Such was the lesson learned by [CuriousMarc] with his recently restored Model 15 Teletype; we covered a similar Model 19 restoration that he tackled. The essential problem is that the five-bit Baudot code that they speak predates the development of ASCII by several decades, making a converter necessary. A task like that is a perfect job for an Arduino — [Marc] put a Mega to work on that — but the interface of the Teletype proved a bit more challenging. Designed to connect two or more units together over phone lines, the high-voltage 60-mA current loop interface required some custom hardware. The testing process was fascinating, depending as it did on an old Hewlett-Packard serial signal generator to throw out a stream of five-bit serial pulses.
The big moment came when he used the Teletype to log into Linux on a (more or less) modern machine. After sorting out the mysteries of the stty command, he was able to log in, a painfully slow process at 45.5 bps but still a most satisfying hack. The ASCII art — or is it Baudot art? — is a nice bonus.
We love restorations like these, and can practically smell the grease and the faint tang of ozone around this device. We’re not thrilled by the current world situation, but we’re glad [CuriousMarc] was able to use the time to bring off a great hack that honors another piece of our computing history.
Readers with not too many years under their belts may recall a time when the classic background sound effect for radio and television news programs included a staccato mechanical beat, presumably made by the bank of teletype machines somewhere in the studio, clattering out breaking stories onto rolls of yellow paper. It was certainly true that teletypes were an important part of the many communications networks that were strung together over the 20th century, but these noisy, greasy beasts had their day and are now largely museum pieces.
Which is exactly where the ancient Model 19 Teletype machine that [CuriousMarc] and company are restoring is destined. Their ongoing video series, six parts long as of this writing, documents in painstaking detail how this unit worked and how they are bringing it back to its 1930s glory. Teletypes were made to work over telephone lines with very limited bandwidth, and the hacks that went into transmitting text messages with a simple 5-bit encoding scheme are fascinating. The series covers the physical restoration of the machine, obviously well-loved during its long service with the US Navy. Of particular interest is the massive power supply with its Thyratron tubes and their mysterious blue glow.
The whole series is worth a watch if you’re even slightly interested in retrocomputing. We’re particularly taken with the mechanical aspects of these machines, though, which have a lot in common with mechanical calculators. [Al Williams] recently covered the non-replacement of the power supply caps for this unit, which is an interesting detour to this restoration.
Visualizing how electronic signals work can be difficult. A physical model can be darn useful in overcoming that difficulty. At a recent workshop entitled “Unboxing Black Boxes” [Julian Hespenheide’s] group created a device to show Baudot Code in operation. This amalgam of wood and Arduino they dubbed émile in honor of Émile Baudot (1845-1903).
Baudot developed his code to transmit telegraph signals from one machine to another, in contrast to Morse code which was principally for human communication. Both codes were used throughout the 20th century. For example, those big clattering, mechanical teletype machines use a minor variation of Baudot code.
Baudot is a fixed length code of 5 bits, as opposed to Morse’s variable length code. Morse has a separate code for each characters while Baudot uses “shift’ codes to change between alphabet and figure characters. For instance, a binary 11 would represent either an ‘A’ or a ‘-‘ depending on the shift state. If the shift code was missed the receiver would get gibberish.
In émile the Baudot code is sent by marbles. That’s right, marbles. There are five marbles, one for each bit in the Baudot code. Each marble rolls in a track toward the Arduino. How does the machine know which marbles to send? “Punch cards”! These are a marvelous aspect of the design.
Each card represents a code. Each position in the card has a gap to allow a marble to pass ( a set bit), or no gap to block the marble (an unset bit). The operator loads 5 marbles and a punch card and launches the marbles via a spring mechanism.