Teletype Model 33 ASR

Teletype Model 33 ASR Restoration Series Playlist:

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The Teletype Model 33 ASR

The Teletype Model 33 is a successor to the earlier 5-bit Baudot Teletypes. The Model 33 is an "8-bit" teletype, using the newly developed ASCII standard. But actually, it is really a 7-bit ASCII teletype: the 8th bit is only used for parity in models that have the parity option, or is always 1 or always 0 in the teletypes that do not have the option. There is no provision for lower case ASCII either, so it's all caps like its predecessors. It also works on the lower 20 mA current loop and is meant for short distance connections to a computer or a modem, although you could also make it work long distance using the 60 mA mode.

The ASR variant stands for Automatic Send Receive, meaning it has an automatic paper tape punch and reader, a keyboard and a printer, and possibly an integrated modem. There were other versions that did not have the tape punch equipment (a module added to the left of the machine), or the ability to send. But the ASR is one of the most popular, particularly as a terminal interface for early 1960's computers, before the advent of so-called glass terminals, or the modern screen terminals as we know them today. That's the type of console on which Unix was developed.

The 33 ASR was made with budget pricing in mind, and is much flimsier than the industrial/military Model 15 and 19 that we previously restored. I mean really a lot flimsier. It's nowhere close to the same league. All the parts are very thin, stamped, and undersized. It uses lots of plastic. Parts hold together precariously and there is a lot of play everywhere. Unlike the Model 15, the little steel used in the 33 actually does rust. It's a miracle of cost cutting and good enough engineering that it works as well as it does. Unlike the Model 15 workhorses, it is not rated for continuous, 24/7 usage. It's maximum usage rating is 20% duty cycle, whatever that means. The 33 has a little bit of electronics in it, but it is somewhat unrefined. It uses a lot of power resistors that heat up like crazy - PCB look burned out, and the thing smells like an overheated piece of old electronics.

Nevertheless, its low price, combined with the fact that you get 4 or 5 instruments in one (a keyboard, a printer, a tape reader, a tape punch, and on the models so equipped a modem), the 8-bit coding scheme, and a 20 mA current loop compatible with a computer, made it an incredibly good deal. It became very popular from the 1960's all the way to the early 1980's.