Q192012: INFO: Troubleshooting Tips for the MSComm Control
Article: Q192012
Product(s): Microsoft Visual Basic for Windows
Version(s): 4.0,5.0,6.0
Operating System(s):
Keyword(s): kbfile kbCtrl kbVBp kbVBp400 kbVBp500 kbVBp600 kbGrpDSVB kbDSupport kbCOMM
Last Modified: 07-MAY-2002
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The information in this article applies to:
- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0
- Microsoft Visual Basic Standard Edition, 32-bit, for Windows, version 4.0
- Microsoft Visual Basic Professional Edition, 16-bit, for Windows, version 4.0
- Microsoft Visual Basic Professional Edition, 32-bit, for Windows, version 4.0
- Microsoft Visual Basic Enterprise Edition, 16-bit, for Windows, version 4.0
- Microsoft Visual Basic Enterprise Edition, 32-bit, for Windows, version 4.0
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SUMMARY
=======
The MSComm control encapsulates much of the functionality provided by the
communications functions of the Windows API. This encapsulation makes these
functions easier to use but does limit the functionality of the MSComm control.
This article provides you with some tips for troubleshooting the MSComm control.
MORE INFORMATION
================
1. If you are using Visual Basic 6.0, the version that ships on the CD is
6.00.8169. If you are using Visual Basic 5.0, there are two versions of the
MSComm control that you can use. Microsoft recommends that you use the
version updated by Service Pack 2 for Visual Basic 5.0. This version is
5.01.4319 dated 7/19/97. The other version, which shipped on the Visual Basic
5.0 CD, is 5.00.3714 dated 1/14/97. The major fix in the Service Pack 2
version is documented in Microsoft Knowledge Base article Q171472, and it
fixes a memory leak that was occurring when the port was repeatedly toggled
open and closed.
2. Use another communications application such as HyperTerminal, supplied with
Windows 95, Windows 98, Windows Me, and Windows 2000, to determine if a
connection can be established independent of Visual Basic. This will
determine if a connection can be made using only the communication functions
in the Windows API. If a connection cannot be made, examine the physical
elements of the connection. Is the proper cable being used? Is it connected
to the correct serial port? If a connection can be made, note the settings
that were used and use the same settings with the MSComm control.
3. Use the Visual Basic VBTerm and Dialer sample applications with the
appropriate settings. If these work, use this code as a starting point for
your application. If they don't work, first try the suggestions below on
these samples. If these suggestions fix the problem, then use the samples as
a starting point. Otherwise, start with the simplest code possible and use
the following suggestions as a starting point.
4. Set the RTSEnable property of the MSComm control to True. Many modems and
other communication devices use the RTS signal for handshaking.
5. Set the RThreshold property to 1. If you set the RThreshold property to any
other value, you may have trouble unless all of your data consists of
fixed-length packets.
6. Set the InputLen property to 0 unless there is an overriding reason not to do
so.
7. Always receive data when data is available. Do not rely on the RThreshold
property to tell you if data is available. Use the OnComm event procedure.
Double-buffer the incoming data. Extract the data from the buffer and process
it. Clean up the buffer to remove the processed data. For example:
Sub MSComm1_OnComm()
Static Buffer As String
' Always buffer incoming data no matter
' what generated the OnComm event.
Buffer = Buffer & MSComm1.Input
' Check if buffer has grown larger than limit.
If Len(Buffer) >= limit Then
' Call procedure to Process the received information.
process Left$(Buffer, limit)
' Clean-up buffer
Buffer = Right$(Buffer, Len(Buffer) - limit)
End If
End Sub
This procedure double-buffers the received data and, unless the program is
bound to fail because of some other performance problems, provides good
security for the received data. It is extremely important to handle every
character as it comes in at high baud rates. If you are looking for a
terminating character sequence instead of length, then the test would use
InStr instead of Len.
8. Whenever possible, use event-driven communications. A template for the OnComm
event can be found in Help under the example tab of the OnComm topic.
Event-driven communication is far more efficient than continuous polling, and
also allows for a wider range of error handling.
========== 16 bit only ==============
1. If you are using Windows for Workgroups 3.11, obtain the updated version of
SERIAL.386.
The following file is available for download from the Microsoft Download Center:
Wg1001.exe
(http://download.microsoft.com/download/vb40pro/Sample/2.0/Win98/En-US/Wg1001.exe)
Release Date: May 31, 2000
For additional information about how to download Microsoft Support files, click
the article number below to view the article in the Microsoft Knowledge Base:
Q119591 How to Obtain Microsoft Support Files from Online Services
Microsoft used the most current virus detection software available on the date of
posting to scan this file for viruses. After it is posted, the file is housed on
secure servers that prevent any unauthorized changes to the file.
2. If the Visual Basic application using the MSComm control is running on a
Windows for Workgroups 3.11 machine with a 16550AF UART chip and the
InBufferSize property is greater than 1024, COMxFIFO=0 must be added to the
[386Enh] section of the System.ini file. This entry disables the FIFO buffer
of COMx's 16550 UART. If a serial port does not have a 16550 UART, this
setting is ignored. The default value is 1 (enabled).
Additional query words: Wg1001
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Keywords : kbfile kbCtrl kbVBp kbVBp400 kbVBp500 kbVBp600 kbGrpDSVB kbDSupport kbCOMM
Technology : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVB500 kbVB600 kbVB400Search kbVB400 kbVB16bitSearch
Version : :4.0,5.0,6.0
Issue type : kbinfo
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