How to access serial and parallel ports by using Visual Basic .NET

For a Microsoft Visual Studio 2005 version of this article, see
904795 .

Summary

This step-by-step article describes how to access serial ports and how to access parallel ports by using Microsoft Visual Basic .NET. This article also contains sample code that illustrates the concepts that are discussed in this article.

Requirements

The following list outlines the recommended hardware, software, network infrastructure, and service packs that you need:
  • Microsoft Windows Server 2003, Microsoft Windows XP, or Microsoft Windows 2000
  • Visual Basic .NET
This article assumes that you are familiar with the following topics:
  • Programming in Visual Basic .NET
  • Platform invoke services in Visual Basic .NET

Use the MSComm control in Visual Basic .NET to access serial ports

Because no Microsoft .NET Framework classes exist to access the communications resources that connected to your computer, you can use the MSComm control in Microsoft Visual Basic 6.0. The MSComm control provides serial communications for your application by enabling the transmission and reception of data through a serial port. To implement basic serial communications by using a modem, follow these steps:
  1. Start Microsoft Visual Studio .NET.
  2. On the File menu, point to
    New, and then click Project.
  3. Under Project Types, click Visual Basic Projects.
  4. Under Templates, click Console Application.
  5. In the Name box, type
    MyConsoleApplication, and then click
    OK.

    By default, Module1.vb is created.
  6. Right-click the MyConsoleApplicationproject, and then click Add Reference.
  7. Click the COM tab, click Microsoft Comm Control 6.0 under Component Name, click
    Select, and then click OK.

    Note To use the MSComm control, you must install the related COM components of Microsoft Visual Basic 6.0 on the same computer that has Microsoft Visual Studio .NET installed.

    For more information about license issues when you use Visual Basic 6.0 controls in Visual Studio .NET, click the following article number to view the article in the Microsoft Knowledge Base:
    318597 Errors when you use Visual Basic 6.0 controls in Visual Studio .NET

  8. Replace the code in Module1.vb with the following code example.
    Imports MSCommLib

    Module Module1

    Sub Main()
    'New a MSComm control
    Dim MSComm1 As MSComm
    MSComm1 = New MSComm
    ' Buffer to hold input string.
    Dim Buffer As String
    ' Use the COM1 serial port.
    MSComm1.CommPort = 1
    ' 9600 baud, no parity, 8 data, and 1 stop bit.
    MSComm1.Settings = "9600,N,8,1"
    ' Tell the control to read the whole buffer when Input is used.
    MSComm1.InputLen = 0
    ' Open the serial port.
    MSComm1.PortOpen = True
    Console.WriteLine("Open the serial port.")
    ' Tell the control to make the Input property return text data.
    MSComm1.InputMode() = InputModeConstants.comInputModeText
    'Clear the receive buffer.
    MSComm1.InBufferCount() = 0
    ' Send the attention command to the modem.
    MSComm1.Output = "ATV1Q0" & Chr(13)
    Console.WriteLine("Send the attention command to the modem.")
    Console.WriteLine("Wait for the data to come back to the serial port...")
    ' Make sure that the modem responds with "OK".
    ' Wait for the data to come back to the serial port.
    Do
    Buffer = Buffer & MSComm1.Input
    Loop Until InStr(Buffer, "OK" & vbCrLf)
    ' Read the "OK" response data in the serial port.
    ' Close the serial port.
    Console.WriteLine("Read the OK response data in the serial port.")
    MSComm1.PortOpen = False
    Console.WriteLine("Close the serial port.")
    End Sub

    End Module
  9. Press CRTL+F5 to build and run this project. You will receive the following output messages:
    Open the serial port.
    Send the attention command to the modem.
    Wait for data to come back to the serial port...
    Read the OK response data in the serial port.
    Close the serial port.

Use platform invoke services to call Win32 API functions in Visual Basic .NET to access serial and parallel ports

To do this, follow these steps:
  1. Start Microsoft Visual Studio .NET.
  2. On the File menu, point to
    New, and then click Project.
  3. Under Project Types, click Visual Basic Projects.
  4. Under Templates, click Console Application.
  5. In the Name text box, type
    MyConsoleApplication, and then click
    OK.

    By default, Module1.vb is created.
  6. Add the following code to Module1.vb before the
    Module Module1 statement:
    Option Strict On

    ' Define a CommException class that inherits from the ApplicationException class,
    ' and then throw an object of type CommException when you receive an error message.
    Class CommException
    Inherits ApplicationException
    Sub New(ByVal Reason As String)
    MyBase.New(Reason)
    End Sub
    End Class
  7. Declare structures, constants, and references to external functions that are in Kernel32.dll

    To call unmanaged functions from your managed Visual Basic .NET application, you must declare references to the structures that you pass as parameters to the unmanaged functions, and you must declare the constants that you pass as parameters to the unmanaged functions. To do this, add the following code to Module1.vb after the Module Module1 statement:
    'Declare structures.
    Public Structure DCB
    Public DCBlength As Int32
    Public BaudRate As Int32
    Public fBitFields As Int32 'See Comments in Win32API.Txt
    Public wReserved As Int16
    Public XonLim As Int16
    Public XoffLim As Int16
    Public ByteSize As Byte
    Public Parity As Byte
    Public StopBits As Byte
    Public XonChar As Byte
    Public XoffChar As Byte
    Public ErrorChar As Byte
    Public EofChar As Byte
    Public EvtChar As Byte
    Public wReserved1 As Int16 'Reserved; Do Not Use
    End Structure

    Public Structure COMMTIMEOUTS
    Public ReadIntervalTimeout As Int32
    Public ReadTotalTimeoutMultiplier As Int32
    Public ReadTotalTimeoutConstant As Int32
    Public WriteTotalTimeoutMultiplier As Int32
    Public WriteTotalTimeoutConstant As Int32
    End Structure

    'Declare constants.
    Public Const GENERIC_READ As Int32 = &H80000000
    Public Const GENERIC_WRITE As Int32 = &H40000000
    Public Const OPEN_EXISTING As Int32 = 3
    Public Const FILE_ATTRIBUTE_NORMAL As Int32 = &H80
    Public Const NOPARITY As Int32 = 0
    Public Const ONESTOPBIT As Int32 = 0

    'Declare references to external functions.
    Public Declare Auto Function CreateFile Lib "kernel32.dll" _
    (ByVal lpFileName As String, ByVal dwDesiredAccess As Int32, _
    ByVal dwShareMode As Int32, ByVal lpSecurityAttributes As IntPtr, _
    ByVal dwCreationDisposition As Int32, ByVal dwFlagsAndAttributes As Int32, _
    ByVal hTemplateFile As IntPtr) As IntPtr

    Public Declare Auto Function GetCommState Lib "kernel32.dll" (ByVal nCid As IntPtr, _
    ByRef lpDCB As DCB) As Boolean

    Public Declare Auto Function SetCommState Lib "kernel32.dll" (ByVal nCid As IntPtr, _
    ByRef lpDCB As DCB) As Boolean

    Public Declare Auto Function GetCommTimeouts Lib "kernel32.dll" (ByVal hFile As IntPtr, _
    ByRef lpCommTimeouts As COMMTIMEOUTS) As Boolean

    Public Declare Auto Function SetCommTimeouts Lib "kernel32.dll" (ByVal hFile As IntPtr, _
    ByRef lpCommTimeouts As COMMTIMEOUTS) As Boolean

    Public Declare Auto Function WriteFile Lib "kernel32.dll" (ByVal hFile As IntPtr, _
    ByVal lpBuffer As Byte(), ByVal nNumberOfBytesToWrite As Int32, _
    ByRef lpNumberOfBytesWritten As Int32, ByVal lpOverlapped As IntPtr) As Boolean

    Public Declare Auto Function ReadFile Lib "kernel32.dll" (ByVal hFile As IntPtr, _
    ByVal lpBuffer As Byte(), ByVal nNumberOfBytesToRead As Int32, _
    ByRef lpNumberOfBytesRead As Int32, ByVal lpOverlapped As IntPtr) As Boolean

    Public Declare Auto Function CloseHandle Lib "kernel32.dll" (ByVal hObject As IntPtr) As Boolean
  8. Before you can access a serial port or a parallel port, you must obtain a handle to the appropriate port and then configure the port communications. To do this, add the following initialization code to Module1.vb after the Sub Main statement.

    Note To establish communication with the LPTx port, you must stop the Print Spooler service. To do this, use the Services tool in Administrative Tools.
    ' Declare the local variables that you will use in the code.
    Dim hSerialPort, hParallelPort As IntPtr
    Dim Success As Boolean
    Dim MyDCB As DCB
    Dim MyCommTimeouts As COMMTIMEOUTS
    Dim BytesWritten, BytesRead As Int32
    Dim Buffer() As Byte

    ' Declare the variables to use for encoding.
    Dim oEncoder As New System.Text.ASCIIEncoding
    Dim oEnc As System.Text.Encoding = oEncoder.GetEncoding(1252)

    ' Convert String to Byte().
    Buffer = oEnc.GetBytes("Test")
    Try
    ' Access the serial port.
    Console.WriteLine("Accessing the COM1 serial port")
    ' Obtain a handle to the COM1 serial port.
    hSerialPort = CreateFile("COM1", GENERIC_READ Or GENERIC_WRITE, 0, IntPtr.Zero, _
    OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, IntPtr.Zero)
    ' Verify that the obtained handle is valid.
    If hSerialPort.ToInt32 = -1 Then
    Throw New CommException("Unable to obtain a handle to the COM1 port")
    End If
    ' Retrieve the current control settings.
    Success = GetCommState(hSerialPort, MyDCB)
    If Success = False Then
    Throw New CommException("Unable to retrieve the current control settings")
    End If
    ' Modify the properties of the retrieved DCB structure as appropriate.
    ' WARNING: Make sure to modify the properties according to their supported values.
    MyDCB.BaudRate = 9600
    MyDCB.ByteSize = 8
    MyDCB.Parity = NOPARITY
    MyDCB.StopBits = ONESTOPBIT
    ' Reconfigure COM1 based on the properties of the modified DCB structure.
    Success = SetCommState(hSerialPort, MyDCB)
    If Success = False Then
    Throw New CommException("Unable to reconfigure COM1")
    End If
    ' Retrieve the current time-out settings.
    Success = GetCommTimeouts(hSerialPort, MyCommTimeouts)
    If Success = False Then
    Throw New CommException("Unable to retrieve current time-out settings")
    End If
    ' Modify the properties of the retrieved COMMTIMEOUTS structure as appropriate.
    ' WARNING: Make sure to modify the properties according to their supported values.
    MyCommTimeouts.ReadIntervalTimeout = 0
    MyCommTimeouts.ReadTotalTimeoutConstant = 0
    MyCommTimeouts.ReadTotalTimeoutMultiplier = 0
    MyCommTimeouts.WriteTotalTimeoutConstant = 0
    MyCommTimeouts.WriteTotalTimeoutMultiplier = 0
    ' Reconfigure the time-out settings, based on the properties of the modified COMMTIMEOUTS structure.
    Success = SetCommTimeouts(hSerialPort, MyCommTimeouts)
    If Success = False Then
    Throw New CommException("Unable to reconfigure the time-out settings")
    End If
    ' Write data to COM1.
    Console.WriteLine("Writing the following data to COM1: Test")
    Success = WriteFile(hSerialPort, Buffer, Buffer.Length, BytesWritten, IntPtr.Zero)
    If Success = False Then
    Throw New CommException("Unable to write to COM1")
    End If
    ' Read data from COM1.
    Success = ReadFile(hSerialPort, Buffer, BytesWritten, BytesRead, IntPtr.Zero)
    If Success = False Then
    Throw New CommException("Unable to read from COM1")
    End If
    Catch ex As Exception
    Console.WriteLine(Ex.Message)
    Finally
    ' Release the handle to COM1.
    Success = CloseHandle(hSerialPort)
    If Success = False Then
    Console.WriteLine("Unable to release handle to COM1")
    End If
    End Try

    Try
    ' Parallel port.
    Console.WriteLine("Accessing the LPT1 parallel port")
    ' Obtain a handle to the LPT1 parallel port.
    hParallelPort = CreateFile("LPT1", GENERIC_READ Or GENERIC_WRITE, 0, IntPtr.Zero, _
    OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, IntPtr.Zero)
    ' Verify that the obtained handle is valid.
    If hParallelPort.ToInt32 = -1 Then
    Throw New CommException("Unable to obtain a handle to the LPT1 port")
    End If
    ' Retrieve the current control settings.
    Success = GetCommState(hParallelPort, MyDCB)
    If Success = False Then
    Throw New CommException("Unable to retrieve the current control settings")
    End If
    ' Modify the properties of the retrieved DCB structure as appropriate.
    ' WARNING: Make sure to modify the properties according to their supported values.
    MyDCB.BaudRate = 9600
    MyDCB.ByteSize = 8
    MyDCB.Parity = NOPARITY
    MyDCB.StopBits = ONESTOPBIT
    ' Reconfigure LPT1 based on the properties of the modified DCB structure.
    Success = SetCommState(hParallelPort, MyDCB)
    If Success = False Then
    Throw New CommException("Unable to reconfigure LPT1")
    End If
    ' Retrieve the current time-out settings.
    Success = GetCommTimeouts(hParallelPort, MyCommTimeouts)
    If Success = False Then
    Throw New CommException("Unable to retrieve current time-out settings")
    End If
    ' Modify the properties of the retrieved COMMTIMEOUTS structure as appropriate.
    ' WARNING: Make sure to modify the properties according to their supported values.
    MyCommTimeouts.ReadIntervalTimeout = 0
    MyCommTimeouts.ReadTotalTimeoutConstant = 0
    MyCommTimeouts.ReadTotalTimeoutMultiplier = 0
    MyCommTimeouts.WriteTotalTimeoutConstant = 0
    MyCommTimeouts.WriteTotalTimeoutMultiplier = 0
    ' Reconfigure the time-out settings, based on the properties of the modified COMMTIMEOUTS structure.
    Success = SetCommTimeouts(hParallelPort, MyCommTimeouts)
    If Success = False Then
    Throw New CommException("Unable to reconfigure the time-out settings")
    End If
    ' Write data to LPT1.
    ' Note: You cannot read data from a parallel port by calling the ReadFile function.
    Console.WriteLine("Writing the following data to LPT1: Test")
    Success = WriteFile(hParallelPort, Buffer, Buffer.Length, BytesWritten, IntPtr.Zero)
    If Success = False Then
    Throw New CommException("Unable to write to LPT1")
    End If
    Catch ex As Exception
    Console.WriteLine(Ex.Message)
    Finally
    ' Release the handle to LPT1.
    Success = CloseHandle(hParallelPort)
    If Success = False Then
    Console.WriteLine("Unable to release handle to LPT1")
    End If
    End Try

    Console.WriteLine("Press ENTER to quit")
    Console.ReadLine()
  9. On the Build menu, click Build Solution.
  10. On the Debug menu, click
    Start to run the application.

    You may receive the following text in the console:
    Accessing the COM1 serial port
    Writing the following data to COM1: Test

    Read the following data from COM1: Serial Data
    Accessing the LPT1 parallel port

    Writing the following data to LPT1: Test


    Press ENTER to quit
    Note Serial Data represents the data that you read from the serial port.
  11. To close the application, press the ENTER key in the console.

Sample code listing (Module1.vb)

Before you use the following code example, replace
COM1 with the name of your serial port, and replace
LPT1 with the name of your parallel port.

Note The following code works only when serial devices and parallel devices are connected to the corresponding ports on the computer. If you do not connect these devices and you run the program, the program waits indefinitely.
Option Strict On

' Define a CommException class that inherits from the ApplicationException class.
' Then throw an object of type CommException when you receive an error message.
Class CommException
Inherits ApplicationException
Sub New(ByVal Reason As String)
MyBase.New(Reason)
End Sub
End Class

Module Module1
'Declare structures
Public Structure DCB
Public DCBlength As Int32
Public BaudRate As Int32
Public fBitFields As Int32
Public wReserved As Int16
Public XonLim As Int16
Public XoffLim As Int16
Public ByteSize As Byte
Public Parity As Byte
Public StopBits As Byte
Public XonChar As Byte
Public XoffChar As Byte
Public ErrorChar As Byte
Public EofChar As Byte
Public EvtChar As Byte
Public wReserved1 As Int16 'Reserved; Do Not Use
End Structure

Public Structure COMMTIMEOUTS
Public ReadIntervalTimeout As Int32
Public ReadTotalTimeoutMultiplier As Int32
Public ReadTotalTimeoutConstant As Int32
Public WriteTotalTimeoutMultiplier As Int32
Public WriteTotalTimeoutConstant As Int32
End Structure

'Declare constants.
Public Const GENERIC_READ As Int32 = &H80000000
Public Const GENERIC_WRITE As Int32 = &H40000000
Public Const OPEN_EXISTING As Int32 = 3
Public Const FILE_ATTRIBUTE_NORMAL As Int32 = &H80
Public Const NOPARITY As Int32 = 0
Public Const ONESTOPBIT As Int32 = 0

'Declare references to external functions.
Public Declare Auto Function CreateFile Lib "kernel32.dll" _
(ByVal lpFileName As String, ByVal dwDesiredAccess As Int32, _
ByVal dwShareMode As Int32, ByVal lpSecurityAttributes As IntPtr, _
ByVal dwCreationDisposition As Int32, ByVal dwFlagsAndAttributes As Int32, _
ByVal hTemplateFile As IntPtr) As IntPtr

Public Declare Auto Function GetCommState Lib "kernel32.dll" (ByVal nCid As IntPtr, _
ByRef lpDCB As DCB) As Boolean

Public Declare Auto Function SetCommState Lib "kernel32.dll" (ByVal nCid As IntPtr, _
ByRef lpDCB As DCB) As Boolean

Public Declare Auto Function GetCommTimeouts Lib "kernel32.dll" (ByVal hFile As IntPtr, _
ByRef lpCommTimeouts As COMMTIMEOUTS) As Boolean

Public Declare Auto Function SetCommTimeouts Lib "kernel32.dll" (ByVal hFile As IntPtr, _
ByRef lpCommTimeouts As COMMTIMEOUTS) As Boolean

Public Declare Auto Function WriteFile Lib "kernel32.dll" (ByVal hFile As IntPtr, _
ByVal lpBuffer As Byte(), ByVal nNumberOfBytesToWrite As Int32, _
ByRef lpNumberOfBytesWritten As Int32, ByVal lpOverlapped As IntPtr) As Boolean

Public Declare Auto Function ReadFile Lib "kernel32.dll" (ByVal hFile As IntPtr, _
ByVal lpBuffer As Byte(), ByVal nNumberOfBytesToRead As Int32, _
ByRef lpNumberOfBytesRead As Int32, ByVal lpOverlapped As IntPtr) As Boolean

Public Declare Auto Function CloseHandle Lib "kernel32.dll" (ByVal hObject As IntPtr) As Boolean

Sub Main()

' Declare local variables that you will use in the code.
Dim hSerialPort, hParallelPort As IntPtr
Dim Success As Boolean
Dim MyDCB As DCB
Dim MyCommTimeouts As COMMTIMEOUTS
Dim BytesWritten, BytesRead As Int32
Dim Buffer() As Byte

' Declare variables to use for encoding.
Dim oEncoder As New System.Text.ASCIIEncoding
Dim oEnc As System.Text.Encoding = oEncoder.GetEncoding(1252)

' Convert String to Byte().
Buffer = oEnc.GetBytes("Test")

Try
' Serial port.
Console.WriteLine("Accessing the COM1 serial port")
' Obtain a handle to the COM1 serial port.
hSerialPort = CreateFile("COM1", GENERIC_READ Or GENERIC_WRITE, 0, IntPtr.Zero, _
OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, IntPtr.Zero)
' Verify that the obtained handle is valid.
If hSerialPort.ToInt32 = -1 Then
Throw New CommException("Unable to obtain a handle to the COM1 port")
End If
' Retrieve the current control settings.
Success = GetCommState(hSerialPort, MyDCB)
If Success = False Then
Throw New CommException("Unable to retrieve the current control settings")
End If
' Modify the properties of the retrieved DCB structure as appropriate.
' WARNING: Make sure to modify the properties according to their supported values.
MyDCB.BaudRate = 9600
MyDCB.ByteSize = 8
MyDCB.Parity = NOPARITY
MyDCB.StopBits = ONESTOPBIT
' Reconfigure COM1 based on the properties of the modified DCB structure.
Success = SetCommState(hSerialPort, MyDCB)
If Success = False Then
Throw New CommException("Unable to reconfigure COM1")
End If
' Retrieve the current time-out settings.
Success = GetCommTimeouts(hSerialPort, MyCommTimeouts)
If Success = False Then
Throw New CommException("Unable to retrieve current time-out settings")
End If
' Modify the properties of the retrieved COMMTIMEOUTS structure as appropriate.
' WARNING: Make sure to modify the properties according to their supported values.
MyCommTimeouts.ReadIntervalTimeout = 0
MyCommTimeouts.ReadTotalTimeoutConstant = 0
MyCommTimeouts.ReadTotalTimeoutMultiplier = 0
MyCommTimeouts.WriteTotalTimeoutConstant = 0
MyCommTimeouts.WriteTotalTimeoutMultiplier = 0
' Reconfigure the time-out settings, based on the properties of the modified COMMTIMEOUTS structure.
Success = SetCommTimeouts(hSerialPort, MyCommTimeouts)
If Success = False Then
Throw New CommException("Unable to reconfigure the time-out settings")
End If
' Write data to COM1.
Console.WriteLine("Writing the following data to COM1: Test")
Success = WriteFile(hSerialPort, Buffer, Buffer.Length, BytesWritten, IntPtr.Zero)
If Success = False Then
Throw New CommException("Unable to write to COM1")
End If
' Read data from COM1.
Success = ReadFile(hSerialPort, Buffer, BytesWritten, BytesRead, IntPtr.Zero)
If Success = False Then
Throw New CommException("Unable to read from COM1")
End If
Catch ex As Exception
Console.WriteLine(Ex.Message)
Finally
' Release the handle to COM1.
Success = CloseHandle(hSerialPort)
If Success = False Then
Console.WriteLine("Unable to release handle to COM1")
End If
End Try

Try
' Parallel port.
Console.WriteLine("Accessing the LPT1 parallel port")
' Obtain a handle to the LPT1 parallel port.
hParallelPort = CreateFile("LPT1", GENERIC_READ Or GENERIC_WRITE, 0, IntPtr.Zero, _
OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, IntPtr.Zero)
' Verify that the obtained handle is valid.
If hParallelPort.ToInt32 = -1 Then
Throw New CommException("Unable to obtain a handle to the LPT1 port")
End If
' Retrieve the current control settings.
Success = GetCommState(hParallelPort, MyDCB)
If Success = False Then
Throw New CommException("Unable to retrieve the current control settings")
End If
' Modify the properties of the retrieved DCB structure as appropriate.
' WARNING: Make sure to modify the properties according to their supported values.
MyDCB.BaudRate = 9600
MyDCB.ByteSize = 8
MyDCB.Parity = NOPARITY
MyDCB.StopBits = ONESTOPBIT
' Reconfigure LPT1 based on the properties of MyDCB.
Success = SetCommState(hParallelPort, MyDCB)
If Success = False Then
Throw New CommException("Unable to reconfigure LPT1")
End If
' Reconfigure LPT1 based on the properties of the modified DCB structure.
Success = GetCommTimeouts(hParallelPort, MyCommTimeouts)
If Success = False Then
Throw New CommException("Unable to retrieve current time-out settings")
End If
' Modify the properties of the retrieved COMMTIMEOUTS structure as appropriate.
' WARNING: Make sure to modify the properties according to their supported values.
MyCommTimeouts.ReadIntervalTimeout = 0
MyCommTimeouts.ReadTotalTimeoutConstant = 0
MyCommTimeouts.ReadTotalTimeoutMultiplier = 0
MyCommTimeouts.WriteTotalTimeoutConstant = 0
MyCommTimeouts.WriteTotalTimeoutMultiplier = 0
' Reconfigure the time-out settings, based on the properties of the modified COMMTIMEOUTS structure.
Success = SetCommTimeouts(hParallelPort, MyCommTimeouts)
If Success = False Then
Throw New CommException("Unable to reconfigure the time-out settings")
End If
' Write data to LPT1.
' Note: You cannot read data from a parallel port by calling the ReadFile function.
Console.WriteLine("Writing the following data to LPT1: Test")
Success = WriteFile(hParallelPort, Buffer, Buffer.Length, BytesWritten, IntPtr.Zero)
If Success = False Then
Throw New CommException("Unable to write to LPT1")
End If
Catch ex As Exception
Console.WriteLine(Ex.Message)
Finally
' Release the handle to LPT1.
Success = CloseHandle(hParallelPort)
If Success = False Then
Console.WriteLine("Unable to release handle to LPT1")
End If
End Try

Console.WriteLine("Press ENTER to quit")
Console.ReadLine()

End Sub

End Module

Troubleshoot

  • When you run the application, you may receive the following error message:
    System.NullReferenceException: Object reference not set to an instance of an object.
    You may receive this error message because your function declarations are incorrect. This error message typically occurs when your declarations contain ByVal parameters instead of ByRef parameters.
  • Your application may wait indefinitely when you invoke the ReadFile function. This behavior typically occurs when you set the read time-outs to zero in the retrieved COMMTIMEOUTS structure. To resolve this issue, modify the properties of the COMMTIMEOUTS structure, as appropriate.

References

For more information, visit the following Microsoft Developer Network (MSDN) Web sites: For more information about AT modem commands, click the following article number to view the article in the Microsoft Knowledge Base:
164660 AT modem command reference

Properties

Article ID: 823179 - Last Review: Nov 26, 2007 - Revision: 1

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