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MAINDEC-08-D2A

December 1965

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Document:	maindec-08-d2aa-d
Order Number:	MAINDEC-08-D2A
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Pages:	10
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OCR Text

IDENTIFICATION

Produc+ Code:

MAINDEC-08-D2AA-D

Product Name:

PDP-8 Teletype Reader Test

Date Created:

April

Maintainer:

Diagnostic Group

Author:

M. Horovitz

Previous Code:

MAINDEC 810

1`2,1965

BEE

MAI.NDEC-08-D2AA-D

ABSTRACT.

MAINbE.C-08-D2AA tests the performance of the Teletype Model 33 Perforated Tape Reader,
using the reader to scan a closed-loop I.est tape punched with alternating groups of the charact.er codes

000 and 377.
Each character is rested for bits either dropped or gained while reading; each group of

characters is checked for characters missed entirely or read more than once.
An Auxiliary Punch program (see paragrciphs 3.3. I .and 5.1.2) is qvailable to produce the

tape used in the test.

`REQulREMENTS

St,orage
MAINDEC-08-D2AA requires the RIM Loader to be in memory locations 7756-7776 octal .
The test progrtan? occupies memory loc.qtions 0001-OT 22 octal .
Equipment

Teletype Model 33 Keyboard/Printer with tape punch and reader.

USAGE

3.1

RIM Loader must be in memory.
Wi+h +he Tape Reacler off, se+ the SWITCH REGISTER +o 7756 oc+al .

Press LOAD ADDRESS,

then START.

Pl.aee,the MAINDEC-08-D2.AA T.est Tape in the Teletype reader, and start the reader.

When loading is completed, turn off the reader and remove the program tape.

3.2

Switch Settings
Starting Addresses (in octal):

0100

Auxiliary punch program

0001

Reader Test program

3.3

St.qr+-up a.nd/8r Entry

3.3.1

Au.;iliary pinch p;ogram -The Teletype reader must be off.

0100 ocfal.s.

Press LOAD ADDRESS, then START.
`

Set the SWITCH REGISTER to

Turn on the Teletype punch.

Allow the program to punch severcil feet of tape, then stop the computer, turn off the punch,

and tecir off the tape .

MAINDEC-08-D2AA-D

3.3.2

Reader Test program -Place the leading end of the test tape, (produced bythe Auxiliary

Punch program) in the Teletype reader.

then START.

Set the SWITCH REGISTER I.o 0001.

Press LOAD ADDRESS,

Turn on t.he reader.

3.4

Errors in usage

3.4.1

Error stops

Error

C(MA)

00250055

Cause of Error

The last character read was incorrect. The AC contains the erroneous
code. The correct character is found on the test tape in the sL££gnd one
pas+ the read sta+ion. If +his is 337, +he AC shows dropped bits as O's.
If the correct code is 000, the AC shows spurious bits as 1 's.

Character count is incorrect.

The program has iust finished reading a

group of 337 codes; the number of characters in this group is not identical
to the number of characters in the preceding group of 000 codes. The number in the AC is the difference between the counts. The error was probably the result of a missed character or a failure in the transmission of a

pulse from the reader buffer +a the AC.

Recovery from Such Errors

Recovery

Error

To rest.art the test, press CONTINUE.

To restart the test, press CONTINUE.

RESTRICTIONS

None.

The test will operate on a PDP-8 of minimum configuration.

DESCRIPTION

5.1

Discussion

5.1.1

The Reader Test program -MAINDEC-08-D2AA tests the perforated tape reader associated

with the Teletype Model 33 Keyboard/Printer.

The program causes the reader lo continuously scan a

specially prepared i.es+ tape and checks for +he accurate transmission of each charac+er from +he read

heads into the AC.

A dropped bit (0 transmitted instead of a 1 ) or a Spurious bit (1 transmitted instead

of a 0) will cause the program to hall.

The erroneous character will appear in the AC.

`.-I

MAINDEC-08-D2AA-D

The loop contains a sequence of character codes, each of which is either 000 (no holes
punched) or 377 (all holes punched).

The characters are arranged as follows: A group of 000 characters

is foHowed by a group containing an identical number of 377's.
groups may vary.

The size of each successive pair of

A sample sequence is shown in section 5. T .2.

The test program counts the number of characters in a group of OOO's and then decreases
+his count while reading the subsequent group of 377's.

reduce to 0.

At the end of that group, the eounr should

If it does not, the test halts; the probable cause of ei.ror is +hat the reader missed a

character entirely.

`..`

If the count is still positive, the missed code was 377; if negative, it -was 000.

In either case, the test will be restarted when the operator presses CONTINUE.

5.1.2

Auxiliarypunch program-This isa separate program within MAINDEC-08LD2AA which

punches a length of tape for use in f.he main test program.` This tape contains the following pattern,,

repeated as many times as desired:

Tape
Pattern

Ocfc'l
Equivalent

There is no halt in the punch program.

Once started, it will run continuously until the

operator stops i+, allowing him I.o produce as long a testtape as he wishes.

5.2

Examples and/or Applications

Although the primary use of MAINDEC-08-D2AA is fo test the `performance of the .tape

read heads and the transmission lines between lhe read heads, reader buffer, and AC, it can also be

used as a quick check of the tape advance mechanism.

If, aft.er a few feet of tape have been read,

there is evidence of wear or of enlarged feed holes, there may be a mechanical malfunction. Normally
the test would be used as a preventive maintenance procedure or when program operation tends I.o.

indicate reader malfunction.

L.I
3

MAINDEC-08-D2AA-D

METHODS

At the start of the lest, the program insures that the tape is positioned at I.he beginning of

a group of 000 characters; it examines the tape until a nonzero character appears, then, continues to
read until the first 000 character is read.

incremented.

The next character is read and tes+ed.

to determine if the two are identical .
and rested.

This character is tested for spurious bits and the counter is

11 is then compared with the previous character

If so, the count is incremented, and the next character is read

This procedure cont.inues until a character is read which differs from t.he previous one.

At this point, two possibilities become apparent:

If the previous character was OOO\and the current one is 377, the character count is decremented, and the next character is read and rested.

The t'est proceeds until the next change of character.

If I.he previous character was 377 and the current one is 000, the count is checked for missed

characters.

If the count is 0, there has been no error and the test continues.

the program halts with the value of the count in t.he AC.

If t.he count is not 0,

If a line was missed while reading OOO`s,

there will be an excess of 377's, and the count will be negative.

If a line was missed while reading

377's, lhere will be a deficiency of these, and the count will be positive.

In each case, the absolute

value of the count corresponds to the number of lines which were missed.

Example:

While reading a group containing I.hree ooo characters, the reader
failed to transmit the third one, so that t.he count remained at 2.

The subsequent group of +hree 377's was reacl correctly, so +hat

when the last of these was read, the count became -I.

The program

stops with 7777 (= -1 ) displayed in the AC lights.

PROGRAM LISTING
/MAINDEC-08-D2AA:

PDP-.8 TELETYPE READER TEST

/TAPE PHASING SEQUENCE
/IS CHARACTER NON-ZERO?
/NO-KEEP LOOKING

/YES-LOOK

FOR

FIRST ZERO

/CHARACTER

MA.INDEC.-08.-D2AA-D

/START

WITH

ZERO

zlNITIALlzE

LINE

/HOLDS

ISZ

LINCT

JIYIS

READ

/INCREMENT

CHARACT,ER

couNTm
LINE

COUNTER

WHEN

'C(TEN)=®

/STORE CHARACTER JUST READ
/TEST
FOR
NON-ZERO CHARACTER
/RESULT SHOULD BE 8 IF C(TEN)=377

DCA

TEN

TiD
IAD

CTI
TEN

SNA

CLA

/IS

Jpr'P

/YES

TAD

TEN

/NO

IT?
-

CHECK

FOR

ZERO

/IT IS

-G0

/ERROR:

CONTAINS

SNA.

JMP
®®25
•®826

HLT
Jlr'1P

/TRY

TAD

TEN

/PFiEPAfiE

SZA

CLA

1©7©

TAD

,OTl+1

1871

3¢55

TAD
DCA

CTl+2
.+2

/IS IT ZERO CHAR.?
/NO -SET NEXT SKIP T0
/IF ZERO,
NEXT SKIP IS

1'075
8©®8

TAD

5947

JMP

®@27

1©72

885®
'89 6 i

7648

®862
©®33
©%54
©®55

@856

0K,

/TEST

TAD

TEN

SNA

CIA

5815
1874

JMP

7 21 4 I

CIA

7©49

CMA

SAME,

TAD

FOR

/EXAil]INE

COM

•76`5a

1©72

/GET

c(pREv)/c(TEii])

STATE
LINE

HERE

ccmp.

SNA
SZA

0F PREVI\OuS

CHAR.

COUNT

ONLY

C(PREV)=C(TIM)

LINCT

3874

DCA 'LINCT

5®14

JMP

D+i

/IF

C(TEN)=8,

/IF

C(TEN)=377,

/NEXT

/GET

TEN
C.LA

ERRONEOUS

AGAIN

/IS

DECREMENT

COUNT

CHARACTER

HERE

ONLY

IF C(PREV)IS

NOT=

COUNT

CONE

C(TEN)=g?

COMr5
LINCT

/YES

/C(LINCT)=Zj,

CONTINUE

HLT

/ERROR:

COUNT

JMPA

/AC

/NO-G0

DECFiEMENT

-CHECK

LINE

CONTAINS

AND

COUNTER

INCOR\RECT.

C(LINCT),

T,RY

AGA.IN

MAINDEC-08-D2AA-D

/CHARACTER

/SAVE
/WAIT

READING

PREVIOUS
LOOP

SUBROUTINE

CHARACTER

.-1
I

READ
/Z'S COMPLEMENT OF 377 FOR CHAR.
/SNA BIT FOR COMPARISON TEST

74®1
1©

SZA

CLA

/COMPARISON

TEST

©®©©
®©®8

TEN,
PREV,

/HOLDS

CURFiENT

/HOLDS

®8@8

LINCT,

/CHARACTER

COUNT

/AUXILIARY

PUNCH

*1©©

INSTRUCTIO(\'

CHARACTERl

CHARACTER

PROGftAM

PAT=TEM
®180

CLL

CLA

Z I .L7 I

CT2
LINCT

/LINE

®1©2

TAD
DCA

91,2)5

TAD

CT2+i

/PATTERN

®1©4

DCA-PAT

81©5

TAD

2„©6

HAL

DCA

COUNT

FOR

TAPE

PATTERN

SELECTOR

PAT

TEN

SZL
/IF
/IF

CMA

.-I

ISZ

8117
•2' i 2 8

JMP 88

®122

PUNCH
PUNCH

CLA

0116

8121

C(L)=l,
C(L)=g,

JMP

7764
25©7

LINCT
AA

/COUNT

CHARACTER

/NOT FINISHED
/FINISHED -REPEAT
/CHARACTER

/PATTEFiN

PA.TTERN

COUNT

SELECTOFi

A
AA

a
88
COM

CTI

CT2
D

ERRI

ERR2
LI N C I
0K
PAT
PREV
READ
SAME

TEN

MAINDEC-08-D2AA-D

DIAGRAMS

8.1

Flow charts

8.I.I

Character Read Routine

8.I.2

Auxiliary

Punch Program

u.I
7

MAINDEC-08-D2AA-D

8.1.3

Reader Test program

•.L
8