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 or modem cards may be set through jumpers on
the card, some may have tiny little switches or it may be set through software. Check all devices to make
sure they are all using different COM settings so they wont conflict with the
COM setting of the modem.
Windows makes things more difficult when choosing COM ports, because it may
give you problems if the COM number is out of sequence. For example,
if the computer is already using COM1 and COM2, and you install a modem
at COM4, Windows may have problems 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.