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Microsoft KB Archive/253257

From BetaArchive Wiki

Article ID: 253257

Article Last Modified on 6/29/2004



APPLIES TO

  • Microsoft Visual Basic 6.0 Professional Edition
  • Microsoft Visual Basic 6.0 Enterprise Edition



This article was previously published under Q253257

SUMMARY

This article demonstrates how to capture the End of File (EOF) character with the MSComm control.

When the OnComm event of the MSComm control is used, it is sometimes necessary to trap the EOF character to detect when the end of a file is reached. The EOF character is defined as CTRL+Z, or ASCII 26.

MORE INFORMATION

This example demonstrates how to create two small sample applications that show how to signal the EOF character. The EOFEnable property must be set to True on the receiving side, and the contents of the MSComm Input buffer must also be read in order for the character to be detected. The following example assumes that a null modem cable is attached between COM1 and COM2. This is only necessary for testing these sample applications. App1 is the COM1 application, App2 is the COM2 application.

Steps to Create App1

  1. Create a new Standard EXE project in Visual Basic. Form1 is created by default.
  2. On the Project menu, select Components, select Microsoft Comm Control, and then click OK.
  3. Add a Command button and an MSComm control to Form1.
  4. Add the following code to the General Declarations section of Form1:

    Private Sub Command1_Click()
       MSComm1.Output = Chr$(26)
    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"
    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 specifying a new Standard EXE project. Form1 is created by default.
  2. On the Project menu, select Components, select the Microsoft Comm Control, and then click OK.
  3. Add an MSComm control to Form1.
  4. Add the following code to the General Declarations section of Form1:

    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
          .EOFEnable = True
       End With
    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.
             InBuff = MSComm1.Input
          Case comEvSend ' There are SThreshold number of
                         ' characters in the transmit
                         ' buffer.
          Case comEvEOF  ' An EOF character was found in
                         ' the input stream.
             MsgBox "EOF character received!"
       End Select
    End Sub
                        

Steps to Run the Applications

  1. Press the F5 key or click the Run button on each project. Position both applications so that you can see them running at the same time.
  2. Make sure you have a standard null modem cable connecting the COM1 and COM2 ports.
  3. Click the Command button labeled Send on App1, and note that a Message Box displaying the message "EOF character received!" appears in App2.


REFERENCES

For additional information, click the article number below to view the article in the Microsoft Knowledge Base:

194923 How To Trap Control Characters Using the MSComm Control


Keywords: kbhowto kbcommport kbctrl KB253257