EXPANSION SLOTS and PORTS

Expansion slots and ports are what connect your computer to the outside world. Expansion slots "expand" the capabilities of your mainboard.

A good example is your video card. It is plugged into an expansion slot and connected to your monitor. If you should want to connect the world's most expensive flat screen monitor to your computer, it would be an easy matter to replace your present video card with one that is specially designed to get the most out of a flat screen monitor. That's just about all there is to it. Upgrading is a snap. Without expansion slots, everything would have to be built into the mainboard and any major upgrade would require its replacement.

Expansion slots also allow flexibility. They allow you to add a SCSI interface, connect to a Network with a Network Interface Card, connect to the outside world with a modem, add more ports, add another monitor, add more printers or scanners or a fax machine, automate your home, control home security - just about anything you can think of. And notice I said "allow', not "require". With expansion slots the computer owner decides what sould be there, not the computer manufacturing company.

The nuts and bolts of expansion slots are not very complicated. They are soldered onto the mainboard, positioned at the back of the board so that one end of the installed cards are accessible to the outside of the rear of the case.

The original IBM-PC had the first expansion slots - ISA, or Industry Standard Architecture - slots designed for an 8 bit bus. That was soon replaced by the 16 bit ISA slot for 286 and later computers. The current standard is the PCI, or Peripheral Component Interconnect, a 32/64 bit expansion slot. Physically, 16 bit ISA slots are almost always made from black plastic, they are about 5 1/2 inches long and are set just over 3/4" from the back of the board. PCI slots are usually made from white plastic, are about 3 3/8" long are are set about 1 1/2" from the back of the board. Both slots are about 3/8" wide and about 5/8" tall.

Todays most common mainboards (for a Pentium microprocessor) will usually have a combination of 16 bit ISA slots and PCI slots - usually 3 of one and 4 of the other. The newest mainboards (for the Pentium III family) might only have PCI slots - and sometimes not very many of those, as many things that were usually on expansion cards might be built-in to the mainboard.

The trend towards building some things into the mainboard is probably best illustrated by the ports a computer uses. A port is a place for the computer to receive or send information and they were originally on expansion cards. The design (and number) of ports soon became pretty standardized and it's now common to find them built-in on all Pentium class mainboards. The most often built-in (and most useful) are the keyboard connector, IDE (hard drive) connectors (usually 2), FDD (floppy drive) connector, a parallel (printer) port and 2 serial ports. There might also be a PS/2 port for a mouse, another for the keyboard, an Infra-Red connector and a couple of USB ports (which are modern, hot-pluggable, PNP type serial ports).

All these expansion cards and ports must have a standardized way to communicate with the processor (and vice versa) - and they do. They use what is generally known as motherboard (mainboard) resources: IRQs, I/O Memory Address locations and DMA Channels.

An IRQ (Interupt Request) is a signal line between the processor and the device. Each expansion device or port has its own assigned IRQ to let the processor know when it has something to send - or to be alerted by the processor when the processor is ready to send something.

The I/O memory address location is an assigned place in RAM memory where the device will always pick-up or deposit it's messages.

A DMA (Direct Mamory Address) Channel is a high speed memory transfer location and would only be used by certain devices - modems or sound cards, for instance, not a keyboard or the mouse.

So you can see that each device has a way to alert (or be alerted by) the processor, a place to leave or pick-up information and a place for high-speed communication, if needed. The only problem is that there are a limited number of each of the resources. For instance, there are only 15 IRQs and 8 of them are used by normal mainboard functions and another 3 are used by the 2 serial ports and the printer (parallel) port. There are only 8 DMA channels. Usually this is not a problem, and the owner doesn't even know (or care!) about IRQ assignments, etc., as the operating system does all the allocations and assignments of these resources during setup. It only becomes a problem if you keep adding devices to your computer - especially if some of your devices are greedy little rascals and want more than their share of the resources. It happens - not all things are designed efficiently. When you run into that problem, things must be adjusted manually, which, fortunately, is sometimes possible. It can be a complex process, so I won't attempt to try to guide you through it here. I'd suggest a good reference ("Upgrading and Repairing PCs, 11th edition" by Scott Mueller is one that I use) or find a tutorial on the web (look at a site dedicated to SCSI, for instance - they sometimes deal with resource conflicts) and, if you are using Win98, study your resources at Start->Programs->Accessories->System Tools->System Information. You'll find them under Hardware Resources. If you're using an operating system earlier than Win98, well - too bad! - I don't have any good suggestions as to how you might dig out this information. This is another good reason to upgrade to Win98.

See Also:
Network Interface Cards
SCSI Cards
I/O Cards
Other Expansion Cards

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