Multibus

Multibus is a computer bus standard used in industrial systems. It was developed by the Intel company and Adopted as the IEEE 765 bus.

The Multibus specification was important because it was a robust, well thought out industry standard with a relatively large form factor so you could design complex devices on it. Being a well defined and well documented industry standard allow a Multibus compatible industry to grow up around it. There were many companies making card cages and enclosures for it. There were many companies making CPU, memory and other peripheral boards for it. In 1982 there over 100 board and systems manufacturers. This allowed complex systems to be built from Commercial off-the-shelf hardware. It also allowed companies to innovate by designing their own specific Multibus board(s) 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 add 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 enhance the systems as a Workstations or a file servers. Other workstation vendors who use Multibus based designs included HP/Apollo and Silicon Graphics IRIS

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 can 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 the Multibus with multiple processors and other DMA devices .

The standard Multibus form factor was 12 inch wide, 6.75 inch deep 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.