DOS and Windows 3.x modem troubleshooting

Updated: 01/24/2018 by Computer Hope

Verify that the connections are correctly configured on the modem before continuing with any of the below troubleshooting.

External computer modem

MS-DOS and Windows 3.x programs will point directly to the modem and require little setup or troubleshooting. Although you will need to know what COM port the modem is currently configured to use. Most older modems that came with MS-DOS and Windows 3.x are legacy hardware devices and have jumpers that physically set the modem to a specific port, such as COM1:IRQ4.

If you are attempting to install a 28.8 k or faster modem, the modem is likely plug and play.


Not all WinModems will work in Windows 3.x.


If your modem software is not communicating with your modem properly, verify that you can echo to the modem by following the steps below.

Exit Windows and get to an MS-DOS prompt. If you are in Windows 3.x from Program Manager click File and click Exit Windows.

At the prompt, type echo atdt12345 > comx (x being the number of your communication port). This command should open up the phone line and dial 12345. If you receive a write fault error message, attempt the other 3 ports to ensure that you modem is not installed on another port.

If all 4 ports receive write fault error messages and you have other devices connected to the serial port other than the modem, remove these devices and echo to the communication port again.

If you continue to experience the same issues, verify that the COM ports are properly setup in CMOS. If the ports are disabled or not installed, the computer will not be able to communicate through these ports. Information about CMOS is located on our CMOS help page.

If you continue to receive the same error message or you do not have any additional serial ports, it's possible there may be a physical issue with computer hardware. It is recommended that you contact the manufacturer of either the modem or your computer.

If you are a user of Windows 3.x or MS-DOS and are having difficulty with a WinModem, you should have software available to test the modem to see if it is working properly. If this software reports that the modem is working properly, it is recommended that you contact the manufacturer of that software to verify it is compatible with WinModems. If your WinModem does not include software to test the modem's functionality, it is recommended you contact the manufacturer of that modem to verify it is working.

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