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COM port made simple with VB.NET

, 4 Oct 2003 CPOL
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Using the com port in windows, using any .NET Language


This article is about controlling the com port in windows. You can't just tell the computer to open the com port like you would with Pascal or C++ in DOS. But there are still some tricks that makes it possible to open and use the com port. The only thing is, you will need to know something about windows - Win2k has a HAL (Hardware Abstraction Layer).

First of all, windows 2000, windows xp and windows 2003 have protected hardware access. This is the HAL. Very useful when it comes to security, but for programmers like me it's a nightmare. I found a solution: Let windows do the job for you.

How did I make this?

I used some basic windows function for my com port controlling functionality:

  • CreateFile
  • ReadFile
  • WriteFile
  • CloseHandle
  • BuildCommDCB
  • SetCommState

You can find these functions in the old VB6 API Viewer program (Or you might take a look in the source of course).

I simply told windows to do the hardware controlling for me. I will trick my program that the com port is a file. This is made possible through the Windows API Functions I use.

The code

The first function of my code is the open function. The function will be trying to make a com port handle. After that I check if the handle is correct. If the handle is ok, I will continue by making a DCB Structure instance ready for use. With this structure we can control all comport settings. (I will only use, speed, parity, stop bits and data bits)

Public Sub Open(ByVal portname As String, _
        ByVal Spd As Integer, ByVal Pty As enumParity, _
        ByVal Dtb As Integer, ByVal Stp As enumStopBits)

        Dim m_CommDCB As String
        Dim m_Baud As String
        Dim m_Parity As String
        Dim m_Data As String
        Dim m_Stop As Byte

        hPort = CreateFile(portname, GENERIC_READ + GENERIC_WRITE, _
              0, 0, OPEN_EXISTING, 0, 0)

        If hPort < 1 Then
            Throw New Exception("Can't open the comport! (Errorcode :" _
                  & GetLastError().ToString() & ")")
        End If

        m_Baud = Spd.ToString()
        m_Parity = PARITYSTRING.Substring(Pty, 1)
        m_Data = Dtb.ToString()
        m_Stop = Stp

        m_CommDCB = String.Format("baud={0} parity={1} data={2} stop={3}", _
              m_Baud, m_Parity, m_Data, m_Stop)

        BuildCommDCB(m_CommDCB, dcbPort)

        If SetCommState(hPort, dcbPort) = 0 Then
            Throw New Exception("kan de compoort niet openen(" & _
               GetLastError().ToString() & ")")
        End If

        m_opened = True
End Sub

The next function is the Write Function. This function controls the writing to the com port. I choose to write one byte at a time to the com port since I wanted to control a microcontroller with my com port. But you can change this of course to a multi-byte write function or add an extra function for multi-byte.

Public Sub Write(ByVal data As Byte)
 Dim dt As Byte()
        Dim written As Integer

        dt(0) = data 'We have a multi-byte buffer, which you can of 
                     'course enable...

        If Opened = True Then
            WriteFile(hPort, dt, 1, written, Nothing)
            Throw New Exception("Comport not opened")
        End If

 dt = Nothing
End Sub
Of course I also want to read from the com port. This is achieved by the read function. For this function the same story as for the Write function with the multi-byte add-on.

Public Function Read() As Byte
        Dim rd As Integer
        Dim ovl As New OVERLAPPED
        Dim dt As Byte()

        dt = Array.CreateInstance(GetType(Byte), 1) 'Initialize the buffer

        If Opened = True Then
            ReadFile(hPort, dt, 1, rd, ovl)
            Throw New Exception("Comport not opened")
        End If

        Return dt(0)
End Function

At last and more important than the open function we have the Close function. This function closes the com port and releases the system handle to it. If we don't do this in NT4/win2k/xp/2003, we get big problems.

Public Sub Close()
        hPort = -1
        m_opened = False
End Sub


I hope my english wasn't too bad and my explaination not too chaotic. But above all, I hope you have fun with my com port driver!


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

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Netherlands Netherlands

Comments and Discussions

QuestionOption Explicit and a Few Gripes Pinmemberbrother.gabriel17-Jul-12 12:09 
QuestionUSB Port programming Pinmemberjamith6-Mar-09 0:46 
QuestionI no nothing about code Pinmemberelmerjfudd19-Jul-08 22:16 
Generalcan't open the comport! (Errorcode :2) [modified] Pinmemberairbase9-Sep-07 23:29 
GeneralRe: can't open the comport! (Errorcode :2) [modified] PinmemberMember 978238924-Jan-13 2:54 
QuestionHow to use this class in VS 2005 PinmemberBrad Wick12-Aug-07 7:47 
Sorry for such a stupid question but I am new with VS 2005. Can you help me with what is needed to put this DLL into a existing application? I added a reference to the .dll and then added "Imports MeinDesign.LPT" to the .vb file. Then in my button I added the following code
Dim LPTPort As New MeinDesign.LPT
LPTPort.Write("Text to send to LPT port")
I get a error that "Kan de parallele poort niet openen" which I am guessing means something that the LPT port was not opened.
AnswerRe: How to use this class in VS 2005 PinmemberWillemM12-Aug-07 8:10 
Generalprocess Byte() code Pinmemberxds200025-Nov-06 21:42 
GeneralRe: process Byte() code PinmemberTrevorWJ1-Mar-07 3:25 
GeneralUsing COM Port with ASP.NET Pinmemberajsri777-Jun-06 22:40 

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