ancient baudot teletype interface

Posted on Feb 5, 2014

This was my first published article, and fell out of my work on the homebrew 8008 system that is now in the Computer History Museum (that link includes four detailed photos). It`s basically a one-way simple UAR/T, though those devices were pricey back then and the design induces the host micro to wait while a character is sent (an interrupt wou

ancient baudot teletype interface
Click here to download the full size of the above Circuit.

ld be better, of course, but my primitive software wasn`t doing anything else at the time). Of course, now one would eschew expensive external hardware and just use a port bit, but this was 1974 and static RAM locations were a scarce resource. This article had the effect of kindling the crazy notion of writing about my projects, and that became a career. I was 22 at the time, living in an apartment in Kentucky, doing all I could to escape employment. A photo of my living room, including the machine and the amazing Model 28 teletype, is below the article. The lengthy software service routine generally required to interface a teletypewriter and an IC microprocessor, such as the Intel 8008, can be eliminated by the circuit shown here. A shift register and some control logic are all that it takes, bringing total component cost to only about $6. 50. In the 8008 system, synchronization with the central processing unit is accomplished through this microprocessor`s READY line, making modification of the teletypewriter itself unnecessary. The hardware configuration given in the figure is designed for a 10 character-per-second Model 28 Teletype, which uses the five-level Baudot code. If the intended application will not easily accommodate data storage in the Baudot code, conversion may be accomplished with a read-only memory, such as National`s MM5221TM. (A Model 33 Teletype presents no decoding problem. ) During the time that the...

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