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The S-100 bus was an early computer bus designed as a part of the Altair 8800, generally considered today to be the first "personal computer". The S-100 bus was the first "industry standard" bus for the microcomputer industry, and S-100 computers, processor and peripheral cards, were produced by a number of manufacturers.
During the design of the Altair, the hardware required to make a usable machine was not available in time for the January 1975 launch date. The designer, Ed Roberts, also had the problem of the motherboard taking up too much room. Attempting to avoid these problem, he placed the existing components in a case with additional "slots", so the missing components could be plugged in later when they became available. The motherboard was split into four separate cards, with the CPU on a fifth. He then looked for a cheap source of connectors, and came across a supply of 100-pin edge connectors. The rest, as they say, is history.
For all intents, the S_100 bus consists of the pins of the Intel 8080 run out onto the backplane. No particular level of thought went into the design, which led to such disasters as various power lines of differing voltages being located next to each other, leading to easy shorting. Another oddity was that the system included two unidirectional 8-bit data buses, but only a single bidirectional 16-bit address bus. A deal on power supplies led to the use of +8v and +18v, which had to be "pulled down" on the cards to TTL (+5v) or RS-232 (+/-12v) levels.
The S-100 bus has a number of variants from different manufacturers, but had eventually been standardized as IEEE-696 towards the end of the 1980s. By this point the S-100 bus had evolved into the standard for all "professional" personal computers, almost all of them running CP/M. The standard was so powerful that many other CPU designs were either modified to "look" like the 8080, or alternately placed on complex converter cards to allow them to be plugged into S-100 machines.
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