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Configuring COM Ports


To configure your system's COM ports, open the Control Panel and choose the System object. Then open the Device Manager property page and expand the Ports branch. Select the port you want to configure and choose Properties to display the Communications Port Properties sheet.

The General property page provides information about the port and enables you to specify with which hardware profiles the port will be used. To specify settings for the port, click on the Port Settings tab to display the Port Settings page.

The settings on the Port Settings page control parameters such as bits per second, stop bits, parity method, and number of data bits, explained earlier. You also can specify the flow control method for the port, explained later in the section "Setting Advanced Options." In the case of a modem, you really don't have to configure the port settings to match the modem's requirements because the modem settings override the port settings. If you occasionally use other devices on the same COM port, however, you need to configure the port according to those other devices' requirements.

To change the IRQ or input/output (I/O) base address of the port, click on the Resources tab to display the Resources page; then specify the new I/O address and IRQ values. Table 19.1 provides a list of common IRQ assignments for your reference in configuring your computer's COM ports.

Note
Although some of the IRQ lines listed in Table 19.1 indicate that they are available, these interrupts might not be available on your system. If your PC contains a network adapter, sound card, or other devices, these IRQs might already be assigned to these devices.


 

Table 19.1. Common IRQ assignments.
IRQ Number Typical Assignment
NMI Nonmaskable interrupt, reports parity errors
0 System timer
1 Keyboard
2 EGA/VGA and cascaded interrupt for second IRQ controller (IRQ9-15)
3 COM2, COM4
4 COM1, COM3
5 LPT2 (printer port 2)
6 Floppy disk
7 LPT1 (printer port 1)
8 Real-time clock
9 Software redirected to IRQ2
10 Available
11 Available
12 Available
13 Math coprocessor
14 Hard disk controller or host adapter
15 Available, or hard disk controller