ASR-33 Teletype

Introduced about 1963, Teletype Corporation's ASR33 was a very popular model of teleprinter. Designed for light-duty office use, it was much flimsier (and cheaper) than its heavy duty cousin, the Model 35ASR.

The printing mechanism was an array of levers, cranks, and a type cylinder wheel on a movable carriage. These mechanical parts printed up to 10 characters per second. Printing was limited to the upper case ASCII character set.

"ASR" stood for "automatic send and receive." The ASR 33 had a built in paper tape reader and tape punch (8 hole ASCII including one parity bit). It could print and read or punch tape at the speed of 10 characters per second. The ASR33 tape reader was purely mechanical; 8 spring loaded fingers would be thrust into the tape (one character at a time) and an assortment of rods and levers would sense how high the finger rose, which told it whether there was a hole in the tape at that position.

The ASR32 was a similar device, but used five hole Baudot code. The otherwise identical KSR33 and KSR32 models ("keyboard send and receive") lacked the paper tape reader and punch. RO33 and RO32 models ("receive only") had neither keyboard nor reader/punch.

More expensive Teletype systems used photo readers that used light sensors to detect the presence or absence of punched holes in the tape. These could work at much higher speeds (hundreds of characters per second). More sophisticated punches were also available that could run at somewhat higher speeds; Teletype's BRPE punch could run at 60 characters per second.

In the photograph, the two holes appearing on the upper right side of the teletype are for an optional acoustic coupler to an internal modem. The paper tape reader is on the left and some paper tape can be seen hanging down below the reader.

In the 1970s early minicomputers frequently had a 20mA current loop interface for connection to a KSR or ASR teletype. As the price of electronic (rather than mechanical) terminals dropped this was gradually replaced by an RS-232 interface.

Basic CRT-based computer terminals which could only print lines and scroll them are often called glass teletypes or dumb terminals to distinguish them from more sophisticated devices. Teletypes were gradually replaced in new installations by dot-matrix printers and CRT based terminals in the mid to late 1970s.

External links

* [http://www.columbia.edu/acis/history/teletype.jpgPhoto of an ASR33]
* [http://keyboards.jargon-file.org/#ASR33 Keyboard layout for Windows that simulates the ASR33 keyboard]