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The two built-in serial ports use 9-pin D-subminiature connectors on the back panel. These ports support devices such as external modems, printers, plotters, and mice that require serial data transmission (the transmission of data one bit at a time over one line).

Most software uses the term COM (for COMmunications) plus a number to designate a serial port (for example, COM1 or COM2). The default designations of your com- puter’s built-in serial ports are COM1 and COM2.

The built-in parallel port uses a 25-pin D-subminiature connector on the computer’s back panel. This I/O port sends data in parallel format (where eight data bits, or one byte, are sent simultaneously over eight separate lines in a single cable). The parallel port is used primarily for printers.

Most software uses the term LPT (for Line PrinTer) plus a number to designate a par- allel port (for example, LPT1). The default designation of your computer’s built-in parallel port is LPT1.

Port designations are used, for example, in software installation procedures that include a step in which you identify the port to which your printer is attached, thus tell- ing your software where to send its output. (An incorrect designation prevents the printer from printing or causes scrambled print.)

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Your computer system has an autoconfiguration capability for the serial ports. This feature lets you add an expansion card containing a serial port that has the same des- ignation as one of the built-in ports without having to reconfigure the card. When the computer detects the duplicate serial port on the expansion card, it remaps (reassigns) the built-in port to the next available port designation.

Both the new and the remapped COM ports share the same IRQ setting, as follows:

COM1, COM3: IRQ4 (shared setting)

COM2, COM4: IRQ3 (shared setting)

These COM ports have the following I/O address settings:

COM1: 3F8h

COM2: 2F8h

COM3: 3E8h

COM4: 2E8h

For example, if you add an internal modem card with a port configured as COM1, the computer then sees logical COM1 as the address on the modem card. It automati- cally remaps the built-in serial port that was designated as COM1 to COM3, which shares the COM1 IRQ setting. (Note that when you have two COM ports sharing an IRQ setting, you can use either port as necessary but you may not be able to use them both at the same time.) If you install one or more expansion cards with serial ports des- ignated as COM1 and COM3, the corresponding built-in serial port is disabled, as described in “Serial Port 1 and Serial Port 2” in Chapter 3.

Hardware Configuration Features B-7