FAQ:Transmitting and Receiving Binary Data with MSComm Control

• Professional and Enterprise Editions of Microsoft Visual Basic, 16-bit and 32-bit, for Windows, version 4.0

SUMMARY

This article covers some of the most Frequently Asked Questions (FAQs) about using the Visual Basic 4.0 Communications (MSComm) control for receiving and transmitting binary data. You can find this and other FAQ articles by querying on the keyword "FAQ." You can find additional general references in the Microsoft Knowledge Base by searching on "article list."

MORE INFORMATION

Q. Can the MSComm control (MSComm16.OCX and MSComm32.OCX) be used to

   transmit and receive binary data?

   data provided neither end of the conversation is using a double byte
   character set (DBCS). The binary data must be converted one byte at a
   time to a character and transmitted. When received, it must be
   converted from a character back into binary data one byte at a time.
   The code fragment below illustrates this process.

   '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.

   'Transmitter Code.
   Dim FileName As String: FileName = "C:\BinFiles\Test.Exe"
   Dim Offset As Long
   Dim FileData As Byte

   Open FileName For Binary Access Read As #1
      For Offset = 1 To FileLen(FileName)
         Get #1, Offset, FileData
         MSComm1.Output = Chr$(FileData)
      Next Offset
   Close #1

   'Receiver Code.
   'Assume That File #1 Has Already Been Opened Elsewhere In The Code
   'And That ByteCount Has Been Dimmed As Long And Initialized To 0.
   Private Sub MSComm1_OnComm()
      Dim TmpStr As String
      Dim StrLen As Long, I As Long
      Dim FileData As Byte

      If MSComm1.CommEvent = comEvReceive Then
         While MSComm1.InBufferCount > 0
            TmpStr = MSComm1.Input
            StrLen = Len(TmpStr)
            For I = 1 To StrLen
               FileData = CByte(Asc(Mid(TmpStr, I, 1)))
               ByteCount = ByteCount + 1
               Put #1, ByteCount, FileData
            Next I
            TmpStr = ""
         Wend
      End If
   End Sub
   'Assume That File #1 Is Properly Closed When All Of The Data Is
   'Received.

   MSComm control (MSComm16.OCX and MSComm32.OCX) to transmit and
   receive binary data?

   uses one of the double byte character sets), 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 (now incorrect) byte
   for the "equivalent" ASCII character. For example, lead byte
   characters, such as &H81 with no trailing byte, are translated to ANSI
   0. While this limitation of the MSComm control does not allow binary
   data to be transmitted and received on DBCS systems, it does allow a
   standard "text" stream to be transferred.

   of the popular binary transfer protocols?

   protocols like XModem/YModem, ZModem, or Compuserve B, for example. You
   would have to code these protocols yourself or purchase a third-party
   communication control that implements the desired protocols.

   arrays?

   review and will be considered for inclusion in a future release. The
   MSComm control's Input and Output properties only work with Strings.

   (MSComm32.OCX)?

   This feature is under review and will be considered for inclusion in
   a future release. However, this feature would be unnecessary if the
   MSComm control would allow the use of byte arrays to transmit and
   receive binary data. (See question 4 above.)

   Visual Basic 4.0 MSComm control (MSComm16.OCX and MSComm32.OCX)?

   the Visual Basic 4.0 MSComm control. Since NULLS can be transmitted
   or received properly, as long as the NullDiscard property is set to
   FALSE, and C programmers can access all of the properties and methods
   of the .OCX controls, there is no longer a need for these functions
   in the MSComm control.

   data at a baud rate greater than 9600 baud?

   including 28,800 baud.