When IBM announced the PC, the company documented the PC internals in their
typically thorough style. The availability of this information, along with a strong desire to
add hardware to the PC, allowed many people to develop plug-in cards so that the PC
could monitor and even control their environment. Initially, the slots inside the IBM PC
didn`t even have a name. The name ISA, for Industry Standard Architecture, was coined
in the early 1980s. There were no official guidelines on how a PC should be expanded,
so cards made by vendor A could not be used at the same time as cards made by
vendor B. It took until the early 1990s for the ISA bus to be documented (
Theory and Operation
by Edward Solari is the classic text for this information). Some
developers took a lower-risk approach and created attachments that plugged into the
serial and parallel ports. But this port resource was quickly depleted, and when more
serial and parallel ports were added via plug-in cards, the internal design of the PC
could not support enough interrupts, Direct Memory Access (DMA) channels, or other
system resources. Sometimes the interactions were subtle and failed only under certain
conditions.The bandwidth of these data paths in and out of the PC was also quite low. A
better solution was required.
The PCI Bus
Many PC and PC component vendors, including Compaq, Digital Equipment, NCR,
IBM,and Intel Corporation, worked together to design a high-bandwidth expansion bus
that was eventually called the Peripheral Components Interconnect bus, or PCI. This bus
definition included configuration information and controls to alleviate the vendor-to-
vendor conflicts of ISA. The PCI bus was included beside the ISA bus inside the PC
host. Software support for automatic configuration was added to Windows so that PCI
cards became easy to install. This was the start of Plug and Play" ,an industry-wide
initiative that governs the way add-in hardware should identify itself.
PCI was an instant success for high-bandwidth devices, but was seen by many as
excessive and complex for the simpler I/O devices. The PCI interface components
typically cost more than a simple I/O device. Another main disadvantage was the
location of the PCI connectors; just like those of their ISA predecessor, they were inside
the PC case.This meant that the case had to be opened, clamping screws undone,
boards plugged exactly right into connectors that were difficult to identify, and then the
system reassembled. This was discouraging for many would-be users because of their
fears that the PC could be easily broken by these actions. PCI was a better solution but
did not address easy expansion of simple I/O.
Several PC-industry leaders worked together to designe an external expansion bus.
Driven by customer demand, it was essential that this bus be simple and low cost.
Consumer focus groups demanded that PC expansion be as easy as connecting a
VCR to a television you should not need to set switches or run software, and you must
be allowed to plug and swap cables while the equipment is turned on.