HOWTO: Trap Control Characters Using the MSComm Control

• Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, versions 5.0, 6.0

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

Keywords : kbIO kbVBp kbVBp500 kbVBp600 kbGrpVB kbCodeSam
Version : WINDOWS:5.0,6.0
Platform : WINDOWS
Issue type : kbhowto