PRB: Accessing WINMODEM with MSComm Control Can Hang Application

• Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, versions 5.0, 6.0
• Microsoft Visual Basic Standard, Professional, and Enterprise Editions, 32-bit only, for Windows, version 4.0

SYMPTOMS

While performing serial communications using the MSCOMM32.OCX with a WINMODEM, upon opening the com port, system resources will be reduced causing the application to hang.

CAUSE

WINMODEM is a software modem. It is different from a standard modem in that it does not include a UART (Universal Asynchronous Receiver Transmitter) chip or a processor chip to process instructions. Instead, it uses the computer's CPU. Necessary software and drivers must be loaded for the WINMODEM to work.

RESOLUTION

Do not open the port until you are ready to output characters to the modem. Immediately following the message MSComm1.PortOpen = TRUE, send a command string or other output to the modem. This can be as simple as an "AT" command string.

MORE INFORMATION

Steps to Work Around Behavior

1. Create a new Standard EXE project. Form1 is created by default.



2. Choose Components from the Project menu, check the "Microsoft Comm Control," and click OK.



3. Add an MSComm control to the form.



4. Add the following code to Form1's code window:

   
         Private Sub Form_Load()
   
            With MSComm1
               .Handshaking = 2 - comRTS
               .RThreshold = 1
               .RTSEnable = True
               .Settings = "9600,n,8,1"
               .SThreshold = 1
               ' Leave all other settings as default values.
            End With
            ' These next 2 lines avoid the problem
            MSComm1.PortOpen = True
            MSComm1.Output = "AT"
         End Sub
    




5. Run the program. Without the MSComm1.Output line, the application will consume resources and may hang. With this line, the application runs fine, as expected.

REFERENCES

For more information on how to use the MSComm control, please see the following article in the Microsoft Knowledge Base:

Q192012 : INFO: Troubleshooting Tips for the MSComm Control

Keywords : kbCtrl kbIO kbVBp kbVBp400 kbVBp500 kbVBp600 kbGrpVB
Version : WINDOWS:4.0,5.0,6.0
Platform : WINDOWS
Issue type : kbprb