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It's almost surprising that many people don't know the difference between COM ports and expansion slots. Well, think of it like a radio station. An expansion slot is like the address or geographical location of the radio transmitter; a COM port is like the station's broadcast frequency. Do you know the address of KRSP? Unless you work there, do you care? Well, the computer doesn't care which connector your modem is plugged into, either. All it wants to know is where to 'tune in.' On the other hand, what happens if two radio stations try to broadcast at the same frequency?
When installing an internal modem or any card that uses COM ports, make sure you know what ports your computer is already using and what is available. Generally computers are capable of addressing four COM ports (COM1, COM2, COM3, and COM4). If the computer's motherboard includes built-in serial ports or a built-in modem, the COM number may be fixed or it may be set through jumpers or the BIOS. Expansion cards are set through jumpers on the card, or some may have tiny little switches. Check all devices to make sure they are all using different COM settings.
Windows makes things more difficult when choosing COM ports, because it can go really wacky if you use numbers out of sequence. For example, if the computer is already using COM1 and COM2, and you install a modem at COM4, Windows may throw up because there is nothing at COM3. For this reason, always fill COM port numbers in order.
One last note: if you are adding an external modem, you don't have to worry about rearranging COM ports. Instead, you have to worry about what the computer's serial port is capable of. Most older systems (including many '486 computers) have a maximum serial port speed of 9600 baud. These can be identified by an 8550 UART chip. Systems using a 16550 UART don't have this limitation.