The ISA bus is actually an 8 bit or 16 bit bus, and operates at 8.25 mhz, or approximately 4MB transfer rate with the 8 bit bus, and 8 mb/s throughput maximum with the 16-bit bus. The expansions slots on a PC's motherboard usually allow for both a 8 or 16 bit ISA card to be connected. This is not the most efficient size, and the ISA standard is being replaced gradually with EISA and PCI buses (Micronics M6Me Dual Pentium processor system for instance, which does not have an ISA expansion slot; its expansions slots are EISA and PCI; but it does have a 64-bit ISA video).

You may notice that most buses operate at a sub-multiple of the actual system clock speed. For instance, 8.25 is one fourth the speed of a 33mhz system. The sub-multiples of speed is how a bus clock and system can maintain synchronous operations.

This graphic shows a ISA 16-bit expansion card and its interface to a 16-bit ISA expansion slot. A 16 bit card has the two sets of connector pins as shown. An 8-bit card would only have the set of pins to the right of the cut in the card.

ISA expansion buses come in Plug-and-Play (PnP) versions. PnP works without modification to an ISA bus. However, most advanced systems with ISA will modify the configuration by adding a single slot specific signal to each bus connector.

Storage Devices