PC Access for Windows Reference Guide

for PC Access for Windows 4.30.158

Checking Your COM Ports with MSD

You can run the MSD program to examine your computer's COM ports and IRQ assignments. This is also useful for seeing whether you have a fast (16550) or slow (8250) UART chip in the COM port your modem uses.

If you are not already running MSD, type:

MSD

at your DOS prompt. Press the "C" key to display a window showing your COM ports. It may look something like the following window:

The above window came from a computer with two COM ports (COM1 and COM2). An external modem happens to be on COM1. The above window doesn't say explicitly that a modem is on COM1, but MSD detected some modem signals on COM1. MSD indicates these with "Yes" entries for COM1 on the rows showing DSR and CTS. If you see these modem signals on a COM port, you probably have a modem connected to it. However, you might not see these signals even if you do have a modem connected to a COM port. Your modem might be turned off, in an unusual state, or your modem cable might be defective (if you have an external modem).

The above window shows fast (16550) UART chips on both serial ports. A fast UART chip greatly improves modem reliability at speeds above 9600 BPS. If you see an 8250 UART on the COM port your modem uses, you should consider upgrading the COM port to use a 16550 UART. This is advisable if you have a V.32bis (14,400 BPS) modem, and absolutely essential if you have a V.FC or V.34 (28,800 BPS) modem.

If you have an 8250, the highest modem speed you can use reliably is hard to predict. It will usually depend on the speed of your computer. For example, if your computer uses a slow 386-SX/20 CPU, your modem may communicate unreliably even at 9600 BPS. If you computer uses a (comparatively fast) 486-DX/2-66 CPU, your modem may communication reliably at 19,200 BPS.

To see your IRQ assignments in MSD, return to the MSD main menu and press the "Q" key. You should see a window something like the following:

The modem happens to use COM1 on this computer, so it monopolizes IRQ 4. That means no other device can use IRQ 4 on this computer. The IRQ assignments for the COM ports in the above window are "standard." You should check your IRQ settings in the Windows Control Panel to insure they agree with the result from MSD.

Last modification date: Friday, January 17, 2003 00:58:49