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The Altair 8800 was based on what came in later years (much to the displeasure of MITS) to be known as the S-100 bus, because it had 100 lines, because MITS found 100-pin edge connectors were available cheaply.

Why were 100-pin edge connectors available cheaply? What were they used for before then? Were they some kind of de facto industry standard, was there some particular machine that used a lot of them, or was it just a random one off event that someone happened to be liquidating a few crates of them?

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    much to the displeasure of MITS Now you've got me curious why.
    – ssokolow
    Commented Nov 11, 2020 at 7:45
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    @ssokolow Ed Roberts, founder of MITS, thought of the system as 'his' and was not pleased by the quick profusion of companies making add-in boards and clone computers; he saw their insistence on calling it the S-100 bus rather than the Altair bus, as adding insult to injury.
    – rwallace
    Commented Nov 11, 2020 at 22:12

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As Wikipedia says (S-100 bus):

"(Author) then looked for an inexpensive source of connectors, and he came across a supply of military surplus 100-pin edge connectors. The 100-pin bus was created by an anonymous draftsman, who selected the connector from a parts catalog and arbitrarily assigned signal names to groups of connector pins."

So the prime reason is "a military surplus". I guess they were used in some army projects in the '70s, like TTL-based mainframes, but I cannot find any evidence for such a claim, as the military computers are not wide-known.

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    In the late 1970s I worked for a So, Calif. based aerospace company and they had a "surplus sales" store on-site. At one point boxes full of 1000s of these 100 pin "S-100" connectors came into the store. As I recall they were quite popular as homebrew was a thing at that point in time. I do also recall seeing them in various pieces of test equipment that were built by the company. I get the impression that these were just generally useful connectors.
    – jwh20
    Commented Nov 11, 2020 at 15:31
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    Computers wouldn't be the only, or likely primary, use of these 100-pin connectors. They would've been used in all sort of electronic devices.
    – user722
    Commented Nov 11, 2020 at 16:13
  • It's still somewhat common to find that overall type of connector (coarse-pitch edge connectors, often with wire wrap or solder eyelet termination) show up in surplus stocks.
    – ikrase
    Commented Mar 25, 2022 at 4:16

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