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

This article has been archived. It is offered "as is" and will no longer be updated.
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 MyConsoleApplication project, 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 MSCommLibModule 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 SubEnd 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 SubEnd 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 UseEnd StructurePublic Structure COMMTIMEOUTS   Public ReadIntervalTimeout As Int32   Public ReadTotalTimeoutMultiplier As Int32   Public ReadTotalTimeoutConstant As Int32   Public WriteTotalTimeoutMultiplier As Int32   Public WriteTotalTimeoutConstant As Int32End Structure'Declare constants.Public Const GENERIC_READ As Int32 = &H80000000Public Const GENERIC_WRITE As Int32 = &H40000000Public Const OPEN_EXISTING As Int32 = 3Public Const FILE_ATTRIBUTE_NORMAL As Int32 = &H80Public Const NOPARITY As Int32 = 0Public 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 IntPtrPublic Declare Auto Function GetCommState Lib "kernel32.dll" (ByVal nCid As IntPtr, _   ByRef lpDCB As DCB) As BooleanPublic Declare Auto Function SetCommState Lib "kernel32.dll" (ByVal nCid As IntPtr, _   ByRef lpDCB As DCB) As BooleanPublic Declare Auto Function GetCommTimeouts Lib "kernel32.dll" (ByVal hFile As IntPtr, _   ByRef lpCommTimeouts As COMMTIMEOUTS) As BooleanPublic Declare Auto Function SetCommTimeouts Lib "kernel32.dll" (ByVal hFile As IntPtr, _   ByRef lpCommTimeouts As COMMTIMEOUTS) As BooleanPublic 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 BooleanPublic 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 BooleanPublic 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 IntPtrDim Success As BooleanDim MyDCB As DCBDim MyCommTimeouts As COMMTIMEOUTSDim BytesWritten, BytesRead As Int32Dim Buffer() As Byte' Declare the variables to use for encoding.Dim oEncoder As New System.Text.ASCIIEncodingDim 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 SubEnd ClassModule 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 SubEnd 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: 12/08/2015 03:18:24 - Revision: 3.4

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  • kbnosurvey kbarchive kbvs2005swept kbvs2005doesnotapply kbserial kbsdk kbkern32dll kbdll kbcommport kbapi kb32bitonly kbinterop kbconsole kbconfig kbappdev kbprogramming kbsample kbhowtomaster KB823179
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