How To Use MSCOMM32.OCX to Transfer Data on DBCS Windows

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SUMMARY

This article illustrates some techniques using the MSComm32.OCX control for receiving and transmitting binary data on double-byte character set (DBCS) Windows.

MORE INFORMATION

The Communications control, MSCOMM32.OCX, provides serial communications for your application by allowing the transmission and reception of data through a serial port where only a string of characters to the transmit buffer is permitted. This feature limits you to transmitting only text- based files.

The binary data must be converted one byte at a time to a character and then transmitted. When received, the data must be converted from a character back into binary data one byte at a time. See the REFERENCES section below for more information.

On double-byte character set Windows, such as Korean, Japanese, and Chinese Windows, binary data will be corrupted if one of the binary values matches a DBCS lead character. The MSComm control will interpret this byte and the following byte as one double-byte character and return only one byte for the equivalent ASCII character. To resolve this problem, which is specific to DBCS Windows, you can implement certain encoding algorithms that can convert binary data into text data and vice versa. This article demonstrates how to convert one byte binary data into ASCII size of three characters to preserve the lead character. On the receiving end, these three ASCII characters are converted back into one-byte binary data. For example:
ASC('a') = 97 ' 97

would be transmitted as three characters:
'0''9''7'

NOTE: The sample code is reading the input file as binary. It is not necessary to convert each character to ASCII value.

If you use a modem instead of RS-232 cable, you need to add some codes to establish the modem connection. For example:
  • MSComm1.Output = 'ATDT 999-9999' for Transmitter

    and -
  • MSComm1.Output = 'AT' for Receiver
Once the connection is established, everything should work the same way.

WARNING: USE OF THE SAMPLE CODE PROVIDED IN THIS ARTICLE IS AT YOUR OWN RISK. Microsoft provides this sample code "as is" without warranty of any kind, either express or implied, including but not limited to the implied warranties of merchantability and/or fitness for a particular purpose.

Sample Code - Transmitter

  1. Start Visual Basic. If Visual Basic is already running, from the File menu, choose New Project. Form1 will be created by default.
  2. Place Microsoft Comm Control on Form1. If Microsoft Comm Control cannot be found in the control bar, go to Tools menu, choose Custom Controls, and then check the Microsoft Comm Control on the List Box.
  3. Add the following code to the General Declarations section of Form1:
          ' General Declaration section
    
           Dim Offset As Long
           Dim FileData As Byte
           Dim FileName As String
    
    						
  4. Add the following code to the Load event of Form1:
          ' Load event of Form
          ' 28800 baud, no parity, 8 data, and 1 stop bit.
          ' In RS-232, maximum speed of 28800 baud can be used
    
          Private Sub Form_Load()
              MSComm1.Settings = "28800,N,8,1"
              MSComm1.InputLen = 1
              MSComm1.CommPort = 2
              MSComm1.PortOpen = True
    
          ' Change the file path to point to the file you want to transfer
              FileName = "d:\sample.hlp"
    
              Offset = 1
              Open FileName For Binary Access Read As #1
          End Sub
    
    						
  5. Add the following code to the OnComm event of MSComm1:
          ' ONCOMM event of Comm OLE Control
          ' The following code is needed to make sure that next set of
          ' characters can be transmitted (CommEvent = 2 is triggered from the
          ' receiver side)
    
          Private Sub MSComm1_OnComm()
              If MSComm1.CommEvent = 2 Then
                  temp = MSComm1.Input
                  If Offset <= FileLen(FileName) Then
                      Get #1, Offset, FileData
                      q = FileData
                      MSComm1.Output = Format(q, "000")
                      Offset = Offset + 1
                  End If
              End If
          End Sub
    
    						
  6. Add the following code to the UnLoad event of Form1:
          ' UnLoad event of Form
    
          Private Sub Form_Unload(Cancel As Integer)
              Close #1
          End Sub
    
    						
  7. Make sure that the Properties of MSComm1 are changed as follows:
          RThreshold = 1     ' triggers when at least one char is on the buffer
          SThreshold = 3
    
    						

Sample Code - Receiver

  1. Start Visual Basic. If Visual Basic is already running, from the File menu, choose New Project. Form1 will be created by default.
  2. Place Microsoft Comm Control on Form1. If Microsoft Comm Control cannot be found in the control bar, go to Tools menu, choose Custom Controls, and then check the Microsoft Comm Control on the List Box.
  3. Add a CommandButton to Form1. Command1 is created by default.
  4. Add the following code to the General Declarations section of Form1:
          ' General Declaration section
    
          Dim ByteCount As Long
          Dim FileData As Byte
    
    						
  5. Add the following code to the Load event of Form1:
          ' Load event of Form
          ' 28800 baud, no parity, 8 data, and 1 stop bit.
    
          Private Sub Form_Load()
              MSComm1.Settings = "28800,N,8,1"
              MSComm1.InputLen = 3
              MSComm1.CommPort = 1
              MSComm1.PortOpen = True
    
          ' Change the file path to point to the file you want to store
              Open "c:\sample.txt" For Binary Access Write As #1
    
          End Sub
    
    						
  6. Add the following code to the OnComm event of MSComm1:
          ' OnComm event
    
          Private Sub MSComm1_OnComm()
              If MSComm1.CommEvent = 2 And MSComm1.InBufferCount > 0 Then
                  FileData = CInt(MSComm1.Input)
                  ByteCount = ByteCount + 1
                  Put #1, ByteCount, FileData
                  MSComm1.Output = Chr$(26)
              End If
          End Sub
    
    						
  7. Add the following code to the Click event of Command1:
          ' CLICK event of button
    
          Private Sub Command1_Click()
              MSComm1.Output = Chr$(26)
          End Sub
    
    						
  8. Add the following code to the UnLoad event of Form1:
          ' UnLoad even of Form
    
          Private Sub Form_Unload(Cancel As Integer)
              Close #1
          End Sub
    
    						
  9. Make sure that the Properties of MSComm1 are changed as follows:

    RThreshold = 3
The Transmitter and Receiver codes should be compiled separately and placed on two separate machines where one machine is the designated transmitter and the other one is the receiver.

The event-driven technique generates an OnComm event when there are characters waiting in the input buffer. Also, the CommEvent property will contain a numeric 2. For the OnComm event to be triggered, you must set the Rthreshold property to a value other than its default of zero. The most common setting for the RThreshold property is 1, meaning that the OnComm event is triggered if a minimum of one character is waiting in the input buffer. In this article, the numeric 3 is being used as a value of RThreshold property because 3 characters at a time are being sent.

REFERENCES

For additional information, please see the following article in the Microsoft Knowledge Base:
194923 How To Trap Control Characters Using the MSComm Control

Properties

Article ID: 158008 - Last Review: July 13, 2004 - Revision: 3.1
APPLIES TO
  • Microsoft Visual Basic 5.0 Control Creation Edition
  • Microsoft Visual Basic 5.0 Learning Edition
  • Microsoft Visual Basic 5.0 Professional Edition
  • Microsoft Visual Basic 5.0 Enterprise Edition
  • Microsoft Visual Basic 4.0 Professional Edition
  • Microsoft Visual Basic 4.0 Professional Edition
  • Microsoft Visual Basic 4.0 16-bit Enterprise Edition
  • Microsoft Visual Basic 4.0 32-Bit Enterprise Edition
Keywords: 
kbhowto kbintldev kbctrl KB158008

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