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It seems that the development of TTL was contemporary ~1961 the maiden moon landing of the Apollo vehicle, while the development of one of the oldest digital signalling standards RS232 using the UART hardware was crystallized later, and that of the advent of the USB hardware, transmission standard and related communication protocols occurred much later.

What was the communication protocol used on the controllers and processers on board the Apollo lander vehicle?

kbakshi314
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    RS232 isn’t a protocol. – Kartman Mar 19 '21 at 04:54
  • @Kartman thanks for bringing the error to my attention. I have edited the text of the question to remove it. – kbakshi314 Mar 19 '21 at 04:57
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    Which protocol? The telemetry via radio or for the internal systems. I’d guess the internal systems were hard wired - switches and lamps for the most part. – Kartman Mar 19 '21 at 04:59
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    “My favorite programming language is solder” -- Bob Pease – gbarry Mar 19 '21 at 05:01
  • @Kartman the question is concerned with the internal systems including the controllers and processors including the CPU. – kbakshi314 Mar 19 '21 at 05:16
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    The question doesn't have anything to do with the tags you've assigned it. – pericynthion Mar 19 '21 at 05:26
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    @kb314 then there's none. There are no independent "communicating" components, just a monolithic computer and a lot of discretes (switches, lights, motors, analog sensors). – hobbs Mar 19 '21 at 05:27
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    The Apollo moon landing was in 1969. RS232 dates to 1960. – Math Keeps Me Busy Mar 19 '21 at 05:34
  • @hobbs wouldn't it be a good guess that controllers and for the various modules such as the orientation, propulsion, attitude controller etc. were independent peripherals feeding to and fro to the CPU? – kbakshi314 Mar 19 '21 at 05:45
  • @MathKeepsMeBusy I realize that, but I'm still unsure that RS232 was indeed used. – kbakshi314 Mar 19 '21 at 05:46
  • @hobbs your comment seems as likely to me as an independently developed system. Is it fair to say that in the absence of series communication a lot of extra wiring would be necessitated alongside higher redundancy requirements for robustness? – kbakshi314 Mar 19 '21 at 05:49
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    The Bell 103 modem, using rs232 came out in 1962. Early ASR33 teletypewriters used current loop (with UART style signals, which were electromechanically generated), but later ones used rs232. Not sure when that started. Perhaps when the Bell 103 modem came out, as the original use for rs232 was connecting teletype and modems. – Math Keeps Me Busy Mar 19 '21 at 06:03

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Dig in here: ftp://ssh.esac.esa.int/pub/ekuulker/Apollo15/The-Apollo-Guidance-Computer-Architecture-and-Operation.pdf

And here: https://arstechnica.com/science/2020/01/a-deep-dive-into-the-apollo-guidance-computer-and-the-hack-that-saved-apollo-14/3/

tl; dr: a bespoke binary stream format for telemetry uplink and downlink.

hacktastical
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Encoding of characters into digital data streams is rather older than Apollo. Émile Baudot's system was patented in 1874, about the time when Jules Verne was writing of the US sending a ship to the moon in the immediate postbellum period.

The USB communication system used for the Apollo mission integrated a bunch of different data and analog streams. Digital commands were encoded with many extra bits so that errors would be detected with a probability of erroneous decoding of \$ \lt 10^{-9}\$. Redundancy and fail-over modes of operation were strong concerns.

Here, for example, from this document is the LEM (Lunar Excursion Module) down link summary:

enter image description here

Terrestrial communication (to remote base stations) used Western Electric 2400 baud modems.

There is quite a bit of fascinating information in that overview document, including a brief description of the console displays which used special data display CRTs with both magnetic and electrostatic deflection (the latter for vector character generation).

Spehro Pefhany
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