What were the applications of 5/6-bit serial port formats?

The Baudot code uses 5 bits, and IIRC at least one of the mechanical teleprinters needed 1.5 stop bits to provide the time for the mechanism to do its thing, this at 45 baud.

RTTY radio comms is probably the one place you still see this stuff in use.

9 bit on the other hand is semi common in RS485 industrial controllers.

Traditional Teletypes and many paper tape storage systems use these. You won't be able to read reels of punched paper tape if you drop support for 5 bit data.

Nor will you be able to interface with Colossus systems, or the Lorentz communications they were designed to decode. At least one of each is apparently still in service - as a museum piece.

Full disclosure : despite being fairly archaic myself, I haven't personally interacted with either of these systems...

Many many years ago, I interfaced an old IBM Selectric Terminal to my CPM machine. It used a 6 bit code that was not supported by my hardware and a 134.5 bit per second data rate also not supported. I wound up bit banging the data out with a software UART. Lucky for me, I was not interested in getting data from the Keyboard as that would have been much more difficult.

In terms of async serial communications the most common use of 6 bits was probably the "TeleTypesetter" (TTS) code: Virtually every newspaper that subscribed to a wire service (like API or UPI) was equipped to receive it.

Like the 5-bit Baudot code (an upper-case-only code primarily used for communication between Teletypes like the Model 15) that preceded it, TTS had "shift in" and "shift out" codes that permitted it to carry far more than the 64 characters you'd expect from 6 bits. The code included upper and lower case alphabets, digits, a large assortment of special characters, plus typesetting-oriented commands like "flush left".

In the old days, news wire services like AP and UPI would send stories to their member newspapers via this code, over leased telephone lines. In each newspaper's "wire room" the copy would be printed on a TeleType Model 20 so the editors could read and select the stories) and also fed to a "reperforator" (paper tape punch). The tape for selected stories would then be fed to a Linotype machine, resulting in cast metal type that could be put on a press, inked up, and printed.

A radio or TV station's news operation would have the model 20 Teletype, but no reperforator or Linotype.

In later years the incoming 6-bit signal was connected directly into a computer's serial port, stored on disk, and made available for review and editing via video terminals. After editing the computer would send the copy to a phototypesetter, resulting in nicely set type on photo paper. (I spent a few years working for a newspaper with such equipment.)

There were also computers that used 6-bit character codes - a lot of them did, in fact, until IBM's System/360 set a de facto standard of 8-bit bytes. The IBM 2741 and 1050 printing terminals were originally used with machines like the 7090 and the 1401, and these terminals used six-bit characters (at 134.5 bit/s, 1.5 stop bits). Later the same terminals were connected to System/360's and the computers had to do the code conversion.

• About teletypesetting and Linotypes
• About the IBM 2741
• About Linotypes