An expansion slot is an opening in a computer where a circuit board can be inserted to add new capabilities to the computer like upgrading the quality of the PC's graphics and sound, connection to a local network, ete. The boards inserted into the expansion slots are called expansion boards, expansion cards, cards, add¬ins, and add-ons. Inside the expansion slots are metallic (typically copper) spring fingers that clamp onto the expansion card when it is inserted into the slot. Each of the fingers matches up with one segment of the card's edge connector to complete one of many different connections of the slot and card combination.

On early PCs, expansion cards were used to add some of the basic functions of the system, including memory, hard disk and floppy disk, display, modems, serial ports, parallel ports, and even the clock and calendar function of the Pc. T oday's PC still add some of these functions through expansion cards, but many of these capabilities are now in-built into the motherboard. In modern PCs, expansion cards are used to improve or add to the capabilities of the system, to add cards for special-purpose hardware, and to connect to a network. Expansion cards now allow a PC to have video capture, sound, fax, scanner, and network capabilities.

A personal computer has expansion slots of different shapes and sizes to support various types of expansion cards. Some popular category of expansion slots are:

• Industry Standard Architecture (lSA)
• Accelerated Graphic Port (AGPI
• Peripheral Component Interconnect (PCI)

The names of expansion cards are after the bus technology used in computers.

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Bus

If the computer is to. be useful, the microprocessor must communicate with all components on the motherboard and expansion cards plugged into the expansion slots. To enable such error free communication, a PC uses Buses. A bus is a collection of wires through which data is transmitted from one part of a computer to another. You can think of a bus as a highway on which data travels within a computer. When used in reference to personal computers, the term bus usually refers a bus that connects all the internal computer components to the CPU and main memory. There's also an expansion bus that enables expansion boards to access the CPU and memory. All buses consist of two parts - an address bus and a data bus.

The data bus transfers actual data whereas the address bus transfers information about where the data should go. The size of a bus, known as its width, is important because it determines how much data can be transmitted at one time. For example, a 16-bit bus can transmit 16 bits of data, whereas a 32¬bit bus can transmit 32 bits of data. Every bus has a speed measured in MHz. A fast bus allows data to be transferred faster, which makes applications run faster. In modern PCs, the old ISA bus is being replaced by faster buses such as PCI.

Three buses used in PCs are: ISA (Industry Standard Architecture), EISA (Extended Industry Standard Architecture), and MCA (Micro Channel Architecture). Note that EISA and MCA are not being used in today's Pc. ISA is also becoming rare and is used only to handle older versions of expansion cards. The buses most common today are PCI (Peripheral Component Interconnect), AGP (Accelerated Graphics Port), and USB (Uni'lersal Serial Bus).