The Industry Standard Architecture or ISA
(Pronounced as separate letters or as eye-sa)
bus began as part of IBM's revolutionary PC/XT and PC/AT released in 1981.
However, it was officially recognized as "ISA" in 1987 when
the IEEE (Institute of Electrical and Electronics Engineers) formally
documented standards governing its 16-bit implementation. AT version of the bus
is called the AT bus and became a de facto industry standard.
History
ISA stands for Industry standard architecture. It was the original
IBM expansion bus and initially no standard was assigned to it. Its first
version was the 8 bit bus and it ran at the speed of about 7 MHz.
In 1984, with the advent of PC
AT (Intel 286), the bus width is increased
to 16 bits and the frequency
successively 6 to 8 MHz, 8.33 MHz and finally, providing a theoretical maximum of 16 MB / s (in practice
only 8 MB / s
as a cycle of two
was used for addressing).
The second generation of PC's used 16 bit ISA expansion bus which
also ran at the same speed i.e. 7 MHz initially. The later cards allow speed of
8.33 MHz for the 16 bit ISA bus. Nowadays the I/O devices are much faster than
their speed but still the ISA connectors are usually included in PC's to make
them is backward compatible with the slower ISA cards.
Current motherboards no longer include ISA bus,
PCI bus replaced
by the faster and Plug &
Play.
ISA bus architecture
ISA bus architecture is the basis of personal computer. 8-bit ISA
bus is used in single user systems with 80386 and 80486 processors. There are
24 address lines and '16 data lines in it. It operates at 8 MHz and 2 to 8
clock cycles are needed to transfer data. The data transfer rate of the system
is less when 8-bit ISA bus is used with 32 bit processor having 32 bit address
and data bus. So, 16 bit ISA bus is used to transfer data. Many peripherals
such as disk controller, printer, and scanner can be connected to ISA bus.
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