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Dead medium: Telegraphic Paper Tape; Digital Paper Tape; Baudot Code; Dead Encoding Formats; ILLIAC; TTY
From: minow@apple.com (Martin Minow)

Source(s): Encyclopedia Britannica, 11th Edition, volume 26, page 521; "A Guide to Illiac Programming" by L. D. Fosdick, Digital Computer Laboratory, University of Illinois, Urbana, Illinois, 1961; personal experience

(((bruces remarks: Martin Minow is a senior software engineer at Apple Computer He is a 35 year programming veteran who worked on the original Illiac computer, as well as TRASK, the PDP-11, the DECtalk speech synthesizer, and many other projects.)))

Paper Tape Oddities

by Martin Minow

Introduction

This note was prepared for Bruce Sterling's Dead Media mailing list. It describes a variety of encoding formats used for computer processing and typesetting from, roughly, 1950 through the early 1970's.

Paper tape formats, especially the five-channel format generally called Baudot, were used for telegraphy since 1874. See the article on Telegraphs in the Encyclopaedia Brittanica, 11th Edition for more details on early telegraphy.

Baudot is a 5-unit start-stop code. (It is described in International Telegraph Alphabet No. 2, and defined in International Telegraph and Telephone Consultative Committee (CCITT) Recommendation F.1, Division C.) In addition to Baudot, I will describe an incompatible variant used by Illiac, a first-generation computer designed in the late 1940's and in production from 1952 through 1962 at the University of Illinois at Urbana- Champaign.

Operation

The Encyclopaedia Brittanica, 11th Edition, vol 26, p. 521 describes the Baudot code in its Telegraph article, here somewhat paraphrased.

The system brought out in 1874 by Emile Baudot and since considerably developed is a multiplex system giving from two to six channels on one wire, each channel giving a working speed of thirty words per minute. Each channel consists of a keyboard and receiver, both electrically connected to certain parts of the distributor (which multiplexes channels).

The keyboard has five keys similar to those of a piano. The letters and figures are obtained by the different combinations, which can be forced by the raised and depressed keys. In the unforced position, a negative battery is connected to the distributor, and in the depressed position, a positive battery. At regular intervals a rotating arm on the distributor connects the five keys of each keyboard to the line, thus passing the signals to the distant station. There the signals pass through the distributor and certain relays, which repeat the currents corresponding to the depressed keys, and actuate electromagnets in the receivers.

Each receiver is provided with five electromagnets, corresponding to the five keys of the keyboard. The armatures of the electromagnets can thus repeat the various combinations for all the signals. When a combination of signals has been received, and the armatures have taken up their respective positions, a certain mechanism in the receiver translates the position of the five armatures into a mechanical movement. This movement lifts a paper tape against a type-wheel. The type-wheel prints the corresponding letter on the paper tape.

The movement for any particular combination of armatures can only take place once per revolution of the type-wheel, and at one particular place. The signals must therefore be sent at regular intervals. To ensure this being done correctly, a telephone or time-tapper is provided at each keyboard. It warns the operator of the correct moment to depress his keys.

The Murray system extended Baudot's design, to use mechanical typewriter-like keyboards, and tape perforators that could control a printer. This printer is purely mechanical, and its speed is very high. An experimental printer constructed about the middle of 1908 by the British Post Office, operated successfully at the rate of 210 words (1260 letters) per minute. (This corresponds to about 21 characters per second.)

Baudot, or International Alphabet

Letter Figure Hex | 16 8 . 4 2 1

BLANK BLANK 00 | .

E 3 01 | . *

LF LF 02 | . *

A - 03 | . * *

SPACE SPACE 04 | . *

S BELL 05 | . * *

I 8 06 | . * *

U 7 07 | . * * *

RETURN RETURN 08 | * .

D $ 09 | * . *

R 4 0A | * . *

J ' 0B | * . * *

N , 0C | * . *

F ! 0D | * . * *

C : 0E | * . * *

K ( 0F | * . * * *

T 5 10 | * .

Z " 11 | * . *

L ) 12 | * . *

W 2 13 | * . * *

H # 14 | * . *

Y 6 15 | * . * *

P 0 16 | * . * *

Q 1 17 | * . * * *

O 9 18 | * * .

B ? 19 | * * . *

G & 1A | * * . *

FIGURE FIGURE 1B | * * . * *

M . 1C | * * . *

X / 1D | * * . * *

V ; 1E | * * . * *

LETTER LETTER 1F | * * . * * *

The Illiac Paper Tape Format

A computer programmer, looking at Baudot, is struck by how the letters and numbers are not ordered by their binary numeric representation. The engineers designing Illiac solved this problem by reassigning the digits, so they had a direct numeric representation on paper tape.

This had several effects. First, the letters were scattered from hell to breakfast. Second, the mechanical teletywriter keyboard layout appeared, for all practical purposes, to be completely random. It was easy to rename the keycaps, but difficult to redesign the teletypewriter mechanism itself.

Furthermore, the hexadecimal digits representing the values 10 through 15 were not the modern A through F but "K, S, N, J, F, L."

The following table describes the Illiac paper tape code. It is taken from "A Guide to Illiac Programming" by L. D. Fosdick, Digital Computer Laboratory, University of Illinois, Urbana, Illinois, 1961, page 99.

The five-channel paper tape has 2 symbol holes, a sprocket hole shown by '.', and three symbol holes.

Associated with each sprocket hole, reading across the width of the tape, is a 5 digit binary number. Three digital positions are to the right of the sprocket hole, and two are to the left. The weight assigned to each position is indicated by the numbers at the head of the table. The fifth, leftmost, digital position is regarded separately, and does not truly have a weight of 16, as the fifth binary position ordinarily would have. The fifth- hole position is used to identify some letters of the alphabet, special symbols, and certain printing operations such as "carriage return and line feed."

Symbol | S 8 . 4 2 1

0 | .

1 | . *

2 | . *

3 | . * *

4 | . *

5 | . * *

6 | . * *

7 | . * * *

CRLF | * *

Symbol | S 8 . 4 2 1

8 | * .

9 | * . *

K | * . *

S | * . * *

N | * . *

J | * . * *

F | * . * *

L | * . * * *

As you can see, the printed digits are directly associated with their binary representation. This greatly simplified the paper tape reader program and Illiac programming. This simplicity was essential for a computer with only 1024 words of memory. The complete paper-tape code is given in Appendix 6 of Fosdick's book (page 272).

In the following table, "letters-shift" and "numbers shift" symbols are shown together. Note, also, that the '$' symbol may also be used as a tab character; however "only one printer has a tabulation mechanism."

Symbol | S 8 . 4 2 1

A ) | * . * *

B ( | * . * *

C : | * * . * *

D $ | * . *

E 3 | . * *

F F | * . * *

G = | * * . *

H ' | * * . *

I 8 | . *

J J | * . *

K + | * . *

L L | * . * * *

M . | * * . *

N N | * . *

O 9 | * . *

P 0 | .

Q 1 | . *

Symbol | S 8 . 4 2 1

R 4 | . *

S - | * . * *

T 5 | . * *

U 7 | . * * *

V , | * . * *

W 2 | . *

X / | * . * * *

Y 6 | . * *

Z W | * * . * *

CRLF | * . *

Letters | * . *

Space | * * . * * *

Delay | * * .

Figures | * * . * *

Delay | * .

|

|

The motivation for the strange Illiac format becomes understandable when you combine the constraints of the paper tape format, the standard Baudot encoding, and the engineer's need to have direct correspondence between the digits 0 through 9. For example, the 'R' is the letter- shift character for digit 4 in both encodings, even though they have different binary encodings.

Other Uses

Baudot, or RTTY, has widely used by amateur radio operators.

Surplus Baudot code teleprinters with built-in modems were also distributed to hearing-impared individuals who could then communicate independently. See the Disability Resources Page for more information on TTY communication. United States telephone corporations are still required by law to provide TTY communication services.

Martin Minow (minow@apple.com)