Traditional PC serial ports, based on members of the 8250/16550 UART families (or their clones within SuperIO chips) support some unusual serial data formats, specifically 5- and 6-bit data, and 1.5 stop bits.

Some USB-Serial adaptors (Prolific) claim to support these formats, some (FTDI) only offer 7 and 8 bit data. I don't think many microcontroller UARTs support 5 or 6 bit data either, though it's time-consuming to research that.

My own experience stretches fairly well back into the distance past, but I cannot recall ever having seen anything use a 5- or 6-bit serial data format. I would say that 7-bit data formats are old-fashioned and tend to come from the 80s or earlier, and that 5/6 bit formats are nothing more than an historical curiosity.

I'd like to recommend to a project I'm involved in which interfaces to serial ports that we drop support (or at least test coverage) for 5- and 6-bit data and 1.5 stop bits. It would be useful to establish whether anyone knows of any application for these which is still in service.

What were the legacy applications for 5 or 6 bit serial data, and do any of them still exist?

  • 4
    a 5 bit telex card circa 1960's qph.ec.quoracdn.net/…
    – JIm Dearden
    Commented Feb 11, 2017 at 12:19
  • Most Atmel 8bit ATMega's support 5,6,7,8 and 9bit UART formats.
    – Tom Carpenter
    Commented Feb 11, 2017 at 12:48
  • 4
    @cbmeeks That's simply 500 millibits more than one bit.
    – tofro
    Commented Feb 14, 2017 at 7:04
  • 11
    Jokes aside: The "stop bits" measure is actually a measurement of time rather than a unit of information. So 1.5 bits is 1.5 bit times, one bit time equalling 1/bit rate s. And 1.5 stop bits means the stop bit signal needs to be stable for 1.5 bit times, i.e. 50% longer than for a "normal" data bit.
    – tofro
    Commented Feb 14, 2017 at 12:18
  • 1
    @tofro is exactly right. "1.5 stop bits" just means that after the last data or parity bit the transmitter holds the line at "marking" condition for at least 1.5 bit times before allowing the "start bit" of the next character to be sent. No actual data is being sent or received (any more than for the start bit) so no storage is needed. It's just a delimiter for the character frame. Some teleprinters required 1.5 stop bits to give them time to complete the operation, esp for a carriage return/line feed. It works perfectly fine to send them two stop bits instead, at a slight loss of throughput. Commented Feb 15, 2017 at 22:39

9 Answers 9


Five-bit teletypewriter codes ("Baudot", etc.)

As far back as the early 1900s (believe it or not) there were teletypewriters. They were intended to replace Morse-style telegraphy, directly printing hardcopy rather than requiring an operator to listen to the Morse code and transcribe the messages by hand.

The first successful such equipment was invented by a man named Baudot. He also invented the five-bit character code that his machines used. (See ["Baudot code"]) Later, a different five-bit (or "five-level") code was developed that made it easier to build a mechanical typewriter-like keyboard that would generate the code. The most successful such code was called ITA2.

These codes provided upper case only. They were primarily used for communication between machines like the Teletype Model 15, in networks like Telex, TWX, and Western Union's "Telegram" service.

These networks have almost completely faded away. Some ham radio operators still use Radio Teletype (RTTY) and they do still use the five-bit codes, partly due to tradition, partly for efficiency (they need fewer bits to send a character than the modern 8-bit codes). But this is most often done using computers as terminals, not ancient teletypewriters like the Model 15. A few of the die-hards do keep some of the old machines running.

Most old Teletype machines with three rows of keyboard keys (instead of the four that were common on typewriters) used these five-bit codes. They only needed three rows on the keyboard because numbers and special characters were shifted from the alphabetic keys. Two keys labeled "Figs" and, I believe, "Ltrs" sent the "shift in" and "shift out" codes. If you hear somebody talking about "three row" teletype machines, this is what they're referring to.

I wrote another answer about how teletypewriters in general, and the 5-level ("Baudot") machines in more detail, here.

The 6-bit TTS code

As for six-bit codes, the most common use (at least in terms of async serial communications) was probably the "TeleTypesetter" (TTS) code. I say this because virtually every newspaper that subscribed to a wire service (like AP or UPI) was equipped to receive it, usually with multiple feeds.

I'm going to say quite a bit about this because, while five-bit teletypewriter info is easy to find out about, there's very little out there about TTS.

The TTS code was a clear descendant of the five-bit codes like ITA2. Despite having six bits it still used "shift in" and "shift out" codes (like the Baudot code family did), permitting TTS to carry over 100 different glyphs (printable characters) and control commands. So it included upper and lower case alphabets, digits, a large assortment of special characters ("Wingdings" - far more than what you'd find on a typical typewriter), plus typesetting-oriented commands like "flush left" (which means "end a paragraph and justify the last line to the left"), center, and flush right.

In the wire room

In the old days, news wire services like AP and UPI (and smaller local ones, like City News Service) would send stories to their member newspapers via this code, over dedicated leased telephone lines. In each newspaper's "wire room" the copy would be printed on a keyboard-less TeleType Model 20 (so the editors could read and select the stories), and also fed to a "reperforator" (paper tape punch). There was one such pair of machines for each wire service the paper subscribed to.

Between each story the wire services would send a bunch of NULs (which would punch essentially blank tape, with only the feed holes), then a bunch of characters that would print as nonsense but would punch out the next-following Story ID on the tape in block letters, followed by more NULs. This made it relatively easy to find the section of tape that corresponded to the following printed copy. It also helped in identifying the correct direction and orientation for the paper tape, as the six-level tape was symmetric about the feed holes! But close inspection of the feed holes gave another way to tell the direction: They "led" the data holes slightly, so the back edge of a feed hole corresponded to the center line of the data holes it went with.

A new hire in the wire room would have the job of tearing off the hardcopy and the tape that went with it as it came out, filing the tape, and distributing the copy to the editors.

The tape for selected stories could then be fed directly to a Linotype machine equipped with a "Teletypesetter Operating Unit", which was a paper tape reader connected to a metal box that was placed on top of the Linotype's keyboard. The box was conceptually very simple: It had a solenoid for each of the Linotype's keys and it simply "pressed the keys" as the tape was read. The result, just as when a human was typing, was cast metal type that could be put on a press, inked up, and printed.

It was possible to edit the story before typesetting by using a TTS Teletype machine, tape reader, and tape punch ("reperforator"). The tape would be duplicated until the desired edit point was reached, then the operator could type additional text. Or to skip things, they would advance the original tape without copying it to the new tape.

A radio or TV station's news operation would have the model 20 Teletype, but no reperforator or Linotype. Copy from the Teletype would be torn off and handed to editors who turned it into the (generally much shorter) stories the anchors would read. In small stations the on-air news readers also did the writing. Eventually the wire services offered feeds already edited for radio or TV, used by smaller stations that didn't want to hire news copy editors.

The Teletype machines and paper tape punches ran nearly continuously, as stories were updated and reposted throughout the day. Ear protection was a good idea in the wire room! To this day, a few "all news" radio stations use the sound of one of those machines pounding out copy as background sound to their live on-air reporters.

TTS code and computer typesetting

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, based on HP 2100 minicomputers. We still had the reperforators but they were turned on only during the hour or so when we ran the nightly backups. Due to repetition on the feeds, almost all stories they wanted to use would be found already in the computer, but if need be they could find the tape for a missed story and read it via a paper tape reader attached to the computer. Incidentally our phototypesetters were made by Merganthaler, the same company that made Linotypes. The software system was called "Text II", from a company called Systems Development Corporation, based in Santa Monica.)

Virtually everything you ever saw printed in a newspaper with "AP", "UPI", or some other news "wire" service in the slug line came into the paper via systems like this. The local newspaper may, however, have edited the stories, sometimes significantly.

TTS phase-out

Around the late 70s, about the same time that I left the paper, AP was planning to offer a higher-speed network using eight-bit codes. So today, nearly 40 years later, I doubt that there's much if any of this six-bit TTS code left in use today - any more than there are five-bit ("three-row") teletype networks.

Well-maintained operating examples of TTS equipment are virtually nonexistent as there is virtually no hobbyist interest in it. One Teletype enthusiast site claims there is one operating example of a Teletype model 20.

Computers and six-bit codes

There were also many computers that used 6-bit character codes internally. Examples were the PDP-8 (a 12-bit machine, so two 6-bit characters could fit in a machine word), IBM 1401 and 7090, CDC 3000 and 6000 series (24-, 48-, and 60-bit words respectively), etc. These were not the same six-bit codes as TTS, and "shifting" was generally not used to expand the code set. Nor did they commonly use six-bit paper tape. Printers and punched cards of the day only supported upper case alphabets, so 64 different glyphs were enough. (See, for example, the Wikipedia article on the IBM 1401.) Some of them did, however, communicate with these six-bit codes over early modems.

IBM's System/360 set a de facto standard of 8-bit bytes (but using IBM's EBCDIC character code, which had very little uptake elsewhere), and several very successful mini-computers with 16-bit words (HP 2100, DG Nova, DEC PDP-11) started using 8-bit characters (generally using ASCII character codes) at around the same time. That pretty much ended the era of six-bit character codes.

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). This was, however, a very different six-bit code from TTS, and in many cases different also from the character code used within the machines they connected to. (Of course.) Like the TTS code, though, they did have "shift in" and "shift out" commands, which corresponded literally to "shifting" the typewriter mechanism (ie rotating the ball 180 degrees). The 2741 and 1050 were based on the IBM Selectric mechanism; the Selectric typeballs of that time had 88 printable characters, already too many for six bits even before other controls are counted. (But not, alas, quite enough for the 94 printable glyphs of seven-bit ASCII.)

Later the same terminals were connected to System/360 and other eight-bit machines and the computers, or purpose-designed interfaces, had to do the code conversion. So if you happen to find a working 2741, yeah, you'll need that six-bit code to talk to it. :) You'll also need to be able to set your serial port to 134.5 bit/s.

  • I wonder if it would have been hard to design a device to translate a pair of consecutive spaces on a Linotype type into a code for an end-of-sentence space? The use of Linotypes has had a deleterious effect on the quality of typography, and it would seem that should have been avoidable.
    – supercat
    Commented Feb 15, 2017 at 23:12
  • 1
    @supercat the Linotype did have keys for en, em and "thin" spaces as well as the expandable-for-justification space band. And TTS code did allow for all four. If the wire services didn't send them, or the Linotype keyboard operators didn't use them, that is hardly Mergenthaler's fault. :) Commented Feb 16, 2017 at 1:07
  • From what I read, Linotype machines tended to behave badly when fed a tape containing two consecutive word-space characters. Requiring that nobody should ever send two consecutive word-space characters seems like a rather extreme solution compared with simply making the machine recognize the combination as an en quad (or whatever amount of space the typesetter deems appropriate).
    – supercat
    Commented Feb 16, 2017 at 17:55
  • 2
    Ah. Word-spaces are the "expandable" space. The "spacebands" are wedge-shaped, and higher than the letterform matrices. Justification was done by a bar that came down against the tops of the spacebands, forcing them further into the stack of matrices, causing them to take more width on the line. I can imagine several reasons why you wouldn't want two or more of these in a row. Commented Feb 16, 2017 at 19:53
  • Yup. Having a larger space after a full stop than after an abbreviation had long been recognized as typographically desirable, and people with typewriters would have made it easy for the Linotype to preserve that distinction. From what I understand, though, the Linotype's inability to handle typists who used "period space space" to indicate a full stop has eliminated what used to generally be a useful typographical semantic distinction (situations where a line ends with a period could sometimes be ambiguous, but most potential ambiguities are resolved, without the reader even having...
    – supercat
    Commented Feb 16, 2017 at 20:00

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.

  • It is supported when you don't insist on an additional parity bit.
    – Janka
    Commented Feb 11, 2017 at 13:14
  • 45 baud... and I thought my Atari's casette recorder at 600 baud was slow! Any decent typist is faster than that!
    – SF.
    Commented Feb 24, 2017 at 14:50
  • 1
    In the days of RTTY and 5 bit "Baudot", we didn't really talk about bits per second. The common description was that they operated at "60 words per minute".
    – gbarry
    Commented Jul 16, 2017 at 6:50

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...

  • Reading "reels of punched paper tape" is not really an issue. The OP is asking about a serial interface. A paper tape reader (or punch) is a parallel device, not serial. Commented Feb 12, 2017 at 9:06
  • I haven't personally interacted with... I have. I burned out the last Teletype left in my University. (Re-purposed it as a serial printer... Of course that typed lines quicker than any human typist. So the stepper-motor overheated and went up in flames :-)) Man, I'm getting old...
    – Tonny
    Commented Feb 12, 2017 at 19:23
  • @JamieHanrahan Tape or punch-card is parallel internally. That's true, but most of those interfaced serially to other devices. I have also seen Shugart, SCSI and GPIB interfaces. I can't recall ever having seen a real parallel one.
    – Tonny
    Commented Feb 12, 2017 at 19:29
  • Hm, well... that's direct opposite to my experience. The paper tape readers and punches I worked with on HP, DEC, and Data General gear all used a very straightforward parallel interface (though not exactly like that of either Centronics or Data Products parallel printers). Note that Shugart, SCSI (in that era), and GPIB are all parallel. Commented Feb 12, 2017 at 21:58
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    WRT the Colossus and Lorenz being "apparently" still in service, there's one rebuilt Colossus in The National Museum of Computing at Bletchley Park which is quite something to behold (the paper fairly whizzes through though the data speed is akin to what one complains about as slow if using dial-up Internet access). Lorenz machines are a bit more plentiful, in that there are a handful surviving, which is more than the single Colossus.
    – Jon Hanna
    Commented Feb 13, 2017 at 2:31

It would be useful to establish whether anyone knows of any application for these which is still in service.

RTTY (FSK modem over radio) still uses 5 bit code at low bitrates 45/50. It's often still used for automatic weather reports.

What were the legacy applications for 5 or 6 bit serial data, and do any of them still exist?

The six bit code was convenient for golf-ball style teleprinters, where it could directly drive the six solenoids that operated the print head without needing additional translation. This is why you'll often see it used with more than one stop bit, as the solenoids need time to get back to their resting position.

You can find lots of information on RTTY and its history here http://www.rtty.com/


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.

  • I still have one of these old Selectric typewriter terminals somewhere in the attic - Last used in ~2000 together with a Sinclair Spectrum (whose IF1 wasn't able to spit out the correct number of data bits and so needed to have a converter box that built 50 bps/6N2 from its 12008N1...)
    – tofro
    Commented Feb 11, 2017 at 14:54
  • "Last used in 2000" - the lubricant has likely turned to something close to chewing gum in consistency, ie no longer a lubricant. Don't expect it to work right! Commented Jan 13, 2018 at 0:23

The most common one still around is probably the International Program Airline Reservation System (IPARS) 6-bit character set used used in the Airline Link Control (ALC) protocol used in the international airline reservation system. Yes, down in the guts of the global air travel system, blocks of 6-bit characters are still being shuffled around to this day all over the world. You can get many of the gory details by looking up 'SITA P1024B'. ALC was traditionally carried over X.25 or a serial bisync connection, but these days I suppose you'll find it encapsulated in IP most of the time (there's an RFC for that).


The IBM 6 bit transcode was one of the serial modes supported by the IBM 2780 amongst other things.

There used to be a (printed) handbook that compared all the then known codes from 5 bit Baudot to 8 bit ASCII (which was rather new at the time). It might be interesting to see if there is a scanned copy somewhere.

I think you will find the 6 bit transcode mentioned in the first edition of The art of electronics.

  • Welcome to Retrocomputing. Thanks for the answer. Do you know the name of this handbook, or remember any of the Baudot codes?
    – wizzwizz4
    Commented Feb 12, 2017 at 17:40
  • @wizzwizz4: Given that Baudot codes date from 1870, they are reasonably easy to find.
    – Chenmunka
    Commented Feb 13, 2017 at 9:13
  • @Chenmunka They are easy to find.
    – wizzwizz4
    Commented Feb 13, 2017 at 9:28
  • 1
    @wizzwizz4 The mentioned handbook, on the other hand, seems more difficult to find. But finding it would be brilliant! (Hint, hint.)
    – cjs
    Commented Sep 19, 2019 at 18:46

Peter Camilerri -- I did exactly the same thing around 1980. I was able to get "letter quality" output while everyone else had "dot matrix". I did this by driving an IBM Selectric terminal using 6-bit serial (with the bit order reversed), using a program I had written in BASIC. This was on an 8-bit Z-80 system running MP/M, the multiuser version of the CP/M OS.


Stock tickers date from 1863 (a variant was Edison's first successful invention in 1869). The text-output ones only printed 32 characters, so would have been a 5-bit application... which is possibly the eldest of the 5-bit-compatible applications of serial data. I don't know the timeline for bit-serial (some tickers operated like a pulse-dial phone, took 'way more than 5 bit-times to print "Z").

Ticker tape went completely obsolete in the 1960s.

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