|Created by||Ed Roberts|
|Width in bits||8|
The S-100 bus or Altair bus, IEEE696-1983 (withdrawn), was an early computer bus designed in 1974 as a part of the Altair 8800, generally considered today to be the first personal computer (or at least the first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, as it was designed for hobbyists rather than the general public). The S-100 bus was the first industry standard expansion 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 common in early home computers 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 to form the single system bus. One early, unanticipated shortcoming was various power lines of differing voltages being located next to each other, resulting in easy shorting. This was addressed in later systems. 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) and ±12 V (typically used on RS-232 lines or disk drive motors). The onboard voltage regulation was typically performed by devices of the 78xx family (for example, a 7805 device to produce +5 volts). These were linear regulators which, depending on the current load, could produce a good deal of heat.
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 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.
A burgeoning industry of "clone" machines followed the introduction of the Altair in 1975. Most of these used the same bus layout as the Altair, creating a new industry standard. These companies were forced to refer to the system as the "Altair bus", and wanted another name in order to avoid naming their competitor when describing their own system. The “S-100” name was coined by Harry Garland and Roger Melen, co-founders of Cromemco, while on a flight to attend the Atlantic City PC '76 microcomputer conference in August 1976. The term first appeared in print in a Cromemco advertisement in the November 1976 issue of Byte magazine. The first symposium on the S-100 bus was held November 20, 1976 at Diablo Valley College with a panel consisting of Harry Garland, George Morrow , and Lee Felsenstein; Jim Warren served as moderator."
George Morrow, with his company Morrow Designs, did a great deal to advance the S-100 technology. Morrow was the first chairman of the S-100 Bus Standards Committee, which by the end of 1983 became the IEEE-696 standard. 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.
Cromemco was the largest of the S-100 manufacturers, followed by Vector Graphic and North Star Computers. Other innovators were companies such as Alpha Microsystems, IMS Associates, Inc., Godbout Electronics (later CompuPro), and Ithaca Intersystems. In May 1984 Microsystems published a comprehensive S-100 product directory listing over 500 "S-100/IEEE-696" products from over 150 companies.
The S-100 bus signals were simple to create using an 8080 CPU, but increasingly less so when using other processors like the 68000. More board space was occupied by signal conversion logic. Nonetheless by 1984 eleven different processors were hosted on the S-100 bus, from the 8-bit Intel 8080 to the 16-bit Zilog Z-8000. In 1986 Cromemco introduced the XXU card utilizing a 32-bit Motorola 68020 processor.
Several other buses were designed that competed with the S-100 bus: the 50-pin "Benton Harbor Bus" used in the Heathkit H8; the SS-50 Bus used in a variety of 6800 and 6809 computers. The 56-pin STD Bus ("STD-80 bus"); used signals of the Z80, which had soon replaced the 8080. The 64-pin STEbus used the far more robust DIN41612 connector, and was not based on any particular processor so it was equally practical to host any processor from a Z80 to a 68020.
As microcomputers got smaller and faster, S-100 became obsolete. The Apple II 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 in 1981 and later extended to 16-bit in 1984 with the IBM PC/AT, the undisputed standard expansion bus for personal computers shortly after. Note that in early S-100 systems, the S-100 bus is not just for expansion; it is a passive backplane that also ties together the essential parts of the system including CPU and memory. The higher chip integration and circuit board density available in later years allowed designers to combine the processor with memory and some I/O functions such as serial ports on one card.
- The S-100 Bus: Past, Present, and Future, InfoWorld, Feb 18, 1980
- Freiberger, Paul; Swaine, Michael (2000). Fire in the Valley: The Making of the Personal Computer (Second ed.). McGraw-Hill. p. 66. ISBN 0-07-135892-7.
- "The Cromemco Story". I/O News 1 (1): 10. September/October 1980. Retrieved 2013-02-22.
- Herbert Johnson, "Origins of S-100 computers", l5 March 2008
- Robert Reiling (December 10, 1976). "Random Data". Homebrew Computer Club Newsletter 2 (11-12): 1.
- Morrow, George; Howard Fullmer (May 1978). "Microsystems Proposed Standard for the S-100 Bus. Preliminary Specification, IEEE Task 696.1/D2". Computer (IEEE) 11 (5): 84–90. doi:10.1109/C-M.1978.218190. ISSN 0018-9162.
- Libes, Sol (September/October 1981). Microsystems 2 (5): 8. "The leaders in the S-100 marketplace are Cromemco ($50M), Vector Graphics ($30M) and North Star ($25M)"
- Libes, Sol (May 1984). "S-100 Product Directory". Microsystems 5 (5): 59–78.
- "New XXU Processor Offers Enormous Speed Advantage". I/O News 5 (4): 1. August/September 1986. ISSN 0274-9998.
- IEEE, "IEEE Standard, 696 Interface Devices", 1983
- "S100 Computers", A website containing many photos of cards, documentation, and history
- ""Cromemco" based, S-100 micro-computer", Robert Kuhmann's images of several S-100 cards
- "Herb's S-100 Stuff", Herbert Johnson's collection of S-100 history
- "IEEE-696 / S-100 Bus Documentation and Manuals Archive", Howard Harte's S-100 manuals collection