purposes and is subject to change without notice.

The following are trademarks of Digital Equipment
Corporation, Maynard, Massachusetts:
DEC
FLIP CHIP
DIGITAL

PDP
FOCAL
COMPUTER LAB

CONTENTS
Page
CHAPTER 1

PC15 HIGH-SPEED PAPER-TAPE READER PUNCH

1.1

Introcl uct ion

1-1

1.2

Paper-Tape Reader

1-1

1 .2. 1

Characteristics and Capabilities

1-1

1.2.2

Operating Modes

1-2

1.2.3

Controls and Indi cators

1-2

1.2.4

Tape Formats

1-3

1.2.5

Instructi ons

1-4

1.2.6

Functional Description

1-4

1.2.6.1

Hardware Readin Operation

1-6

1.2.6.2

Program-Controlled Operati on

1-6

1.3

Paper-Tape Punch

1-7

1.3. 1

Characteristics and Capabilities

1-7

1.3.2

Operating Modes

1-7

1.3.3

Controls and Indicators

1-8

1.3.4

Tape Formats

1-8

1.3.5

Instructi ons

1-8

1.3.6

Functional Description

1-8

1.4

Programming Considerations

1-9

1.4.1

High-Speed Paper-Tape Reader

1-9

1.4.2

Hi g h-Speed Paper-Tape Punc h

1-9

1.5

Programming Examples

1-10

1.5. 1

Paper-Tape Reader/Punch Handlers

1-10

1.5.2

Paper-Tape Reader Programming Example

1-10

1.5.3

Paper-Tape Punch Programming Example

1-11

1.5.4

Programming With API or PI

1-12

1.5.4 •. 1

Program Interrupt Example

1-12

1.5.4.2

API Example

1-13

CHAPTER 2

THE DECDISK SYSTEM

2. 1

Introclucti on

2-1

2.1.1

System Descri pti on

2-1

2.1.2

Storage of Digital Data on Fixed-Head Rotating Disks

2-1

2.1.3

Storage of Data in a Serial Format

2-2

iii

CO NTE NTS (Cont)
Page
2.1.4

Random Accessi ng of Data

2-2

2.1.5

Data Accessing at Selectable Speeds

2-2

2.1.6

Data Protection from Over-Writing

2-2

2.2

DECdisk Operation

2-3

2.2.1

Disk Surface Recording Format

2-3

2.2.2

DECdisk Architecture

2-5

2.2.2.1

The Control Secti on

2-5

2.2.2.2

The Data Transfer Section

2-9

2.2.2.3

Maintenance Section

2-15

2.3

The Operator's Controls

2-17

2.3.1

Transfer Rate Selection

2-19

2.3.2

Disk Address Selection Jacks

2-19

2.3.3

Write Lockout Switches

2-21

2.4

The Operator's Indicators

2-21

2.5

Programming Examples

2-23

2.5.1

Calling Sequence Table

2-23

2.5.2

Disk Flag Tests

2-24

2.5.2.1

Use of IORS

2-24

2.5.2.2

Skip Chain

2-25

2.5.3

Error Flag Tests

2-26

2.5.4

Programming with the ADS Register

2-26

2.5.5

Programming Multiple-Disk Systems

2-30

2.5.6

Using DECdisk in a System

2-30

2.6

Summary of DECdisk Characteristics

2-31

CHAPTER 3

THE DECT APE SYSTEM

3. 1

Introducti on

3-1

3.2

DECtape Format

3-1

3.2.1

Timing Track

3-3

3.2.2

Mark-Track Format

3-3

3.2.3

Data Blocks

3-7

3.3

TU55 and TU56 DECtape Transports

3-8

3.4

TC15 DECtape Control

3-8

3.5

DECtape Instruction Set

3-9

CO NTE NTS (Cont)
Page

3.6

Data Flow

3-12

3.7

DECtape Programming Considerations

3-12

3.7.1

Control Functi ons

3-12

3.7.2

MOVE Function

3-13

3.7.3

SEARCH Function

3-13

3.7.4

READ DATA Functi on

3-14

3.7.5

READ ALL Function

3-14

3.7.6

WRITE DATA Functi on

3-15

3.7.7

WRITE ALL Function

3-15

3.7.8

WRITE TIMING and MARK TRACK Function

3-15

3.7.9

Disable Interrupt

3-16

3.7.10

Error Conditions

3-16

3.7.10.1

Timing Error

3-17

3.7.10.2

Parity Error

3-17

3.7.10.3

Sel ect Error

3-17

3.7.10.4

End of Tape (EaT)

3-17

3.7.10.5

Mark Track Error

3-17

3.8

Programming Examples

3-18

3.8.1

Auto-Search

3-18

3.8.2

Read Data

3-19

3.8.3

Bootstrap Loading Technique

3-20

3.8.4

Writing and Reading in Opposite Directions

3-21

3.9

Programming Notes

3-23

3.9.1

Modification of Individual Data Words

3-23

3.9.2

Data Transfer - Upper Boundary Protecti on

3-23

3.9.3

Speci al Formats on Tape

3-23

3.9.4

Turnaround Commands

3-23

3.10

DECtape Summary

3-24

3.10.1

DECtape Function Summary

3-24

3.10.2

DECtape Error Summary

3-25

3.10.3

DECtape Ti ming Data on Standard Format (Certifi ed) Tape

3-26

CONTENTS (Cont)
Page
CHAPTER 4

TELETYPE CONTROLS

4. 1

Introducti on

4-1

4.2

LT15 Single Teletype Control

4-1

4.2.1

Transmitter

4-1

4.2.2

Receiver

4-2

4.2.3

Instruction Set

4-2

4.3

LT19D Multi-Station Teletype Control

4-2

4.3.1

LT19D Multiplexer

4-2

4.3.2

LTl9E Teletype Control

4-3

4.3.3

LT19F EIA Line Adapter

4-3

4.3.4

LT19H Cable Set

4-3

4.4

The Operation of the LTl9 Multi-Station Teletype Control

4-3

4.4.1

LT19D Multiplexer

4-3

4.4.2

LT19E Teletype Control

4-4

4.4.3

The LT19F EIA Line Adapter

4-4

4.4.4

The LT 19H Cable Set

4-4

4.5

The Instructi on Set

4-8

4.5. 1

Programming Examples

4-11

CHAPTER 5

LINE PRINTERS

5.1

Introducti on

5-1

5.2

Channel and Buffer Setup

5-2

5.3

Data Word Formats

5-2

5.3.1

lOPS ASCII

5-2

5.4

Single Line Operation

5-3

5.5

Multi-Line Operation

5-4

5.6

LP15C Control Characters

5-4

5.6. 1

Horizontal Tab (HT)

5-4

5.6.2

ALT MODE and Carriage Return (CR)

5-4

5.6.3

Vertical Format Unit (VFU) Characters

5-4

5.7

Control Characters for LP15, F, H, J, and K

5-5

5.8

lOT Instructi ons and Flags

5-5

5.8.1

Error Flag

5-5

5.8.2

LP Alarm Flag

5-6

CO NTE NTS (Cont)
Page
5.8.3

Line Overflow Flag

5-6

5.8.4

Illegal Horizontal Tab (ILL HT)

5-7

5.8.5

Busy Flag

5-7

5.8.6

Done Flag

5-7

5.8.7

Interlock Flag

5-7

5.9

Programming Example

5-7

ILLUSTRA nONS
Figure No.

Title

Art No.

Page

1-1

Tape Format and Accumulator Bits (Alphanumeric Mode)

15-0232

1-3

1-2

HRI Tape Format and Accumulator Bits (Binary Mode)

15-0233

1-5

2-1

DECdisk System Configuration

15-0234

2-1

2-2

Disk Surface Recording Format

15-0235

2-4

2-3

DECdisk Control Section

09-0413

2-7

2-4

DECdisk Data Transfer Section

09-0358

2-10

2-5

Simulating the Disk Surface with the Maintenance Logi c

09-0393

2-16

2-6

Simulating the RS09 with the Maintenance
Logic

09-0359

2-18

2-7

AC Bit Usage for lOT DGSS

09-0288

2-19

2-8

AC Bit Usage for lOT DGHS

09-0360

2-19

2-9

Transfer Rate Selection Switch and Disk Address Select Jacks

2-20

2-10

Write Lock Out Switches

2-20

2-11

Indi cator Panel

2-21

2-12

Calculating Fast Access Calling

09-0420

2-28

2-13

Flow Diagram of the Subroutine Tha'~ Uses
the ADS Register

09-0421

2-29

3-1

DECtape Format

3-2

Mark-Track Format

09-0112

3-5

3-3

Bidirectional Reading and Writing

15-0236

3-6

4-1

LT19E Multi-Station Teletype Control Block
Diagram

15-0237

4-5

4-2

LT19E, F, H Teletype Control Interface &
Communications

15-0238

4-7

3-2

vii

ILLUSTRATIONS (Cont)
Title

Art No.

Page

5-1

Data Buffer Header Format

15-0419

5-2

5/7 ASCII Packing Scheme

15-0420

5-3

IMAGE ALPHA Format

15-0421

5-3

Figure No.

TABLES
Table No.

Title

Page

1-1

Indi(!ators Associated with Paper-Tape Reader

1-2

2-1

The Function Register Bit Configuration

2-6

2-2

Status Register Bit Functions

2-11

2-3

The DECdisk Instruction Set

2-13

2-4

Maintenance lOTs

2-16

2-5

The Indicator Panel

2-22

2-6

Adjusted ADS Register for Medium and Low Transfer Rates

2-28

2-7

Disk Data Checks

2-30

3-1

Mark Track Coding

3-4

3-2

TC 15 Control lOT Instructi ons

3-9

3-3

3-10

3-4

Status Register B Bit Assignments

3-11

4-1

LT15 lOT Instructi ons

4-2

lOT Assignments for One LT19

4-8

4-3

lOT Assignment for Two LT19s

4-9

4-4

lOT Assignments for Three LT 19s

4-9

4-5

lOT Assignments for Four LT 19s

4-10

5-1

Line Printer Characteristi cs

5-1

5-2

Control Character Assignments

5-5

5-3

LP 15 lOT Instructi ons

5-6

viii

Chapter 1
PC15 High-Speed
Paper-Tape Reader Punch

1.1 INTRODUCTION
The PC 15 High-Speed Paper Tape Reader/Punch is used to input perforated paper-tape programs into
core memory, or to punch core memory programs or data on paper tape. Information is punched on
8-channel fanfolded paper tape in the form of 6- or 8-bit characters at a maximum rate of 50 characters/second. Information is read at a maximum rate of 300 characters/second. The PC15 consists
of a PC05 Paper Tape Reader/Punch with interface and control logic for using the reader/punch with
a PDP-15.

1.2 PAPER-TAPE READER
1 .2.1 Characteristics and Capabilities
Data can be read from tape and transferred to the PDP-15, using the computer hardware readin logic
or using program-controlled transfers. For hardware readin operation, the hardware readin logic
supplies inputs for selecting the operating mode, starting tape motion, and implementing transfers.
For program-controlled transfers, the computer issues input/output transfer (lOT) instructions that
select the operating mode, advance the tape, and implement the transfer. To maintain a maximum
rate of 300 characters/second, a new select lOT must be issued within 1 .67 ms of the last reader flag.

If not, the reader operates start-stop and reads characters at a 25 character/second rate. The requirements for maximum character rate are described in detail in Programming Considerations,
Parag raph 1 .4. 1 •
The reader interfaces with the automatic priority interrupt (API) facility, the program interrupt facility,
and the input/output skip chain. For API operation, the reader is assigned API level 2; a unique entry
address of 50 is assigned to its service routine.
8
The reader contains a no-tape sensor and flag (character ready for transfer) circuits. If a no-tape
condition is detected, the reader flag is set, and a program interrupt is initiated whenever a reader
select lOT is given. The states of the reader flag, the reader API 2 level, PI request and skip request

1-1

devices are displayed on an indicator panel at the top of Cabinet H963E (Bay 1 R). In additi on, this
panel displays the reader buffer contents and the I/o address (API unique entry address). These items
and the reader controls are described in Controls and Indicators, Paragraph 1.2.3.
Reader mechanical facilities include a right-hand bin for supply for tape being read, a left-hand bin
for receiving the tape, and a feed-through mechanism to control passage of the tape into the receiving
bin. A snap-action retainer on the feed-through mechanism facilitates simple loading of the tape.

1.2.2 Operating Modes
The PC 15 reader operates in either an alphanumeric or binary mode.

For program-controlled transfers,

the operating mode is selected by lOT instructions. For hardware readin operation, control logic in
the reader automatically selects the binary mode.
When alphanumeric mode is selected, one S-bit character (in ASCII code) is read and transferred to
the PDP-15 accumu lator. In the bi nary mode, the reader reads three 6-bit characters (three frames
with channels 7 and S ignored) from tape and assembles them into an lS-bit word for transfer to the
accumulator.

1 .2.3 Controls and Indicators
Two front panel controls are provided for the PC15 Paper-Tape Reader: ON LINE/OFF LINE and
FEED. The ON LINE position places the reader under computer control. The OFF LINE position,
which is used for loading paper tape, raises an out-of-tape flag and places the reader under local
control. The indicators associated with reader operation are located on an indicator panel at the top
of cabinet H963E (Bay lR). Table 1-1 lists the indicators and their functions.

Table 1-1
Indicators Associated with Paper- Tape Reader

Indicator

Function

READER BUFFER 00-17

Indicates the contents of the paper-tape reader buffer.

API 2 RDR

Denotes API level 2 is active as the result of a reader
interrupt.

I/o ADDRESS

Indicates the unique trap address associated with I/o
devi cesi address 50S for paper-tape reader.

RDR FLG

Denotes information has been read from tape and is
avai lab I e for transfer from reader buffer.

1-2

Table 1-1 (Cont)
Indicators Associated with Paper-Tape Reader

Function

Indicator

PI RQ

Denotes one of the I/o devices (including paper-tape
reader) handled by the BA 15 Peripheral Expander has
generated an interrupt request.

SKIP RQ

Denotes one of the I/O devices (including paper-tape
reader) handled by BA 15 has responded to a skip lOT
instruction.

1 .2.4 Tape Formats
The format of the perforated paper tapes for the alphanumeric (ASCII usage) mode is shown in Figure 1-1 •
In addition, tape channels are related to the PDP-15 accumulator stages. The leader and trai ler portions of the tape are used to introduce or conclude a paper-tape program. Only the feed hole is
punched for the leader/trailer portions. Note that each character is read by one lOT instruction.

8
\ 0 \1

CHANNEL

12131415161718191101'11'21'31'41'51'61171 ACCUMULATOR
~

'-y-J

'-v-'

UNUSED

CHANNEL

TAPE CHANNEL

87654 321
LEADER
(FEED HOLE ONLY)

DIRECTION
OF TAPE
MOVEMENT

__--l--- FEED HOLE

1
•

•

••

•

••

•

••

3378 _ READ BY ONE rOT
2778 INSTRUCTION
3038
}

TRAILER
(FEED HOLE ONLY)

L..-_ _ _ _..I

o=HOLE POSITION
uHOLE PUNCHED
15-0232

Figure 1-1

Tape Format and Accumulator Bits (Alphanumeric Mode)

1-3

The paper-tape format for binary mode using hardware readin (HRI) is shown in Figure 1-2 as well as
the relationship of accumulator stages for the lS-bit word.

Note that only the feed hole is perforated

for the leader/trailer portion and that channel S is always punched in the program portion of the tape.
Any character without hole S punched will be ignored. Channel 7 punched in the last character indicates the last lS-bit instruction is to be executed by the computer. This instruction can halt machine operation or can transfer machine control to another part of the program. When using this format,
channel 7 must be punched using the alphanumeric mode.

1 .2.5 Instructions
The PDP-15 lOT instructions used for program-controlled loading of paper-tape data are listed below.
Refer to Volume 1 of this handbook for lOT instruction format.
Mnemonic

Octal Code

RSF

700101

Skip next instruction if reader flag is a 1.

RCF

700102

Clear reader flag. Read reader buffer, inclusively OR contents of reader buffer with AC,
and deposit result in AC.

RRB

700112

Read reader buffer and clear reader flag. Clear
AC and transfer contents of reader buffer to AC.

RSA

700104

Select alphanumeric mode and place one S-bit
character in reader buffer. Clear flag before
character is read from tape. Set reader flag to
1 when transfer to reader buffer is complete.

RSB

700144

Select binary modes. Assemble three 6-bit
characters in reader buffer. Clear reader flag
during assembly and set flag when assembly is
complete.

Operati on Performed

The paper-tape reader responds to an input/output read status (laRS) instruction by supplying the status
of its device flags and no-tape flags to the accumulator. The reader device flag (reader interrupt)
interfaces wi th bit 01 of the accumulator. The reader no-tape flag interfaces with bits OS of the
accumulator.

1.2.6 Functional Description
The PC 15 reader consists of an electromechanical tape feed system, a light source and photo cells for
sensing tape perforations, a buffer register for storing and assembling data, and control logic for computer interface, tape advance, and transfer operations. These circuits can be used with the PDP-15
hardware readin logic, or can be used for program-controlled transfers, as described in the following
paragraphs.
1-4

FIRST CHAR READ
CHANNEL

,...,...,

,............

r-"-.

10 1

........

,............

THIRD CHAR READ
\

S-

I"""-.

,...A-,

1213\4\ 5\6\ 7\ 819\10 \11 \12\13\14\15\16\17\

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

CHANNEL

SECOND CHAR READ
\I 6

'-y-J~

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

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

ACCUMULATOR

'-y-J~

T APE CHANNEL
87654

32
....- ; - - FEED HOLE

LEADER
(FEED HOLE ONLY)
8 CHANNEL PUNCH- ..........
eoooo_ooo
ED FOR EACH CHAeooOo.ooo
RAcTER

DIRECTION
OF TAPE
MOVEMENT

.0000_000

eoooo_ooo
.0000.000

eoooo_ooo
eOOOOeOOO

eoooo_ooo
___ O O O e O O O

CHANNEL 7 PUNCH-._..__ED CAUSES LAST -INSTRUCTION TO
BE EXECUTED
TRAILER
(FEED HOLE ONLY)

-FIRST CHARACTER READ }
-SECOND CHARACTER READ
FIRST INSTRUCTION
- THIRD CHARACTER READ
READ BY ONE lOT OR
INITIATED BY READIN
} NEXT INSTRUCTION

LAST THREE FRAMES MUST BE
} PUNCHED USING ALPHANUMERIC
CODE TO EFFECT THE CHANNEL
7 PUNCH.

•
•
0= HOLE POSITION
• = HOLE PUNCHED

15-0233

Figure 1-2

HRI Tape Format and Accumulator Bits (Binary Mode)

1-5

1.2.6.1

Hardware Readin Operation - The PC15 reader can be used with PDP-15 hardware readin

logic to load programs from paper tape at a rate of 300 characters/second. For this operation, the
desired tape is installed in the high-speed reader, and the program loading address is selected, using
the console ADDRESS switches. The console RESET key is then pressed to initialize the computer and
paper-tape reader. A readin operation is started by pressing the READIN key on the console.
With this key action, a Readin (RI) condition is stored in the reader, and the binary mode is selected.
The reader then advances the tape, reads three characters from tape, assembles them into an lS-bit
word in the reader buffer, and signals the hardware readin logic with a program interrupt. The hard,ware readin logic, in turn, transfers the 18-bit word to the accumulator under I/O processor and computer timing. The word is subsequently loaded into core memory by forcing a DAC instruction. The
first lS-bit word is stored at the address specified by the console ADDRESS switches. Subsequent lSbit words are stored in sequential memory locations.
The readin operation continues until a perforated hole 7 is detected. This condition is inserted in the
last character of the last lS-bit instruction. When this condition is detected, the reader supplies the
hardware readin logic with a skip request. As a result, the hardware readin logic causes the last instruction to be loaded into the Memory Input register for execution. This instruction can halt machine
operati on (HALT) or can transfer program control to another part of the program (JMP). When using
the readin feature with the MP15 Memory Parity option, the last instruction on the paper tape (which
will be executed by the processor) will not be written into the next sequential memory location. That
location, however, will be loaded with data that may contain wrong parity. Therefore, that location
should be re-stored by the program before an attempt is made to read from it. Otherwise, a parity error wi II occur.

1.2.6.2

Program-Controlled Operation - The PC15 reader operates in the binary or alphanumeric

mode depending on the select lOT instructions issued by the computer. On decoding a reader select
alphanumeric (RSA) mode lOT (700 104S) , the reader advances the tape one character, loads this character into the reader buffer, and sets the reader device flag. The reader then signals the computer
that data are avai lable by providing a reader interrupt to the API or PI, or by responding to an RSF lOT
instruction. If the API facility is being used, program control is transferred to the reader service routine where the computer services the request, and an RCF (700102 S) or RRB (700112S) instruction is
issued. If the API facility is not being used, the computer issues an RSF instruction, and the reader
returns a skip request whenever its flag is set. The skip request causes the next instruction (normally
a JMP . -1 in wait loops) to be skipped so that the character can be transferred to the accumulator by
issuing an RCF or RRB instruction. The RCF or RRB instructi on transfers the reader buffer character to
the I/O bus and loads it into the least significant bits (10 through 17 for S-bit alphanumeric character)
of the accumulator. The character is subsequently stored in a core memory location designated by the
program. The read reader buffer (RRB) instruction also clears the reader flag for the next read operation.
For binary mode operation, the computer issues a reader select binary (RSB) mode instruction (octal
700144). On decoding this instruction, the reader clears its device flag, advances the tape three

1-6

characters, reads these characters from tape, and assembles them into an 18-bit word in the reader
buffer. The reader also counts the number of characters with hole 8 punched read from tape and, when
a count of three is reached, generates an interrupt request. The control functions for transfer of the
18-bit word to the accumulator is the same as that described for the alphanumeric mode.

1.3 PAPER-TAPE PUNCH
1 .3.1 Characteristics and Capabilities
The PC 15 paper-tape punch consists of a tape feed system, a mechanical punch assembly, a buffer
register, and control logic for mode selection and activation of the tape feed and punch mechanism.
Tape advance, mode selection, and transfer of information to the punch are controlled by lOT instructions. Tape is perforated at a rate of 50 characters/second. When the punch is selected by an
lOT instruction, data from the PDP-15 accumulator (AC10-AC17) are transferred to the punch buffer.
Then, without further inputs, a character is perforated on tape.
The punch contains a device flag that denotes punch status for transfers. This device flag interfaces
with the PI faci I ity and I/o skip chain. The status of the punch flag is displayed on an indicator
panel at the top of Cabinet H963E (Bay 1 R). An out-of-tape switch is located on the punch mechanism. This switch initiates action that stops punch operations when approximately one inch of unpunched tape remains.
Power for the punch operation is available whenever the PDP-15 power is on. The punch runs when
selected by an lOT instruction or when the FEED switch is pressed.
Punch mechanical features include a magazine for unpunched tape and a container for tape chad.
Both are accessible when the reader-punch drawer is extended from the cabinet.

1 .3.2 Operating Modes
The PC15 Punch operates in the alphanumeric or binary mode as designated by lOT select instructions.
One of these instructions is required for each character punched for mode change. In the alphanumeric mode, an 8-bit character (in ASCII or modified ASCII code) is punched for each accumulator
transfer to the punch. For the binary mode, one 6-bit datel character is perforated for each accumulator transfer. Hole 8 is always punched, and hole 7 is never punched. Three of these characters,
however, form one computer word for readin operations.

1-7

1 .3.3 Controls and Indicators
The pe15 Punch has a front panel FEED control. This control is used to advance the tape from the
punch as required for leader or trailer. The punch also has one indicator (PUN FLG) directly associated with its operation. Thi:; indicator, located on an indicator panel at the top of Cabinet H963E
(Bay 1 R), indicates the status of the device flag and, shows that the punch is avai lable for a punch
operation when lit. The punch also shares the PI RQ and SKIP RQ indicators on this panel with other
I/O devices.

1 .3.4 Tape Formats
Tape formats are shown in Figures 1-1 and 1-2.

1 .3.5 Instructions
The PDP-15 lOT instructions used for punching of paper tape under program control are I isted below.
Refer to Volume 1 of this handbook for lOT instruction format.
Mnemonic

Octal Code

PSF

700201

Skip next instruction if punch flag is a 1 •

PCF

700202

Clear punch flag and punch buffer.

PSA

700204

Select alphanumeric mode and punch one character. Set punch flag when punch is complete.

PSB

700244

Select binary mode and punch one 6-bit character. Set punch flag when punch is complete.

Operation Performed

The punch responds to the 10RS instructi on (Volume 1, Paragraph 3.7. 1) by supplying the status of its
device flag and no-tape flag to the accumulator. The device flag interfaces with bit 02 of the accumulator, and the no-tape flag interfaces with bit 09.

1 .3.6 Functional Description
The PC15 Punch operates in the alphanumeric or binary mode, depending on whether a PSA or PSB
instruction is issued. When one of these instructions is decoded, information is loaded into the punch
buffer from bits 10 through 17 of the accumulator and is punched onto tape. During the interval the
punch operation is in progress, the punch flag is cleared to indicate the punch is busy. When the
punch operation is complete:, the punch flag is set to 1 to indicate it can accept another input
character.

1-8

The operating sequence for punch operations normally begins with a PSF instruction to test the device
flag. If the device flag is 1, a skip request is returned to the computer, and the computer issues a
PCF instruction. This instruction clears the device flag and the punch buffer. The computer then
issues a PSA or PSB instruction. On decoding a PSA instruction, the reader loads the accumulator
input into its buffer, advances the tape, and punches one character. For the alphanumeric mode
channel 8 is punched as a function of bit AC 10. For the alphanumeric mode channel 7 is perforated
as a function of bit AC 11. After the character is punched, the reader sets its device flag, and the
process is repeated. This operation, performed by the PCF and PSA instructions, can be combined
by microprogramming the two instructions to form octal 700206.

The same principles are used for punching a binary character; however, a PSB instruction is used in
place of the PSA instruction. On decoding a PSB, the punch perforates channel 8 and inhibits the
punching of channel 7. The remaining six channels are punched as a function of AC12 through AC17,
and represent one 6-bit character of a computer word.

1.4 PROGRAMMING CONSIDERATIONS
1 .4. 1 High- Speed Paper-Tape Reader
To use the reader at the transfer rate of 300 cps, a select lOT (RSA or RSB) must be issued within
1 .67 ms after each flag. This action is required because a 40 ms reader stop delay is present. When
this delay is activated, it overrides the select lOT input and subsequently stops the tape. Thus, if a
new select lOT is not received within 1.67 ms of the setting of the flag, the reader operates start-stop
and reads characters at 25 cps rate.

No data is lost.

The RSA (octal 700104) and RCF (octal 700102) can be microprogrammed to form an octal 700106 instruction. This instruction reads the character, transfers the character to the accumulator, and advances the tape in one operation. An RSF (octal 700101) and RRB (700112) cannot be microprogrammed.

1.4.2 High-Speed Paper-Tape Punch
Channel 7 can be punched using only the alphanumeric mode. Therefore, when punching the last
character of a tape for hardware readin operation, the last character must be punched in the alphanumeri c mode.
The PCF instruction can be microprogrammed with a PSA or PSB instruction to form octal 700206 or
700246. This instruction clears the punch flag and buffer, sel ects the appl icabl e mode, loads the

1-9

punch buffer, advances the tape, and perforates the character on tape. After completi ng the punching,
the punch flag is set to denote the punch can accept another character. Microprogramming the pcr
and PSF instructions is not allowed.

1.5 PROGRAMMI NG EXAMPLES
1.5. 1

Paper-Tape Reader/Punch Handlers

All PDP-15 Systems are supplied with standard I/O device handler subroutines for the paper-tape
reader/punch hardware. For PDP-1S/10 Systems with 4K core, the COMPACT software includes
paper-tape handler routines such as PTLIST and PTDUP. The Basic I/O Monitor, supplied with
PDP-1S/l0E Systems with 8K core or greater, include standard I/O device handlers for the high-speed
paper-tape reader and punch. These standard device handlers operate in systems with or without API
and are upward compatible with all other monitors on the PDP-1S/20 Software System. Complete instructions on use of standard paper-tape reader and punch handlers and their modification for special
applications are provided in the PDP-1S/l0 Software System Manual, DEC-1S-GR1A-D.

1.S.2

Paper-Tape Reader Programming Example

The following subroutine illustrates the use of programmed lOT instructions to read a group of binary
words from paper tape. Twenty-five 18-bit words are read and stored in a table starting at ADDRES:'i.
NOTE
This example is for instructional purposes only and is not
to be considered a complete, fully tested soft'NOre system
segment.
SUBRTE

READLP

0
LAW
DAC
LAC
PAX
10RS
AND
SZA
JMP*
RSB
RSF
JMP
RRB
DAC
AXR
ISZ
JMP
JMP*

-31
WDCNT
(ADRESS

/2S DECIMAL WORDS

(1000

/IS THE PAPER TAPE READER EMPTY?
/yES IF NO N-ZE RO •
/EXIT •••• ITS EMPTY
/NO. START READING A WORD.

SUBRTE

.-1
O,X
1
WDCNT
READLP
SUBRTE

/WAIT FOR IT.
/GET IT FROM HARDWARE BUFFER.
/POINT TO NEXT LOC AT ADDR.
/HAVE 2S WORDS BEEN READ?
/NO ••• CONTINUE LOOPING.
/yES. EXIT.

1-10

1 .5.3

Paper-Tape Punch Programming Example

The following subroutines illustrate some paper-tape punch programming considerations. Their purpose
is to unpack successive 6-bit ASCII characters from a table, convert them to 7-bit ASCII, and punch
them on paper tape. The starting address of the table is placed in a location named ADDRESS. The
number of words in the table is placed in WORDCNT. After these parameters have been deposited,
the subroutines are entered by a JMS to PNCHOUT •
NOTE
This example is for instructional purposes only and is not
to be considered a complete, fully tested software system
segment.
PNCHOUT

LAC
TCA
DAC
CLX
NXTWORD

NXTCHAR

WORDCNT

/THIS INITIALIZATION
/ROUTINE STORES 2 1 S
/COMPLEMENT WORDCNT
/AND CLEARS XR.

LAW
DAC

-3
COUNT

/SET UP A COUNTER FOR
/3 CHARACTERS.

LAC
RAL

ADDRESS,X

fUSE XR TO GET EACH WORD.
/AC HOLDS 3 6-BIT ASCII
/CHARS. ROTATE INTO LINK.
/ROTATE WORD 6 PLACES
/THRU LINK. THE NEXT
/6-BIT CHAR. IS IN AC12-17.

SAVEAC
(77)

(40)

/SAVE REMAINING CHARS.
/rHIS ROUTINE CONVERTS
/THE 6-BIT ASCII IN AC12-17
/TO 7-BIT ASCII IN
/ACl1-17.

JMS
PPCHAR
SAVEAC
LAC
COUNT
ISZ
NXTCHAR
JMP
1
AXR
ISZ
WORDCNT
JMP* PPASCII
JMP* PNCHOUT

/READY TO PUNCH CHAR.
/RESTORE SHIFTED AC.
/LAST CHARACTER?
' /NO. DO NEXT CHARACTER.
/POINT TO NEXT WORD.
/LAST WORD?
INO. DO NEXT WORD.
/YES . RE TU RN TO PROG RAM.

RTL
RTL
RTL
DAC
AND
TAD
AND
TAD

PPCHAR

WORDCNT

(40)

(77)

DAC
IORS
AND
SZA
JMP

STORE
(400)

EOT

/SAVE CHAR. FOR INO TAPE I
/TEST.
/LOAD PUNCH STATUS INTO AC.
/TEST NO PUNCH TAPE BIT.
/SKIP IF TAPE OK.
/GO TO END OF TAPE RTE.

1-11

LAC
PSA
PSF
JMP
PCF
JMP*

1 .5.4

STORE

/LOAD AC WITH CHARACTERS
/SELECT ALPHA MODE & PUNCH
/WAIT FOR FLAG.

.-1
PPCHAR

/RETURN TO SUBPROGRAM.

Programming With API or PI

The standard device handlers for the high-speed paper-tape reader and punch include complete inter· ..
rupt subroutines for both API and PI service. Details on how the Program Interrupt Control (PIC) skip
chain and the Automatic Pri ority Interrupt (API) channels are set up and provided in Part III of the
PDP-15/10 Software System Manual. The following example of a hypothetical interrupt service subroutine is provided for general understanding of interrupt servicing.
NOTE
This example is not a complete, fully-tested interrupt
service handler.

1 .5.4. 1 Program Interrupt Example
RSB

.LOC 0
0
JMP

SKPCHN

/ISSUE READER SELECT
/BINARY lOT WITH PI ENABLE.
/REST OF USER PROGRAM.

SKPCHN

SPFAL
SKP
JMP* INT6
RSF
SKP
JMP* INT2
PSF
SKP
JMP* INT3

/SAVE PC, LINK, EXTEND MODE
/& MEM. PROT. BITS AT LOC O.
/GO TO SKIP CHAIN.
/POWER FAIL FLAG TEST.
/GO TO NEXT TEST.
/GO TO POWER FAIL SUBRTE.
/PAPER-TAPE READER DONE?
/GO TO NEXT TEST.
/GO TO PTR INTERRUPT.
/PAPER-TAPE PUNCH DONE?
/GO TO NEXT TEST.
/GO TO PTP INTERRUPT.
/OTHER TESTS

/INT6, INT2, AND INT3 ARE PART OF A TABLE
/OF INTERRUPT SERVICE ROUTINE STARTING ADDRESSES.
/ AN EXAMPLE OF INT2 FOLLOWS:

1-12

INT2

PTRPIC

/15-BIT ADDRESS OF PAPER
/TAPE READER SERVICE
/ROUTINE.
/OTHER I/O SERVICE ROUTINE
/POINTERS

PTRPIC

DAC
LAC*

PTRAC
(0

/SAVE AC.
/SAVE PC, LINK, BANK MODE
/AND USER MODE IN PTROUT.
/REST OF INTERRUPT HANDLED.

1.5.4.2

API Example
RSB

/SELECT READER IN
/BINARY MODE
/REST OF INTERRUPT
/HANDLED.

PTRINT

.LOC
JMS

50
PTRINT

0
DAC
LAC

PTRAC
PTRINT

/API ENTRY. SAVE AC.
/SAVE PC, LINK, BANK
/MODE & USER MODE BITS.

DAC

PTROUT

/PAPER TAPE READER
/API ENTRY LOCATION

1-13

Chapter 2
The DECdisk System

2.1 INTRODUCTION
The DECdisk system is a computer peripheral that stores digital data on fixed-head rotating disks in
serial format.

The data can be randomly accessed at selectable speeds and, when necessary, protected

from overwriting.

2. 1 . 1 System Description
The DECdisk system comprises an RF15 DECdisk Controller and from one to eight RS09 Disk Drives.
The controller connects to the computer I/O Bus and communicates with the central processor for control and status information.

For data information, the controller communicates with memory through

the data channel. Each disk drive connects to the controll er through a parall el disk bus. Both control
and data information pass through the parallel disk bus.

Figure 2-1 illustrates the DECdisk System.

2.1.2 Storage of Digital Data on Fixed-Head Rotating Disks
Each RS09 Disk Drive consists of a rotating disk, a hysteresis synchronous motor, a matrix of 128 fixed
read/write heads, and the electronics required to drive the heads.

PDP- 15

I/O BUS

RF15

CONTROLLER
DISK BUS

I !sor -~--~ -R~09 I
0 1 7
DISK DRIVES
15-0234

Figure 2-1

DECdisk System Configuration

2-1

The 128 magnetic read/write heads ride on the surface of the rotating disk, which is nickel-cobalt
plated. Each read/wri te head covers a separate track on the nickel-cobal t surface; thus, disk action
is simi lar to the operation of many circular tapes running simultaneously in continuous loops.
Each track on the disk can store 2048 eighteen-bit data words. As a track fi lis, the system automatically moves to the next track. The disk rotates at 1800 rpm (60 Hz power) and can, therefore,
transfer a word every 16 f..IS. The storage capacity of each disk is 262,144 words (2048 words x 128
heads). Total system capacity is 2,097,152 words (8 disk drives x 262,144 words).

2.1.3 Storage of Data in a Serial Format
The DECdisk System stores the data on each disk in a serial format. The serial format causes the bits
of each word to be recorded one at a time along a single track, rather than all at once across eighteen
tracks. Therefore, only 1 of a possible 128 data heads is actively reading or writing data at a single
time.

2.1.4 Random Accessing of Data
The DECdisk is a random-access storage system. Each disk is logically segmented into 2048 sl ices or
words, and each slice is preassigned a number or address from 1 to 3777 . The controller, in response
8
to the computer, can select at random any track of a disk and any address along that track to read or
write a word.

2.1.5 Data Accessing at Selectable Speeds
There are three speeds (switch-selected by the operator) at which data can be transferred between the
disk surface and the computer. The highest speed transfers a data word with each successive address,
covering a track in one revolution. The medium speed transfers every second word of a track in the
first revolution, and then transfers the alternate words on the same track during the second revolution.
The slowest speed takes four revolutions to cover a complete track. Once the operator has selected
the desired speed, the control! er hardware controls proper interleaving of the words. However, the
data should be read back at the same speed at which it was written to avoid scrambl ing the data.

2.1.6 Data Protection from Over-Writing
Sixteen switches are avai lable on each RS09 Drive to protect disk-stored data.
hibit the computer from overwriting on eight separate tracks.

2-2

Each switch can in-

2.2 DECDISK OPERATION
Information flow within the DECdisk System is determined by the recording format on the disk surface
and the internal architecture of the controller. The following paragraphs describe the operation of
the disk recording format and the system architecture.

2.2.1

Disk Surface Recording Format

As previously described, 128 read/write heads covering 128 concentric tracks ride on each disk surface.

The circumference of each disk is logically divided into 2048 data segments or addresses, and

in each segment of any track a complete 18-bit computer word can be stored. A 2049th segment
call ed a gap is provided to give the heads time to switch tracks. This segment has no address and
stores no data or timing tracks. It is used as a marker to notify the controller each time a revolution
has been compl eted .
Each data segment must store, in addition to its data word, six control bits; and each disk, in addition
to its data tracks, must contain six control tracks. The control bits are recorded with the data bits;
the control tracks are prerecorded on the disk surface at the factory.

Figure 2-2 illustrates the loca-

tion of these bits and control tracks.
Data are recorded seri ally on each track in 24 bit words; '18 data bits, one parity bit, four guard bits,
and one data control bit.

Each 24-bit word unit is identified by an address that is prerecorded on a

special track before the disk is connected to the computer in the plant. This address is recorded
serially on the B track (see Figure 2-2) exactly one word before the word with which it is associated.
The controller can then assp-mble and identify the address before the heads reach the word itself.
Each address is 13 bits long; 11 bits supply addressing data, 1 bit is a control bit, and 1 bit is a
parity bit.
There are five additional prerecorded tracks on the disk surface. The A track is a prerecorded track
with pulses 660 ns apart that are used to strobe data into or out of the data tracks. The C track is a
track used to delimit each word unit. The controller relies on the C track to signal when a word has
been assemb I ed or wri tten. The control I er can then notify the computer to accept the word read or to
supply another word to be written.

Each of the three prerecorded tracks described - the A, B, and C

tracks - are copied on three spare tracks that are used if one of the original tracks is accidentally
erased in the field. If the spare tracks are damaged, all the timing tracks can be rewritten in the
field with a special timing track writer.

2-3

660n5

IA I I I I I I I I I I I I I I I I I I I I I
PREFORMATTED
TRACKS

SECTION OF'S" OISK
I'V

S_ "RESS~ \ I \ I I I

.J:=...

EillJ
T T
P N

DATA
TRACK

I-

DATA FOR 1250

Note; ,
A- Timin9 Track
B- Addre.. Track
C - Delimitt. Track
0" Sample Data Track
CT- Control Bit
G= Guard Bit
p. Parity Bit
DCTL - Data Cootrol Bit

I
~ I

8 Se],:
.c So
(6 • 1'1i J:' 4
"'-lieo4· o4C.t •
~I'(I1'e l).....
S

/f~l)S)

Note:2
The heads are built in groups of 8 (called shoesl and mounted
around the disk surfaci.

1~8l)
(1~8 /i.'41'o4 1'1i
~o4l).....~ '4Clrs
'lit1'e It

~o4l)S)
Hi-023l1

Figure 2-2

Disk Surface Recording Format

2.2.2 DECdisk Architecture
In this manual, the DECdisk System architecture is presented in three parts; the Control section, the
Data Transfer section, and the Maintenance section (shown in Figures 2-3, 2-4, and 2-5 and 2-6,
respectively). Through the Control section, the Software Operating System initializes the controller
by sel ecting the disk drive (RS09) to be used, the track address within that drive (Data Track Matrix)
to be used, and the first address within the track to be used.

One of three functions is then selected:

READ the disk; WRITE on the disk; or WRITE CHECK what has already been read or written. The Data
Transfer section assembles the word off the selected track for a READ operation, or writes the word bit
by bit onto the track during a WRITE operation. This section also notifies the computer when it has
assembled a word or needs another word to write, and the data is transferred through the three-cycle
data channel. When the last word has been transferred, the computer issues an overflow pulse to the
controller. An interrupt then occurs, and transfers are stopped. The Maintenance section simulates
either the disk surface head signals or RS09 output signals and is used exclusively for testing the
DECdisk System.

2.2.2. 1 The Control Section - The block diagram of the Control section in Figure 2-3 shows 11
relatively independent sections. Some of these sections contain registers, and the bits of these registers are numbered according to the position they occupy when they are read from or into the accumulator of the Central Processor.
Three of these registers - the Disk Number, the Track Address, and the Word Address - are set by the
software system to select the disk (one of a possible eight), the track within that disk (the read/write
head matrix), and the starting address within the track.

Each time a word is transferred, the word

address is automatically incremented by one to prepare for the next word. When the Word Address
Register overflows, the track address is automatically incremented; and when all tracks have been
exhausted, the Disk Number Register is incremented. These registers continually step from word to
word, track to track, and disk to disk unti I the system has been covered.
NOTE
Incrementing occurs during a val id operation only.

After the system has been covered, the computer is notified that it has run out of disks. The dead
space (gap) shown in Figure 2-2 is used to give the controller time to switch tracks when it needs to
do so.
The Word Address Register is constantly being compared to the contents of the Segment Register, which
in turn is sampling the "B" or address track. When the "C" or delimiter track indicates that a valid

2-5

address in the Segment Register, the word address is compared with the assembled address; and if the
two match, an ADDRESS OK signal is passed to the data transfer log ic. This signal informs the data
transfer logic that the data it wants to read is presently passing over the read head of its selected
channel, or that the space in which the data transfer logic wants to write is about to come under the
read/write heads.
The interlace logic is used by the operator to reduce the transfer rate of the disk to either a medium
or a low speed. The medium speed cuts the rate in half by adjusting the final address of the Disk
Segment Register so that only every second address is used in the first revolution of the disk, and the
alternate addresses are picked up from the same track on the subsequent revolution. The low speed
cuts the transfer rate by four.

Each address is then adjusted to require four revolutions of the disk

before a complete track is fi lied. Bits X4 and X2 indicate low and medium speed, respectively, and
are set if these speeds are selected by the operator. The flag BZ sets whenever a valid operation is
under way, and WB sets when a data 11111 is to be written. All of these bits can be read into the accumulator under program control.
The ADS Register receives each val id current segment address from the Segment Register. The current
segment address is then available to the accumulator in the ADS Register under program control.

Note

that the ADS Register receives the current address, and not the adjusted address for low or medium
speed transfers.
There are three bits in the Function Register, which is double buffered. Bits 15 and 16 specify the
function that is to be performed by the controller. The function is loaded into the first buffer, and
an execute lOT (DSCN) is issued to load it into the second buffer far execution. At the end of an
operation, or if an error occurs, the second buffer is cleared and execution stops. The operation can
then be continued by issuing a DSCN lOT execute. Table 2-1 shows the bit configuration needed to
select each function.

Bit 17, also contained in the Function Register, enables the program interrupt

and API logic of the control.
Table 2-1
The Function Register Bit Configuration
Function

Bit 15

Bit 16

No effect

Read

Write

Write Check

2-6

NOTES:
i. The READ/WRITE data heads are mounted on shoes in groups of a and each shoe
is mounted on a card. The cards are mounted around the underside of the disk so
that each head covers a different track. The timing card has I shoe.
2. The cards are cabled from the RS09 READ/WRITE logic and selection matrix ,which
in turn cable to the controller.
3. Each RS09 has both input and output cable slots. The signals are cabled in
parallel from drive to drive to a maximum of a.
4. The track address register selects the head according to the folowing bit
config uration:

/A -D~"08-M

RS09

HEADCABLES--+~-----------,'/~<\;>~' DATA

CARDS

)

~_______\\-T_t:>~8 '),E:~E6

HEAD

TIMING CARD
1 SHOE

HEAO CABLES

-..,-'

. ~AT~IX ~A:T.~IX
#0

HEAD CABLE
CONNECTORS

OISK

r-\;--.

A,B,C SIGNALS

cp
OOUT

CONTROL SECTION

SELECT

DATA SIGNALS

DATA TRACK MATRICES
SELECT AND
READ/WRITE LOGIC

RFI5

TIM I NG TRACKS
READ/WRITE
LOGIC

(~')
-r---

oR-

DATA SIGNALS TO
DATA SECTION Figure 2-4

I
ADS REGISTER

1
SELECTION
PANEL
B (ADDRESS J
TRACK SIGNALS--

IORS - - -

01 SK
FLAG

OVERFLOW ___

STATUS REGISTER

DISK NUMBER
REGISTER

I 0 11 12 I 31 4 1 5 1 6 1 7 lal 91 10 1

1151 16 1 17 1

1151161171

707001
Ol-------..t

rop

PULSES
AND
DEVICE
AND
SUBDEVICE
CODES

707021

DEVICE
AND
SUBDEVICE
DECODING
AND
rOT TRAP
LOGIC

I/O BUS
CABLE
PDP-15

SEGMENT ADDRESS
REGISTER

TRACK ADDRESS
REGISTER

INCREMENT

I--- ~~iER EACH

WORD

ADDRESS O. K.

SKIP IF DISK FLAG
CLEAR CONTROL

707022 -

OR TRACK AND WORD ADDRESS INTO AC

707062 -

OR DISK NUMBER INTO AC

I
WORD ADDRESS
REGISTER

INTERLACE BUSY
AND WRITE

9 110 111 1'21'31'41'51 16 1'7

1..-_ _....1

LJ
o

I
FUNCTION
REGISTER

/7 la I

BI NARY-OCTAL
DECODER

A e.C TRACK SIGNALS

TIMING
GENf:tTOR
CONTROL

~ TIMING

CONTROL
SIGNAL TO

..J..... ALL REGISTERS

707024 - - LOAD AC INTO TRACK AND WORD ADDRESS
707064 -

LOAD AC INTO DISK NUMBER

707041 -

CLEAR FUNCTION REGISTER

707042 - - XOR AC TO FUNCTION REGISTER
707044 __ EXECUTE FUNCTION REGISTER
707202 - - OR ADS REGISTER INTO AC
707242 -

I/O BUS

CLEAR STATUS REGISTER

1-_ _ _--'_7_0_7_2_6_2-1- OR STATUS REGISTER INTO AC

00-17
o...,-----e>t

I/O BUS
DRIVERS AND
RECEIVERS

09-0413

Figure 2-3

DECdisk Control Section

2-7

The tim ing generator and control logi c receive the A and C track signals and generate all of the
system timing and control pulses necessary to carry out the various macro operations (such as shifting
the Segment Register and incrementing the Word Track and Disk Address Registers).
The lO-bit Status Register reflects the state of the system after it has performed its specified operation.
Any timing or parity errors that have occurred during the operation are indicated here. Table 2-2 summarizes the function of each bit.

2.2.2.2 The Data Transfer Section - The data transfer section, shown in Figure 2-4, has 4 subunits;
two l8-bit registers and two controls. During a READ operation, a word is assembled into the Shift
Register. If the word has been assembled from the selected address (ADDRESS OK), and the C track
indicates that a valid word has been assembled; the contenl's of the Shift Register are then jammed into
the Buffer Register. The computer is notified that a word is ready for transfer, and a multi -cycle
data break occurs. At the same time, the Shift Register is assembling the next word. The word count

(We) and current address (CA) for the DECdisk are in location 36 and 37 , respectively.
8
8
During the WRITE operation, the computer transfers the word to be written into the Buffer Register
where it waits for the ADDRESS OK signal. When this signal arrives, the word is immediately transferred to the Shift Register and is serially shifted from there onto the selected track.
During a WRITE CHECK operation, which is designed to allow the programmer to compare data in
memory with corresponding data on the disk, the memory word is fed into the Buffer Register and into
the Shift Register, where it is compared bit by bit with the corresponding word being read directly
from the disk. If a discrepancy or a parity error exists, the DISK flag is posted.
The instruction set, listed in Table 2-3, allows the computer to clear, load, or read from each of
seven registers in the control section. The following points should be noted:
a.

The DISK flag is posted under two conditions;

(1) at the end of the operati on, and
(2)

if one of the six error conditions occur.

The DISK flag causes either a PI or an API interrupt, if these interrupts are enabled
in both the control! er and the computer.
b.

Whenever the DISK flag is posted, the second buffer of the Function Register is
cleared, and the operation stops. The first buffer does not clear; and the operation
can either be continued by issuing the execute lOT, or altered by changing its code
and then issuing the execute instruction.

2-9

I/O BUS

00-17

CONTROL
LINES

........

MULTI CYCLE
DATA CHANNEL
PROGRAM INTERRUPT
AND
AUTOMATIC
PRIORITY INTERRUPT
CONTROL

API
or
PI
FLAG

OVERFLOW

DCH
FLAG

'"
---o
I

BUFFER REGISTER

I 0 l' I 2 I 3 I 4 I 5 I 6 I 7 I I 1'0 1'1 1'21'31'41'51'61'71
8

t
i
READ

f
~
SHIFT REGISTER

10 l' I 2 I 3 I 41 5 161 7 I 8 19 110 1'1 1'21'31'41'51'61'71

WRITE
I---

T O/FROM
-DATA
SI GNALS
09 - 0358

Figure 2-4

DECdisk Data Transfer Section

The ADS Register reflects the current position of the disk and not the adjusted address.
A program can read its contents and calculate the nearest possibl e address to which it
could transfer its first word (taking into account the speed setting), set the address into
the Address Register and, thereby, reduce the initial latency time. (The ADS Register
can be one address late.)

The disks are not synchronized. When the control transfers from disk to disk, the con'"
trol itself has no way of knowing the next disk location in its revolution. The ADS
Register locates the next disk.

During an operation, the Disk, Track, and Word Address registers automatically increment as the system rotates from word to word, track to track, and disk to disk. At all
other times, these registers remain constant.

Table 2-2
Status Register Bit Functions

Bit

Flag Name

Function

ERR

This ERRor flag is the logical OR of the error conditions of bits
1 to 7. When this bit is set, it causes an interrupt and conditions the skip lOT. It also inhibits the current operation until
a continue lOT is issued.

HDW

The disk HarDWare Error is set if the control detects missing bits
from the A, B, or C track. A set HDW causes the control to
freeze for further evaluation. (During a "freeze ll condition,
writing is stopped and the A timing pulses are inhibited.)
A freeze is disabled with an I/o RESET, a CAF, or the DECdisk clear lOT.

APE*

The Address Parity Error flag is set if a parity error occurs when
the address is being assembl ed, provided that the control has
been programmed to READ/WRITE or WRITE CHECK. This flag
does not set if the disk is idling. APE also freezes the control.

MXF

A Missed X (Trans) Fer flag is set if the disk requested a data
transfer from the computer and did not get it for 2-3 revolutions.
A 130 ms timer triggers to post the MXF flag. Either a data
channel failure or a data channel overload initiates this flag.
When analyzing an MXF error, the following points should be
considered:
a.

The computer increments its current address in the cycle
before it transfers its data.

The controller increments its disk or track address when
it requests a transfer during a read operation, but only
after a transfer is acknowledged during a WRITE or
WRITE/CHECK operation.

* Note that the hardware is designed to allow only the first of these three errors to set
during an operati on.

2-11

Table 2-2 (Cont)
Status Register Bit Functions

Bit

Flag Name

Function

WCE

When the Write Check Error flag is set, the controller has
discovered during a WRITE CHECK that the word from memory
differs from its corresponding word on the disk. The error flag
is raised and all further checking is stopped. The word being
checked is in disk location WA-1 (Word Address minus 1),
and its corresponding word is in memory address CA-1 (Current
Address minus 1).

DPE*

The Data Parity Error status bit is set whenever the data parity
bit does not agree with the computed parity of the data word
just read. The control transfers the data word containing the
parity error and raises the error flag. No further transfers
occur unti I the program intervenes. The WA-1 contains the
disk address of the word in error. The CA contains the memory
address of the word in error.

WLO*

The Write LockOut error bit is set when an attempt is made to
write into a protected region on the disk. READ or WRITE
CHECKING a protected area is permitted.

NED

If a disk which does not exist is called for under program control or sequenced into during data transfers, the Non Existence
Data flag is raised to signal the error.

DCH

The Data CHannel Timing Errors status bit is set whenever the
processor has not completed a DCH transfer before the disk
control is ready to transfer data. No error flag is raised. This
status bit is intended as a warning that the DCH channel is
overburdened.

PGE

The ProGramming Error status bit is set whenever the program
issues an illogical command to the disk. Furthermore, if the
command directly conflicts with the operation of the control,
the command is ignored. No error flag is raised. This status
bit is provided as a warning to the programmer.

XFC

When the job requested via the program (either READ, WRITE,
or WRCHK) is finished, the (X) TransFer Complete flag indicated by this bit interrupts the processor and conditions the
SKIP lOT.

* Note that the hardware is designed to allow only the first of these three errors to set
during an operation.

2-12

Table 2-3
The DECdisk Instruction Set

Code

Mnemonic

Descripti on

707001

DSSF

Skip if Disk Flag. The Disk flag is raised for either an
error condition (ERR) or when transfer is complete (XFC).
This flag is indicated on bit 13 of the Input/Output Read
Status (IORS) facility. If the Program Interrupt (PIE)
and/or Automatic Priority (API) is enabled, the DSSF
flag causes the program to be interrupted.

707021

DSCC

Clear the Disk Control and disable the IIfreeze ll status
of the control. This lOT is the only command honored
by the control when a IIfreeze ll is caused by either a
timing track hardware or an Address Parity Error and
forces the control to abort the operation in progress. It
effectively Power Clears the DISK CONTROL.

707022

DRAL

OR the contents of the Address Pointer 0 (APO) into the
AC. Bits 0 through 6 contain the track address and bits 7
through 17 contain the word address of the next word to
be transferred.

707062

DRAH

OR the contents of the Disk Number (APl) into the AC.
Bits 15, 16, and '17 contain the Disk Number. Bit 14 is
read back if a dal'a transfer has exceeded the capacity
of the Disk Control (causes a NED error status).

707024

DLAL

Load the contents of the AC into the APO.

707064

DLAH

Load the contents of the AC (l5, 16, 17) into the Disk
Number (APl).

707041

DSCF*

Clear the Function Register, Interrupt Mode.

707042

DSFX*

XOR the contents of AC bits 15-17 into the Function
Register (FR). The use of each bit is the same as described for bits 15-17 of the Status Register.

707044

DSCN*

Execute the condition held in the FR. Since the AP
contains the next available word (because it is incremental), this lOT can be used to continue after having
changed the Word Count (WC) and Current Address (CA)
held in core memory, or it can be microcoded with the
Clear (DSCF) and XOR (DSFX) instructions to execute
a new function at different address.

707202

DLOK

OR the contents of the 11-bit Disk Segment Address (ADS)
into the AC. The ADS Register contains the real-time
segment address, which is useful for minimizing access
times. The address read always indicates the physical
position of the disk (that is, one address of 2048 for one
revolution (360 0 ) of the disk, independent of the transfer
rate being used).

*These instructions may be microcoded in any combination.

2-13

Table 2-3 (Cont)
The DECdisk Instruction Set

Code

Mnemonic

Description

707202
(Cont)

ADDRESS OF DISK
SEGMENT (ADS)

When reading the ADS Register, the most significant
four bits contain the status condition explained below.
AC Bit
---

Name

Busy. The disk has been commanded
to transfer data and it is not finished.
When reading the ADS Register, this
is an indication that if the Address
Pointer is used by the programmer to
determine the Track Address (TA), the
Track Address may not be valid if the
ADS Register contains 3777 (since the
TA may be changing at this time).

The control is set to transfer every
fourth word. The effective transfer
rate is, therefore,64f.ls per word.

The control is set to transfer every
other word. The effective transfer
rate is I therefore I 32 fJS per word.

Function

If neither X4 nor X2 is set I the control is operating at its
highest rate or 16 fJS per word.
AC Bit
---

Name

Function

Write Bit. This bit is used primarily
for maintenance purposes. It is the
intermediate storage location for the
data being transferred to the disk
during WRITE.

707242

DSCD

Clear the Status Register and Disk Flag.

707262

DSRS

OR the contents of the Disk Status Register with the Accumulator (AC). Status at the point of interrupt is as
follows:
AC

o
1

2-14

Error (ERR)
Disk Hardware Error (HDW)
Address Parity Error (APE)

Table 2-3 (Cont)
The DECdisk Instruction Set

Code

707262
(Cont)

Description

Mnemonic

3
4
5
6
7

8
9
10

Missed Transfer (MXF)
Write Check Error (WeE)
Data Parity Error (DPE)
Write Lockout (WLO)
Non Existent Disk (NED)
DCH Timing Error (DCH)
Program Error (PG E)
Transfer Complete (XFC)

AC 15,16, and 17 Function Register states are as fo"ows:
(If Bit 17 is a 1, the API and PI logic in the controller is
enabled .)
Bit 15 (FO)

Bit 16 (Fl)

0
0
1
1

0
1
0
1

No Effect
READ
WRITE
WRCHK

2.2.2.3 Maintenance Section - The Maintenance section provides a means to test each unit of the
DECdisk System without running the other units. Signals that usually come from the read/write heads
of the disk surface can be simulated by the controller under lOT control with the logic shown in
Figure 2-5.

Similarly, signals from the RS09 output cables can be simulated by the controller with

the logic shown in Figure 2-6. In this way, the controller can be tested without the disk drive, and
the RS09 electronics can be tested without the disk surface.
The Buffer Register, which is normally available to the data channel alone, can be accessed from the
Central processor under the control of maintenance lOTs.
A design feature of the control is that signals transmitted over cables between the controller and the
RS09 disks perform active functions while they are themselves active. Therefore, if a wire in the
cable is broken, a function is disabled rather than uncontro"ably activated.
Table 2-4 lists the maintenance lOTs, and Figures 2-5 and 2-6 show simplified versions of some of the
maintenance logic for the simulator section.

2-15

(

TO RS09 HEAD CABLE
) CONNECTOR S LOTS

SPECIAL HEAD SIMU LATOR CABLE
G789-G790B

(

ATT

BTT CTT DAT A

~
DEVICE
AND
SUBDEVICE
DECODING

lOP PULSES
AND
SELECT CODE S

707204

707224
I/O BUS
00-17

D30 OF CONTROLLER

rlj13j9jsll

DGHS

r----+- DGSS (Figure

)

I/O BUS
RECEIVERS

NOTES:
1. TOG is complemented when DGHS is released.
2. Bits 13,9,and 5 are set from corresponding AC bits.
3. The letters ABeD indicate which track is simulated.
D is for all data trocks.

Figure 2-5

Simulating the Disk Surface with the Maintenance Logic

Table 2-4
Maintenance lOTs

Code

Mnemonic

Descripti on

707204

DGHS

Generate Simulated Head signals. This maintenance
lOT causes the control to generate analog signals that
simulate the disk head signals, as received di rectly
from the head. The AC is used to determine the sequence of pulses to be generated, and the bit rate is
controlled by the diagnostic program. Each lOT, in
effect, is treated as though it were one cell space on
the disk. The function of the AC bits is shown in
Figure 2-5.

2-16

Table 2-4 (Cont)
Maintenance lOTs

Code

Mnemonic

707204

DGHS

Descri pti on

The bits are arranged as shown to provide for data packing,
since only the bits that appear in the AC in bit cell 1
position are used when the lOT is generated. An RAR can
then be used to position the data for the next Simulated
Head Signal.
When either of the maintenance lOTs (707204 or 707224)
is used, a Maintenance Control flip-flop is set that inhibits the effect of control delay timeouts, which are a
result of the lower data rates encountered under program
simulati on. If subdevice Bit 0 (MB 12) is used when issuing
the above lOTs, the Maintenance Control flip-flop is
cleared.

707224

DGSS

Generate Simulated Disk signals. This lOT causes the
control to generate Simulated Disk Interface signals within the control. No disk is necessary. The AC is used to
determine the sequence of pulses to be generated and the
bit rate is controlled by the diagnostic program. Each
lOT, in effect, is treated as though it were one cell space
on the disk. The function of the AC bits is shown in
Figure 2-8.
The bits are arranged as shown to provide for data packing,
since only the bits which appear in the AC in bit cell 1
position are used when the lOT is generated. An RAR can
be used to position the data for the next Simulated Head
Signal.

707002

DRBR

OR the contents of the Buffer Register with the AC. This
is a function normally performed by the data channel.

707004

DLBR

Load the contents of the AC into the Buffer Register. This
is a function normally performed by the data channel.

2.3 THE OPERA TORI S CONTROLS
There are three groups of operator controls on the disk system. These include a three-position switch
to sel ect the transfer rate, a jumper panel to assign the address of each disk, and a seri es of write
lockout switches to protect regions from being written onf'o each disk. The first two controls are part
of the controller itself, and the write lockout switches are avai lable on each disk drive.

2-17

OR OF SIMULATED SIGNALS
AND RS09 SIGNALS

SIGNALS TO
REGISTERS

------~~---------~y --------~--------"\

(
lOT DGSS

SYNC
SCOPE

LO BUS

D21 N2

3 --+----1
DATA SIGNALS
POSITIVE AND NEGATIVE
DTP

5--+----1

7 --+----1

CTN=D--C TRACK
SIGNALS

9 --+----1

11--+----1

B TRACK
SIGNALS
BTP

13--+----i

15--+----i
ATNM

A TRACK
SIGNALS

LO BUS

17 --+----1
ATPM
NOTE:
SYNC provides a point on which to synchronize on oscilloscope at any place needed
in the program.The probe location is D21 N2 on D-BS-RF09-0-09.
09·0359

Figure 2-6

Simulating the RS09 wHh the Maintenance Logic

2-18

BIT CELL 2
09-0388

Figure 2-7

AC Bit Usage for lOT DGSS

BIT CELL 1

BITCELL2

Figure 2-8

2.3.1

AC Bit Usage for lOT DGHS

Transfer Rate Selection

The operator can select a high, medium, or low transfer rate by positioning the rate selection switch
at HI, MED, or LOW.
At the high speed, data are transferred to or from each word on a disk channel every 16 fJS.
At the medium speed, the rate is halved to every 32 fJS, not by slowing down the disk, but by requiring
the disk to rotate twi ce in order to fi II a channel completely. During the first rotation, every second
address is read from or written into; in the second rotati on the remaining addresses are used. All this
is done automatically without extra coding.

However, the programmer must ensure that the disk is

read at the same speed at which it was written, or the data become unintelligible.
At the low speed, every fourth address is used on the first revolution, and the remaining addresses are
picked up on the successive three turns. The transfer rate is one word every (4 x 16) 64 fJS. The programmer is not required to do any extra coding, as the hardware completes the operation. The programmer must, however, ensure that the data are read and written at the same speed.

2.3.2 Disk Address Selection Jacks
The jacks shown in Figure 2-9, which are part of the controller, are used to assign selecti on numbers
to the RS09 Disk Drive. The select wire of each drive is wired to an individual plug in the DISK bank.

2-19

The select decoder of the controller is wired to the DISK SELECTION jacks. Each disk can be assigned
any address by plugging the appropriate DISK SELECTION jack into that disk's plug of the DISK bank.
Any jacks that are not assigned should be plugged into one of the NONEXISTENT DISK plugs; a selected
nonexistent-disk error can then be detected.

2.3.3 Write Lockout Switches
There are 16 lockout switches on each disk drive {labeled 00 through 74). Each switch protects 8
tracks on the disk. Switch 00 protects tracks 0 to 78' switch 04 protects tracks 10 through 178 , and

so on, up to track 177 • When anyone of these switches is set to DISABLED, the 8 tracks that they
8
protect cannot be written on. If the program tries to write in such a protected area, the WLO flag
(Write LockOut) is posted and writing is inhibited. Figure 2-10 shows the lockout switches.

Note

that switch 00 actually protects the first head of each shoe, switch 04 the second head, etc.

For the

programmer, this translates into successive tracks in blocks of 8

2.4 THE OPERATOR'S INDICATORS
The operator has at his disposal an extensive indicator panel that reflects the state of the DECdisk
System (see Figure 2-11). If a I ight on the panel is I it, the bit it reflects is set. Table 2-5 summarizes
the meaning of each light.

Figure 2-11 Indicator Panel

2-21

Table 2-5
The Indicator Panel

Indi cator Group

STATUS

Ind i cator Name

Indication When Lit

ERR, HDW,
APE, MXF,
WCE, DPE,
WLO, NED,
DCH, PGE,
XFC

The Status Register bits described in Table 2-2
are set.

Three bits of the Function Register decoded in
Tab I e 2-1 are set.

FUNCTION

Three bits of the Disk Selection Register are set.

. DISK
ADDRESS
POINTER

The first 7 bits from left to right indicate the
contents of the Track Address Register, and the
following 11 bits indicate the content of the
Word Address Reg i ster .
DISK

FLAG
{

PARITY

DATA
OFlO

The computer has overflowed its Word Count
Register and has set this flag to stop further
transfers.

ADDR

A parity error on the B or address track has been
detected.
A parity error on the current data track has been
detected.

{ DATA

The disk is presently BUSY and engaged in a
data transfer.

When this bit is set, the operator has selected
the LOW transfer rate, and every fourth bit is
being transferred. (ITL = interlace.)

{. X2

Same indication as X4, except that the operator
has selected the MED transfer rate.

~ DOFL

During a transfer, the control sequenced into the
ninth disk, which does not exist. (NED = Non
Existent Disk.)

SEQ

During a transfer, the control sequenced into a
disk unit that does not exist. The difference between DOFL and SEQ is that SEQ a disk could
be added, i.e., the system capacity was not exceeded. With DOFL, the control asked for a
disk address greater than 78.

ITL

NED

This level is the logical OR of the two conditions
that cause an API or PI break - the ERROR flag
and the TRANSFER COMPLETE flag.
The flag that requests a multicycle data break is
set.

2-22

Table 2-5 (Cont)
The Indicator Panel

Indicator Group

NED
(Cont)

Indi cator Name

Indication When Lit

PSL

A nonexistent disk unit was specified by the
program. It was not sequenced under a transfer;
the error was a direct programming mistake.

WRITE

A WRITE operation is taking place.

RUN

The control is busy and properly synchronized.

A missing negative pulse or extra positive pulse
from the ATT track bipolar signal pair was detected.

". MP

A missing positive or extra negative pulse from
the A TT track bipolar signal pair was detected.

HDWR ERR

BT
<

Any pulse of the bipolar signal pair from the
BTT track was detected as missing or extra.

Any pulse of the bipolar signal pair from the
CTT track was detected as missing or extra.

... DT

Any pulse of the bipolar signal pair from the
addressed data track was detected as missing
or extra.

MAINT

The controller is in Maintenance mode.

2.5 PROGRAMMING EXAMPLES
2.5.1 Call ing Sequence Table
The following program can be used to read, write, or write check any number of the words that can
be accommodated in core memory. The program is set up from a call ing sequence table that I ists the
word count, current address, disk number, track number, the address of the first word in the track,
and the function to be performed. The execute subroutine that follows enters these variables in their
respective registers and commands the disk to execute.
In this sample program, a pointer (DO) is set into auto-increment register 10. Each time the register
is indirectly addressed, it is initially incremented by one. The effective address for the first entry
is the WC; for the second, the CA; and so on, down the call ing sequence table to the FR. With this
technique, the execution subroutine sets up the disk and the multicycle data break to carry out the
prescribed operation.

2-23

/Call ing Sequence Table
CAL TAB

JMSDO

o (WC)

o (CA)
o (APO)
o (AP1)
o (FR)

LAC DO
DAC 10
LAC* DO
DAC 36
LAC* 10
DAC 37
LAC* 10
DLAL
LAC* 10
DLAH
LAC* 10
DSCF
DSFX
{ DSCN
JMP* 10

/ Jump to execute subroutine
/2 1 s complement of number of words to be transferred
/Start of memory core data table less 1
/Disk starting word address and track
/Disk number
/Functi on (read, write check) desired
/Continue program sequence
/Execution Subroutine
/Enter execution subroutine
/Fetch pointer
/Deposit pointer in auto index register
/Fetch word count
/Deposit in word count register
/Fetch current address
/Deposit it in CA register
/Fetch disk starting word and track address
/Deposit it into its registers
/Fetch disk number
/Load into disk number register
/Fetch the functi on
/Clear the function register
/XOR the function register
/Execute the condition held in the function register
/Exit the disk subroutine

* Note that these instructions are usually mi croprogramrned into 707047.

2.5.2 Disk Flag Tests
The disk flag is posted if an error occurs during the transfer or when the transfer is successfully completed. The disk flag causes a PI or API break (if they are enabled) to locations 0 or 63 , respectively,
8
8
and the program tests for an error or sets up the next transfer from the selected location.

2.5.2.1 Use of laRS - The input/output read status (laRS) facility provides for programmed interrogation of I/O device status (e.g., DECdisk) by groups. When the laRS instruction is executed, the
states of a" device flags are entered into specific bit positions of the AC. The DECdisk flag is entered into AC bit position 13. Testing the AC contents by groups saves time in locating a specific
flag. The following subroutine i "ustrates the use of IORS to test for the DECdisk flag.
FLAGS

o
laRS
DAC
AND
SNA
JMP
JMP

SAVE
MSKl
GRP2
TSTl

/Enter flags into AC
/Save flags
/Test with group 1 mask
/Skip if flag is located
/Go to group 2 device test
/Test group 1 device flags

2-24

TSTl

MSKl
DMSK

LAC
AND
SNA
JMP
JMS

SAVE
DMSK
GRP1A
DISK

000777
000020

/Get group 1 flags
/Mask AC with disk mask
/Skip if disk flag is located
/Test next subgroup lA
/Hand led isk
/Group 1 mask
/DECdisk mask

2.5.2.2 Skip Chain - The disk flag can be tested by the DSSF instruction if PI or API are not used.
The following subroutine I ists this procedure.
PI
FLGS

/Store the I ink, extend mode (PDP-9) protect and PC + 1

JMP
FLGS
lOT SKPA
SKP
JMS
DEVA
lOT
SKPB
SKP
JMS
DEVB

/
/Skip if device A flag
/Go to next device
/Handle device A
/Skip if device B flag
/Go to next device
/Handle device B

DSSF
SKP
JMS

DISK

/Skip if disk flag
/Go to next device
/Handle disk

ION
JMP*

/Turn PIE on
/Return to main program

The program is now aware that the disk flag has been set. To determine if a successful transfer has
taken place, read the status word into the AC by the DSRS lOT. AC bit 0 is the logical OR of all
significant error conditions, and it can be quickly tested by the skip on positive accumulator (SPA)
instruction. If no skip occurs, an error exists; the next step is accompl ished to determine the error
and take the required action. The following program illustrates these points.
DISK

o
DSRS
DAC
SPA
JMP
JMP

ER
XFC

DBR
JMP*

DISK

SAVE

/Store PC + 1, link, EXD (PDP-9) and protect
/Disk read status
/Save the status
/Test for an error condition
/Go to error routine
/Go to transfer complete test. Set program flag

/ API debreak and restore command

/
2-25

The API and PI subroutines usually differ in that the API is kept as short as possible so that it does not
tie up the API channel and delay other devices. Techniques for programming the API are explained
in the appropriate manual.

For simplicity, the same handler for both PI and API is used in this manual.

2.5.3 Error Flag Tests
The error flags that cause an interrupt or API break are classified in three categories, according to
the action that should be initiated when they occur. HDW (APE, MXF, and timing errors such as BT,
CT, etc.) indicates hardware malfunctions. WLO and NED show that either an operator error was
made when the system was initiated, or that the data transferred exceed the capacity of the system to
store it. The operator can correct this situation. If WCE or DPE occur, the program itself can take
corrective action by determining if the error persists, and subsequently rewriting the erroneous data.
Only if a parity error persists should the operator be notified.
The first two classes of error flags should be tested first. If they caused the interrupt (HDW, APE, and
MXFi WLO and NED), the program is stopped, and the status is left in the accumulator for the operator
to interpret. If the last set of errors occurs, further action can be expected from the program.
EXAMPLE:
ER

LAC STATUS
AND (346000
SNA
JMP REWRIT
LAC STATUS

/Get the status
/Mask out all but the first two classes
/Skip if an error occurs
/It was a soft error, go to rewrite
/It was a hard error, store the status and notify
/the operator

Note that the error flags are arranged in descending order of importance so that they can also be
tested by successive RTLs skips on link and SMAs.
REWRIT

/The parity error was discovered during a read or a write check.
/The program can either halt, go back and repeat the operation
/several times to see if the error is still there, or go back and
/rewrite all the data that has been written erroneously and
/then retest it I or both.

2.5.4 Programming with the ADS Register
The contents of the ADS register reflect the current position of the disk surface. This information is
available to the program through the lOT instruction DLOK and can reduce file transfer time between
the disk and core memory. Consider the following example, which is illustrated in Figure 2-12.

2-26

Example:
Assume that a file 17778 words long is to be transferred from core memory onto a
disk. Let the current address (CA) and the word count 0NC) be 20008 and the 2's
complement of 17778 (102410), respectively. Let address pointer 0 (APO) = 050000
and let address pointer 1 (APl) = O. The function is set to WRITE and the transfer
rate to HIGH.
Assume further that after the call ing sequence has been set up by the program, the
ADS register is read into the AC and found to be set to location 6608. The program
would then determine the nearest address to which it could begin transferring data,
taking into account the amount of coding that must be processed before the start lOT
is issued and the time it takes to switch tracks, if a track must be switched (200 ~s),
plus set up time. About 240 ~ or 1510 addresses later is a reasonable figure. In
this example, the projected ADS address (PADS) is, therefore, 6778.
The PADS falls within the area on the disk where the fi Ie is to be transferred. The program can now
make one of two decisionsj it can wait unti I the disk rotates to location 0 before it starts to transfer
data, or it can begin transferring the file at location 677 8 • One way to manage the transfer is to
divide the file in core into two subfiles. The first subfile starts at core location 2677 and transfers
8
to disk location 677 ; it overflows 1101 words later. The second subfile starts at the original CA
8
8
location 2000 , transfers to disk location 0, and overflows 6778 words later. This procedure transfers
8
the fi Ie in one revolution in its proper sequence I and the time saved from the previous method is approximately the time it takes for one-quarter of a revolution. (See Figure 2-12.)
The more general problem of calculating the two subfiles and determining the PADS address for all
three transfer rates is somewhat more complicated. Recall that the ADS Register does not give the
adjusted address of the Disk Segment register during medium- or low-speed transfers. During medium
transfer rates, this adjusted address can be found by rotating the ADS Register (the 11 least significant
bits) one to the right. During low-speed transfers, the adjusted address is calculated by rotating the
11 least significant bits two places to the right. If the first address calculated does not fall into one
of the revolutions where the data is stored for this fi Ie, the next address should be tried, and if the
transfer rate is LOW, then the following two addresses. This continues unti I all four revolutions are exhausted, or unti I a val id PADS is determined. The flow diagram of Figure 2-13 illustrates the process.
Assume, for example, that a PADS is calculated and falls into the section shown in Table 2-6. When
the transfer rate is HIGH, any address from 74 to 105 is acceptable to determine whether the PADS
falls within the file area. When the transfer rate is medium, then it is possible that only every second
address belongs to the fi Ie. The second Iine converts the high-speed addresses to their appropriate
medium-speed address. If, for example, address 75 converted to 2036 does not fall within the data fi Ie,
then the program must go on to address 76. Converted, this address is 37 to the medium-speed transfer.

If 37 does fall within the file area, the program can begin transferring its file at that point. When lowspeed transfer rates are used, four addresses, one for each revolution, may have to be tested for val id
PADS points.

2-27

Table 2-6
Adjusted ADS Register for Medium and Low Transfer Rates
Transfer
Rate

Address

HIGH

100

101

102

103

104

105

MEDIUM

2036

2037

2040

2041

2042

LOW

1017

2017

3017

1029

2020

3020

1021

TRANSFER RATE SET TO HIGH
APO

6608 ADS
6778 PADS

17778
(A) THE FILE ON THE DISK

20°°8

2676 8
26778

3777 8

(S) THE FILE IN CORE
09- 0420

Figure 2-12

Calculating Fast Access Ca" ing

2-28

SET API
READ ADS

YES

GENERATE THE
FOUR POSSIBLE
PADS, ONE FOR
EACH REVOLUTION

GENERATE THE
TWO POSSIBLE
PADS,ONE FOR
EACH REVOLUTION

COMPARE EACH
PADS WITH THE
DISK FILE ADDRESS

YES

USE THE INITIAL
CALLING SEQUENCE

ESTABLISH THE
CALLING SEQUENCE
FOR THE TWO
SUBFILES AND
BEGIN TO
TRANSFER

09-0421

Figure 2-13

Flow Diagram of the Subroutine That
Uses the ADS Register

2-29

2.5.5 Programming Multiple-Disk Systems
Sequencing from track-to-track and disk-to-disk is program-transparent except for the latencies that
occur when switching from disk-to-disk. The disks are not synchronized. The latency can be reduced
by using the ADS register and the techniques described in Paragraph 2.5.4. Ensure that the ADS
register is read and the correct disk has been selected; i.e., that address pointer 1 has been properly
set up (see Figure 2-13).
If API is set to a disk that does not exist in the system, or if the program sequences into a nonexistent
disk (such as disk 9 in an eight disk system), then an error flag is posted.

Note that the eighth disk

does not overflow and wrap around to disk O.

2.5.6 Using DECdisk in a System
DECdisk is as reliable as the main core memory and considerably more reliable than industry compatible
tape. The disk should always be supported by another bulk memory unit, typically DECtape or industrycompatible tape. It takes about 30s to fi II a DECtape reel from DECdisk, and each disk surface fills
two such reels. As data files are generated they should be regularly dumped from the disk into its
support memory. How often this is done depends on the application; in most systems, this job can become a background activity except when very important files are under construction. DECdisk may
9
cause approximately one irretrievable error in 2 x -10 bits transferred. With most information, this is
not a problem. However, if the error occurs during the transfer of system software or the accumulation
of a payroll file, the result could be disastrous.

For this reason several error detecting techniques have

been devised. These are I isted with short explanations in Table 2-7.

Table 2-7
Disk Data Checks

Explanation

Name

Lateral Parity Checking

This test is automatically performed by the hardware
each time a word is read, written, or write checked.

WRITE CHECK

This function checks the disk itself. It compares the
file in core with the file as it should have been written in memory. The checking is done at the controller,
however; consequently, consistent errors in the data
paths are not detected.

2-30

Table 2-7 (Cont)
Disk Data Checks

Name

Explanation

WRITE then READ

This technique tests the disk, the data paths, and core
memory itself. It copies the file onto the disk, and
then reads it back into core into a different area of core
memory. The two files are then checked for consistency. (The overhead is high.)

Longitudinal Parity Check

This technique tests the disk and the data paths and
core. When a table or file is built, a longitudinal
checksum is calculated with it. Whenever the table
is transferred, the checksum is recalculated and compared with the original. (The overhead is very high.)

Error Detecting and
Correcting Codes

Hamming codes that can be generated for each word
automatically and correct some errors when they occur.
The overhead when this is done with software is usually
prohibitive.

One additional short test can be run on a file after it has been transferred: add the original word
count to the original APO and compare the result to the APO just after the transfer.

The two values

should be identical.

2.6 SUMMARY OF DECDISK CHARACTERISTICS
The following is a summary of the DECdisk System characteristics:
a.

Storage Information

(1) fixed head
(2) serial, random access

(3)

8 disks per controller

(4)

128 data tracks per disk

(5)

2048 eighteen-bit words per track

(6)

262,144 eighteen-bit words per disk

(7)

2,097,152 eighteen-bit words per disk system

System Transfer Rates
Three switch-sel ectable speeds:

16 !JS per word;
32 IlS per word;
64 IlS per word.

Protection
Tracks on each disk are protected from a write operation in groups of eight (a total of
16,384 words).
2-31

Access
(1)

16.7 ms (average) when the ADS register is not used

(2) 200 fJS if the ADS register is used
e.

Rei iabi lity
Six recoverab Ie errors and one nonrecoverab Ie error in 2 x 109 bi ts transferred.
(A recoverable error is defined as an error that occurs only once in four successive
reads .)

Core Locations
Automatic Priority Interrupt
Data Channel

63 on level
36, 37

2-32

Chapter 3
The DECtape System

3.1 INTRODUCTION
The DECtape System is a magnetic tape data storage faci lity. The system, consisting of up to four
TU56 Dual DECtape Transports, or eight TU55 single DECtape Transports, and a TC 15 DECtape Control,
stores information at fixed positions on magnetic tape as in magnetic disk or drum storage devices,
rather than at unknown or variable positions in conventional magnetic tape systems. This feature allows
replacement of blocks of data on tape in an ordered fashion without disturbing other previously recorded
information. In particular, during the writing of information on tape, the system reads format (mark)
and timing information from the tape and uses this information to determine the exact position at which
to record the information to be written. Similarly, in reading, the same mark and timing information
is used to locate data to be played back from the tape.
This system has a number of features to improve its reliability and make it exceptionally useful for
program updating and program editing appl ications. These features are: phase- or polarity-sensed
recording on redundant tracks, bidirectional reading and writing, and a simple drive mechanism that
uses aerodynamically lubricated tape guiding (the magnetic tape surface floats on air and does not
touch any meta I surfaces except the head).

3.2 DECTAPE FORMAT
DECtape uti I izes 10-tracks with a read/write head for each track. Tracks are arranged in five nonadjacent, redundant channels: a timing channel, a mark channel, and three information channels
(see Figure 3-1). Redundant recording of each character bit on nonadjacent tracks materially reduces
bit drop-out and minimizes the effect of skew. Series connection of corresponding track heads within
a channel and the use of Manchester phase recording techniques, rather than amplitude sensing techniques, virtually el iminate dropouts. The timing and mark channels are preformatted prior to recording
all normal data read and write functions.
A reel of DECtape is 260 ft long and is divided into two end zones of 10 ft each and a recording zone
of 240 ft. The end zones are merely used as leader-trailer to wind the tape around the heads and onto

3-1

10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0I

TIMING TRACK I

(17

MARK TRACK I
INFORMATION TRACK I

INFORMATION TRACK 2

INFORMATION TRACK 3
INFORMATION TRACK lA
(Samea.,Tll

INFORMATION TRACK 2A
(Same as IT 2)

)

c,:

a a

:~:~e~~~~I~N TRACK 3rt a

0
0

[

Track Allocation Showing Redundantly Paired Tracks

TIMING TRACK
MARK TRACK
91

121

DATA OR
4 CONTROL
WORD

INFORMATI ON
TRACKS

I
141

REDUNDENT
TRACKS
NOT
SHOWN

B':Jsic Six Line Tape Unit

~--

ONE COMPlE TE REEL - 260 FT 4096 BLOCKS (MAX.)

r?'oc'B:coc'~EBBEJB~~'I~"Ffj
~---- ONE SLOCK 26 6\0'S BI T WORD LOCATIONS
TIMING TRAC
MARK TRACK

DATA 3

lIJ

§£
" ~ ~
~

DATA 1
DATA 2

K f f - - f---- -

Z
0

f4--- C~~~~~L

Z
0

: I)V

--+--

f-- r -

~
UJ

Z
0

256 \0 DATA WORDS

~ §

Z
0

.. \.

DECtape Format

3-2

~;:

Control and Data Word Assignments

Figure 3-1

--.

Z
0

-~-

~ ;:
0

C~~~~~L _ _

--- i - - 1--

3/4"

REDUNDANT

'LlJ

~~~~~~:;;~A doD 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0

)

REOUNDANT
TRACKS
NOT
SHOWN

TRACKS

the take-up reel. Timing and mark channels' are recorded prior to recording the data. The 24D-ft
recording zone is divided into blocks. Each block, which contains a specified number of data words,
is separated by a number of control words on the mark track. The number of data words per block is
determi ned by i nformati on on the mark track when the tape is preformatted. The standard PDP-15
DECtape format is 576 blocks of 256 18-bit data words. Each 18-bit data word uses six data lines,
and each data line contains three bits. The total length of tape is equivalent to 884,736 data lines
which can be divided into any number of blocks up to 4096. Blocks are normally limited to a total of
4096 to provide compatibility with 12-bit computers such as the PDP-8.

3.2. 1 Timing Track
The timing of operations performed by the tape drive and some control functions are determined by the
information on the timing channel. Therefore, wide variations in the speed of tape motion do not
affect system performance.

3.2.2 Mark-Track Format
The mark track is reserved for control codes (see Table 3-1 and Figure 3-2) which are stored serially.
The codes are 6-bits long and are used for initiating controls to raise flags in the program, requesting
data breaks, detecting block mark numbering and block ends, and protecting control portions of tape.
The TC15 DECtape Controller automatically identifies these codes to control transmission of data. The
mark track also provides for automatic bidirectional compatibi lity, variable block formatting, and
end-of-tape sensing.
During a" tape processing functions except recording of the- timing and mark track, a sing Ie mark-track
bit is read from each line of tape, regardless of whether the information is being read from or written
onto the data tracks; and each tape line in both the information and mark tracks is positioned at the
center of the correct polarization in the timing track, as shown in Figure 3-1. The six lines on the
tape that contain the mark code in the mark track are designated as a mark frame. A mark frame is a
group of six tape lines. The six bits in the mark track are called a mark code, as shown in Figure 3-1.

A given change of polarization on tape read in one direction produces a pulse opposite in polarity to
that produced by the same change when tape is read in the opposite direction. Consequently, a mark
code read in reverse of the direction in which it was recorded has the order of bits reversed and the
bits complemented. For example, a mark code read forward as 100010 is read as 101110 in reverse
(see Figure 3-3). This correspondence is termed the complement obverse or the complement image.
Every 6-bit code has one and only one complement obverse, which is constructed by complementing
a" bits and reversing their order.

3-3

Table 3-1
Mark Track Coding

Mark

Octal Code

Function

Reverse End
Mark

Indi cates the tape is progressing from the end zone
toward the control words and data blocks.

Expand Mark

Allows DECtape compatibility between 18- and 36-bit
machines.

Forward Block
Mark

Signifies the start of a block. Block number is stored
in memory, and, when the TC15 controller decodes
code 26, the computer obtains the forward block
number.

Reverse Guard
Mark

Allows time for the computer to decide what to do with
the forward block number.

Lock Mark

Indicates first of four octal 10 cells and allows for computer-decision time.

Reverse PCC
Mark

Indi cates last cell before a data word, and is used to
initiate parity checksum logic. For write operations,
the first 18-bit word to be written onto tape should be
transferred from the computer to the controll er duri ng
reverse PCC mark.

Reverse Final
Mark

Indicates first data word.

Reverse Prefinal
Mark

Indicates second data word.

Data Mark

Indicates a data word is contained in data track.

Prefi na I Mark

Indi cates next to last data word of block.

Final Mark

Indicates last data word in block and shows that next
cell begins with the forward longitudinal parity check.

PCC Mark

For a write operation, the parity checksum is deposited
in this cell. For a read operation, the value written in
this cell is compared with the value of the checksum due
to the read operation.

Reverse Lock
Mark

Provides additiona I time to protect records in the event
of mark track errors.

Guard Mark

Performs same function as reverse lock mark (code 73).

Reverse Block
Mark

Complement obverse of forward block mark and is used
when tape is travelling in reverse direction.

Expand Mark

Same as expand mark previously described.

End Mark

Identifies end zone and indicates transport is running out
of tape.

3-4

MOTION OF TAPE

1'4
I

MARK TRACK

<.n

C
C

INTERBLOCK SYNC. MARK

W
I

NEXT
I BLOCK
I MARK

0
I I 101 I 11 101 I I I 101 I I I 101 I 10 100 I 100 I 0 I 01010 I 010110

CH. I
CH.2
CH.3
REVERSE
END MARKS

DATA

010101 0101 10 o I 1010 001000 0010 IocooloOO 001000 I I 1000 I I 1000

0100101010010

ONE BLOCK

-'-----_._-

I_U

FORWARD
END MARKS

INTERBLOCK SYNC. MARK
REVERSE BLOCK MARK (Nil

FORWARD BLOCK MARK (Nil

GUARD MARK

REVERSE GUARD MARK

REVERSE LOCK MARK

LOCK MARK

PCC MARK

REVERSE PCC MARK

FINAL MARK

REVERSE FINAL MARK

PRE-FINAL MARK

REVERSE PRE - FINAL MARK

ADDITIONAL DATA MARKS

DATA MARK SHOWING BIT ORIENTATION OF lB BIT WORD ON TAPE

PCC = 6 BIT PARITY CHECK CHARACTER (HARDWARE COMPUTEDl
09-0112

Figure 3-2

Mark-Track Format

TIME

DATA READ IN SAME DIRECTION

DATA READ IN OPPOSITE DIRECTION

AS RECORDED

FROM THAT RECORDED

TyE

I 00 0 I 0

lONE CHANNEL
I

DATA....JI ~ HEAD

DIRECTION OF TAPE MOTION

---- DIRECTION OF TAPE MOTION

TIME

~ ONE CHANNEL

TyE
1 000 1 0

\.. DATA

HEAD

4 0 1 0 1 0 1 0 I 0 1 0 1 SHIFT REGISTER

0 1 0 I 0 I 0 I 0 I 0 I SHIFT REGISTER

t. NOTE THAT THE 1 READ BACKWARDS

BECOMES A ZERO (FLUX REVERSES)

----

~+loioIOiol'

o0 0 t 0

L]J

~ '10101010101

tOO 010

L]J

~Oll 101010101 LtJ

1 000 1 0

~, I' I0I0I0I0I

1 000 t 0

L]j

-+
4

L.J

1 000 1 0

L]J

100010

L.[1

L]j

I' I' I I I I
0

I
4

----

1 000 1 0

0 1010111 0 1 0 1

{

10001 0

Y ll
O

11 11 1 0 1 0 1

.....
6

~010111010101
i

-+
3

1 0 0 0 1 0

L]J

Yl101010lti01
42 8

I C

1 000 1 0

L]J
0

1'1'1' 1

}
6

56 8
15-0236

Figure 3-3

Bidirectional Reading and Writing

3-6

There are eight octal codes of two digits that are their own complement obverses: 07, 13, 25, 31,
46, 52, 64, and 70. All other possible combinations of two octal digits (there are 56) are different
from their complement obverses.
Because the DECtape System allows reading and writing in both directions of tape motion, the mark
track must be coded to present the same information when entering a block from either direction. The
marks at the end of a block are the complement obverses of the marks at the beginning, in reverse
order. For exampl e, assume the control reads the marks 25, 26, and 32 as the first three marks beginning a block in forward moti on. In reverse motion, the control sees the obverse complement of
the contents of the mark track. The first information when reading the block in reverse is therefore,
also read as 25, 26, and 32. Mark codes to be read in when tape is moving in reverse direction are
recorded in obverse comp Iement • Thus, the mark track appears symmetri ca I .
All mark codes used in the standard DECtape format are listed in Table 3-1. Only 10 valid codes
exist even though a given code may have different designations, such as 10 and 73.
The standard mark track uses the serial code of 6-bit characters to divide the tape into words. Codes
are written on the mark track opposite word locations to identify the type of information stored at that
location on tape. Block addresses are written for both forward and reverse directions and identified
by two types of mark codes (the second is the complement obverse of the first). A checksum is written
at each end of the block. The hardware computed checksum is the 6-bit logi cal equivalent (i .e., the
complement of the exclusive OR) of each six bits written on tape, plus the reverse checksum previously
recorded. By including the reverse checksum in the computation, the block may be read in either
direction at a later time without an error. The control uses the final marks to establish synchronism and
raise block-end flags.

Da~a marks locate data words.

3.2.3 Data Blocks
A tape contains a series of data blocks that can be of any length that is an even number of 18-bit
words. Block length is determined by information on the mark channel. Usually a uniform block
length (256

for the PDP-15) is established over the entire length of a reel of tape by a program that
10
writes mark and tim ing information at specifi c locations. The abi lity to write variable-length blocks is
useful for certain data formats.

For example, small blocks containing index or tag information can be

alternated with large blocks of data. The maximum number of bloc.ks addressable is 4096 when 12-bit
machine compatibi lity is required. Otherwise, the number of blocks can be increased to 13,928.
A block is identified by a number recorded during tape formatting on the data tracks just before and
after the area where data are stored in the block. This number is recorded at either end of a block;
thus, it can be read from either direction.
3-7

Block numbers normally occur on tape in sequence from 0 to N-1, where N is the number of blocks.
The total length of tape is equivalent to 884,736 data lines per tape, which can be divided into any
number of blocks by prerecording of the mark track. However, 576

blocks of 256

words are con-

sidered to be standard format for a PDP-15 DECtape.

3.3 TU55 AND TU56 DECTAPE TRANSPORTS
The TU55 DECtape Transport is a bidirectional magnetic-tape transport consisting of a read/write head
for recording and playback of information on DECtape. The TU56 Transport is similar to the TU55 but
is a dual transport (two transports mounted side-by-side on a single chassis). Connections from the
read/write heads in the transport are made directly to the TC15 DECtape Control, which contains the
associated read and write amplifiers. The logic circuits in the DECtape Transports control tape movement in either direction over the read/write head.
Tape drive motor control is accomplished by regulating the torque of two motors that transport the tape
across the head according to the establ ished function of the device, i.e., go, stop, forward, or reverse.
In normal tape movement, full torque is appl ied to the forward or leading motor, and a reduced torque
is applied to the reverse or trailing motor to keep proper tension on the tape. Tape motion is bidirectional; as a resul t, each motor serves as either the leading or trai ling drive for the tape, depending on
the forward or reverse control status of the DECtape transport.
Tape movement can be controlled by commands originating in the computer and applied to the TU55
or TU56 through the TC15 DECtape Control, or can be controlled by commands generated by manual
operation of rocker switches on the front panel of the transport. Manual control is used to mount new
reels of tape on the transport, or to perform a quick maintenance check for proper operation of the
control logic in moving the tape.

3.4 TC15 DECTAPE CONTROL
A maximum of eight TU55 DECtape or four TU56 transports can be connected to one TC15. Of the
four data channels avai lable, DECtape is assigned to channel 0 (i .e., core memory locations 30
and 31).
C(30) = Word Count (in 2 1 s complement form) - WC
C(31) = Current Address Register - CA
Data transfers can occur to or from only one transport at any given time at a rate of one word every
200 jJS ± 60 jJS (1 biock of 256

words every 53 ms), after the desired block has been found.

3-8

The CA is incremented before the data transfer (except in SEARCH where the CA is not incremented);
thus, the initial contents should be set to the desired initial address minus one. The WC is also incremented before each transfer and must be set to the 2 1 s complement of the desired number of data
words to be transferred. In this way, the word transfer that causes the word count overflow is the last
transfer to take place.

3.5 DECTAPE INSTRUCTION SET
The number of lOTs required for the TC15 is minimized by the scheme of transferring all necessary
DECtape control data (i.e., unit, function, mode, direction, etc.) from the AC to Status Register A
in the DECtape Controller, using one set of lOTs. Similarly all status information (i .e., all above
information plus status bits, error flags, etc.) can be read into the AC from Status Register B in the
DECtape Controller via a second set of lOTs (see Table 3-2).
A bit of each status register is set or cleared by its corresponding bit in the AC when the proper load
status lOT is issued. Similarly, when the read status lOT is issued, each bit of the status register addressed is read into its corresponding accumulator bit.

Table 3-2
TC15 Control lOT Instructions

Mnemonic

Octal Code

Description

DTCA

707541

Clear status register A. The DECtape control and error flags
are undisturbed (DTF and EF).

DTRA

707552

Read status register A. The AC is cleared and the content of
status register A is ORed into the accumulator.

DTXA

707544

XOR status register A.. The exclusive OR of the content of
bits 0 through 9 of the accumulator and status A is loaded into
status register A, and bits 10 and 11 of the accumulator are
sampled to control clearing of the error and DECtape flags,
respectively. Any time this command is given with AC bits
0-4 set to 1, the select delay of 120 ms wi II be incurred.

DTLA

707545

Load status register A. Combines action of DTCA and DTXA
to load ACO-9 into status register A. Bits 10 and 11 control
clearing of error and DECtape flags, respectively.

DTEF

707561

Skip on error flag. The state of the error flag (EF) is sampled.

If it is set to 1 the contents of the PC is incremented by one to
skip the next s,equential instruction.

3-9

Table 3-2 (Cont)
TC15 Control lOT Instructions

Mnemonic

Octal Code

Description

DTRB

707572

Read status B. The AC is cleared and the content of status B is
ORed into the accumulator.

DTDF

707601

Skip on DECtape flag. The state of the DECtape flag (DTF) is
sampled. If it is set to a 1, the content of the PC is incremented by one to skip the next sequential instruction.

All 10 comand bits (O to 9) of status register A may be sensed I set, or changed via lOTs (see Table 3-3
for Status Register A bit assignments). Bits 10 and 11 of the AC are not retained by status A, but enable or disable the clearing of the DECtape and ERROR flags. To issue a DECtape command, the
command bits 0-9 of status register A are set as desired by bits 0-9 of the AC with bits 10 and 11 set
to O. The bits in status register B can be sensed and cleared by lOTs (see Table 3-4 for Status Register B
bit assignments). Bit 11 of register B is set when a DTFoccurs and must be cleared before the next DTF
to avoid a timing error. If any error occurs, bit 0 of register B and the corresponding bit (1-5 depending on the error) will be set. This bit must be cleared to avoid further interrupts on the same condition.
All error flags (i .e., status register B) are cleared by issuing a DTXA instruction with AC bit 10 set "to

o. The DONE flag is cleared by issuing a DTXA instruction with AC bit 11 set to O.
Table 3-3
Register A Bit Assignments

Function

Bit

Transport un i t
select (decodes
one of eight
DECtape transports)

0-2

Motion (determines forward
or reverse motion)

STOP/START

Conditions

Octal Code
000
001
010
011
100
101
110
111

o = Forward (FWD)
1 = Reverse (REV)

o = Stop (STOP)
1 = Start motion (GO)

3-10

Transport Unit
8 or 0
1
2
3
4
5
6
7

Table 3-3 (Cont)
Register A Bit Assignments

Function

MODE

Bit

Conditions

o = Normal Mode (NM)

1 = Continuous Mode (CM)

Function

6-8

Interrupt
Disable

1 = Enables DECtape flag or error flag to initiate PI or API
interrupt.
o = DECtape flag or error flag inhibited from initiating PI
or API interrupt.

Error Flag

o = Clear all error flags

Operations
Move
Search
Read Data
Read All
Write Data
Write All
Write Timing
Unused (causes
sel ect error)

Octal Code
000
001
010
011
100
101
110
111

1 = Error flags undisturbed

DECtape flag

o = Clear DECtape DONE flag

1 = DECtape DO NE flag undisturbed

Table 3-4
Status Register B Bit Assignments

Function

Conditions

Bit

Error Flag

Set if error occurred (i nclusive OR of bits 1-5)

Error Indi cators

1-5

Bit 1 = Mark Track Error
Bit 2 = End of tape error
Bit 3 = Select Error
Bit 4 = Parity Error
Bit 5 = Timing Error

6-10

Not used

DECtape flag set at completion of selected function

DECtape Flag

3-11

3.6 DATA FLOW
Data are transferred between the central processor and DECtape via a double-buffered (DECtape buffer
and data buffer) arrangement in the TC 15 DECtape Controller. To write data onto DECtape, a threecycle data channel request is initiated, which causes the 18-bit data word to be transferred from the
central processor to the DECtape buffer and then to the data buffer. The data word is subdivided into
groups of three bits - each bit going to a specific data track write head. Longitudinal parity is generated as the data are transferred to the write heads and written onto the tape by the hardware.
To transfer data from DECtape to the central processor, the reverse process occurs. The three read
heads transfer the data in three-bit bytes to the data buffer. The contents of the data buffer, when
full, are transferred to the DECtape buffer. A three-cycle data channel request is initiated to transfer the contents of the DECtape buffer to the central processor. This transfer must take place within
200 fJS (± 60 fJS) or the contents of the data buffer will be transferred to the DECtape buffer before
the DECtape buffer has transferred the preceding word.

3.7 DECTAPE PROGRAMMING CONSIDERATIONS
3.7. 1 Control Functions
The seven functions available with the TC15 and their octal codes as specified by the bits 6-8 of the
AC are as follows:
Function

Octal Code

MOVE
SEARCH
READ DATA
READ ALL
WRITE DATA
WRITE ALL
WRITING TIMING AND MARK TRACK
Unused at present (select error if given)

o
1

2
3
4
5
6
7

All functions take place in either direction and in either normal mode (NM) or continuous mode (CM).
NM differs from CM only in the fact that the DECtape flag (DTF) occurs at more frequent intervals in
NM. The DTF settings that occur in NM are el iminated in the CM unti I word count overflow (yVC)
has occurred.

3-12

3.7.2 MOVE Function
The MOVE function, with the GO bit set to 1, sets the selected unit in motion (forward or reverse).
NM and CM have no meaning and are ignored in this function. When the tape enters either end zone*
(i .e., beginning of tape (BOT) and end of tape (EaT)) and the unit in questi on is selected, then:
a.

The error flag (EF) is set.

The EaT bit (bit 2 of status register B) is set.

An interrupt occurs** .

A program check on the forward/reverse motion bit (AC bit 3) of the status register will determine
whether EaT or BOT occurred. However, if the unit is deselected while in motion, the tape runs off
the reel with no flags raised and no interrupt. To stop a selected unit at any time, the GO bit (AC
bit 4) must be set to o. When setting the GO bit to 0, the forward/reverse motion bit and unit selection bits should not be changed from their current status. The hardware accepts the change in the
TC15 (i .e., status A bit 3 changes from its former state) without error indication, but does not pass
this change on to the transport. Programming confusion can result. After a unit is deselected, status
information pertaining to that unit is no longer accessible unless it was saved by the program prior to
deselection.

3.7.3 SEARCH Function
The SEARCH function permits random access of data blocks on OECtape. This function is used to locate the number of block to (or from) which data transfer wi" occur. In normal mode at each block
mark unti I EaT occurs, the OTF is raised and an interrupt occurs. The block number is automati cally
transferred by the hardware into the memory location specified by the CA. The CA must have been
set previously by the program, but the contents are not incremented. The WC is incremented at each
OTF; the program must clear the OTF bit in the status register and check the block number until the
desired one is found.
In continuous mode, the WC is set to the 2 1s complement of the number of block numbers to skip. At
each block mark, the block number is read into the memory location specified by the CA which is not
incremented. The OTF is raised only at the block mark at whi ch the WC overflows. At that time, an
interrupt occurs and the block whose number was just read may be read or written. Continuous mode
provides a virtually automatic OECtape search.
*If either end zone is entered during turn around or during stopping of tape, the EaT bit is not set
and no interrupt occurs.
**AII references to the occurrence of interrupts assume both:
1. The program interrupt is on.
2. The OTF and EF have been enabled to the program interrupt or API (i.e., bit 9 of status register A is set to a 1). If either of these is not true, flags are raised and status bits are set
(and may be sensed and/or cleared), but no interrupt occurs.

3-13

3.7.4 READ DATA Function
READ DATA is used to transfer blocks of data into core memory with the transfer controlled by the
standard tape format.

The standard block length is 256 18-bit words. For this and all following

functions, the CA register must be set initially to the transfer memory location minus one because
the CA register is incremented just before each word transfer. The WC register is also incremented
pri or to each word transfer sc' it must be set to the 2s compl ement of the number of words to be transferred prior to the transfer. Data may be transferred in forward or reverse direction.
Any number of words equal to or less than one block may be transferred in NM. The DTF is raised
and an interrupt occurs at the end of each block. The DTF must be cleared before the beginning of
the next block (1.7 ms) to avoid an erroneous timing error, (see Summary). When partial blocks are
transferred, data transmission ends with WC overflow. The word that causes the WC overflow is the
last one transferred. However, the remainder of the block is read and parity checked before the DTF
and interrupt occur. Tape motion continues until the GO bit is reset to 0 by the program. If the GO
bit is not reset to a 0 or a new function is specified before the end of the next block, a timing error
will occur. READ DATA in NM is intended primarily for single, 256-word, block transfers. If any
other number of words is to be transferred, it is advantageous to use CM. However, if the programmer
uses NM for any other number of words, the program must check for WC overflow at each interrupt
because there is no other way to determine when to stop the tape or change to another function. When
the WC overflow occurs, it is essential that the functi on be changed or the GO bit set to

o. Other-

wise, transfer begins again (the lOT to clear the DTF implicitly specifies the same function again) at
the next block (or next word for the ALL functions) because WC equals o.
Any number of words may be transferred in CM. However, the DTF and an interrupt occur only once
after a WC overflow and an end of block. The comments concerning tape continuation apply in CM
as well as NM.

3.7.5 READ ALL Function
The READ ALL function allows information to be read from a tape that is not in the normal format.
This function reads all data channels recorded on DECtape, regardless of the mark track value. During
the READ ALL function 1 the DECtape control does not distinguish between different marks recorded on
the mark track, except to check for mark track errors (MKTK).
In normal mode (NM), the DTF is raised and causes an interrupt at the end of each 18-bit word transfer.

Data transfer stops after WC overflow, but tape motion continues unti I the GO bit is set to 0 or

a new function is specified (in both NM and CM). If the DTF is not cleared after each word transfer,
a timing error occurs at the end of the next word (200 IlS later).

3-14

For continuous mode, the DTF is raised and causes an interrupt at WC overflow only. When this
interrupt is ignored, no more data transfers occur but tape motion continues to EOT.

3.7.6 WRITE DATA Function
The WRITE ENABLE switch on the transport must be in WRITE ENABLE position for all WRITE functions.
All the details of the READ DATA function description apply with the following exceptions.
a.

In normal mode, the DTF is set to a 1 at the end of each block. If WC overflow did not
occur in the block just ended and no new function is specified, the next block will be
written, provided the DTF has been cleared. If WC overflow occurred in the block just
ended and no new function is specified, the tape continues to move but the writers are
disabled. In both CM and NM, when partial blocks are written, data transfer from core
to DECtape stops at WC overflow. OOOOOOs are written in the remaining data words of the
block and the parity check character is computed over the entire block and recorded.

In continuous mode, the DTF is set at the end of the block in whi ch WC overflow occurred.
Therefore, if no new function is specified, the tape continues to move, but the writers
are disabled.

A six-bit parity check character (PCC) is computed (the XOR of the reverse parity check
character and every six bits of every data word) and recorded by the DECtpae control for
every block of data recorded during the WRITE DATA function. It is used for automatic
parity checking during the READ DATA function.

3.7.7 WRITE ALL Function
All the details of the READ ALL function description apply. The WRITE ALL function is used to write
an unusual format (such as block numbers on DECtape after timing and mark tracks have been recorded).
The word which causes WC overflow is the last one written in NM or CM. The tape continues to move,
but the writers are disabled.
NOTE
Change of function must be delayed for 90 tJS to ensure recording of last word. Alternative method: set WC to 1
greater than desired number of word transfers, and change
function within 40 tJS after WCO.

3.7.8 WRITE TIMING and MARK TRACK Function
This function and only this function may be performed with the selector switch on write timing and
mark track (WRTM) on the maintenance control panel. The timing track is actually hardware-recorded
during execution of this function. The mark track is generated and recorded by program. The value
written in the mark track is determined by bits 0,3,6,9,12, and 15 of the l8-bit word being written

.e., the same bits assigned to channel 1).
3-15

CM may be used for this function, because the hardware WC provides an automatic counter and
interrupt at WC overflow only; in NM, the DTF and interrupt occur at every word until WC overflow.
In NM, after WC overflow, if the GO bit or DECtape flag is not cleared, a timing error occurs and
no more data are recorded. After WC overflow in CM, if the GO bit is not set to 0, zeros are written
on tape.

3.7.9

Disable Interrupt

The disable interrupt feature allows the program to effectively remove DECtape from the interrupt
line.
When command bit 9 in the status register is set to a 1, the TC 15 is connected to the interrupt system.
If this bit is 0, the DTF in the TC15 cannot cause an interrupt even if the interrupt facility in the PDP-15
is ON. Similarly, any of the five error conditions will cause an interrupt if bit 9 is set to 1 in the status
register, but cannot cause a program interrupt if bit 9 is a O.
Whether this bit is set or not does not influence the setting of status bits 0-5 of the status register B on
receipt of an error flag (EF) or DTF. Similarly, the result of the I/O skip instruction is independent of
the condition of this bit.

3.7.10 Error Conditions
Five types of errors can be detected in the use of DECtape:
a.

Timing Error

Parity Error

Sel ect Error

End of Tape

Mark Track Error

For all errors the EF is raised, a bit is set in the status register, and an interrupt occurs (if the enableto-interrupt bit has been set). The DTEF instruction skips on the inclusive OR of those error bits; hence,
each status bit must be checked to determine the ki nd of error. For all but the parity error, the
selected transport is stopped, and the EF is raised at the time of error detection.

No DTF occurs. For

a parity error, the GO bit remains 1 (i.e., motion continues), and the EF is raised simulataneously
with the DTF in NM.

Only 1 interrupt occurs; hence, the program must check the EF.

3-16

A parity error in CM raises the EF at the end of the block in which the parity occurs causing an
interrupt (if enabled). If no program action is taken (e.9., stop transport or reverse and re-read),
data transfer continues and the DTF is raised. The DTF causes an interrupt at WC overflow and end
of final block read.

3.7.10.1 Timing Error - A timing error (program malfunction) is a 'data miss' or program failure to
clear DTF status bit. A timing error occurs also if the program switches to READ or WRITE DATA
function whi Ie the DECtape is currently passing over a data area on tape.

3.7.10.2 Parity Error - A parity error occurs only during the READ DATA function for a hardware
computed parity check character (PCC) fai lure.

3.7.10.3 Select Error* - A select error results from any of the following conditions:
a.

Attempt to select unit when two or more DECtape transports are set to the same unit number and both are on REMOTE.

Attempt to write on DECtape transport with WRITE ENABLE/WRITE LOCK switch in the
WRITE LOCK position.

Attempt to select unit for any function with DECtape transport REMOTE/OFF/LOCAL
switch in the OFF or LOCAL (off-line) position.

Attempt to write timing and mark tracks with the DECtape controls switch in any position
other than write timing and mark track.

Attempt to perform any function other than write timing and mark tracks with the DECtape
control switch in the write timing and mark track position.

Attempt to perform any function other than read all with DECtape control switch in the
read mark track positi on.

Attempt to execute unused functi on (7).

3.7.10.4 End of Tape (EOT) - An EOT error occurs when the DECtape enters either end zone with the
GO bit = 1 and the forward/reverse direction bit set to continue in the same direction. In NM and
CM, data transfer stops at the last legitimate block, the EF is raised, the tape transport stops, and an
error interrupt occurs.

3.7.10.5 Mark Track Error - A mark track error occurs when the DECtape control fails to recognize
a legitimate mark on the mark track. The error may occur in all but the move or write timing and mark
track functions. In both CM and NM, the EF is raised, the tape transport stops, and an interrupt
occurs.
* No-tape or tape-run-off-reel conditions are not detectable.

3-17

3.8

PROGRAMMING EXAMPLES

The following exam pi es illustrate how several DECtape functions can be programmed. In the first
example, a specific block is searched out and found, and in the second example, data is read from
this block. The subroutines are written in PDP-15 Basic Symbolic Assembler language. Since no indirection is used, the exampl e is intended for Page 0 operation.

3.8.1

Automatic Search

BEGIN

LAC (CBLK

LAC (JMP SEARCH
DAC SWITCH
LAC (321400
DTLA

/GET THE ADDRESS WHERE THE
/BLOCK NO. GOES
/PUT IT INTO THE CA
/ZERO OUT THE we TO AVOID
/OVERFLOW
/GET THE EXIT POINT
/PUT IT INTO SWITCH
/GET THE STATUS A
/LOAD STATUS A REGISTER

JMS DECTAP

/API SETUP

DECTAP

DTDF
SKP

/SAVE PC, LINK, EXTEND MODE
/AND MEM. PROTECT BITS
/SAVE THE AC
/SKIP 0 N ERROR FLAG
/SKIP
/GO TO ERROR ROUTINE (NOT
/SHOWN)
/SKIP ON DECTAPE FLAG
/SKIP

SWITCH

JMP SEARCH

/GO TO SEARCH ROUTINE

LAC (BLK
AND (007777

/GET THE CURRENT BLOCK NUMBER
/MASK OUT ALL BUT THE LAST
/12 BITS
/SKIP IF DIFFERENT FROM
/DESIRED BLOCK
/SAME BLOCK, GO TO READ DATA
/EXAMPLE
/DIFFERENT BLOCK, CALCULATE
/THE DIFFERENCE BY SUBTRACTING
/THE DESIRED BLOCK FROM THE
/CURRENT BLOCK. IF THE RESULT
lIS NEGATIVE, THEN THE TAPE
/DRIVE IS MOV ING TOWARD THE
/DESIRED BLOCK. THE RESULT OF
/THIS CALCULATION IS THE TWO'S
/COMPLEMENT OF THE COUNT TO
/THE DESIRED BLOCK.
/GO TO TEST IF TAPE WAS REVERSED

DAC 31
DZM 30

DAC ACSAV
DTEF
SKP
JMP DECER

SAD RBLK
JMP RBLKS
TCA
DAC TEMP
LAC RBLK
TCA
ADD TEMP
SMA

JMP TEST

3-18

DAC 30
LAC (010000
DTXZ

DBR
JMP* DECTAP
TEST

ISZ TAG
JMP REV
TCA
JMP GO
DZM TEMP
ISZ TEMP
JMP .-1

REV

LAC (361400
DTLA
CLC
LAC TAG
JMP* DECTAP

TAG

/MOTION OK SET UP WC
/GET STATUS A CHANGE
/XOR CHANGE TO STATUS A
/(PUT INTO CONTINUOUS)
/THE XOR INSTRUCTION IS USED INSTEAD OF THE CLEAR
/AND LOAD TO AVOID SETTING THE SELECT DELAY I WHICH
/COULD PREVENT ANY ACTION FOR 120MS
/DEBREAK AND RESTORE
/RETURN TO MAIN PROG RAM
/CHECK TO SEE IF THE TAG WAS SET
/IT WAS NOT; GO AND REVERSE
/THE TAPE
/IT WAS I COMPLEMENT THE AC
/TO GENERATE WC
/GO BACK AND SET UP THE WORD
/COUNT
/SET UP A DELAY WHICH WILL
/ALLOW THE TRANSPORT TO MOVE
/SEVERAL BLOCKS PAST THE CURRENT
/ONE.
/GET THE NEW STATUS A WORD
/WHICH REVERSES THE TAPE.
/LOAD THE NEW STATUS A WORD
/CLEAR AND COMPLEMENT THE AC
/PUT RESUL TS IN TAG TO REMEMBER
/THAT REV HAPPENED.
/RETURN TO MAIN PROG RAM UNTIL
/THE NEXT NUMBER IS PICKED
/UP WHILE THE TAPE IS
/TRAVELLING IN THE REVERSE
/DIRECTION.

Note that it is necessary to remember that the tape had been reversed I and was travelling towards the
desired block. In this way the previous subroutine could be used when the tape again found the current block while travelling in the reverse direction.

3.8.2

Read Data

RBLKS

LAC ADDR
DAC 31
LAC WDCNT
TCA
DAC 30
LAC (003000
DTXA
LAC (JMP REDE
DAC SWITCH

/GET THE ADDRESS
/PUT IT INTO CA REGISTER
/GET THE WORD COUNT
/TWO'S COMPLEMENT IT
/PUT IT INTO THE WC REGISTER
/GET THE UPDATED FUNCTION
/XOR IT INTO THE CONTROLLER
/GET THE EXIT POINT
/PUT IT INTO SWITCH

3-19

LAC ACSAV
DBR
JMP* DECTAP
ADDR

ADDRESS

WDCNT

IGET THE AC AND RESTORE IT
IDEBREAK AND RESTORE
IRETURN TO MAIN PROGRAM

After the block has been found, the program begins to read the data in the block. The RBLKS subroutine sets up the word count and current address, and XOR's the new function. It also resets the
interrupt chain, priming it to jump to REDE, the subroutine which is to handle the data once it was
read (not given here) .

3.8.3

Bootstrap Loading Technique

The data channel facility and the design of CONTINUOUS MODE allow for linked loading of data
from DECtape, during which the first two DECtape data words determine the core location and amount
of data which follows. The address into which data is loaded is specified by CAi thus, if the CA
points to the WC-l, the first word transferred specifies the number of words to be transferred. After
the first data word transfer, the CA points to itself, and the second word transferred specifies where
the following data is to be loaded. No program interrupts, timing, or computation is required to
locate the data. Only the TC15 and DCH features are used.
Problem:
Load x data words beginning at DECtape block 4 of uni t 3 into core locations M to M+X-l, assuming
tape has been positioned at block 4.
IBOOTSTRAP EXAMPLE

IOTD,

DZM*(30
LAC (27
DAC*(31
LAC IOTD
DTLA

ITO ENSURE NO WC OVERFLOW
ITO BEGIN LOADING AT REGISTER 30
ICA
IIOT DATA
ILOAD STATUS REGISTER

332400

IREAD ON UNIT #3, CM, FORWARD, GO (1)
/WITH INTERRUPT ENABLED

3-20

The following represents the format of the data on the tape starting at block 4.
WORD 0 ... eORE LaC 30
WORD 1 ... eORE LaC 31

WC (2 1s Comp.)
m-l
Data

BLOCK 4

BLOCK 5

{

BLOCK 4+ «X/256)-1)

X Data
Words

Data
-z-

-t..

Where X is an integral multiple of 256.

3.8.4 Writing and Reading in Opposite Directions
As mentioned earlier, it is possible to read data from a DECtape in the opposite direction from which
the data was written via program manipulation. However, a re-ordering of both the entire block and
individual words is required.
a.

Block Re-ordering: A block of words xl "'Xn recorded in one direction is loaded into
core as xn -+- x 1 when read in the opposite direction.

Word Unscrambling: Data read in backwards comes into core memory locations from
the TC02 in the following 18-bit format:
Bits:
15 16 17 12 13 14 9 10 11 6 7 8 3 4 5 0 1 2
In bit positions:
O~

.. 17
NOTE

If data is to be re-ordered on the fly, the routine is limited
to 140 IJS since the word transfer rate = 200 IJS (±30%). The
probabi Ii ty of such a routine not working is very high if interrupts from other devices are encountered.

3-21

The following subrouti nes may be used for re-ordering the block of .words and unscrambl ing each
individual word.

/ROUTINE TO UNSCRAMBLE DECTAPE WORD ON REVERSE TRANSFER
/CALLING SEQUENCE:- LAC WORD; JMS UNSCR
/EXIT WITH NEW WORD IN AC
/37 LOCATIONS USED; EXECUTION TIME 47 ~SECS.
UNSCR

USTl
UST2

XX
CMA
DAC
RCL
RTL
AND
DAC
LAC
AND
RCR
RTR
XOR
DAC
RAR
RTR
RTR
RTR
AND
DAC
LAC
AND
RCL
RTL
RTL
RTL
XOR
DAC
LAC
AND
XOR
JMP*
XX
XX

usn

/COMPLEMENT 2 HOLD WORD
/SWITCH PAIRS OF OCTAL DIGITS

(707070
UST2

usn

(707070
UST2

usn

/MOVE LAST PAIR

(770000
UST2

usn

/MOVE FIRST PAIR

(770000

UST2
UST2

usn

/COMBINE ENDS
/ ... 2 ADD TO MIDDLE

(7700
UST2
UNSCR
/COULD USE OTHER TEMP. STORAGE

3-22

3.9 PROGRAMMING NOTES
3.9. 1 Modification of Individual Data Words
The only way to modify individual data words within a block is to:
1•

Read bloc kin to core buffer

Modify Respective Core locations

Write core buffer back into original block on tape

3.9.2 Data Transfer - Upper Boundary Protection
The WC controls all data transfers. After WC overflow, no more data transfers take place. Thus, to
protect memory when reading a block of unknown length, the we is set to the 2 ' s complement of the
difference between the initial address where data is transferred and the upper boundary.
Simi lar action prevents writing beyond a predetermined point on tape when transferring an unknown
number of words from core.

3.9.3 Special Formats on Tape
The user is cautioned always to specify an even number of words in his spec ial format. If he does not,
the control will indicate parity errors where none exist.

3.9.4 Turnaround Commands
Programming Note: When a turnaround command is issued (i.e., complement the direction bit whi Ie
the GO bit remains set to 1), the tape may not be up to speed when the point at which the command
was issued is passed (in the new direction). The tape wi II be up to speed one standard 256-word block
length after the turnaround point. Therefore, to find a block in the opposite direction, it is sufficient
to delay the turn around one block as shown in the following example:
HEAD

TAPE MOTION
REV.

+-.

STANDARD 18 81T, 256 WORD, TC 15,
PDP-9/15 BLOCK FORMAT

LI.

r<l

DATA

...
N

r<l

iii

----

FWD.

DATA

l
NOTES:

LI.

iii

r<l

1.) CONSIDER HEAD FIXED WITH TAPE MOVING PAST IT

2.) 31-R-R REVERSE BLOCK If (ie RECOGNIZED ONLY IN REV.)
3. ) 31 -F - F FORWARD BLOCKN (ie RECOGNIZED ONLY IN FWD.)
TO FIND BLOCK 32 FORWARD:
1.) SEARCH REVERSE TO BLOCK 30

2.) TURN AROUND AND SEARCH FORWARD FOR BLOCK 32
3.) BLOCK 31 MAY BE FOUND BUT BLOCK 32 IS
GUARANTEED TO BE FOUND.

3-23

09-0106

With this turn-around specification, finding blocks next to the end zones requires special handling.
Block 0 forward may be found if the tape is backed into the end zone twice before turning around.
To prevent this special end zone handling, a new formatting program must be written which provides
one block length of inter-block zone marks (no-op marks) so that the program can bounce off the end
zone and find block 0 (if the tape has the new format on it). The end zone problem is also solved for
either format by not using the block next to the end zones (block 0, 1101).
When using non-standard format tape (i .e., not 1102 blocks of 400 words), a length of tape equal
8
8
to one 18 bit, 256
(400 ) word block must pass the head before the turn-around command is issued.
10
8
This length is approximately five in. of tape; however, when calculating the required delay for a nonstandard format tape, it should be computed in equivalent standard block lengths.
Example: Turn-around delay calculation for blocks of 94

words.
10
256 words/block x 1 block delay = 2.7 block delay required
92 words block

3.10 DECTAPE SUMMARY
3.10. 1 DECtape Function Summary

O. Move

1. Search

DTF:
CA:
WC:

No Interrupt
Ignored
Ignored

DTF:

Interrupt at each block
mark
Not incremented
Incremented at each
block mark

CA:
WC:

2. Read Data

DTF:
CA:
WC:

3. Read All

Continuous Mode (CM)

Normal Mode (NM)

Function

DTF:
CA:
WC:

Same as NM

Interrupt at end of
each block
Incremented at each
word transfer
Incremented at each
word transfer
Interrupt at each
word transfer
Incremented at each
word transfer
Incremented at each
word transfer

3-24

DTF:
CA:
WC:

Interrupt at block mark where
WC overflows
Not incremented
Incremented at each block mark

Interrupt at WC overflow and
end of block
Incremented at each word
transfer
Incremented at each word
transfer

DTF:

Interrupt at WC overflow

CA:

Incremented at each word
transfer
Incremented at each word
trcinsfer

WC:

Continuous Mode (CM)

Normal Mode (NM)

Function
4. Write Data

Same as 2. Read Data

5. Write All

Same as 3. Read All

6. Write Timing
& Mark Tracks

Same as 3. Read All

7. Unused*

-------

*If used by mistake, the control gives a Select Error (SE).

3.10.2 DECtape Error Summary
Norma I Mode or
Conti nuous Mode

Function
Move

Sel ect Error
EOT

Sel ect Error
EOT
Ti mi ng Error
MK TRK Error

Read Data

Sel ect Error
EOT
Timing Error
Par i ty Error
MK TRK Error

Read All

Select Error
EOT
Timing Error
MK TRK Error

Write Data

Se Iec tError
EOT
Timing Error
MK TRK Error

Write All

Sel ect Error
EOT
Timing Error
MK TRK Error

Write Timing
& Mark Trac ks

Se I ec tError
Ti mi ng Error

3-25

3.10.3 DECtape Timing Data on Standard Format (Certified) Tape
Operation

Time

Time to answer data channel request

Up to 200 tJS* ,

Word Transfer Rate

1 18-bit word every 200 tJS*

Block Transfer Rate

1 256 word block every 53 ms*

Start Time

375 ms (±20%)

Stop Time

375 ms (±20%)

Turn Around Time (see Paragraph 3.9.4)

375 ms (±20%)

Search - Read Data Function change for present block

Up to 400 tJS*

Search -Write Data Function change for present block

Up to 400 tJS*

Read ->- Search Function change for next block number

Up to 1000 tJS*

Write ->- Search Function change for next block number

Up to 1000 tJS*

DTF to beginning of next data block

1 .7 ms*

DTF Occurrence:
Move: NM, CM

Never

Search: NM
Read Data: NM
Write Data: NM

Every 53 ms*

Search: CM

(We) X53 ms*

Read Data: eM
Write Data: CM

(# blocks) X53 ms*

Read All: NM
Write All: NM
Write Timing & Mark Tracks: NM

Every 200 tJS*

Read All: eM
Write All: eM
Write Timing & Mark Tracks: eM

(WC) X200 tJS*

* (±30%)

3-26

Chapter 4
Teletype Controls

4.1

INTRODUCTION

There are three Teletype controls avai lable in a PDP-15 System:
a.

An internal Teletype control which supports the console Teletype and is provided as part
of the PDP-15 Processor.

The LT15A Single-Teletype Control, which supports one additional Teletype and is typically used on PDP-15/30 and PDP-15/40 Background/Foreground Systems. The LT15A
Control plugs into the BA 15 Option Panel, which also houses the paper tape reader/punch
control and the VP 15 Display Control.

An LT19 Multi-Station Teletype Control which handles from one to five Teletypes. The
LT19 is more flexible than the other two in that it will also drive EIA compatible devices.
This control requires that a DW15A Positive to Negative Bus Converter be on the PDP-15
System to which it interfaces, because it interfaces to the negative logic bus.

These three Teletype controls support DEC-modified Teletype Models 33 or 35 KSR under lOT control.
The internal control and the LT19 Multi-Station Teletype Control also operate with Teletype Models
33 and 35 ASR. Each control operates in full-duplex with an 8-bit code which has one-unit start and
two-unit stop codes.
NOTE
The console terminal normally operates as full-duplex
with local copy. *
Code variations tolerated by the LT19 Control are described in Paragraph 4.3.

4.2

LT15 SINGLE-TELETYPE CONTROL

The LT15 Single-Teletype Control consists of two functional sections, the transmitter and the receiver.

4.2.1

Transmitter

The transmitter accepts the 8-bit parallel code from the computer I/O bus, converts it to serial form,
and sends it to the Teletype printer.
Each time the transmitter has finished serializing the data, it raises its flag and forces an interrupt or
either API channel 3 at trap address 74, or on the program interrupt.

* A term often confused with and equated to "half-duplex".
4-1

4.2.2

Receiver

The receiver accepts the serial code from the Teletype keyboard, converts it to a parallel code, and
makes it avai lable to the I/O bus to be fetched under lOT command by the CPU.
Each time the receiver has finished converting the serial data to parallel code, it also raises a flag
that requests an interrupt on API channel 3 trap address 75 or a program interrupt.

4.2.3

Instruction Set

The instruction set summarized in Table 4-1 is identical to that for each of the controllers in the LT19.
For programming examples, refer to the programming section of the LT19.

Table 4-1
LT 15 lOT Instructi ons

Mnemonic

Octal Code

Descri pti on

Transmitter lOTs
TSF1
TCF1
TLS1

704001
704002
704004

Skip on transmitter (Teleprinter) flag.
Clear transmitter flag.
Load transmitter buffer and transmit.

Receiver lOTs
KSF1
KRB1

4.3

704101
704102

Skip on receiver flag.
Clear receiver flag and read buffer.

LT19D MULTI-STATION TELETYPE CONTROL

The LT19D Multi-Station Teletype Control interfaces up to five Model 33 or 35 Teletypes or signalcompatible EIA devices to the PDP-15 Computer.
The LT19 connects to the PDP-15 I/O bus through a DW15A Positive to Negative Bus Converter. Operation is through the CPU under lOT control. Each LT19 consists of four subsystems as described below.

4.3.1

LT19D Multiplexer

The LT19D contains multiplexing logic for a total of 5 Teletype controls. The LT19D is also the logic
shell into which the other options plug.

4-2

4.3.2 LT19E Teletype Control
The LT19E Teletype Control operates with the following characteristics:
a.

It uses five- or eight-bit character codes. Eight is standard.

It uses a one-unit start code.

It uses 1-, 1 .5-, or 2-unit stop codes. Two is standard.

It operates in full duplex.

Transmission and reception speeds are variable, (screwdriver adjustment) to 30,000 baud.

Each LT19E will control ASR, KSR RO or SO Teletype units as supplied by DEC.

Up to five LT19E Teletype Controls can be handled by a single LT19D.

4.3.3 LT19F EIA Line Adapter
The LT19F is a group of modules which plugs into the LT19D and converts negative logic LT19E levels
to EIA levels.
Each LT19E can support one LT19F. The LT19F supplies EIA logic levels which are compatible with
certain types of Dataphones§, such as the Bell 103A. Although the LT19E, F combination supplies
the necessary data signals to the Dataphone@, it does not supply control signa Is.

4.3.4 LT19H Cable Set
The LT 19H is a cable and a set of instructions for interconnecting an LT 19D, E, F combination to another LT19D, E, F, or an equivalent PT08. The LT19H takes advantage of the LT19F Bus Drivers and
special terminating techniques to provide for a low cost interprocessor data link between PDP-9/15
and PDP-8 computers. The cable comes in five lengths:
LT19HA -

50 ft

LTl9HB -

100 ft

LT19HC -

150 ft

LT19HD -

200 ft

LT19HE -

250 ft

4.4 THE OPERATION OF THE LT19 MULTI-STATION TELETYPE CONTROL
4.4.1 LT19D Multiplexer
Figure 4-1 illustrates the structure of the LT 19D Multiplexer. The multiplexer supports up to five
LTl9E, F, and H options, which plug into their appropriate slots. The LTl9D supplies API, program
9Dataphone is a registered trademark of Bell Systems.
4-3

interrupt (PI), and skip facilities for each of the Teletype controllers. All transma flags are ORed
together and will cause interrupts on either the PI or API facility. The API traps to location 74 on
level 3. Each receive flag will also cause a PI or an API interrupt. The API traps to location 75 on
level 3. Skip requests are all ORed '~ogether to the skip line.

4.4.2 LT19E Teletype Control
Figure 4-2 illustrates the structure of each LT19E Teletype Control. Each LTl9E is capable of servicing an ASR, KSR 33 or 35 Teletype, or an equivalent peripheral. Operation is full-duplex. Each
LT19E decodes its own transmit or receive lOT and accepts or deposits parallel data onto the I/O bus
for transfer to or from the AC. When data is transmitted to the Teletype, the parallel data is strobed
into the transmitter, converted into the appropriate serial code, and passed to the printer through its
cable. When the controller is receiving data from the Teletype, the serial incoming signal is converted to an appropriate parallel code and presented to the I/O bus lines for transfer under lOT to the AC.
Whenever a word is ready to be transferred into the AC from the receive logic, the appropriate flag is
raised and an API or PI interrupt is requested. The program must identify the requesting device in a
skip chain.
Simi larly, as soon as a word has been transmitted, the transmitter raises a flag to request another word.
This flag, when it causes an API or PI request, must also be identified by the program through a skip
chain.

4.4.3 The LTl9F EIA Li ne Adapter
The addition of the LT19F option to an LT19E simply adds level converters and a driver to the output of
the LT19E. Another cable slot is available to take advantage of the EIA levels and increased drive.
These levels are compatible with those accepted by such data sets as the 103A; however, the 103A
usually requires additional control logic to be controlled by the computer. This logic must either be
supplied by another devi ce or its need removed by special wiring. The appropriate dataphone manual
should be consulted.

4.4.4 The LTl9H Cable Set
The LT19 System can be used as a low cost interprocessor communications link by interconnecting two
LT19D, E, F, or equivalent (PT08) Controllers together with the LT19H. The H option is simply a cable
and instructions on how to use it. The cable pi ugs into the output of an LT 19F. The maximum baud
rate that such a system can operate is dependent on the entire system. DEC recommends that the total
rate handled by a complete LT19 System be no more "than 30,000 baud.
4-4

TO PTOB OR LT19
DATA COMMUNICATIONS
INTERFACE L T19H

TO PTOB OR L T19
DATA COMMUNICATIONS
INTERFACE L T19H

TO PTOB OR L T19
DATA COMMUNICATIONS
INTERFACE LT19H

TO DATAPHONE
SUCH AS
MODEl: 103A

DATA COMM.~
CABLE 7005B91

}

'--_-,-_-' A25

'--_-,-_-' A27

A 26

,..----'--~

' - - - - r - - - - ' A2B

A29

r EIATELETYPE ....,
L _C~NNE1:...! _ .J

I INTERFACE L T19F I

r--I---,

r--I--,
r--I---,
r--I--,
r--I--l
I
I
I
I
I (
I

I INTERFACE L T 19F I

013

I (
)
I TELETYPE CABLE SLOT:
, TELETYPE CONTROL
UNIT CHANNEL 1
LT19E

'
L
PARALLEL RECEIVE DATA B ITS

'-----r----'

DATA SET CABLE 7005717

10-~

r ---

r- EiATELETYPE - ,

L _ C~NNE~ _--1
021

A02

)

I TELETYPE CABLE SLOT'
I
I TELETYPE CONTROL I
I UNIT CHANNEL 2 I
LT19E

--1

l-l -

r- EIATELETvPE - ,

I INTERFACE L T 19F I
L _ C~NNE':..:. _ -.J

(

)

I TELETYPE CABLE SLOT I

I TELETYPE CONTROL I

UNIT CHANNEL 3
LT19E

.J L l-~

r- -

r-- -

r ~IATELE-:ryPE --,
I INTERFACE L T19F I
L

_C~NNE~

(

Bl0

_-.J

)

I TELETYPE CABLE SLOT I

I
,
I TELETYPE CONTROL I
UNIT CHANNEL 4
I
LT19E
I

L 1-

-~-I r--

r ~IATELETYPE II
INTERFACE LT19F
L _C~NNE~I_ .J

...J

(

B18

)

I TELETYPE CABLE SLOT'

I
I
I TELETYPE CONTROL I
I

UNIT CHANNEL 5
LT19E

THE LOGICAL OR
OF ALL SKIP
REQUESTS FROM
LT 19E'S

PDP-9, 9/L
OR
PDP-15 I/O BUS

API AND
PI REQUEST
FOR
TRANSMIT

API 3 RQ
(LOC 74)
PROG INT RQ

API AND
PI REQUEST
FOR
RECEIVE

API 3 RQ
(LOC 75)
PROG INT RQ

DEVICE AND SUBDEVICE SELECT LINES

....-~

CD
5,6
7,B

SKIP RQ

L l-~r--~ ...J
THE LOGICAL OR
OF ALL TRANSMIT
FLAGS FROM
LT19E'S

OUT

SKIP
REQUEST

DEVICE
SUBDEVICE
AND
DATA LINE
RECE IVERS

WITH lOP'S 1,2,4
PARALLEL TRANSMIT

THE LOGICAL OR
OF ALL RECE IVE
FLAGS FROM
LT19E'S

DATA BITS 10-17

_04

CD
1,2
3,4

'-- SKIP RQ
' - - - API 3 RQ
PROG INT RQ
15-0237

Figure 4-1 LT19E Multi Station Teletype
Control Block Diagram

4-5

LT19 H DATA COMMUNICATIONS INTERFACE
CABLE TO PTOe OR ANOTHER L T 19F FOR
INTER PROCESSOR COMMUNICATIONS.
LT19HA· 50 FT. CABLE
L T19HB ~ 100 FT. CABLE
LT19HC = 150 FT. CABLE
L T19 HD =200 FT. CABLE
L T19 HE =250 FT. CABLE
A25 DATA SET CABLE SLOT

~ TRANSMIT ~

LEVEL
: CONVERTE R :

LA~D .ERIV~R .J

r RECEIVE- ,
1
LEVEL
I
I CONVERTER I
AND
I
I
L ~E£EI'~E!! .J

EIA LT19F TELETYPE INTERFACE
CHANNEL 1

013 TELETYPE CABLE SLOT

READER
RUN

TELETYPE TRANSMITTER

SKIP ON
TRANSMIT FLAG

TRANSMIT
FLAG

CLEAR
TRANSMIT
FLAG

CABLE ENABLE

TELETYPE RECEIVER

CLEAR KEYBOARD
FLAG AND READ

LOAD
AND
TRANSMIT

KEYBOARD (RECEIVE) FLAG
SKIP ON KEYBOARD
(RECEIVE

TRANSMIT
IOT DECODER AND TRAP

FROM BUFFERED
DEVICE, SUBDEVICE B ITS AND
IOP PULSES

FROM BUFFERED
tC BITS 10-17

TO I/O BUS DATA
BITS 10-17
L T19D TELETYPE CONTROL
UNIT CHANNEL 1
15-0238

Figure 4-2

LT19E, F, H Teletype Control Interface & Communications

4-7

4.5 THE INSTRUCTION SET
Each transmitter and receiver of each LT19E in the system has a unique set of instructions listed below:
a.

Transmitter lOTs
(1)

(2)
(3)
b.

Skip on transmitter flag (also called Teleprinter flag).
Clear transmitter flag.
Load transmitter buffer and transmit. Flag is set when the word has been transmitted.

Receiver (keyboard) lOTs
(1)
(2)

Skip on receiver flag.
Clear receiver flag and read the receiver buffer.

The PDP-15 System Software has allocated enough lOTs to accommodate up to 16 LT19Es in four LT19Ds.
Tnblp.s 4-2 through 4-5 list the lOTs that are assigned to each controller in the four possible configurations. Note that if there is an LT15A already on the system when the first LT19 is added, LT19E Unit
# 1 is not used becaUSe the LT 15 has used its lOTs.

Table 4-2
lOT Assignments for One LT19
lOT Code
Unit No.

Function

Transmitter

Receiver

704001
704002
704004

704101

SKIP
CLEAR
LOAD
CLEAR & READ
SKIP
CLEAR
LOAD
CLEAR & READ

704021
704022
704024

SKIP
CLEAR
LOAD
CLEAR & READ

704041
704042
704044

SKIP
CLEAR
LOAD
CLEAR & READ

704061
704062
704064

SKIP
CLEAR
LOAD
CLEAR & READ

704201
704202
704204

4-8

704102
704121

704122
704141

704142
704161

704162
704301

704302

Table 4-3
lOT Assignment for Two LT19s

Unit No.

LT1<J#1
lOT Code
Transmitter
Receiver

Function
SKIP

CLEAR
LOAD
CLEAR & READ

SKIP

CLEAR
LOAD
CLEAR & READ

SKIP

CLEAR
LOAD
CLEAR & READ

SKIP

CLEAR
LOAD
CLEAR & READ

SKIP

CLEAR
LOAD
CLEAR & READ

704001
704002
704004

704101

LT19#2
lOT Code
Transmitter
Receiver
704201
704202
704204

704301

704102
704021
704022
704024

704121

704302
704221
704222
704224

704122
704041
704042
704044

704141

704322
704241
704242
704244

704142
704061
704062
704064

704161

704501

704341
704342

704261
704262
704264

704162
704401
704402
704404

704321

704361
704362

704421
704422
704424

704502

704521
704522

Table 4-4
10 TAssi gnments for Three LT19s

Unit No.

Function
SKIP

CLEAR
LOAD
CLEAR & READ
SKIP

3·

CLEAR
LOAD
CLEAR & READ

LTl9# 1
lOT Code
Transmitter Receiver
704001
704002
704004

704101

LT19#2
lOT Code
Tra nsm itter Receiver
704201
704202
704204

704102
704021
704022
704024

704121

704141
704142

4-9

704401
704402
704404

704302
704221
704222
704224

704122
704041
704042
704044

704301

LT19#3
lOT Code
Transmitter Receiver

704321

704502
704421
704422
704424

704322
704241
704242
704244

704341
704342

704501

704441
704442
704444

704521
704522
704541
704542

Table 4-4 (Cont)
lOT Assignments for Three LT19s

Unit No.

Function

CLEAR
LOAD
CLEAR & READ

SKIP

CLEAR
LOAD
CLEAR & READ

LT19#1
lOT Code
Transmitter Receiver
704061
704062
704064

LTl9#2
lOT Code
Transmitter Receiver

704161

704261
704262
704264

704162
704601
704602
704604

704701

LTl9#3
lOT Code
Transmitter Receiver

704361

704461
704462
704464

704561
704562

704362
704621
704622
704624

704721

704641
704642
704644

704741
704742

704722

704702
Table 4-5
lOT Assignments for Four LT19s

LT19#4
LT19#3
LT19#2
LTl9#1
lOT Code
lOT Code
lOT Code
Unit
lOT Code
No. Function Transmi tter Receiver Transmitter Receiver Transmitter Receiver Transmitter Receiver
SKIP

CLEAR
LOAD
CLEAR
& READ
SKIP

CLEAR
LOAD
CLEAR
& READ

704001
704002
704004

704101

704201
704202
704204

704121

704221
704222
704224

704122
704041
704042
704044

704141

704161

704241
704242
704244

704701

704702

704421
704422
704424

704341

704261
704262
704264

704361

704441
704442
704444

704461
704462
704464

704721

704722

4-10

7040

704521

7040
70407040-

7040

704541

7040
70407040-

7040

704561

7040
70407040-

704562
704641
704642
704644

7040

704542

704362
704621
704622
704624

7040
70407040-

704522

704342

704162
704601
704602
704604

704321

704501

704502

704322

704142
704061
704062
704064

704401
704402
704404

704302

704102
704021
704022
704024

704301

704741

704742

704661
704662
704664

704761

704762

4.5.1

Programming Exampl es

The following program assumes that the system has one LTl9D and two LTl9Es. It also assumes that
API is available.
PI

o
JMP FLGS

/STORE LINK, PAGE/BANK MODE, PC &
/MEM. PROT. BITS.

lOT SKPA
SKP
JMS DEVA

/SKIP IF DEVICE A FLAG
/GO TO NEXT DEVICE
/HANDLE DEVICE A

lOT SKPT
SKP
JMS MULTT
lOT SKPR
SKP
JMS MULTR

/SKIP IF LTl9 TRANSMITTER FLAG
/GO TO NEXT DEVICE
/HANDLE TRANSMITTER FLAG
/SKIP IF LTl9 RECEIVER FLAG
/GO TO NEXT DEVICE
/HANDLE RECEIVER FLAG

ION
RES
JMP* PI

/TURN PIC ON
/RESTORE
/RETURN TO MAIN PROGRAM

JMS MULTR

/RECEIVER API SERVICE

JMS MUL TT

/TRANSMITTER API SERVICE

MULTR

0
DAC RAC
lOT SKPR1
SKP
JMS TTR1
lOT SKPR2
SKP
JMS TTR2
LAC RAC
DBR
JMP* MULTR

/STORE LINK, PAGE/BANK MODE I MEM. PROT. & PC.
/SAVE AC
/SK IP IF RECEIVER 1 FLAG
/GO TO NEXT DEVICE
/HANDLE RECEIVER 1 FLAG
/SKIP IF RECEIVER 2 FLAG
/GO TO NEXT DEVICE
/HANDLE RECEIVER 2 FLAG
/RESTORE AC
/DEBREAK AND RESTORE
/RETURN TO MAIN SKIP CHAIN

TTR1

/STORE LINK, PC I PAGE/BANK MODE & MEM.
/PROT. BITS.
/CLEAR FLAG AND READ RECEIVER 1
/PUT CHARACTER INTO FILE
/INCREMENT POINTER
/RETURN TO LTl9 SKIP CHAIN

lOT CLRR1
DAC* PTR1
ISZ PTR1
JMP* TTRl
TTR2

lOT CLRR2
DAC* PTR2
ISZ PTR2
JMP* TTR2

/STORE LINK, PC, PAGE/BANK MODE & MEM.
/PROT. BITS.
/CLEAR FLAG AND READ RECEIVER 2
/PUT CHARACTER IN FILE
/INCREMENT FILE POINTER
/RETURN TO LTl9 SKIP CHAIN

4-11

After determining that the interrupting flag belonged to an LTl9 receiver, the program then identifies
the particular receiver. This operation must be done wi th another skip chain, which then branches the
program to the appropriate subroutine to fetch and store the character in a file referred to by the p<)inter
PTR1 or PTR2. The pointer is incremented for the next word. In a more comprehensive program, e,]ch
file would also need a counter which would overflow when the file filled. The program would then
dump the file on tape, disk, or a similar device, or pack the words and then dump them.
MULTT

o
DAC TAC
lOT SKPTl
SKP
JMS TTT1
lOT SKPT 2
JMS TTT2
LAC TAC
DBR
JMP* MULTT

TTTl

o
LAC* PTTl
ISZ PTTl
lOT CLDTl
JMP* TTTl

TTT2

o
LAC* PTT2
ISZ PTT2
lOT CLDT2
JMP* TTT2

/STORE LINK, PC, PAGE/BANK MODE & MEM.
/PROT. BITS
/SAVE AC
/SKIP IF TRANSMITTER 1 FLAG
/GO TO NEXT DEVICE
/HANDLE TRANSMITTER 1 FLAG
/SKIP IF TRANSMITTER 2 FLAG
/HANDLE TRANSMITTER 2 FLAG
/RESTORE AC
/DEBREAK AND RESTORE
/RETURN TO MAIN SKIP CHAIN
/STORE LINK, PC, PAGE/BANK MODE & MEM.
/PROT. BITS.
/GET TRANSMITTER 1 CHARACTER
/INCREMENT PLOTTER
/DEPOSIT WORD IN TRANSMITTER AND
/CLEAR TRANSMITTER FLAG
/RETURN TO LTl9 SKIP CHAIN
/STORE LINK, PC, PAGE/BANK MODE & MEM.
/PROT. BITS.
/GET TRANSMITTER 2 CHARACTER
/INCREMENT POINTER
/DEPOSIT WORD IN TRANSMITTER 2
/ AND CLEAR TRANSMITTER FLAG
/RETURN TO LTl9 SKIP CHAIN

This routine parallels the previous coding for the receiver flag. Only the pointers and lOTs have
been changed to protect the fj I es.

4-12

Chapter 5
Line Printers

5.1

INTRODUCTION

The line printers provide the PDP-15 with a selection of hard-copy output devices. The characteristics of the five options are listed in Table 5-1.

Table 5-1
Line Printer Characteristi cs

Option

Lines/Min

Characters/Line

LP15C
LP15F
LP15H
LP15J
LP15K

1000
356
253
245
173

132
80
80
132
132

Number of Printing
Characters

64
64
96

64
96

All of these printers may use the same systems programs, allowing for the differences in line lengths
and that the 64 character printers convert all lower case character codes to upper case before printing.
The diagnostic program for the LP15C differs from the others because of differences in the printer and
its control. The LP15C differs from the other printers in the way it handles vertical format characters
(see Paragraphs 5.6 and 5.7). For most applications, however, these differences do not affect the
user.
Once started by an LPPl or LPPM command, the line printers use the 3-cycle data channel facility of
the PDP-15 to access a character buffer in core. This buffer contains up to 256 lines of characters,
each line terminated by a line feed or other control character. In this way, up to 256 lines may be
printed without further attention from the program.

5-1

5.2

CHANNEL AND BUFFER SETUP

The line printer is assigned to data channel locations 34 and 35. The word count location, 34, is not
used and can be ignored. The current address location, 35, should be initialized to the address irrmediately precedi ng the start of the data buffer. Location 35 wi II be modified as printing progress,:~s.
The data buffer area must begin with a 2-word header in the format shown in Figure 5-1.

16 17

~~~~E~ ,-:-1-LI---L--L-..L.I.....II-...L-I--I1'---L.---lIL......&..I---I-1-1-1---1-1-LI---J.-I---L-..J..I.....I

LINE COUNT

NOT USED BY
LP 15 INTER FACE

NOT USED BY
LP15 INTERFACE

0= IOPS ASCII
1 = IMAGE ALPHA

I I I I I I I I I I
NOT USED BY LP 15 INTERFACE

Figure 5-1

I I
15-0419

Data Buffer Header Format

Bit 0 and bits 9-16 of header word 1 are not used by the hardware. Bit 0 indicates the line printer
mode of operation to the program. It is set in multi-line mode and cleared in single-line mode. Bits
9-16 contain flags and parity bits used by the Advanced Monitor System software. The line count IS
used in multi-line mode (see Paragraph 5.5), and bit 17 of the first header word selects the format of
the data words to follow.

5.3

DATA WORD FORMATS

Following the header words are words containing the characters to be printed in one of the two formats
as indicated by the header.

5.3. 1

lOPS ASCII

In the lOPS ASCII format, five 7-bit character codes are contained in two consecutive words, as
shown in Figure 5-2.
In IMAGE ALPHA format, a single 7-bit character code, right justified, is located in each 18-bit
word, as shown in Figure 5-3.

5-2

13 14

I I I I I I
~

____

-J~

______

1st CHARACTER

I I

~_~

2nd CHARACTER

I I I I
3rd

I I I I I I I
3 rd

I I I I I I LO

4th CHARACTER

CHARACTER
NOT USED

15-0420

Figure 5-2

5/1 ASCII Packing Scheme

I I I I,I I

II I I

1 st CHARACTER

NOT USED

10 11

I I I I I I I I

III I

2 nd CHARACTER

NOT USED

15-0421

Figure 5-3

5.4

IMAGE ALPHA Format

SINGLE LINE OPERATION

On receiving the LPP 1 lOT (706541 ), the printer prints a single I ine of text. First, two consecutive
8
data channel requests are used to obtain the header words. The I ine count is ignored, but the data
format selection bit is saved. The data channel is again used to bring in the first two data words (for
efficiency, even IMAGE ALPHA mode words are obtained in pairs). According to the selection format, the characters are unpacked and sent one-by-one to the printer.
When the data buffer is exhausted, another pair is brought in and the process continues.
As each character is unpacked, it is checked for being one of the vertical control characters (such as
Line Feed) listed in Paragraphs 5.6 and 5.7. When one is found, it terminates the current line,
causes the appropriate control action (advancing the paper), and sets the Done flag. If more characters are received than the line can hold (80 or 132), with no control characters, the Line Overflow
flag is set, indicating the error (see Paragraph 5.8.3).

5-3

5.5

MULTI-LINE OPERATION

On receiving the LPPM lOT (706521 ), the printer enters multi-line mode. This mode of operation is
8
similar to single-line operation, except that the line count field of the header word is stored in the
line counter. Each control character encountered causes the counter to decrement, and only when it
reaches 0 is the Done flag raised and printing terminated. Thus, up to 256 lines may be printed from
one LPPM.
Note that only lines of print are counted. Every control character (except horizontal tab) counts a~
one line, even though it may advance the form from an entire page to not at all.

5.6

LP15C CONTROL CHARACTERS

The LP15C recognizes HT, CR, LF, FF, OLE, DCl through DC4, and ALT MODE as control characters. Their functions are as follows:

5.6. 1

Horizontal Tab (HT)

The HT control character does not end a line or decrement the line counter. Permanent tab stops ar,~
located every eight columns, starting with column 9. Receiving an HT causes the controller to genu
erate spaces until the next stop is reached. At least one space will be generated. Thus, if the sequence A HT B is received, and the A is printed in column 7, the B will be printed in column 9. If
the A appears in column 8, however, the B cannot be in column 9, and is put instead in column 17.

If more than 128 characters have been sent to the printer when the tab is received, there are no more
stops on the I ine and the ILL HT flag is raised to signal the error (see Paragraph 5.8).

5.6.2

ALT MODE and Carriage Return (CR)

ALT MODE and CR have identical effects; they terminate the current line, start another at the left
margin, but do not advance the paper. This allows for overstriking. The line counter is decremented.

5.6.3

Vertical Format Unit (VFU) Characters

All the remaining control characters use the VFU located in the printer to govern form advance. This
unit contains a punched paper tape loop, synchronized with the paper feed. Each of the various control characters selects one of the eight channels across the paper tape, and the form is advanced until
a hole is sensed in that channel. Line Feed, for instance, uses channel 8 which normally has a hole
in every line except near the paper perforation which should be skipped over. See Table 5-2 for other
assignments. All of these characters decrement the line counter by 1.

5-4

Table 5-2
Control Character Assignments

ASCII

Character

VFU Channel

Conventional Meaning*

012
013
014
020
021
022
023
024

LF Line Feed
VT Vertical Tab
FF Form Feed
OLE Devi ce Control
DC 1 Device Control
DC2 Devi ce Control
DC3 Devi ce Control
DC4 Devi ce Control

8
7
1
2
3
4
5
6

1 line
1/3 page
Top of next page
1/2 page
2 lines
3 lines
1 line
1/6 page

* Using "normal" VFU tape, other spacing may be obtained by using specially prepared
VFU tape.

5.7

CONTROL CHARACTERS FOR LP15, F, H, J, AND K

Because these printers lack a mechanical VFU, some of the control characters must be handled in a
different manner. HT, CR, and ALT MODE have identical effects to those described in Paragraph
5.5. Form Feed causes the paper to advance to the top of the next page. The rest of the control
characters cause the form to advance a fixed number of lines:

5.8

Character

Number of Lines

LF Line Feed
OLE Devi ce Control
DC 1 Device Control
DC2 Device Control
DC3 Device Control
DC4 Device Control
VT Vertical Tab

1
30
2

3
1
10
20

lOT INSTRUCTIONS AND FLAGS

Table 5-3 describes the lOTs used in normal printer operations. Other lOTs, useful only during maintenance, are described in the LP15C and LP15F Maintenance Manuals.

5.8.1

Error Flag

This flag is raised by an inclusive OR of the LP Alarm, Line Overflow, Illegal HT, and Interlock
flags. When the Error flag is raised, the printer prints out all characters received before the error, if
possible, and the Done flag is raised.

5-5

Table 5-3
LP15 lOT Instructions

Mnemonic

Octal Code

Descri pti on

LPP1

706541

The line printer prints a single line of text, as
described in Paragraph 5.4.

LPPM

706521

The line pri nter enters mu Iti -I i ne mode, as descri bed in Paragraph 5.5.

LPSF

706501

Skip if done or error. Skips the following instructi on if the printer's Done or Error flag is
set.

LPEI

706544

Enable interrupt system. Connects the printer to
the PDP-15 priority interrupt system. Either the
Done or Error flag will cause an interrupt if enabled. If the API is in use, the interrupt wi II be
on level 3, channel 56.

LPCD

706621

Clear Done flag.

LPCF

706641

Clear status register and Error flag.

LPRS

706642

Read status register. This lOT reads into the AC
a register made up of the following system flags:
Bit

Flag

0
1
2
3
4
5
6

Error
LP Alarm
Line Overflow
Illegal HT
Busy
Done
Interlock

The following paragraphs describe each of the status register flags.

5.8.2

LPAlarmFlag

This flag indicates some error condition in the printer itself. This may be due to printer power off,
insufficient paper supply, printer off line, printer yoke open, or some electrical or mechanical malfunction.

5.8.3

Line Overflow Flag

This flag is set when more than 132 (or 80) characters are sent in a single line (i .e., without vertical
control characters). A software error is indi cated .

5-6

5.8.4

Illegal Horizontal Tab (ILL HT)

This flag indicates that an HT has been sent after the last tab stop on the line has been passed. This,
too, is a software error.

5.8.5

Busy Flag

This flag is set at the start of a print operati on and is cleared by raising the Done flag.

5.8.6

Done Flag

This flag is set by encountering the first control character in single-line mode, the final control character in multi-line mode, or completing printing following an error. It is cleared by lOT 6621
(LPCD).

5.8.7

Interlock Flag

This flag is set if there is no printer connected to the controller. Check the printer cable connections.

5.9

PROGRAMMING EXAMPLE

This programming example does not use the interrupt system, and assumes that interrupts are disabled.
At START, the Form Feed contained in IBUFF is printer. This ensures a fresh form, and causes the
Done flag to raise upon completion. PRINTM may now be called any number of times; its argument is
a pointer to the buffer containing the characters to be printed. After calling PRINTM, the program
may continue without waiting for the printer to complete its ')peration. PRINTM waits, if necessary,
for the last task to finish, checks for errors, then starts the new print operation.
.LOC 35
IBUFF-1
START

/INITIALIZE DATA CHANNEL ADDRESS

.LOC 200
LPP1

/PRINT FORM FEED FROM IBUFF

JMS PRINTM
CBUFF-1

/CALL TO MULTIPLE PRINT ROUTINE
/POINTER TO CHARACTER BUFFER

HLT

5-7

IBUFF
CBUFF

000001
000000
000014
033000
000000

/HEADER- IMAGE ALPHA FORMAT
/HEADER
/CODE FOR FORM FEED
/HEADER, 33 LINES, lOPS ASCII FORMAT
/HEADER

/DATA WORDS--33 PRINT LINES

PRINTM

o
LPSF
JMP .-1
LPRS
SPA
JMS ERR
LAC* PRINTM
DAC* (35
LPPM
ISZ PRINTM
JMP* PRINTM

/MUL TIPLE LINE PRINT ROUTINE
/CHECK IF PRINTER DONE
/NO, WAIT
/DONE, CHECK FOR ERROR (BIT 0)
!rYPE OUT ERROR MESSAGE
/NO ERROR, GET BUFFER POINTER
/LEAD DATA CHANNEL ADDRESS
/ST ART PRI NTER .
/RETURN TO CALL +2

5-8