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Bringing the Model 33 to the 21st Century - Part 1


Veteran Member
Oct 22, 2008
Kamloops, BC, Canada
Before the advent of the modern CRT or dot matrix based computer terminals of the 70’s the world of the terminal was almost exclusively in the domain of teletypewriters. Large and heavy electromechanical devices that best resembled a typewriter but with a few modifications that enabled a remote device such as a computer to send it text and receive input from the keyboard or an auxiliary input like a paper tape reader.
Of all the different makes and models of teletypewriters that were sold, the model 33 from the Teletype Corporation became the single most identifiable machine in existence.


The first machines left the assembly line in 1962 at a cost of $700 and before they finally discontinued them in 1981 more than 600000 machines had been sold. Over the years there were many, many options made available for the base machine. Some could only act as printers (Receive Only) while more commonly they acted as full terminals with a paper tape I/O under the model ASR33. (Automatic Send and Receive) Other options included customized keyboard layouts, sprocket fed paper, large buckets for the paper “chads” the tape punch would eject and alternate methods to communicate either with other teletypes or computer hosts. By the mid-70’s many teletype users had started switching to video display terminals which were rapidly becoming more affordable and many machines began to see almost dedicated uses in the pioneering days of the microcomputer. Famously Paul Allen and Bill Gates received their first true experience with a computer when their school installed a timeshared teletype machine.
Typically in this era a teletype would interface with the host over a current loop connection but when you were miles from the machine and could not afford a private line your only other choice was to use a modem.
Through the life of the 33, modems were still a Wild West. Bell had almost a dictatorship over what you did with their lines. While the easiest solution was to use an acoustic coupler to get around this limitation (and it was available as an optional CCU) the noise of a teletype in typical situations made this option less than ideal. Because of the modularity of the model 33 and how the power supply, selector magnet driver and the communications interface were all one removable unit (typically called the Call Control Unit) you were given the additional options (among private line, console-only and leased lines) of an internal modem which was authorized by Bell for use on their lines, or a CCU that could interface directly with one of Bell’s Dataphones.


These often absurdly large devices were the first commercial modems anyone could get their hands on. Typically they were leased from the phone company but operated on the basic fundamentals of taking a serial data connection and modulating it. No compression. No error correction. Nothing special. While Bell went off and had a half dozen standards of their own one that stuck good and hard was Bell 103. At speeds up to 30 characters per second (300 baud) it was backwards compatible with the 10 characters per second (110 baud) that the model 33 was mechanically locked to. It was a very good combination and many teletypes spent the rest of their useful life as remote terminals for various computer systems.
We are now almost 35 years past the point since the last model 33 rolled off the production line. What machines remain today are prized for their popularity and are often paired with famous machines such as the Altair 8800 or DEC’s PDP-8/e. When you decide now is the time to try returning it partially back to it’s old roots as a remote terminal you suddenly discover that nobody has really given a damn about any use beyond a local terminal for a long, long time. This is where my problem began.


I had been searching unsuccessfully for a model 33 for some time already when I went into my local electronics supply store in the summer of 2013. At the time I was working on my paper tape reader which was one of my other projects. While we discussed possible ways to approach what I wanted I was asked the question on why I didn’t just get a teletype and use that instead. Stating I didn’t own one he then stopped me dead and mentioned he had one and it was for sale. He brought it in a week later and after we fiddled with a touchy selector magnet for an hour we negotiated on a few hundred dollars for the machine AS-IS. It wasn’t perfect. It was completely rebranded by the Canadian firm Marsland Engineering, the stand and chad bucket were missing and some of the computer specific keys like BREAK were not installed but hey, I wasn’t going to complain. It even came with a full set of wiring diagrams and the basic cleaning, lubrication and adjustment documentation.





Among the other things that came with it was an additional Call Control Unit. This one was a TWX controller for Telex service. This particular CCU while it fit in my machine was not actually for my machine. The model 32 Teletype was visually identical to the 33 with the exception that instead of using ASCII for communications it used BAUDOT, which was completely incompatible. Added to that you couldn’t even get Telex service anymore so you had nothing to connect it to. It was totally useless.

At the time my primary use of the teletype was talking to any of the various machines I had in my collection. None of them support the old current loop standards so to bring the Teletype into the 80’s we had to make an adapter to convert 20ma current loop to RS-232. There are a number of ways you can do this but personally I liked the use of optocouplers and after the modification of an existing schematic to supply power to the loop and switch the TxD and RxD lines we were chattering away.