Teletype Machines

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Teletype machines from the Teletype Corporation, Skokie, Illinois. Left: ASR33 without built-in telephone. Right: The Teletype Corporation ASR 33 Teletype (1967). 110 bps, 8-level ASCII encoding (uppercase only); 4-row Automatic Send Receive, 101C Dataset (modem); Bell System TWX service. Teletypes were nearly 100% mechanical with no electronics to speak of and required regular maintenance and lubrication.

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The ASR33 (pictured) above was by far the most common Teletype model, although we did have an ASR37 in the machine room for some time (NEED PHOTO). Paper is roll-fed. The paper tape device would be used for sending recorded keysrokes or other data, or it could be used to capture incoming material (thus Automatic Send Receive). The KSR models (Keyboard Send Receive) lacked the paper tape reader/punch. Most non-IBM computers of the 1960s until the mid-1970s -- such as DEC PDP-xx minicomputers -- came with a Teletype console terminal. The 33 and 35 models were uppercase only; the 37 model had upper and lower case. To this day certain characteristics of the Telepype live on in the 110-"baud" 2-stop-bits configuration required to synchronize with the Teletype printing mechanism, still supported by most communication devices and software.

Pushing the keys was good exercise; the keys traveled a good half inch before making contact, and resistance was considerable. The Answerback reply was programmed by breaking teeth off a plastic gear. Teletypes in one form or another go back to about 1907. They were used originally as automatic Telegraph and Telegram machines. Teletypes reached their familiar mature form in the 1920s.

Third Photo: Carl Friend (and also correct identification of the Model 37). Other photos: found on the Web.

CLICK HERE for a large color photo from the Catholic University of Leuven. CLICK HERE to view Dennis Ritchie and Ken Thompson using Teletype consoles of PDP-11 computers during the early development of the Unix operating system.


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Frank da Cruz / fdc@columbia.edu / Columbia University Computing History / Jan 2001 - Jan 2003