The Hardware Book

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ISA (Technical)

This file is designed to give a basic overview of the bus found in most IBM clone computers, often referred to as the XT or AT bus.

The AT version of the bus is upwardly compatible, which means that cards designed to work on an XT bus will work on an AT bus.

This bus was produced for many years without any formal standard. In recent years, a more formal standard called the ISA bus (Industry Standard Architecture) has been created, with an extension called the EISA (Extended ISA) bus also now as a standard.

The EISA bus extensions will not be detailed here. This file is not intended to be a thorough coverage of the standard.

It is for informational purposes only and is intended to give designers and hobbyists sufficient information to design their own XT and AT compatible cards.

Physical Design:

ISA cards can be either 8-bit or 16-bit. 8-bit cards only use the first 62 pins, and 16-bit cards uses all 98 pins. Some 8-bit cards use some of the 16-bit extension pins to get more interrupts.

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