World Library  
Flag as Inappropriate
Email this Article

S-100 bus

Article Id: WHEBN0000152036
Reproduction Date:

Title: S-100 bus  
Author: World Heritage Encyclopedia
Language: English
Subject: Cromemco OCTART, IMSAI 8080, Bus (computing), Cromemco Bytesaver, Micromation
Collection: 1974 Introductions, Computer Buses, Early Microcomputers, Ieee Standards
Publisher: World Heritage Encyclopedia

S-100 bus

S-100 bus
Year created 1974 (1974)
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The S-100 bus or Altair bus, IEEE696-1983 (withdrawn), was an early computer bus designed in 1974 as a part of the Altair 8800, generally considered today to be the first microcomputer designed for hobbyists rather than end users. 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).


  • Architecture 1
  • History 2
  • IEEE-696 Standard 3
  • Retirement 4
  • References 5
  • External links 6


Harry Garland and Roger Melen, co-founders of Cromemco, holding S-100 backplane (1981)

The S-100 bus is a passive backplane of 100-pin printed circuit board edge connectors wired in parallel. Circuit cards measuring 5 × 10-inches serving the functions of CPU, memory, or I/O interface plugged into these connectors. The bus signal definitions closely follow those of an 8080 microprocessor system, since the Intel 8080 microprocessor was the first microprocessor hosted on the S-100 bus. The 100 lines of the S-100 bus can be grouped into four types: 1) Power, 2) Data, 3) Address, and 4) Clock and control.[1]

Power supplied on the bus was unregulated +8 V and ±16 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 commonly were mounted on heat sinks.

The bi-directional 8-bit data bus of the Intel 8080 was split into two unidirectional 8-bit data buses. Later these two 8-bit busses would be combined to support a 16-bit data width for more advanced processors.

The address bus was 16-bits wide in the initial implementation and later extended to 24-bits wide. A bus control signal could put these lines in a tri-state condition to allow direct memory access. The Cromemco Dazzler, for example, was an early S-100 card that retrieved digital images from memory using direct memory access.

Clock and control signals were used to manage the traffic on the bus. For example the DO Disable line would tristate the address lines during direct memory access. Unassigned lines of the original bus specification were later assigned to support more advanced processors. For example the Zilog Z-80 processor had a non-maskable interrupt line that the Intel 8080 processor did not. One unassigned line of the S-100 bus then was reassigned to support the non-maskable interrupt request.


The Cromemco XXU processor board, introduced in 1986. At 16.7 MHz it was the fastest CPU ever developed for the S-100 bus. It used a Motorola 68020 processor with 68881 co-processor and 16 Kbytes of high-speed cache memory. This CPU was used in the Cromemco CS-250 computer, widely deployed by the U.S. Air Force.

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 an inexpensive 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.[2]

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 referring to their competitor when describing their own system. The “S-100” name was coined by Lee Felsenstein.[6] Just one year later the S-100 Bus would be described as "the most used busing standard ever developed in the computer industry."[7]

Cromemco was the largest of the S-100 manufacturers, followed by Vector Graphic and North Star Computers.[8] 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.[9]

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.[9] In 1986 Cromemco introduced the XXU card, designed by Ed Lupin, utilizing a 32-bit Motorola 68020 processor.[10]

IEEE-696 Standard

As the S-100 bus gained momentum there was a need to develop a formal specification of the bus to help assure compatibility of products produced by different manufacturers, and to extend the bus so that it could support processors more capable than the Intel 8080 used in the original Altair Computer. In May 1978

  • 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

External links

  1. ^ Garland, Harry (1979). Introduction to Microprocessor System Design. New York: McGraw-Hill. pp. 159–169.  
  2. ^ The S-100 Bus: Past, Present, and Future, InfoWorld, Feb 18, 1980
  3. ^  
  4. ^ "The Cromemco Story". I/O News 1 (1): 10. September–October 1980. Retrieved 2013-02-22. 
  5. ^ Herbert Johnson, "Origins of S-100 computers", l5 March 2008
  6. ^ Robert Reiling (December 10, 1976). "Random Data". Homebrew Computer Club Newsletter 2 (11-12): 1. 
  7. ^ Zaks, Rodnay (1977). Microprocessors - From Chips to Systems. Sybex. p. 302. 
  8. ^ 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) 
  9. ^ a b Libes, Sol (May 1984). "S-100 Product Directory". Microsystems 5 (5): 59–78. 
  10. ^ "New XXU Processor Offers Enormous Speed Advantage". I/O News 5 (4): 1. August–September 1986.  
  11. ^ Morrow, George; Fullmer, Howard (May 1978). "Proposed Standard for the S-100 Bus" (PDF). Computer (IEEE Computer Society) 11 (5): 84–90.  
  12. ^ Elmquist, Kells A.; Fullmer, Howard; Gustavson, David B.; Morrow, George (July 1979). "Standard Specification for S-100 Bus Interface Devices" (PDF). Computer (IEEE Computer Society) 12 (7): 28–52.  
  13. ^ a b c "An American National Standard: IEEE 696 Standard Interface Devices". 
  14. ^ Libes, Sol (May 1984). "S-100 Product Directory". Microsystems 5 (5): 59. However there is no doubt that the S-100 market can now be considered a mature industry with only moderate growth potential, compared to the IBM PC-compatible market. 
  15. ^ Breeding, Gary (January–February 1984). "Cromemco Systems Network Transactions at Chaotic Exchange". I/O News 3 (6): 20.  
  16. ^ "USAF will equip its tactical fighter squadrons with a mission planning system". Aviation Week & Space Technology 126 (22): 105. June 1, 1987. 
  17. ^ Libes, Sol (May 1984). "S-100 Product Directory". Microsystems 5 (5): 59. Whereas the early growth of the S-100 marketplace relied mainly on hobbyists and early personal computer users, the industry is now concentrating on OEM multiuser systems, and applications requiring more computer power. 
  18. ^ "CME Taps Datacode To Distribute Quotation Data To Floor Traders". WatersTechnology. January 27, 1992. 


The market for S-100 bus products continued to contract through the early 1990s, as IBM computers became more capable. In 1992 the Chicago Mercantile Exchange, for example, replaced their S-100 bus computers with the IBM model PS/2.[18] By 1994 the S-100 bus industry had contracted sufficiently that the IEEE did not see a need to continue supporting the IEEE-696 standard. The IEEE-696 standard was retired on June 14, 1994.[13]

As the IBM PC products captured the low-end of the market, S-100 machines moved up-scale to more powerful OEM and multiuser systems. Banks of S-100 bus computers were used, for example, to process the trades at the Chicago Mercantile Exchange; the United States Air Force deployed S-100 bus machines for their mission planning systems.[15][16] However throughout the 1980s the market for S-100 bus machines for the hobbyist, for personal use, and even for small business was on the decline.[17]

IBM introduced the IBM Personal Computer in 1981 and followed it with increasingly capable models: the XT in 1983 and the AT in 1984. The success of these computers cut deeply into the market for S-100 bus products. In May 1984 Sol Libes (who had been a member of the IEEE-696 Working Group) wrote in Microsystems: “there is no doubt that the S-100 market can now be considered a mature industry with only moderate growth potential, compared to the IBM PC-compatible market.”[14]

Racks of Cromemco S-100 Systems at the Chicago Mercantile Exchange in 1984


The American National Standards Institute (ANSI) approved the IEEE standard on September 8, 1983. The computer bus structure developed by Ed Roberts for the Altair 8800 computer had been extended, rigorously documented, and now designated as the American National Standard IEEE Std 696-1983.[13]

In July 1979 Kells Elmquist, Howard Fullmer, David Gustavson, and George Morrow published a “Standard Specification for S-100 Bus Interface Devices.”[12] In this specification the data path was extended to 16 bits and the address path was extended to 24 bits. The IEEE 696 Working Group, chaired by Mark Garetz, continued to develop the specification which was proposed as an IEEE Standard and approved by the IEEE Computer Society on June 10, 1982.[13]


This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.

Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.