Serial Port Communications in Visual C++

Posted on 2003-03-24
Medium Priority
Last Modified: 2008-03-17
hi, I've been writing an app to try and communicate with a device over the com ports, I've managed to open the port and send information to the device, but I can't work out how to recieve information from the devices.  I need to write code that waits for some input, then recieves it and acts upon it.  I think I need to use somethign liek waitforcommevent (or something like that) but I'm not sure how it works.

// create com port
m_sComPort = "COM3:";
m_hCom = CreateFile(
                         GENERIC_READ | GENERIC_WRITE,
                         0,                                        // exclusive access
                         NULL,                                   // no security
                         0,                                        // no overlapped I/O
                         NULL);                                   // null template

// configure serial port
m_bPortReady = GetCommState(m_hCom, &m_dcb);
m_dcb.BaudRate = 112500;
m_dcb.ByteSize = 8;
m_dcb.Parity = NOPARITY;
m_dcb.StopBits = ONESTOPBIT;
m_dcb.fAbortOnError = TRUE;
m_bPortReady = SetCommState(m_hCom, &m_dcb);

// Enable PC Control
bWriteRC = WriteFile(m_hCom, "AT*EAM=\"PC\"\r",12,&iBytesWritten,NULL);

// Wait for input

BYTE  Byte;
DWORD dwCommModemStatus, dwBytesTransferred;
CString Input = "";

SetCommMask (m_hCom, EV_RXCHAR | EV_CTS | EV_DSR | EV_RLSD | EV_RING);

WaitCommEvent (m_hCom, &dwCommModemStatus, 0);

do {
// Read the data from the serial port.

ReadFile (m_hCom, &Byte, 1, &dwBytesTransferred, 0);

// Display the data read.

if (dwBytesTransferred == 1)
{Input += Byte;}
} while (dwBytesTransferred == 1);

Thats the code I've got at the moment, but I sort of just butchered the routines from somewhere else. can anyone help?
Question by:gb115
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Accepted Solution

DidierT earned 200 total points
ID: 8196772
Hi !

You must use IO_COMPLETION in a WaitForMultipleObjectFunction. Here is a short code :

int MySerPort::read()
   OVERLAPPED  overlapped  ;
   int         waitReport  ;
   int         res , nbBytesRead ;
   // Read completion structure
   overlapped.Offset = 0;
   overlapped.OffsetHigh = 0;
   overlapped.hEvent = NULL;// Filled w nb of bytes read
   // Read on the port
   // give Completion routine "OnReceive"
   res = ReadFileEx( m_hComm, buffer, size,
                     &overlapped, onReceive) ;
   // Wait for completion (Infinite !)
   if ( res )
      waitReport = WaitForSingleObjectEx(
                      m_killWaitingData ,
                      INFINITE          ,
                      true              ) ;// wait flag
      // Completion test
      switch (waitReport)
        case -1 :     // kill
        err = GetLastError() ;
        case WAIT_OBJECT_0 :       // killed !
        case WAIT_IO_COMPLETION : // Read completed
            nbBytes = (unsigned long)overlapped.hEvent ;
            break ;
      // .... Something wrong !
   return ( nbBytes ) ;

Create a completion CallBack routine as follow :

static void CALLBACK onReceive(  DWORD dwErrorCode      ,
                                 DWORD dwNbOfBytesTransf,
                                 LPOVERLAPPED lpOverlp      )
   if (dwErrorCode == NO_ERROR)
      // Set the number of bytes received
      lpOverlp->hEvent = (HANDLE)dwNbOfBytesTransf ;

When read is completed, the callback routine is called and then your process exit the waitForMultipleObject wait.

You can also do as well with a time-out different from INFINITE. The m_killWaitingData event is used to unblock your reception thread if necessary (on destructor for example).


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