S-100 bus

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.

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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. The rest, as they say, is history.

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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.

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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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Computer bus: In computer architecture, a bus is a subsystem that transfers data or power between computer components inside a computer or between computers. Unlike a point-to-point connection, a bus can logically connect several peripherals over the same set of wires. Each bus defines its set of connectors to p...

Altair 8800: The MITS Altair 8800 was a microcomputer design from 1975, based on the Intel 8080A CPU. Sold as a kit through Popular Electronics magazine, the designers intended to sell only a few hundred to hobbyists, and were surprised when they sold over ten times that many in the first month. Today the Altair...

Microcomputer: Although there is no rigid definition, a microcomputer (sometimes shortened to micro) is most often taken to mean a computer with a microprocessor (?P) as its CPU. Another general characteristic of these computers is that they occupy physically small amounts of space....

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