S-100 bus
Encyclopedia
The S-100 bus or Altair bus, IEEE696-1983 (withdrawn), was an early computer bus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 the zenith of the microcomputer world until the advent of the IBM PC
(which some of them outperformed).
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).
on a fifth. He then looked for a cheap source of connectors, and he came across a supply of military surplus 100-pin edge connector
s.
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. Although the exact details are unclear, some time in 1976 the "S-100 bus" name was agreed on by the major third-party vendors, apparently for "Standard 100 pin bus".
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.
Several other buses were designed with minor improvements on 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"); the 32-pin STEbus
; etc.
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.
Computer bus
In computer architecture, a bus is a subsystem that transfers data between components inside a computer, or between computers.Early computer buses were literally parallel electrical wires with multiple connections, but the term is now used for any physical arrangement that provides the same...
designed in 1974 as a part of the Altair 8800
Altair 8800
The MITS Altair 8800 was a microcomputer design from 1975 based on the Intel 8080 CPU and sold by mail order through advertisements in Popular Electronics, Radio-Electronics and other hobbyist magazines. The designers hoped to sell only a few hundred build-it-yourself kits to hobbyists, and were...
, generally considered today to be the first personal computer
Personal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
(or at least the first microcomputer
Microcomputer
A microcomputer is a computer with a microprocessor as its central processing unit. They are physically small compared to mainframe and minicomputers...
, as it was designed for hobbyists rather than the general public). The S-100 bus was the first industry standard expansion bus
Expansion bus
An expansion bus is made up of electronic pathways which move information between the internal hardware of a computer system and peripheral devices. It is a collection of wires and protocols that allows for the expansion of a computer.- History :The first kit-built microcomputers used a bus design...
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
Homebrew Computer Club
The Homebrew Computer Club was an early computer hobbyist users' group in Silicon Valley, which met from March 5, 1975 to December 1986...
) implemented drivers for CP/M
CP/M
CP/M was a mass-market operating system created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc...
and MP/M
MP/M
MP/M was a 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 using a separate terminal....
. 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
IBM PC
The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...
(which some of them outperformed).
Architecture
The S-100 bus essentially consisted of the pins of the Intel 8080Intel 8080
The Intel 8080 was the second 8-bit microprocessor designed and manufactured by Intel and was released in April 1974. It was an extended and enhanced variant of the earlier 8008 design, although without binary compatibility...
run out onto the backplane
Backplane
A backplane is a group of connectors connected 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 backbone to connect several printed circuit boards together to make up a complete...
to form the single system bus
System bus
A system bus is a single computer bus that connects the major components of a computer system. The technique was developed to reduce costs and improve modularity....
. One early, unanticipated shortcoming was various power lines of differing voltages being located next to each other, resulting in easy shorting
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....
. This was addressed in later systems. The system included two unidirectional 8-bit
8-bit
The first widely adopted 8-bit microprocessor was the Intel 8080, being used in many hobbyist computers of the late 1970s and early 1980s, often running the CP/M operating system. The Zilog Z80 and the Motorola 6800 were also used in similar computers...
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
Voltage regulator
A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. A voltage regulator may be a simple "feed-forward" design or may include negative feedback control loops. It may use an electromechanical mechanism, or electronic components...
on the cards to +5 V (used by TTL
Transistor-transistor logic
Transistor–transistor logic is a class of digital circuits built from bipolar junction transistors and resistors. It is called transistor–transistor logic because both the logic gating function and the amplifying function are performed by transistors .TTL is notable for being a widespread...
) and ±12 V (typically used on RS-232
RS-232
In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
lines or disk drive motors).
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 was split into four separate cards, with the CPUCentral processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
on a fifth. He then looked for a cheap source of connectors, and he came across a supply of military surplus 100-pin edge connector
Edge connector
An edge connector is the portion of a printed circuit board 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 single discrete female connector , and they also tend to be...
s.
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. Although the exact details are unclear, some time in 1976 the "S-100 bus" name was agreed on by the major third-party vendors, apparently for "Standard 100 pin bus".
Another designer who did a great deal to push the S-100 technology forward was George Morrow
George Morrow (computers)
George Morrow was part of the early microcomputer industry in the United States. Morrow promoted and improved the S-100 bus used in many early microcomputers...
, 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.
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 first product launch was the IMSAI 8080 in 1975. One...
, Cromemco
Cromemco
Cromemco was a Mountain View, California microcomputer company known for its high-end Z80-based S-100 bus computers in the early days of the home computer revolution. The Cromemco Dazzler was the first color graphics card available for personal computers....
, Godbout Electronics (later CompuPro
Bill Godbout
Bill Godbout was an early computer pioneer and entrepreneur known for manufacturing and selling computer equipment, parts and Electronic kits in Silicon Valley, before the time of the Apple II....
), and Ithaca Intersystems
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 worked in a small rented space in the Collegtown neighborhood adjacent to the university campus. They...
. 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
CP/M
CP/M was a mass-market operating system created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc...
. The standard was so powerful that many other CPU designs were either made to "look" like the 8080 (most notably the Zilog Z80
Zilog Z80
The Zilog Z80 is an 8-bit microprocessor designed by Zilog and sold from July 1976 onwards. It was widely used both in desktop and embedded computer designs as well as for military purposes...
), or otherwise placed on complex converter cards to allow them to be plugged into S-100 machines.
Several other buses were designed with minor improvements on the S-100 bus:
the 50-pin "Benton Harbor Bus" used in the Heathkit H8
Heathkit H8
Heathkit's H8 was an Intel 8080-based microcomputer sold in kit form starting in 1977. The H8 was similar to the S-100 bus computers of the era, and like those machines was often used with the CP/M operating system on floppy disk...
; the SS-50 Bus
SS-50 Bus
The SS-50 bus was an early computer bus designed as a part of the SWTPC 6800 Computer System that used the Motorola 6800 CPU. The SS-50 motherboard would have around seven 50-pin connectors for CPU and memory boards plus eight 30-pin connectors for I/O boards...
used in a variety of 6800 and 6809 computers; the 56-pin STD Bus
STD Bus
STD Bus is a computer bus popular with industrial control applications but has also been used in computing applications. The STD Bus has been called "STD-80" as well: this has created a popular confusion that the bus contains 80 pins; however, it refers to its bus being related to the Zilog Z80...
("STD-80 bus"); the 32-pin STEbus
STEbus
The STEbus is a non-proprietary, processor-independent, bus with 8 data lines and 20 address lines. It was popular for industrial control systems in the late 1980s and early 1990s before the ubiquitous IBM PC dominated this market....
; etc.
As microcomputers got smaller and faster, S-100 became obsolete. The Apple II
Apple II
The Apple II is an 8-bit home computer, one of the first highly successful mass-produced microcomputer products, designed primarily by Steve Wozniak, manufactured by Apple Computer and introduced in 1977...
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.
External links
- 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