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Configuring an Internal Modem


Internal modems are on expansion cards that fit into a computer’s expansion bus. To that end, the modem you install must be designed to work in the type of expansion bus slot in that computer. Configuring an internal modem is much like configuring any other hardware device. You must set the IRQ, I/O address, and virtual COM port so that they don’t conflict with other devices.


The only one of these configuration parameters that you haven’t seen before is the virtual COM port. A virtual COM port is a logical designation given by the operating system for a serial port. A computer can have only


two physical serial ports, but it can have as many as four logical ports. Each physical port must be associated with a logical port so that the operating system can use it. The same is true for modems (since they are, in fact, serial devices). You must set a modem to use a COM port that is not being used by any other device.


You usually configure the IRQ, I/O addresses, and virtual COM port automatically with Plug and Play. Jumpers, DIP switches, or software programs can be used on legacy devices. See Chapter 6, «Network Installation and Upgrades,» for information on configuring expansion cards.


Each COM port shares an IRQ address with another port. COM 1 and COM 3 share IRQ 4, and COM 2 and COM 4 share IRQ 3. If two devices are set to different COM ports (COM 1 and COM 3, for example) but have the same IRQ, the modem may not function properly. It is usually best to set an internal modem to COM 4, because COM 1 is most likely to be in use, while COM 2 (which shares the IRQ with COM 4) is less likely to be used. Table 7.1 lists the virtual COM ports, their associated IRQs, and default I/O addresses. Note which COM ports share IRQ addresses.

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