bus : Computer Hardware Buyers’ Glossary


The various I/O cards (video controller, internal modem, sound card, SCSI (Small Computer System Interface) controller etc.) inside the computer communicate with each other, the RAM (Random Access Memory) and the CPU (Central Processing Unit) 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 (Advanced Technology) 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 (International Business Machines) ’s MCA (Micro-Channel Architecture) bus, the VESA (Video Equipment Standards Association) bus and the EISA (Extended Industry Standard Architecture) bus, ISA. All of these fizzled in the market place. Intel then invented the PCI (Peripheral Component Interconnect) bus PCI which uses a great many tiny connector pads. Such slots are called PCI 32-bit slots. A 64-bit PCI bus is in the works. Older PCs (Personal Computers) have 3 or more PCI slots plus 3 or more 16-bit ISA slots. Not so old machines have PCI slots plus some sort of video slot, e.g. AGP (Accelerated Graphics Port). The PCI slot evolved into the PCI-X (Peripheral Component Interconnect — extended) and later into the PCI-E (Peripheral Component Interconnect — Express) (Express). The latest machines have a mixture of PCI and PCI-E slots.

The original PC (Personal Computer) 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. A 32-lane PCI-E can pump through 500 MBits/s on each lane. A PCI-E-x1 has one lane; a PCI-E-x8 has 8 lanes etc.

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