bus
bus topology
computer bus
A set of electrical conductors
(wires, PCB tracks or connections in an integrated circuit)
connecting various "stations", which can be functional units
in a computer or nodes in a network. A bus is a
broadcast channel, meaning that each station receives every
other station's transmissions and all stations have equal
access to the bus.
Various schemes have been invented to solve the problem of
collisions: multiple stations trying to transmit
at once, e.g. CSMA/CD, bus master.
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 (/pub/misc/omnibus.html).
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 is usually specified in bits and
is the number of parallel connectors. This and the clock
rate determine the bus's 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.
See also bus network.
<Ukranian>.
(2010-07-10)
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