The ASR-33 was an electromechanical terminal that let a person type to a computer, print incoming text, and read or create punched-paper tape. Its name means “Automatic Send and Receive”; unlike a modern terminal, it worked at a measured 10 characters per second and combined keyboard, printer, tape reader, and tape punch in one machine.
What an ASR-33 terminal did
Teletype introduced the Model 33 commercially in 1963, following an original U.S. Navy design. A University of Queensland Physics Museum example is dated circa 1970. The ASR version integrated four functions: an uppercase keyboard, an impact page printer, a punched-tape reader, and a tape punch. It could send characters from the keyboard or tape, and receive data by printing it, punching it onto tape, or doing both.
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“Automatic” did not mean autonomous computing. The ASR’s answer-back mechanism could respond to an inquiry character so a sender could check the recipient and confirm an unbroken transmission. The University of Queensland describes the Model 33 as one of the early products to use seven-bit uppercase ASCII, and notes its widespread use with early minicomputers. University of Queensland Physics Museum
How the ASR-33 worked
Keyboard, printer, and paper tape
The operator typed characters on the keyboard. In remote use, a connected computer could direct the terminal to print received text, perforate tape, or print and perforate simultaneously. The tape reader could send a previously prepared sequence to the computer. This made the terminal both a live input/output device and a way to store and replay data without typing it again.
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The one-inch tape recorded characters as holes in a paper strip. The period technical manual specifies 72 characters per line, 10 characters per inch, six printed lines per inch, and automatic carriage return and line feed. The printer produced uppercase text, not the mixed-case output readers expect from a modern screen.
Local preparation and remote communication
Local mode let an operator prepare program tapes from the keyboard, with a printed copy, or read and copy a tape while producing a page record. Once ready, the tape could be sent in remote mode. The distinction was practical: compose or edit offline, then transmit a prepared sequence to the computer.
The period Teletype Corporation Bulletin 310B specifies seven-bit 1967 ASCII for the printer and keyboard, even-parity handling, and 115-volt AC operation. It says each code element took 9.09 milliseconds, yielding 10 characters per second at 110 baud. Teletype Corporation Bulletin 310B
ASR, KSR, and RO: what differs
| Model 33 variant | Keyboard | Tape reader | Tape punch | Typical capability |
|---|---|---|---|---|
| ASR (Automatic Send and Receive) | Yes | Yes | Yes | Type or send tape; print received text or punch it to tape |
| KSR (Keyboard Send and Receive) | Yes | No | No | Keyboard input and printed output, without the ASR tape hardware |
| RO (Receive Only) | No | No | No | Receive and print output, without keyboard or tape functions |
These are functional distinctions, not cosmetic trim levels. Check the hardware actually present on a particular machine: an incomplete or modified example may not match its label.
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Why teletypes shaped Unix and networking
Teletypes made interactive computing possible before inexpensive displays were commonplace. The Science Museum Group describes their move from office communications into computer interfaces, and their role in early time-sharing and computer networks, including user access to the ARPANET through terminals. Science Museum Group: Teletype Model 33
Unix retained the legacy in the name tty, derived from “TeleTYpe,” for terminal devices and pseudo-terminals. KU Leuven also connects 110-baud operation with the terse design of early Unix commands: slow printing output made brevity useful. The original Unix erase character was #, a choice that made sense on a printing terminal, where editing was unlike correcting text on a screen. KU Leuven Computer Museum: Teletype Model 33 ASR
Can you restore or operate one today?
Yes, but an ASR-33 is a mechanical restoration and operating project, not simply a keyboard that needs a cable. Its motor, relays, gears, printer, tape reader, and punch all need to function together. KU Leuven warns that worn worm gearing can prevent the connected computer from starting, so powering a questionable machine while it is connected can put the host system at risk. Start with inspection and documentation rather than assuming the terminal is safe to attach.
- Identify the variant and verify that keyboard, printer, reader, and punch are present and move freely.
- Inspect the paper path, gears, and mechanical wear; do not assume a machine that powers up is operating correctly.
- Use the period technical manual to understand controls, operating modes, timing, tape routing, and electrical interfaces before testing.
- Confirm the interface and current-loop requirements of both terminal and host before connecting them.
- Decide whether the goal is a static display, occasional demonstration, or sustained operation; those goals place different demands on condition and parts.
The Soemtron archive lists Model 33 keyboard, tape-reader, operating, and schematic documents, useful starting points for identifying and servicing a set. Soemtron archive: Teletype documentation
What to look for in an example
Condition and completeness matter more than the ASR label alone. Check whether tape mechanisms are present, whether the keyboard and printer work, and whether the paper path and gearing show wear. Also establish the electrical interface before planning a computer connection: KU Leuven records a 60-milliampere current loop and 110-baud operation for its described Model 33. The Science Museum Group gives a representative ASR/KSR33 size of 900 × 570 × 530 mm and a weight of 31 kg without the paper tray, so plan for a substantial, heavy machine. Science Museum Group collection record
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