Hardware Reference
In-Depth Information
Figure 2.1
A typical PC expansion scheme
The most obvious method of expanding the PC bus is simply to provide a
number of access points to the bus on the system motherboard. This approach
was followed by IBM (and countless manufacturers of clones and compatibles)
as a means of connecting essential items of peripheral hardware (such as displays
and disk drives) via controllers fitted to
adapter
(or
option
)
cards.
This same
method of connection can also be employed for more specialized applications
such as analogue data acquisition, IEEE-488 bus control, etc.
Several PC expansion bus schemes have evolved over the past two decades.
The original and most widely used standard is based on
Industry Standard
Architecture
(ISA). This standard is also referred to as the
8-bit expansion bus
or simply as the
PC expansion bus.
This original PC expansion scheme was
based on a single 62-way direct edge connector which provided access to the
8-bit data bus and the majority of control bus signals and power rails (Figure 2.1).
In order to provide access to a full 16-bit data path available from the PC/AT
standard a further 36-way direct edge connector was later added. This provided
access to the remaining data bus lines together with some additional control
bus signals. Cards that required only an 8-bit data path and a subset of the
PC's standard control signals were able to make use of
only
the first 64-way
connector. Cards that needed the full 16-bit data path (not available on the early
PC and XT machines) required
both
connectors. This enhanced standard is
often referred to as
Extended Industry Standard Architecture
(EISA), the
16-bit
expansion bus
, or simply the
PC/AT expansion bus
.
With the advent of PS/2, a more advanced expansion scheme has become
available. This expansion standard was known as
Micro Channel Architecture
(MCA) and it provided access to the 16-bit data bus in the IBM PS/2 Models
50 and 60 whereas access to a full 32-bit data bus was made available in the
Model 80 (which was fitted with an 80386 CPU).
