ISA bus

ISA

Industry Standard Architecture
One 8-bit and five 16-bit ISA slots on a motherboard
Year created	1981; 36 years ago (1981)
Created by	IBM
Superseded by	PCI (1993)
Width in bits	8 or 16
No. of devices	Up to 6 devices
Style	Parallel
Hotplugging interface	no
External interface	no

Industry Standard Architecture (ISA) is a retronym term for the 16-bit internal bus of IBM PC/AT and similar computers based on the Intel 80286 and its immediate successors during the 1980s. The bus was (largely) backward compatible with the 8-bit bus of the 8088-based IBM PC, including the IBM PC/XT as well as IBM PC compatibles.

Originally referred to as the PC/AT-bus it was also termed I/O Channel by IBM. The ISA concept was coined by competing PC-clone manufacturers in the late 1980s or early 1990s as a reaction to IBM attempts to replace the AT-bus with its new and incompatible Micro Channel architecture.

The 16-bit ISA bus was also used with 32-bit processors for several years. An attempt to extend it to 32 bits, called Extended Industry Standard Architecture (EISA), was not very successful, however. Later buses such as VESA Local Bus and PCI were used instead, often along with ISA slots on the same mainboard. Derivatives of the AT bus structure were and still are used in ATA/IDE, the PCMCIA standard, Compact Flash, the PC/104 bus, and internally within Super I/O chips.

Compaq created the term "Industry Standard Architecture" (ISA) to replace "PC compatible". The ISA bus was developed by a team led by Mark Dean at IBM as part of the IBM PC project in 1981. It originated as an 8-bit system. The newer 16-bit standard, the IBM AT bus, was introduced in 1984. In 1988, the "Gang of Nine" PC compatible manufacturers, including Compaq, proposed the 32-bit Extended Industry Standard Architecture (EISA) standard and in the process retroactively renamed the AT bus to "ISA" to avoid infringing IBM's trademark on its PC/AT computer. IBM designed the 8 bit version as a buffered interface to the external bus of the Intel 8088 (16/8 bit) CPU used in the original IBM PC and PC/XT, and the 16-bit version as an upgrade for the external bus of the Intel 80286 CPU used in the IBM AT. Therefore, the ISA bus was synchronous with the CPU clock, until sophisticated buffering methods were developed and implemented by chipsets to interface ISA to much faster CPUs.