Multibus is a computer bus standard used in industrial systems. It was developed by Intel Corporation and was adopted as the IEEE 796 bus.[1]

The Multibus specification was important because it was a robust, well-thought out industry standard with a relatively large form factor so complex devices could be designed on it. Being a well-defined and well-documented industry standard allowed a Multibus-compatible industry to grow around it. There were many companies making card cages and enclosures for it. Many others made CPU, memory, and other peripheral boards. In 1982 there were over 100 Multibus board and systems manufacturers.[2] This allowed complex systems to be built from commercial off-the-shelf hardware. It also allowed companies to innovate by designing a proprietary Multibus board and then integrating it with other vendors' hardware to create a system. A good example of this is Sun Microsystems with their Sun 1 and Sun 2 workstations. Sun built custom designed CPU, memory, SCSI, and video display boards and then added 3com Ethernet networking boards, Xylogics SMD disk controllers, Ciprico Tapemaster 1/2 inch tape controllers, Sky Floating Point Processor and Systech 16 port Terminal Interfaces to configure the system as a workstation or a file server.[3] Other workstation vendors who used Multibus-based designs included HP/Apollo[4] and Silicon Graphics IRIS.[5]


Multibus architecture

Multibus is an asynchronous bus that accommodates devices with various transfer rates while maintaining maximum throughput. It had 20 address lines so it could address up to 1 Mb of Multibus memory and 1 Mb of I/O locations. Most Multibus I/O devices only decoded the first 64 Kb of address space.

Multibus supported multi-master functionality that allowed it to share the Multibus with multiple processors and other DMA devices.[6]

The standard Multibus form factor was a 12-inch-wide (300 mm), 6.75-inch-deep (171 mm) circuit board with two ejection levers on the front edge. The board had two buses. The wider P1 bus which pin assignment was defined by the Multibus specification. A second smaller P2 bus was also defined as a private bus.

Multibus standards

Multibus includes the following buses:

  • Multibus System Bus - adopted as IEEE 796
  • iSBX (I/O Expansion Bus) - adopted as IEEE P959
  • iLBX (Execution Bus)
  • Multichannel I/O Bus


Multibus I

IEEE-796: Microcomputer System Bus; First released by Intel in 1974. The cards did not use front panels, and they used card edge fingers as the connectors (similar to ISA/PC-AT cards). Companies like Northwest Technical still provide "End of Life" products for Multibus. This bus is obsolete.

  • IEC 796-1:1990 Microprocessor system bus—8-bit and 16-bit data (MULTIBUS I) -- Part 1: Functional description with electrical and timing specifications
  • IEC 796-2:1990 Microprocessor system bus—8-bit and 16-bit data (MULTIBUS I) -- Part 2: Mechanical and pin descriptions for the system bus configuration, with edge connectors (direct)
  • IEC 796-3:1990 Microprocessor system BUS I, 8-bit and 16-bit data (MULTIBUS I) -- Part 3: Mechanical and pin descriptions for the Eurocard configuration with pin and socket (indirect) connectors

Multibus II

IEEE-1296 32-bit/10MHz bus, at 40Mbyte/s. Card sizes are 3U x 220mm, and 6U x 220mm. These cards are larger than the VME Eurocard sizes which are 3U/6U x 160mm. Uses TTL ('Fast' series) gates for drivers and the Backplane Connectors are DIN41612 type C. Multibus II is not yet considered obsolete, but considered mature; however it is not recommended for new designs. IEEE-STD-1296: High-performance synchronous 32-bit bus: MULTIBUS II, released in 1987, and 1994. Also as ISO/IEC 10861.

  • ISO/IEC 10861:1994 Information technology—Microprocessor systems—High-performance synchronous 32-bit bus: MULTIBUS II

Historical uses

Multibus II hardware running Intel iRMX operating system is used in the majority core systems on CLSCS the London Underground Central Line signals control system was supplied by Westinghouse now Invensys and commissioned in the late 1990s. The Central Line is an automatic train operation line. Automatic train protection is by trackside and train borne equipment that does not use Multibus. It is the automatic train supervision elements that use a mix of iRMX on Multibus, and Sun Solaris operating system on Sparc computers. 16 Multibus local site computers are distributed along the Central Line together with 6 central Multibus computers at the control centre. All 22 Multibus computers are dual redundant. CLSCS Multibus continues in full operation in 2011.

Oslo Metro or Oslo Tunnelbane uses a similar although less complex Westinghouse supplied Multibus hardware control system through the central Common Tunnel or Fellestunnelen tracks but was expected to be decommissioned in 2011.

See also


  1. ^ "Ieee Standard Microcomputer System Bus". Retrieved 2011-11-21. 
  2. ^ The SUN Workstation Architecture, Andreas Bechtolsheim, Forest Baskett, Vaughan Pratt, Stanford University Computer systems Laboratory Technical Report No. 229, March 1982
  3. ^ The Sun Hardware Reference[dead link]
  5. ^ Silicon Graphics IRIS 2000/3000 FAQ
  6. ^ Sun 68000 Board User's Manual, Sun Microsystems, Inc, February 1933, Revision B

External links

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  • Sun-1 — was the first generation of UNIX computer workstations and servers produced by Sun Microsystems, launched in May 1982. These were based on a CPU board designed by Andy Bechtolsheim while he was a graduate student at Stanford University and funded …   Wikipedia

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