S-100 Bus - History

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 CPU on a fifth. He then looked for a cheap source of connectors, and he came across a supply of military surplus 100-pin edge connectors.

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 varied between manufacturers, but was eventually standardized as IEEE-696 towards the end of 1983. By this time it was a common standard for small computers (almost all of them running CP/M).

The bus signals were simple to create using an 8080 CPU, but increasingly less so when using the Z80 and even more unrelated processors like the 68000. More board space was occupied by signal conversion logic.

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"); used signals of the Z80, which had soon replaced the 8080. The 64-pin STEbus used the far more robust DIN41612 connector, and was not based on any particular processor so it was equally practical to host any processor from a Z80 to a 68020.

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.