bus : Computer Hardware Buyers’ Glossary

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bus
The various i/o cards (video controller, internal modem, sound card, SCSI controller etc.) inside the computer communicate with each other, the RAM and the CPU via a common set of wires embedded in the motherboard. The original XT used 62 conductors. Slots are the holes in the motherboard where you insert the i/o cards. Slots with only the original 62 connections are called ISA (Industry Standard Architecture) 8-bit slots because they allow the devices to transfer one character (8 bits) at a time, one bit over each wire. It sounds crazy that of 62 conductors, only 8 carry data and the rest are for housekeeping, but that’s the way it is. The AT computer introduced an additional 36 connections for a total of 98. Slots with 98 connections are called ISA 16-bit slots. They can transmit data 2 characters (16 bits) at a time. Then evolved various experiments at extending the bus further including IBM’s MCA bus (Micro channel Architecture), the VESA bus (Video Electronics Standards Association) and the EISA bus (Extended Industry Standard Architecture). All of these fizzled in the market place. Intel then invented the PCI bus (Peripheral Component Interconnect) which uses a great many tiny connectors. Such slots are called PCI 32-bit slots. A 64-bit PCI bus is in the works. Most modern PCs have 3 or more PCI slots plus 3 or more 16-bit ISA slots.

The original PC bus ran at 32 MHz. The basic PCI bus runs at 100 MHz. They typically run around 133 MHz and run up to 200 Mhz in theory.

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