bus

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A common digital pathway between resources and devices. In a computer, there are two major types: the system bus and peripheral bus. The system bus, also known as the "frontside bus" or "local bus," is the internal path from the CPU to memory and is split into address bus and data bus subsets. Addresses are sent over the address lines to signal a memory location, and data are transferred over the data lines to that location.

System buses transfer data in parallel. In a 32-bit bus, data are sent over 32 wires simultaneously. A 64-bit bus uses 64 wires.

Peripheral Buses

The peripheral bus is the pathway to the peripheral devices such as a disk or printer. PCI and PCI Express are widely used peripheral buses. Devices connect to these parallel buses with cables to controller cards that plug into slots on the motherboard. Another common bus is USB, and devices are cabled to ports on the computer. USB is a serial bus, in which data travels over one wire.

Other peripheral buses have been used, including ISA, EISA, Micro Channel, VL-bus, NuBus, TURBOchannel, VMEbus, MULTIBUS and STD bus. The CAN bus and FlexRay bus are automotive buses.

Why a Bus?

Thus far, we have not found out who originally coined the term or why. However, electronic buses were originally shared pathways, in which all devices receive the same signals. Subsequently, buses with point-to-point topologies were developed that send signals to only one device. In either case, there is no relationship to a vehicle that stops at bus stops, one after the other. The only data transfer technology somewhat similar to a real bus is a Token Ring network. See bus network, software bus, serial bus, PCI, PCI Express, USB and AGP.

System and Peripheral Buses

This illustration shows how chips, memory and peripherals in a PC interconnect via the system bus (top) and peripheral buses (AGP, PCI and PCI Express).

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Bus	System Bus
bus	Frontside Bus