ISA (Industry Standard Architecture) was the name given to the 8-bit expansion slots in the IBM PC & XT (ISA-8), and the 16-bit slots introduced in the IBM PC AT in 1984. The ISA bus is used by millions of legacy expansion cards throughout the world. All ISA-bus computers, however, have a common bottleneck: no matter how fast the microprocessor in a PC, the ISA bus is limited to a maximum data rate of approximately 500 Kbytes per second for all the cards on the bus. This limitation can be a severe problem in high-speed data acquisition and graphics applications, where multi-megabyte files are common. Speeding up the processor does little good if the application is bottlenecked by the limited throughgput of the PC's ISA bus.

Large numbers of legacy and custom cards have kept the ISA bus alive much longer than expected, with cards readily available at comparatively low costs. ISA cards are slow, they are not Plug and Ply, and are becoming obsolete.

ISA card form factors (H x L, maximum dimensions):

• ISA (XT Bus): 4" x 13.2" card with a 64-pin edge connector. 5V signaling, 8-bit bus width, 4.77-8MHz bus clock signal, and a 8MB/s maximum transfer rate.
• ISA (AT Bus): 4" x 13.2" card with a 100-pin edge connector, 5V signaling, 16-bit bus width, 8MHz bus clock signal, and 16 MB/s maximum transfer rate. Most ISA slots are 16-bit slots.