Model ASR33 Teletype

From Trammell Hudson's Projects

Model ASR33 Teletype

ASR33 teletype printing a calendar
ASR33 with top cover removed

A friend of mine has an excellent condition Teletype Model 33 ASR that I've built an 110 baud 7-bit current loop adapter with an auto-on power relay. We've used it to read paper tapes and as a serial console for a Raspberry Pi. The bit encoding and timing is purely mechanical with a rotor very reminscent of the forty year older Model 15 Teletype.


The Teletype Model 33 is an electromechanical teleprinter designed for light-duty office. It is less rugged and less expensive than earlier Teletype machines. The Teletype Corporation introduced the Model 33 as a commercial product in 1963 after being originally designed for the US Navy. There are three versions of the Model 33:

  • the Model 33 ASR, (Automatic Send and Receive), which has a built in 8-level punched tape reader and tape punch;
  • the Model 33 KSR (Keyboard Send and Receive), which lacks the paper tape reader and punch;
  • the Model 33 RO (Receive Only) which has neither a keyboard nor a reader/punch.

The Model 33 was one of the first products to employ the then new ASCII code. A companion Model 32 used the more established five-level Baudot code. Because of its low price and ASCII-compatibility, the Model 33 was widely used with early minicomputers.

Print head

ASR33 type wheel

Selector bars

ASR33 rails

Very similar to the design of the Model 15 teletype.

Paper tape reader

ASR33 papertape reader / puncher

The paper tape punch is visible in the background.