S-100 bus
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The S-100 bus, IEEE696-1983 (withdrawn), was an early computer bus designed in 1974 as a part of the Altair 8800, generally considered today to be the first "personal computer".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. The S-100 bus formed the basis for homebrew computers whose builders (e.g. the Homebrew Computer Club) implemented drivers for CP/M and MP/M. These S-100 microcomputers ran the gamut from hobbyist toy to small business workstation and were the zenith of the microcomputer world until the advent of the IBM PC (which some of them outperformed).
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 problems, 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.
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 +8 V and +18 V, which had to be "pulled down" on the cards to TTL (+5v) or RS-232 (+/-12 V) levels.
Another designer who did a great deal to push the S-100 technology forward was George Morrow, with his company Morrow Designs. Morrow was the first chairman of the S-100 Bus Standards Committee, which later became IEEE-696. Other innovators were companies such as Cromemco, Godbout Electronics (later CompuPro) and Ithaca Intersystems. The standards committee introduced the 16 bit data bus to S-100, previously 8 bit data, by using the two separate uni-directional data buses as a single bi-directional bus.
The S-100 bus has a number of variants from different manufacturers, but had eventually been standardized as IEEE-696 towards the end of 1983. 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 made to "look" like the 8080 (notably the Zilog Z80), or alternately placed on complex converter cards to allow them to be plugged into S-100 machines.
