(^222) PC Hardware: A Beginner’s Guide
The challenges of working with expansion cards, beyond choosing the right one, are
installation, configuration, and operation, with the emphasis on the first two. A personal
computer is configured and balanced to a pretty exact set of parameters when it is manu-
factured. The established hardware standards are for the most part generally accepted
and supported, but standards are open to interpretation and not all devices work the
same in different manufacturers’ PCs. Adding new functions to the PC may create
conflicts among the assignable resources and introduce problems in areas that were
perfectly fine before the new device was added. Expansion cards exist in a world of system
resources that is made u pof IRQs, DIP switches, jum per blocks, and system resources.
Understanding how the CPU interacts with an expansion card and the role of the system
resources and their assignments is the key to success with expansion cards and PCs.
However, before I get any further into that, let’s review the fundamental components,
concepts, and technology behind expansion cards and their use with PCs.
Expansion Buses
Every expansion card, whether it is a video adapter, modem, or network interface card, is
designed to communicate with the motherboard and CPU over a single communications
and interface standard that is called a bus. A PC usually supports at least two different
expansion buses and often more, and more is always better. An expansion bus, which is
also called a bus architecture, defines a specific interface that consists of how much data it
carries, how fast it transfers it, how it connects to the motherboard, and how it interacts
with the CPU or RAM.
Since the beginning, the PC has not used all that many types of expansion buses. In
fact, the standard used on the original PCs is still available on most motherboard designs.
On the other hand, several that sought to improve on the original have passed into history,
leaving essentially only a few. Here are the PC bus structures that have been the most
popular over the years: ISA (Industry Standard Architecture) The ISA expansion bus (which is
pronounced as the letters “eye-ess-aye,” not “ice-a”) is now generally obsolete,
but most motherboards still have at least one ISA slot to provide backward
compatibility for older hardware. You can still buy ISA expansion cards, but
they are becoming hard to find. On most motherboards, the ISA bus slots are
16-bit that will also support 8-bit cards. Figure 11-3 shows a drawing of a 16-bit
ISA card. An 8-bit card would have only the left-most half of the edge connector
on the bottom edge of the card. The ISA slot, as illustrated in Figure 11-4, is
divided into two sections. The 16-bit card occupies both sections and the 8-bit
card inserts into only one of the sections.
Some newer ISA cards are Plug-and-Play compatible, but for the most part they
are not. This means that ISA devices require at least some manual configuration
and setup. The ISA bus is also called the AT bus, for the IBM PC AT on which it