Troubleshooting Communications Under Windows 3.0 (82136)

SUMMARY

The following table depicts the standard settings for the four communications ports that Microsoft Windows version 3.0 and Microsoft MS- DOS version 4.01 support. This information is useful for troubleshooting communications problems under Windows 3.0.

A brief description of each column appears below the table. NOTE: In the table below, Windows 3.0 is abbreviated as WIN3.

           WIN3 COMM.DRV       WIN3  Default        WIN3 Desired
            Procom Plus      SYSTEM.INI Settings  SYSTEM.INI Settings
   Port      I/O Range  IRQ      I/O Range           I/O Range
   ----    -----------  ---  -------------------  ------------------

                             [386Enh]             [386Enh]
   COM1      3F8h        4   COM1BASE=3F8h        COM1BASE=3F8h
   COM2      2F8h        3   COM2BASE=2F8h        COM2BASE=2F8h
   COM3      3E8h        4   COM3BASE=2E8h        COM3BASE=3E8h
   COM4      2E8h        3   COM4BASE=2E0h        COM4BASE=2E8h

The first column lists the ports. The second column describes the settings for the ports that both the Windows COMM.DRV and most popular communications packages use by default. The third column shows what Windows 3.0 sets by default and the fourth column shows what should be set in the [386Enh] section of the SYSTEM.INI file for proper functioning of the ports under 386 enhanced mode Windows 3.0.

NOTE: The default values specified for COM3 and COM4 ensure PS/2 compatibility.

There is a problem with using the Windows 3.0 SYSTEM.INI file setting COM(x)IRQ= in the [386Enh] section. Changing the COM port interrupt is NOT be recognized by Windows applications. However, MS-DOS applications running with Windows 3.0 in 386 enhanced mode do recognize this COM(x)IRQ= setting. Therefore, Windows applications that use the COM ports are limited to hardware interrupts 3 and 4 for COM2 and COM1, respectively.

MORE INFORMATION

In 386 enhanced mode, Windows 3.0 accesses the communication ports using a device called the virtual communications driver (VCD). MS-DOS applications running from Windows in 386 enhanced mode access COM ports by using the VCD. Changing the COM(x)IRQ= setting in the SYSTEM.INI file allows the use of a COM port that has been set to a different hardware interrupt, but this works only for MS-DOS applications running in 386 enhanced mode.

Windows applications use the VCD as well; however, they must also communicate through a second layer, the Windows real/standard mode COMM driver. The real/standard mode COMM driver does not recognize changes made to the VCD; therefore, Windows applications cannot use COM ports assigned to any interrupts other than industry standard ones.

The following information about COM(x)IRQ is from the SYSINI2.TXT file:

   COM1Irq=<number>
   COM2Irq=<number>
   COM3Irq=<number>
   COM4Irq=<number>

   Default: COM1Irq=4; COM2Irq=3; COM3Irq=4; COM4Irq=3

   Purpose: Specifies which interrupt line is being used by
            the device on the specified communications port.
            Check your hardware documentation for the
            appropriate value. Setting a value to -1 disables
            input for that COM port. You would do this only
            if there is a hardware conflict between ports.

Microsoft has confirmed this to be a problem in Microsoft Windows version 3.0. We are researching this problem and will post new information here as it becomes available.

Troubleshooting Communications Problems Under Windows 3.0

You can simultaneously use two communication ports that share the same interrupt (for example, COM1 and COM3, or COM2 and COM4) only if the hardware is capable of doing so. The ability to share communications port interrupt request lines (IRQs) is hardware dependent. Currently, the only hardware that you can be sure that IRQ sharing is supported on is Micro Channel Architecture (MCA) hardware. Although Extended Industry Standard Architecture (EISA) hardware does have IRQ sharing as part of its specifications, implementation on current machines is random. If you cannot successfully share an IRQ with the COMIrqSharing switch set to true, the hardware does not support IRQ sharing and Windows 3.0 is not able to overcome the lack of support for this feature using software.

Note: On some implementations of EISA, there is some configuration of the system hardware that must take place before IRQ sharing can occur. An example of this is Compaq's EISA machines. By default, they are not configured to IRQ share. Be aware that you may need to take additional steps to configure such machines.
COM3 and COM4 may not be reliable under Windows 3.0 (in standard or real mode) unless both COM1 and COM2 are first activated. If you use only COM3 and/or COM4, you may experience problems (with printing, communications, or your mouse). If you use COM1 and COM3 without using COM2, you also may have problems. The easiest way to remember this is to not use a higher serial port (2, 3, or 4) unless all lower number ports (1, 2, and 3) are first activated (or in use).
Standard and real mode Windows 3.0 use COMM.DRV directly, whereas 386 enhanced mode Windows 3.0 serializes the ports using the VCD. For this reason, serial communication can theoretically be considered more reliable under standard and real mode because there can be no miscommunication between the VCD and COMM.DRV. In cases where you are using multiple communications ports under Windows 3.0 386 enhanced mode, verify that the base port addresses are set as described in the table earlier in this article.

NOTE: IRQ sharing is possible under 386 enhanced mode Windows 3.0 only if you make the necessary changes to the [386Enh] section of the SYSTEM.INI file as outlined above.