Standard computer bus system
developed by MITS
for their Altair 8800 computer

General description

The S-100 bus, originating in 1974 with the Altair 8800 from MITS, was pivotal in the early personal computing landscape. MITS designed this 100-pin edge connector backplane to enable expansion for the Altair 8800, giving rise to a burgeoning community of enthusiasts.

The Altair 8800, showcased in Popular Electronics in 1975, garnered attention for its simplicity and openness. This model’s success catalyzed the adoption of the S-100 bus by other computer manufacturers, creating a diverse ecosystem.

This bus system facilitated extensive expansion capabilities. Users could add components such as memory boards, storage controllers, and communication interfaces, fostering a culture of innovation and experimentation. The S-100 bus became synonymous with the do-it-yourself ethos, attracting hobbyists eager to customize their computing experience.

Digital Research’s CP/M operating system found widespread use with S-100 systems. Its standardization streamlined software compatibility, contributing to the popularity of S-100-based computers.

Beyond the Altair 8800, various S-100 computers gained prominence, including the IMSAI 8080, Cromemco systems, and NorthStar Horizon. Each system added its unique flavor to the S-100 ecosystem, expanding its reach.

However, the S-100 bus faced challenges as the computing industry matured. Newer bus standards like ISA and PCI emerged, offering enhanced features and widespread adoption. This transition marked the decline of the S-100 bus in mainstream computing.

Despite its eventual obsolescence, the S-100 bus holds historical significance. It symbolizes the nascent era of microcomputing, characterized by community-driven innovation and the birth of personal computing. The S-100’s legacy lives on as a testament to the foundational role it played in shaping the trajectory of computer history.

S-100 Bus Boards Examples

In my collection, I have several printed circuit boards in the S-100 bus standard:

S100 IMS MPU-A 8080A board

The MPU-A board is the processor board for the IMSAI 8080 Microcomputer System. It is designed using the 8080 microprocessor chip. The bus arrangement and board connector has been chosen to be 100% compatible with the PUTS Altair M8800. Microcomputer system so that all boards are 100% interchangeable between the Altair system and the IMSAI 8080 system.

The IMSAI MPU-A board is structured around the 8080A microprocessor chip, and much of the MPU-A board is wired to support the 8080A device. The MPU-A board provides interfacing between the 8080A chip and the data and address busses, clock and synchronization signals, and the voltage regulation necessary for the 8080A and other chips. The internal functioning of the 8080A is thoroughly described in the Intel 8080 Microcomputer System User’s Manual.

S100 Cromemco ZPU Z80 board

The Cromemco ZPU is an S-I00 bus CPU (Central Processing Unit) using the powerful Z-80 microprocessor. This CPU is used in Cromemco computers and is also a plug-in replacement for the CPU card used in the Imsai 8080 and Altair 8800 computers.

The Cromemco ZPU has an exclusive set of features to increase the power of your total computing system. Most important, the ZPU was designed from the outset for reliable operation at 4MHz clock rate – twice the speed of most microcomputer systems. The ZPU also offers an on board wait state generator, optional independent selection of M1 wait states, power on jump capability, address mirroring circuitry, refresh address mirroring circuitry, and other features all discussed in this

S100 SD-System SBC-200 board

The SBC-200 is a single board microcomputer designed around the powerful Z80 microprocessor. The board operates on the industry standard S-100 bus. The Single Board Computer SBC-200 is suited for data processing, industrial and process control applications.


  • S-100 Bus Compatible
  • Z-80 Central Processing Unit
  • 1024 Bytes of Random Access Memory
  • 8K Bytes of EPROM using 2716
  • Serial Input/Output Port (with Asynchronous and Synchronous Operation)
  • Parallel Input and Output Ports
  • Four Channel Counter/Timer (Z80-CTC)
  • Software Programmable Baud Rate Generator
  • No Front Panel Required For Operation
  • 4 Mhz Operation

S100 Vector Graphics ZCB CPU board

The Vector Graphic ZCB Single Board Computer provides the capability of a complete computer system on a single board. The ZCB contains what you would normally find on a CPU board, a PROM/RAM board and an I/O board, all contained on one 5-100 bus compatible board.

The ZCB contains a Z-80A microprocessor, 1024 bytes of static RAM memory, sockets for up to 12K of PROM, one serial I/O port and three 8-bit parallel ports. Circuitry is provided to support static or dynamic memories. All S-100 8080 signals are also generated making the ZCB compatible with 8080 based systems. On-board power regulation and filtering is provided using IC regulators and heat sinks for power dissipation. Input loading on the bus is 1 low power TTL load while output fanout is 10 TEL loads.

While the ZCB does replace 3 boards in a typical system, it is not a complete stand alone microcomputer on one board- A terminal device of some sort is needed as well as additional RAM memory, for most applications, The ZCB could, however, be used as a complete microcomputer in many process control applications. In this case, preprogrammed PROMs (up to 12K bytes worth) could be used on a ZCB board that was equipped with a power supply. Sense data could arrive by an input parallel port, be processed by the CPU section of the board and any data output (control signals, for instance) could be communicated to the equipment via the output parallel port. The data input and a summary of actions taken could be communicated to a central data processing location by the serial port. Thus the ZCB might be used as an intellegent node on a complete process control system, which could decide and provide output immediately and provide information to a higher level data processing system.

S100 Vector Graphics Z-80 board

The Z-80 Board is designed for operation in an S-100 system. It generates ALL the. S-100 signals originally generated by 8080 CPU boards following the S-100 definition, and accepts as input the S-100 signals intended as input to an 8080 processor board, with two exceptions:

1) Vectored interrupt request lines (5-100 lines 4 – 11) are not received or handled in any way by the board.

2) The timing of some signals, particularly PSYNC and the status signals, is slightly different than that produced by 8080 boards, but the differences are usually inconsequential.

Also, a few additional lines are used, required for dynamic memory in a Z-80 based system.

S100 SD-System VDB-8024 video board

VDB-8024 is a video display board which provides a high quality means for interfacing a monitor and keyboard to the S-100 bus. The display is interfaced through the S-100 bus using a single output port and provides an input port through which a keyboard can be interfaced to the S-100 bus. The output generated by the board is both composite video and separate TTL levels for horizontal sync, vertical sync and video input. This provides an easy interface to most standard monitors. The video display board, with the addition of a keyboard and a monitor, functions as a complete terminal.

The VDB utilizes a Z80 microprocessor to perform the control functions of a video terminal. These functions inlcude LF, CR, full cursor control, home, clear, scroll and back space. In addition to the basic function several special additional features are provided. One of these features provides for specified fields of characters to be enhanced. This field can be enhanced in any of the following ways:
Underline, Reverse, Blink, Protect or combinations. Another feature of this board is the programmable character generator. This is a 7 x 8 dot character matrix with 2 bits of descenders to provide upper and lower case characters in a 7 x 10 dot field. 128 characters are typically available while 256 characters can be made available using a 2K x 8 PROM. These characters are displayed in a field of 80 characters in length and 24 lines per page. The VDB processor runs only during H & V sync to eliminate any disturbance of the display due to the processor running during the raster scan. There are several software controllable functions. These include a speed control similar to a baud rate control and also a scroll up or scroll down. In addition, a set of 32 special characters are available.

S100 Cromemco 4KZ board

The Cromemco 4KZ memory board incorporates 4096 8-bit memory locations with on-board address generation and a control port for memory bank select. The 4KZ mates with the Cromemco ZPUTM to provide virtually full speed operation at 4mHz with reliable low-power static RAM chips. The 4KZ is also compatible with 8080-based systems and may be used in any 5-100 bus computer.

S100 Cromemco 16KZ board

The Cromemco 16KZ is a high-speed 16K RAM card designed for the S-100 microcomputer bus. The 16KZ can operate at 2MHz or 4 MHz system clock rates with no wait states whatsoever. A special Bank Select feature is incorporated on the 16KZ that allows memory space to be organized into as many as 8 banks of 64K each. The memory banks in which the 16KZ resides are selected by switches on each 16KZ card; the active banks of memory are selected under software control. The 16KZ also contains Direct Memory Access Override Circuitry that allows any DMA device to access different banks of memory during DMA operations than otherwise.

With its high-speed capability and memory bank expandability, the Cromemco 16KZ is designed to be a card of lasting value. It is particularly well-suited for use with the Cromemco line of S-100 bus computer equipment. It is also compatible with other S-100 bus computers including theAltair 8800 and the Imsai 8080.

S100 SD-Computer VERSAFLOPPY board

VERSAFLOPPY the versatile floppy disk controller board from S.D. COMPUTER PRODUCTS, provides a low cost means of interfacing to many of the available floppy disk drives, including: Shugart SA400/SA450; Shugart SA800/SA850; MFE 700/750; Persci 70; Persci 277; GSI GS-105. VERSAFLOPPY operates with many 8080 and Z80 CPU boards on the S-100 Bus. However, VERSAFLOPPY was designed to be used optimally with S.D.’s SBC-100, single board computer, and 32K Expandoram Boards to form a complete,low cost disk based, computer.

At the heart of VERSAFLOPPY is the powerful Western Digital FD1771B-1 NMOS LSI controller chip. This device performs most of the timing and control functions required by floppy disk drives.

S100 Solid State Music PB1 ROM board


  • S-100 bus computer systems.


  • 2 separate programming sockets for 2708 or 2716 (5V) EPROMs.
  • Meets all manufacturers data sheet requirements for programming.
  • Programming voltage generated on board — no need for an external power supply.
  • Programming sockets are DIP switch addressable to any 4K boundary.
  • Software control of 2708/2716 programming selection–no hardware reconfiguration required.
  • Provisions for 2 optional ZIP sockets from Textool for easier insertion and removal of EPROMs being programmed.
  • Special safety features to prevent accidental programming include LED indicator for programming mode and an on-off switch for programming voltage.


  • 4 separate sockets for 4K of 2708 or 8K of 2716 EPROMs.
  • Addressable by DIP switch to any 4K or 8K boundary above 8000 Hex.
  • Unused EPROM sockets do not enable data bus drive so the board is never committed to the full 4K or 8K of memory.
  • Jumper selectable wait states (0 to 4) for fast or slow EPROMs.


  • Complete subroutines for checking EPROM erasure, programming and verification.


  • Address and data lines fully buffered.
  • Solder masked PC board with gold plated edge connector contacts.
  • Low profile sockets provided for all ICs.
  • Power requirements:, +8V @ 500ma, +16V @ 25ma (less EPROM), -16V @ 5ma (less EPROM).

S100 Solid State Music IO-4 board


  • S-100 bus computer systems


  • Two serial 110 ports (2 in & 2 out) with full handshaking status from UART’s.
  • Status sense easily reversible; Status bit position jumper selectable.
  • 20/60 mA current-loop interface with optical isolators or EIA interface.
  • Independent baud-rate selection from 55 to 9600 baud.
  • Regulated +5V, +12V, & -12V outputs provided on both serial headers.
  • Number of stop bits, parity even or odd, word length and Status/Data port reversible by DIP switch control.


  • Two latched I/O ports (2 in & 2 out) using 8212 latch-buffers.
  • Regulated +5V & -12V outputs provided on all 4 parallel headers.


  • Independent DIP switches for setting address of the serial and parallel port blocks;
  • Serial I/O to any four port boundary, Parallel I/O to any two port boundary.


  • Interrupt capability provided for on serial and parallel I/O ports.
  • All jumpers on (C headers for easy I/O reconfiguration).
  • Two spare 16-pin DIP patterns provided.
  • Solder masked PC board with gold-plated edge connector contacts.
  • Low profile sockets provided for all ICs. Ribbon cable assemblies included.
  • Power requirements — +8V @ 0.95A, +16V @ 0.6A, & -16V @ 80mA typical.

S100 Mountain Hardware PROM board

Mountain Hardware’s PROROM board is a general purpose PROM  (programmable read only memory) module compatible with the S-100 computer bus. The board has provisions for 71/2 kilobytes of ultraviolet erasable proms (EPROMs). There are also sockets for 512 bytes of random access memory (RAM). The board has a built in programmer that will program any of the proms on the board. The programmer does not need any special software. Any number of bytes may be programmed at a time.

The PROROM board is supplied in several forms. Typically it includes one 6834 EPROM (512 bytes) and one 3539 RAM (256 bytes). A monitor routine is pre-programmed into the EPROM, which allows the user to perform basic functions with the host computer.

S100_Micro-Disk Controller MDC-A2 board

The North Star MICRO-DISK System (MDS) controller is an integrated system of hardware and software. It is designed specifically to provide a complete, compact and economical disk drive controller for use with S-100 bus 8080 or Z80 microcomputer systems.

The MDS controller is implemented entirely from meditim and small scale TTL integrated circuits and PROM memory. The entire controller fits on a,single 5″x10″ printed circuit card. The following diagram.shows the general organization of the MDS controller.

S100 Motherboard