The Teletype Model ASR-33 was a very popular model of teleprinter. Introduced about 1963 by Teletype Corporation and designed for light-duty office use, it was less rugged and less expensive than earlier Teletype machines or its heavy-duty cousin, the Model 35-ASR.
The Model 33's printing mechanism was an array of levers, cranks, and a type cylinder on a movable carriage. The Model 33 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 and used eight-bit ASCII code including one parity bit. It could print and read or punch tape at the speed of ten characters per second. The ASR-33 tape reader was purely mechanical; eight spring-loaded probes would be thrust into the tape, one character at a time, and an assortment of rods and levers would sense how high the probe rose, which told it whether there was a hole in the tape at that position.
The paper tape reader is on the lower left and the paper tape punch is directly above it. As it exits the machine, the tape passes under a triangular lip that allows the tape to be easily torn by lifting against the sharp edge of the lip.
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The ASR-32 was a similar device, but used five-hole Baudot code and had a three-row keyboard. The otherwise identical KSR-33 and KSR-32 models ("Keyboard Send and Receive") lacked the paper tape reader and punch. RO-33 and RO-32 models ("Receive Only") had neither keyboard nor reader/punch.
The Model 33 could accommodate an internal modem with an optional acoustic coupler to the right of the keyboard. This was replaced by a blank panel on units operating on a current loop.
In the 1960s and early 1970s minicomputers typically had a 20mA current loop interface for connection to a 33 ASR teletype used as a console terminal and paper tape program loader. A Model 33 cost about $700, much less than early video terminals such as the Tektronix 4010. As the price of electronic terminals dropped in the late 1970s, the current loop was gradually replaced by an RS-232 interface.
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.
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.
