bus
Device on a computer's motherboard that provides a data path between the CPU and attached devices (keyboard, mouse, disk drives, video cards, etc.). Like a vehicular bus that stops at designated stations to pick up or drop off riders, a computer bus receives a data signal from the CPU and drops it off at the appropriate device (for example, the contents of a file in RAM are sent, via the bus, to a disk drive to be stored permanently). Conversely, data signals from devices are sent back to the CPU. On a network, a bus provides the data path between the various computers and devices. See also USB.
bus
Large motor vehicle designed to carry passengers usually along a fixed route according to a schedule. The first gasoline-powered bus was built in Germany in 1895 and carried eight passengers. The first integral-frame bus was constructed in the early 1920s in the U.S. In the 1930s diesel engines were introduced, providing greater power and fuel efficiency to larger buses. With the development of highway systems, transcontinental bus lines became common in North America. Double-decked buses are used in some European cities; articulated buses pull trailers with flexible joints. Trolley buses, whose electric motors draw power from overhead wires, are now used mostly in European cities.
bus
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.
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| System and Peripheral Buses |
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| 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). |
bus
| (architecture) | bus - One of the sets of conductors (wires, PCB
tracks or connections in an integrated circuit) connecting
the various functional units in a computer. There are
busses both within the CPU and connecting it to external
memory and peripheral devices. The data bus, address bus
and control signals, despite their names, really constitute a
single bus since each is useless without the others. The width of the data bus, i.e. the number of parallel connectors, and its clock rate determine its data rate (the number of bytes per second which it can carry). This is one of the factors limiting a computer's performance. Most current microprocessors have 32-bit busses both internally and externally. 100 or 133 megahertz bus clock rates are common. The bus clock is typically slower than the processor clock. Some processors have internal busses which are wider than their external busses (usually twice the width) since the width of the internal bus affects the speed of all operations and has less effect on the overall system cost than the width of the external bus. Various bus designs have been used in the PC, including ISA, EISA, Micro Channel, VL-bus and PCI. Other peripheral busses are NuBus, TURBOchannel, VMEbus, MULTIBUS and STD bus. Some networks are implemented as a bus at the physical layer, e.g. Ethernet - a one-bit bus operating at 10 (or later 100) megabits per second. The term is almost certainly derived from the electrical engineering term "bus bar" - a substantial, rigid power supply conductor to which several connections are made. This was once written "'bus bar" as it was a contraction of "omnibus bar" - a connection bar "for all", by analogy with the passenger omnibus - a conveyance "for all". More on derivation. |
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