Q194923: HOWTO: Trap Control Characters Using the MSComm Control
Article: Q194923
Product(s): Microsoft Visual Basic for Windows
Version(s): 5.0,6.0
Operating System(s):
Keyword(s): kbcode kbIO kbVBp kbVBp500 kbVBp600 kbGrpDSVB kbCodeSam
Last Modified: 06-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
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SUMMARY
=======
It is often necessary to look for and react to special unprintable characters
when using the MSComm control. These characters can include XOn, XOff, STX, ETX
and others.
MORE INFORMATION
================
In order to trap these types of characters, you must look for the ASCII value of
the character, not the string representation of it. Unprintable characters in
Visual basic are all represented in the same manner, so there is no way to
differentiate between them without looking at the ASCII value. This is easily
accomplished by utilizing the Visual Basic Chr$() function.
The following example assumes that you have a null modem cable attached between
COM1 and COM2. This is only necessary to for testing. App1 is the COM1
application, App2 is the COM2 application.
Steps to Create App1
--------------------
1. Create a new Standard EXE project. Form1 is created by default.
2. Choose Components from the Project menu, check the "Microsoft Comm Control,"
and click OK.
3. Add an MSCOMM control to the form.
4. Add two CommandButtons to the form.
5. Add the following code to Form1's code window:
Const Xon = &H11
Const Xoff = &H13
Private Sub Command1_Click()
MSComm1.Output = "123456789" & Chr$(Xoff)
End Sub
Private Sub Command2_Click()
MSComm1.Output = "987654321" & Chr$(Xon)
End Sub
Private Sub Form_Load()
Form1.Caption = "App1"
With MSComm1
.Handshaking = 2 - comRTS
.RThreshold = 1
.RTSEnable = True
.Settings = "9600,n,8,1"
.SThreshold = 1
.PortOpen = True
End With
Command1.Caption = "&Send Xoff"
Command2.Caption = "Send &Xon"
End Sub
Private Sub Form_Unload(Cancel As Integer)
MSComm1.PortOpen = False
End Sub
Steps to Create App2
--------------------
1. Start a new instance of Visual Basic.
2. Create a new Standard EXE project. Form1 is created by default.
3. Choose Components from the Project menu, check the "Microsoft Comm Control,"
and click OK.
4. Add an MSCOMM control to the form.
5. Add a TextBox to the form. Change the MultiLine property of the TextBox to
True. Enlarge the TextBox so it will cover most of the form, as you will be
displaying all the received data in it.
6. Add a Label to Form1.
7. Add the following code to Form1's code window:
Const Xon = &H11
Const Xoff = &H13
Private Sub Form_Load()
Form1.Caption = "App2"
With MSComm1
.CommPort = 2
.Handshaking = 2 - comRTS
.RThreshold = 1
.RTSEnable = True
.Settings = "9600,n,8,1"
.SThreshold = 1
.PortOpen = True
End With
Text1.Text = ""
Label1.Caption = "No input yet"
End Sub
Private Sub Form_Unload(Cancel As Integer)
MSComm1.PortOpen = False
End Sub
Private Sub MSComm1_OnComm()
Dim InBuff As String
Select Case MSComm1.CommEvent
' Handle each event or error by placing
' code below each case statement.
' This template is found in the Example
' section of the OnComm event help topic
' in VB help.
' Errors
Case comEventBreak ' A Break was received.
Case comEventCDTO ' CD (RLSD) Timeout.
Case comEventCTSTO ' CTS Timeout.
Case comEventDSRTO ' DSR Timeout.
Case comEventFrame ' Framing Error
Case comEventOverrun ' Data Lost.
Case comEventRxOver ' Receive buffer overflow.
Case comEventRxParity ' Parity Error.
Case comEventTxFull ' Transmit buffer full.
Case comEventDCB ' Unexpected error retrieving DCB]
' Events
Case comEvCD ' Change in the CD line.
Case comEvCTS ' Change in the CTS line.
Case comEvDSR ' Change in the DSR line.
Case comEvRing ' Change in the Ring Indicator.
Case comEvReceive ' Received RThreshold # of chars.
Label1.Caption = "Input"
InBuff = MSComm1.Input
Call ParseChars(InBuff)
Case comEvSend ' There are SThreshold number of
' characters in the transmit
' buffer.
Case comEvEOF ' An EOF character was found in
' the input stream.
End Select
End Sub
Sub HandleInput(InBuff As String)
' This is where you will process your input. This
' includes trapping characters, parsing strings,
' separating data fields, etc. For this case, you
' are simply going to display the data in the text
' box.
Text1.Text = Text1.Text & InBuff
End Sub
Sub ParseChars(ByVal InString As String)
Dim temp As String
Dim x As Long
Dim OutString as String
For x = 1 To Len(InString)
temp = Mid$(InString, x, 1)
If temp = Chr$(Xoff) Then
Label1.ForeColor = vbRed
Label1.Caption = "Xoff received"
temp = ""
ElseIf temp = Chr$(Xon) Then
Label1.ForeColor = vbGreen
Label1.Caption = "Xon received"
temp = ""
End If
OutString = OutString & temp
temp = ""
Next x
Call HandleInput(OutString)
End Sub
Steps to Run the Applications
-----------------------------
1. Press the F5 key or click the run button on each project. You will need to
move the apps around so you can see them both running at the same time.
2. Make sure you have a standard null modem cable between COM1 and COM2.
3. Click the CommandButton labeled "Send Xoff" on App1. You should see the
string "Xoff received" appear in red in App2's label. The string "123456789"
will be added to the TextBox, followed by an unprintable control character.
4. Click the CommandButton labeled "Send Xon" on App1. You should see the string
"Xon received" appear in green in App2's label. The string "987654321" will
be added to the TextBox, followed by an unprintable control character.
Additional query words: mscomm serial xon xoff
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Keywords : kbcode kbIO kbVBp kbVBp500 kbVBp600 kbGrpDSVB kbCodeSam
Technology : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVB500 kbVB600
Version : :5.0,6.0
Issue type : kbhowto
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