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Troubleshooting modems

Updated: January 21, 2005

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Troubleshooting modems

What problem are you having?

The modem diagnostics indicate that an external serial modem is not receiving commands.

Cause:  If the modem diagnostics indicate that the modem is not receiving commands, the modem cabling may be faulty.

Solution:  Try connecting the modem with a new cable.

The modem cable is good, but the modem still does not receive commands.

Cause:  The modem is installed incorrectly.

Solution:  Check the modem's documentation to make sure that you installed it correctly.

The modem is installed correctly but the diagnostics indicate that it is not responding correctly.

Cause:  The incorrect model and make were specified during installation in Windows or an obsolete installation (.inf) file was used to install the modem.

Solution:  Check the modem documentation for the correct model and make or a compatible model and make, and reinstall the modem. You can check with the manufacturer to see if there are more recent installation files available for installing the modem.

According to the modem diagnostics, the modem works but you still cannot make a connection.

Cause:  The modem is connected incorrectly to the phone line or there is a problem with the phone line.

Solution:  Check the connection to the phone line. If the modem is connected correctly, have a telephone professional check your phone line.

You cannot connect or are having problems connecting to another computer or an online service provider.

Cause:  The data connection parameters (see Hardware settings) for the two modems are not the same. Data connection parameters for two modems need to be identical for them to connect successfully.

Solution:  Check the hardware settings for both computers. Typical settings are eight data bits, None for parity, one stop bit.

If you are connecting to an online service provider, refer to the documentation provided by your service provider for the correct settings. These are typically eight data bits, None for parity, one stop bit. Bulletin boards and most service providers use these settings. If these do not work, try seven data bits, even parity, and one stop bit. A few online service providers use these settings. Other settings are extremely rare.

Your PCMCIA modem card was not detected automatically when you inserted it.

Cause:  The card's built-in COM port is not configured.

Solution:  Use Add Hardware in Control Panel to configure the card's built-in COM port. You can then install the PCMCIA modem by using the Phone and Modem Options in Control Panel. (See Install a modem.)

An RS-232 serial device is functioning poorly or not at all.

Cause:  Some RS-232 serial devices now support speeds faster than the 115.2 Kbps currently supported by the standard serial ports installed on most computers. (For example, some serial devices support 230 Kbps.)

Solution:  If the Port speed in the connection preferences is faster than 115.2 Kbps, then try reducing the speed to 115.2 Kbps. You can leave the Maximum Port Speed in the modem properties at the higher setting. If the serial port on your computer supports speeds faster than 115.2 Kbps, then set the Port speed to whichever is slower, the serial port speed or the device speed.

A connection in Network Connections reports that a port is in use or not configured for remote access.

Cause:  There are several possible causes:

  • When you started Windows, a Plug and Play modem was not connected or it was turned off, so it was not detected correctly.

  • The port is in use by another program, such as Fax.

  • The connection may not have been updated to reflect changes in communications equipment, such as new serial ports or modems.

Solution:  Use one of the following solutions:

  • If the modem was disconnected or turned off, reconnect or turn on your modem, and then either restart Windows or use Device Manager to scan for new hardware.

  • If the communications port is already in use by another connection, hang up the connection, or modify the connection to use a different port.

  • If you have changed your hardware (such as a new serial port or modem), you must reconfigure the connection.

See also:  Configure a connection to a remote network; Reinstall a Plug and Play device


  • To diagnose and correct modem problems with the interactive modem troubleshooter, see List of troubleshooters and click Modem. For modem diagnostics, see Test a modem. Refer to the modem manufacturer's documentation or customer service representative for the correct modem commands and responses.

  • It is usually a good idea to note changes to modem settings and switches as you make them. This enables you to return the modem to its previous state, if necessary.

  • If you have modem problems, you may want to report them to the modem manufacturer or your supplier. In your report, include your modem log, which records information about your attempts to connect to other systems. For information about creating a modem log, see Log and view modem commands.

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