Serial Device Stops Working After Installing New Serial Device

The information in this article applies to:

• Microsoft Windows operating system versions 3.1, 3.11
• Microsoft Windows for Workgroups versions 3.1, 3.11
• Microsoft Windows 95

SYMPTOMS

After you install a new serial device in your computer, an existing serial device may stop responding when you use the new device. For example, you may experience problems with a serial mouse on COM1 after you install an internal modem on COM3.

CAUSE

Serial ports COM1 and COM3 share a common interrupt request line (IRQ), IRQ 4, and serial ports COM2 and COM4 share IRQ 3. Because COM1 and COM3 share the same IRQ, the two serial ports cannot be used simultaneously.

This behavior most often occurs when an internal modem is installed on COM3 or COM4 with a serial mouse already installed on COM1 or COM2. When you use the modem, the mouse stops responding (hangs). This is caused by the modem taking control of the IRQ the mouse uses to communicate with the computer.

RESOLUTION

To resolve this situation, remove the new serial device from your computer to determine if the existing serial device regains its functionality. If the remaining serial device functions correctly, reinstall the new serial device, but configure the new device to use a different IRQ. For information about how to configure the new serial device, consult the documentation included with the new device, or contact the device's manufacturer.

MORE INFORMATION

Some computers and serial port adapters support IRQ sharing. This is common on Micro Channel Architecture (MCA) bus computers, such as IBM PS/2 computers, and Extended Industry Standard Architecture (EISA) bus computers, provided the full EISA standard is implemented.

NOTE: IRQ sharing generally does not work when one of the devices is in constant use (such as a serial mouse).

In Windows 3.x, it is possible to configure serial ports to use the same IRQ (for example, COM1 and COM3 on IRQ 4, and COM2 and COM4 on IRQ 3) one after the other, but not at the same time unless the serial adapter supports IRQ sharing. Windows 3.1 and Windows for Workgroups 3.1x support IRQ sharing for serial adapters. To use the adapters properly, make sure that the IRQ values are set correctly. To do so, follow these steps:

1. Double-click Control Panel in Program Manager, and then double- click Ports.

2. Double-click the port you want to configure, and then click Advanced.

3. In the Interrupt Request Line (IRQ) box, click the correct interrupt.

4. Click OK, click OK, and then click Close.

5. Use any text editor (such as Notepad) to open the System.ini file in the Windows folder.

6. Add the following line to the [386Enh] section of the System.ini file:

         COMIrqSharing=True
   

7. Save and then close the System.ini file.

8. Restart your computer.

For more information about the "COMIrqSharing=" setting for Windows 95, please see the following article in the Microsoft Knowledge Base:

   ARTICLE-ID: Q130533
   TITLE     : Using Previous Windows Communications Switches in Windows
               95

The serial ports of Plug and Play modems do not appear in Device Manager in the Ports section as you might expect. To view the modem's resources, click the modem in Device Manager and then click Properties.