Hardware Reference
In-Depth Information
An important advantage of MCA was that it permitted data transfer at sig-
nificantly faster rates than was possible with ISA. In fairness, the increase in
data transfer rate may be unimportant in many applications and also tends to
vary somewhat from machine to machine. As a rough guide, when a standard
AT machine is compared with a PS/2 Model 50, data transfer rates could be
expected to increase by around 25% for conventional memory transfers and by
100% (or more) for DMA transfers.
Since MCA interrupt signals were shared between expansion cards, MCA
interrupt structure tends to differ from that employed within ISA where interrupt
signals tend to remain exclusive to a particular expansion card. More import-
antly, MCA provided a scheme of bus arbitration in order to decide which
of the 'feature cards' had rights to exercise control of the MCA bus at any
particular time. The arbitration mechanism provided for up to 15 bus masters,
each one able to exercise control of the bus. As a further bonus, MCA provided
an auxiliary video connector and programmable option configuration to relieve
the tedium of setting DIP switches on system boards and expansion cards.
Despite its advantages over ISA/EISA, MCA was a relatively short-lived
standard and it was never widely adopted by the industry. Instead, a new
(and much enhanced) standard was introduced. This bus expansion standard is
referred to as
Peripheral Component Interconnect
(PCI) and it quickly became
the dominant standard leading to the rapid obsolescence of the PC and PC/AT
bus standards. That said, many ISA/EISA cards are still in use today and so we
shall begin by describing these standards in some detail before moving on to
more modern bus standards.
PC ISA/EISA
expansion bus
The PC ISA/EISA expansion bus is based upon a number of
expansion slots
,
each of which is fitted with a 62-way direct edge connector together with an
optional subsidiary 36-way direct edge connector. Expansion or option cards
may be designed to connect only to the 62-way connector or may, alternatively,
mate with both the 62- and 36-way connectors. Since only the 62-way connector
was fitted on early machines (which had an 8-bit data bus), cards designed for
use with this connector are sometimes known as
8-bit expansion cards
or
PC
expansion cards
. The AT standard, however, provides access to a full 16-bit data
bus together with additional control signals and hence requires the additional
36-way connector. Cards that are designed to make use of
both
connectors are
generally known as
16-bit expansion cards
or
AT expansion cards
.
The original PC was fitted with only five expansion slots (spaced approxi-
mately 25 mm apart). The standard XT provided a further three slots to make
a total of eight (spaced approximately 19 mm apart). Some cards, particularly
those providing hard disk storage, required the width occupied by two expan-
sion slot positions on the PC-XT. This was unfortunate, particularly where the
number of free slots was often at a premium!
All of the XT expansion slots provided identical signals with one notable
exception; the slot nearest to the power supply was employed in a particular
IBM configuration (the IBM 3270 PC) to accept a keyboard/timer adapter.
This particular configuration employed a dedicated
card select
signal (B8 on
the connector) which was required by the system motherboard. Other cards
which
would
operate in this position included the IBM 3270 Asynchronous
Communications adapter.
