S-100 bus

The S-100 bus, IEEE696-1983 (withdrawn), was an early computer bus designed in 1974 as a part of the Altair 8800, generally considered today to be the first personal computer (or at least the first "microcomputer", insofar as it was designed for hobbyists rather than the general public). The S-100 bus was the first industry standard bus for the microcomputer industry. S-100 computers, consisting of 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).


The S-100 bus essentially consisted of the pins of the Intel 8080 run out onto the backplane. No particular level of thought went into the design, leading to such disasters as various power lines of differing voltages being located next to each other, resulting in easy shorting. The system included two unidirectional 8 bit data buses, but only a single bidirectional 16 bit address bus. Power supplies on the bus were unregulated +8 V and +/- 18 V, designed to be regulated on the cards to +5 V (used by TTL) +/- 12 V (typically used on RS-232 lines for disk drive motors).


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 backplane taking up too much room. Attempting to avoid these problems, he placed the existing components in a case with additional "slots", so that the missing components could be plugged in later when they became available. The backplane was split into four separate cards, with the CPU on a fifth. He then looked for a cheap source of connectors, and he came across a supply of 100-pin edge connectors.

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 IMS Associates, Inc., Cromemco, Godbout Electronics (later CompuPro), and Ithaca Intersystems. The standards committee introduced the 16-bit data bus to the S-100, which had up to then transferred only 8 bits at a time, 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 (most notably the Zilog Z80), or otherwise placed on complex converter cards to allow them to be plugged into S-100 machines.

As microcomputers got smaller and faster, S-100 became obsolete. The Apple II, for example, in 1977 had expansion cards about a quarter of the size of an S-100 card. The popularity of IBM's first personal computers made the ISA bus, first used on the IBM PC/AT in 1984, the undisputed standard expansion bus for personal computers shortly after. Note that in an S-100 system, the S-100 bus is not just for expansion; it also ties together the essential parts of the system, for example CPU, memory, and interrupt controller. In later systems, those connections happen elsewhere, where they are cheaper and faster than on a backplane.

Notable S-100 bus computer users

Among the most notable users of S-100 bus computers was science fiction writer and Byte magazine columnist Jerry Pournelle, who wrote many of his early novels on an S-100 bus machine running CP/M he dubbed "Zeke", which is now on display in the Smithsonian.


External links

  • Marcus Bennett's S-100 Manuals Collecion Focusing on Cromemco and other S-100 manufacturers.
  • Howard Harte's S-100 Manuals Collection

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