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The S-100 bus or Altair bus, IEEE 696-1983 ''(withdrawn)'', is an early
computer bus In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This ex ...
designed in 1974 as a part of the
Altair 8800 The Altair 8800 is a microcomputer designed in 1974 by MITS and based on the Intel 8080 CPU. Interest grew quickly after it was featured on the cover of the January 1975 issue of Popular Electronics and was sold by mail order through advertiseme ...
. The bus was the first industry standard
expansion bus In computing, an expansion card (also called an expansion board, adapter card, peripheral card or accessory card) is a printed circuit board that can be inserted into an electrical connector, or expansion slot (also referred to as a bus slo ...
for the microcomputer industry. computers, consisting of processor and peripheral cards, were produced by a number of manufacturers. The bus formed the basis for homebrew computers whose builders (e.g., the
Homebrew Computer Club The Homebrew Computer Club was an early computer hobbyist group in Menlo Park, California, which met from March 1975 to December 1986. The club had an influential role in the development of the microcomputer revolution and the rise of that aspec ...
) implemented drivers for
CP/M CP/M, originally standing for Control Program/Monitor and later Control Program for Microcomputers, is a mass-market operating system created in 1974 for Intel 8080/ 85-based microcomputers by Gary Kildall of Digital Research, Inc. Initial ...
and
MP/M MP/M (Multi-Programming Monitor Control Program) is a discontinued multi-user version of the CP/M operating system, created by Digital Research developer Tom Rolander in 1979. It allowed multiple users to connect to a single computer, each us ...
. These 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 The IBM Personal Computer (model 5150, commonly known as the IBM PC) is the first microcomputer released in the IBM PC model line and the basis for the IBM PC compatible de facto standard. Released on August 12, 1981, it was created by a team ...
.


Architecture

The bus is a
passive backplane A backplane (or "backplane system") is a group of electrical connectors in parallel with each other, so that each pin of each connector is linked to the same relative pin of all the other connectors, forming a computer bus. It is used as a backbo ...
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 The Intel 8080 (''"eighty-eighty"'') is the second 8-bit microprocessor designed and manufactured by Intel. It first appeared in April 1974 and is an extended and enhanced variant of the earlier 8008 design, although without binary compatibil ...
microprocessor was the first microprocessor hosted on the bus. The 100 lines of the bus can be grouped into four types: 1) Power, 2) Data, 3) Address, and 4) Clock and control. Power supplied on the bus is bulk unregulated +8 Volt DC and ±16 Volt DC, designed to be
regulated Regulation is the management of complex systems according to a set of rules and trends. In systems theory, these types of rules exist in various fields of biology and society, but the term has slightly different meanings according to context. Fo ...
on the cards to +5 V (used by
TTL TTL may refer to: Photography * Through-the-lens metering, a camera feature * Zenit TTL, an SLR film camera named for its TTL metering capability Technology * Time to live, a computer data lifespan-limiting mechanism * Transistor–transistor lo ...
ICs), -5 V and +12 V for
Intel 8080 The Intel 8080 (''"eighty-eighty"'') is the second 8-bit microprocessor designed and manufactured by Intel. It first appeared in April 1974 and is an extended and enhanced variant of the earlier 8008 design, although without binary compatibil ...
CPU IC, ±12 V
RS-232 In telecommunications, RS-232 or Recommended Standard 232 is a standard originally introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between a ''DTE'' (''data terminal equipment'') such a ...
line driver ICs, +12 V for disk drive motors. The onboard voltage regulation is typically performed by devices of the
78xx 78xx (sometimes L78xx, LM78xx, MC78xx...) is a family of self-contained fixed linear voltage regulator integrated circuits. The 78xx family is commonly used in electronic circuits requiring a regulated power supply due to their ease-of-use and ...
family (for example, a 7805 device to produce +5 volts). These were
linear regulator In electronics, a linear regulator is a voltage regulator used to maintain a steady voltage. The resistance of the regulator varies in accordance with both the input voltage and the load, resulting in a constant voltage output. The regulating circ ...
s which are commonly mounted on heat sinks. The bi-directional 8-bit data bus of the Intel 8080 is split into two unidirectional 8-bit data buses. The processor could use only one of these at a time. The
Sol-20 The Sol-20 was the first fully assembled microcomputer with a built-in keyboard and television output, what would later be known as a home computer. The design was a combination of an Intel 8080-based motherboard, a VDM-1 graphics card, the 3P+S ...
used a variation that had only a single 8-bit bus and used the now-unused pins as signal grounds to reduce
electronic noise In electronics, noise is an unwanted disturbance in an electrical signal. Noise generated by electronic devices varies greatly as it is produced by several different effects. In particular, noise is inherent in physics, and central to the ...
. The direction of the bus, in or out, was signaled using the otherwise unused DBIN pin. This became universal in the market as well, making the second bus superfluous. Later, these two 8-bit buses would be combined to support a 16-bit data width for more advanced processors, using the Sol's system to signal the direction. The address bus is 16-bits wide in the initial implementation and later extended to 24-bits wide. A bus control signal can put these lines in a tri-state condition to allow direct memory access. The
Cromemco Dazzler The Cromemco Dazzler was a graphics card for S-100 bus computers introduced in a ''Popular Electronics'' cover story in 1976.Les Solomon"Solomon's Memory", in ''Digital Deli'', Workman Publications, 1984, It was the first color graphics card avail ...
, for example, is an early card that retrieved digital images from memory using direct memory access. Clock and control signals are used to manage the traffic on the bus. For example, the ''DO Disable'' line will 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 Zilog, Inc. is an American manufacturer of microprocessors and 8-bit and 16-bit microcontrollers. It is also a supplier of application-specific embedded system-on-chip (SoC) products. Its most famous product is the Z80 series of 8-bit microp ...
Z-80 The Z80 is an 8-bit microprocessor introduced by Zilog as the startup company's first product. The Z80 was conceived by Federico Faggin in late 1974 and developed by him and his 11 employees starting in early 1975. The first working samples were ...
processor has a
non-maskable interrupt In computing, a non-maskable interrupt (NMI) is a hardware interrupt that standard interrupt-masking techniques in the system cannot ignore. It typically occurs to signal attention for non-recoverable hardware errors. Some NMIs may be masked, but ...
line that the Intel 8080 processor does not. One unassigned line of the bus then was reassigned to support the non-maskable interrupt request.


History

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 is split into four separate cards, with the
CPU A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, and ...
on a fifth. He then looked for an inexpensive source of connectors, and he came across a supply of military surplus 100-pin
edge connector An edge connector is the portion of a printed circuit board (PCB) consisting of traces leading to the edge of the board that are intended to plug into a matching socket. The edge connector is a money-saving device because it only requires a sing ...
s. The 100-pin bus was created by an anonymous draftsman, who selected the connector from a parts catalog and arbitrarily assigned
signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
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 referring to their competitor when describing their own system. The "" name, short for "Standard 100", was coined by
Harry Garland Harry T. Garland (born 1947) is a scientist, engineer, author, and entrepreneur who co-founded Cromemco Inc., one of the earliest and most successful microcomputer companies. He received the B.A. degree in mathematics from Kalamazoo College, and ...
and
Roger Melen Roger Douglas Melen (born 1946) is an electrical engineer recognized for his early contributions to the microcomputer industry, and for his technical innovations. Dr. Melen was co-founder of Cromemco, one of the earliest microcomputer companies. ...
, co-founders of
Cromemco Cromemco was a Mountain View, California microcomputer company known for its high-end Zilog Z80, Z80-based S-100 bus computers and peripherals in the early days of the personal computer revolution. The company began as a partnership in 1974 betwe ...
. While on a flight to attend the Atlantic City PC '76 microcomputer conference in August 1976, they shared the cabin with Bob Marsh and
Lee Felsenstein Lee Felsenstein (born April 27, 1945) is an American computer engineer who played a central role in the development of the personal computer. He was one of the original members of the Homebrew Computer Club and the designer of the Osborne 1, the ...
of
Processor Technology Processor Technology Corporation was a personal computer company founded in April 1975 by Gary Ingram and Bob Marsh in Berkeley, California. Their first product was a 4K byte RAM board that was compatible with the Micro Instrumentation and Telemet ...
. Melen went over to them to convince them to adopt the same name. He had a beer in his hand and when the plane hit a bump, Melen spilt some the beer on Marsh. Marsh agreed to use the name, which Melen ascribes to him wanting to get Melen to leave with his beer. The term first appeared in print in a Cromemco advertisement in the November 1976 issue of
Byte magazine ''Byte'' (stylized as ''BYTE'') was a microcomputer magazine, influential in the late 1970s and throughout the 1980s because of its wide-ranging editorial coverage. "''Byte'' magazine, the leading publication serving the homebrew market ..." '' ...
.Herbert Johnson
"Origins of computers"
15 March 2008
The first symposium on the bus, moderated by Jim Warren, was held November 20, 1976 at
Diablo Valley College Diablo Valley College (DVC) is a public community college with campuses in Pleasant Hill and San Ramon in Contra Costa County, California. DVC is one of three public community colleges in the Contra Costa Community College District (along with ...
with a panel consisting of
Harry Garland Harry T. Garland (born 1947) is a scientist, engineer, author, and entrepreneur who co-founded Cromemco Inc., one of the earliest and most successful microcomputer companies. He received the B.A. degree in mathematics from Kalamazoo College, and ...
, George Morrow, and
Lee Felsenstein Lee Felsenstein (born April 27, 1945) is an American computer engineer who played a central role in the development of the personal computer. He was one of the original members of the Homebrew Computer Club and the designer of the Osborne 1, the ...
. Just one year later, the Bus would be described as "the most used busing standard ever developed in the computer industry."
Cromemco Cromemco was a Mountain View, California microcomputer company known for its high-end Zilog Z80, Z80-based S-100 bus computers and peripherals in the early days of the personal computer revolution. The company began as a partnership in 1974 betwe ...
was the largest of the manufacturers, followed by
Vector Graphic Vector graphics is a form of computer graphics in which visual images are created directly from geometric shapes defined on a Cartesian plane, such as points, lines, curves and polygons. The associated mechanisms may include vector display a ...
and
North Star Computers North Star Computers Inc. (later styled as NorthStar) was an American computer company based in Berkeley, California existing between June 1976 (when according to popular rumor it was formed as "Kentucky Fried Computers") and 1989. Originally a mai ...
. Other innovators were companies such as
Alpha Microsystems Alpha Microsystems, Inc., often shortened to Alpha Micro, was an American computer company founded in California in 1977. The company was founded in 1977 in Costa Mesa, California, by John French, Dick Wilcox and Bob Hitchcock. During the dot-com ...
,
IMS Associates, Inc. IMS Associates, Inc., or IMSAI, was a microcomputer company, responsible for one of the earliest successes in personal computing, the IMSAI 8080. The company was founded in 1973 by William Millard and was based in San Leandro, California. Their ...
, Godbout Electronics (later CompuPro), and
Ithaca Intersystems Ithaca Intersystems was a microcomputer manufacturer in the 1970s and 1980s, located in Ithaca, New York. The early years drew on engineering talent from Cornell University when the founders, including Steven Edelman, worked in a small rented spa ...
. In May 1984, ''Microsystems'' published a comprehensive product directory listing over 500 "/IEEE-696" products from over 150 companies. The 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 bus, from the 8-bit Intel 8080 to the 16-bit Zilog Z-8000. In 1986, Cromemco introduced the XXU card, designed by Ed Lupin, utilizing a 32-bit
Motorola Motorola, Inc. () was an American Multinational corporation, multinational telecommunications company based in Schaumburg, Illinois, United States. After having lost $4.3 billion from 2007 to 2009, the company split into two independent p ...
68020 The Motorola 68020 ("''sixty-eight-oh-twenty''", "''sixty-eight-oh-two-oh''" or "''six-eight-oh-two-oh''") is a 32-bit microprocessor from Motorola, released in 1984. A lower-cost version was also made available, known as the 68EC020. In keeping ...
processor.


IEEE-696 Standard

As the 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. There was also a need 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, George Morrow and Howard Fullmer published a “Proposed Standard for the Bus” noting that 150 vendors were already supplying products for the Bus. This proposed standard documented the 8-bit data path and 16-bit address path of the bus and stated that consideration was being given to extending the data path to 16 bits and the address path to 24 bits. In July 1979 Kells Elmquist, Howard Fullmer, David Gustavson, and George Morrow published a "Standard Specification for Bus Interface Devices." In this specification the data path was extended to 16 bits and the address path was extended to 24 bits. The
IEEE The Institute of Electrical and Electronics Engineers (IEEE) is a 501(c)(3) professional association for electronic engineering and electrical engineering (and associated disciplines) with its corporate office in New York City and its operation ...
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 The Institute of Electrical and Electronics Engineers (IEEE) is a 501(c)(3) professional association for electronic engineering and electrical engineering (and associated disciplines) with its corporate office in New York City and its operati ...
on June 10, 1982. The
American National Standards Institute The American National Standards Institute (ANSI ) is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organi ...
(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.


Retirement

IBM introduced the
IBM Personal Computer The IBM Personal Computer (model 5150, commonly known as the IBM PC) is the first microcomputer released in the IBM PC model line and the basis for the IBM PC compatible de facto standard. Released on August 12, 1981, it was created by a team ...
in 1981 and followed it with increasingly capable models: the XT in 1983 and the AT in 1984. The success of these computers, which used IBM's own, incompatible bus architecture, cut deeply into the market for 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". As the IBM PC products captured the low-end of the market, machines moved up-scale to more powerful OEM and multiuser systems. Banks of bus computers were used, for example, to process the trades at the Chicago Mercantile Exchange; the United States Air Force deployed bus machines for their mission planning systems. However throughout the 1980s the market for bus machines for the hobbyist, for personal use, and even for small business was on the decline. The market for bus products continued to contract through the early 1990s, as IBM-compatible computers became more capable. In 1992, the Chicago Mercantile Exchange, for example, replaced their bus computers with the IBM model PS/2. By 1994 the 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.


References


External links


"S100 Computers"
A website containing many photos of cards, documentation, and history

Robert Kuhmann's images of several cards

Herbert Johnson's collection of history
"IEEE-696 / Bus Documentation and Manuals Archive"
Howard Harte's manuals collection {{Computer-bus Computer buses *S-100 IEEE standards Computer-related introductions in 1974 Cromemco S-100 machines