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Industry Standard Architecture (ISA) is a computer bus specification used for 8-bit IBM-compatible systems. An ISA bus provides a basic route for peripheral devices that are attached to a motherboard to communicate with different circuits or other devices that are also attached to the same motherboard.
Peripheral component interface (PCI) started replacing the ISA bus in the mid-'90s. New motherboards were manufactured with fewer ISA slots, and preference was given to PCI slots.
Initially, an ISA bus was the best option for Intel machines. However, eventually a faster and wider bus was required, and an issue of incompatibility arose. The manufacturers relied on the same ISA bus but added 16-bit characteristics.
The new ISA bus was flexible in that it could connect multiple devices. It supported 16-bit peripheral devices. Therefore, five devices with 16-bit interrupt request (IRQ) could be connected at the same time. Also, three additional devices could be connected parallel to five devices with 16-bit IRQ and a 16-bit direct memory access (DMA) channel. The CPU clock speed varied from 16 to 20 MHz.