In the early days of the PC, very little support was included in the motherboard for,
peripheral devices. The controllers and adapters used to drive and interface any pe¬
ripheral devices, such as the monitor, hard disk, floppy disk, and so on, had to be
added to the motherboard's circuitry through expansion cards, which are also known as
expansion boards, adapters, add in cards, and daughterboards. These days, much of the
support for peripherals is built into the motherboard, but on older PCs, adding a new
peripheral device usually means adding an expansion card.
Expansion cards are also added to the very latest PCs, often to upgrade the quality or speed of the PC's graphics and sound or to connect to nearby computers or printers or the outside world. Expansion cards can be used to improve the video performance, add or improve the sound system, add additional or new ports or connectors, provide a network connection, and many other functions. They can add a completely new function or capability or augment or replace an existing one.
It may sound obvious, but expansion cards are inserted into expansion slots. These slots are located on the PC's motherboard and are receptacles that provide an interconnection for
the card into the system bus structures. An expansion slot contains metallic, typically copper, springy fingers that clamp onto the expansion card's edge connectors when the card is inserted into the slot. The edge of an expansion card has metal connectors attached that will match up to the fingers of the slot and complete the connections that connect the card to the motherboard and its bus structure through the slot. As I will discuss later, the card and slot have to be the same type of interface. Figure 11 2 shows a card being inserted into a slot. Notice the card's edge connectors and how they match up to the expansion slot.
USING EXPANSION CARDS
Expansion cards were used to add basic functions to older PCs, including memory, hard disk and floppy disk controllers, video controllers, serial and parallel ports, modems, and even the clock and calendar functions. Today's PCs add only a few of these functions through expansion cards since most of these capabilities are built into the motherboard or chipset. Typically, expansion cards are now used to improve or add to the capabilities of a PC, such as controllers and adapters for special purpose hardware and network interfaces. Through expansion cards a PC can become a sound system, a graphics workstation, a movie theatre, or a member of a global network.
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 manufactured. 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 up of IRQs, DIP switches, jumper 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.
