Slots on the PC Motherboard

Bus and I/O Information for the various types of slots on personal computer motherboards.

ISA Bus (Industry Standard Architecture)

The first incarnation of this type of slot was the early 8-bit version called the XT bus. It was proprietary controlled by IBM. The original IBM Personal Computer introduced in 1981 included this 8-bit subset of the ISA bus.

In 1984, IBM introduced the PC-AT which was the first full 16-bit implementation of the ISA bus. The second incarnation was created by IBM called the AT bus, which was 16-bit and also proprietary at first. Due to a lack of industry specifications at first, not all of the original ISA bus devices were compatible and to this day no uniform standards has ever been set.

The ISA bus has enjoyed the longest run of any other card slot type. The ISA slot has since been abandoned in favor of more current technology.

Microchannel (MCA)

IBM created this proprietary 16-bit or 32-bit parallel computer bus in 1987. Due to the fact that IBM no longer dominated the PC industry, and they had control over Microchannel, it never caught on beyond implementation in IBM computers. It was first used in the PS/2 workstations, and the architecture has granted us today the small keybaord and mouse connectors. The Microchannel motherboard slots used only a single cable, combining power and data. Patents on important Micro Channel features, such as Plug and Play automatic configuration, were not granted to IBM until after PCI had replaced MCA in the marketplace.

EISA Bus (Extended Industry Standard Architecture)

IBM created a proprietary bus technology called Microchannel. PC clone manufacturers countered with EISA, an alternative to Microchannel in 1988. EISA incorporates many of the benefits of the Micro Channel bus while maintaining compatibility with legacy ISA expansion boards. EISA connectors are a superset of the 16-bit connectors used on ISA system boards. ISA 8-bit and 16-bit expansion boards can be installed into the ISA compatible portion of the EISA slot. EISA expansion boards use the signals on the ISA compatible portion of the connector as well as additional signals that provide enhanced function and performance.