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4/8/2010

An application must always set communication timeouts using the COMMTIMEOUTSstructure each time it opens a communication port.

If this structure is not configured, the port uses default timeouts supplied by the driver, or timeouts from a previous communication application.

By assuming specific timeout settings when the settings are actually different, an application can have read/write operations that never complete or complete too often.

When read/write operations time out, the operations complete with no error values returned to the ReadFileand WriteFilefunctions.

To determine if an operation has timed out, verify that the number of bytes transferred is fewer than the number of bytes requested.

For example, if the ReadFilefunction returns TRUE, but fewer bytes were read than requested, the operation has timed out.

To configure timeouts for a serial port

  1. Initialize the COMMTIMEOUTSstructure by calling the GetCommTimeoutsfunction or by setting the members manually.

  2. Specify the maximum number of milliseconds that can elapse between two characters without a timeout occurring with the ReadIntervalTimeoutmember.

  3. Specify the read timeout multiplier with the ReadTotalTimeoutMultipliermember.

    For each read operation, this number is multiplied by the number of bytes that the read operation expects to receive.

  4. Specify the read timeout constant with the ReadTotalTimeoutConstantmember.

    This member is the number of milliseconds added to the result of multiplying the total number of bytes to read by ReadTotalTimeoutMultiplier.

    The result is the number of milliseconds that must elapse before a timeout for the read operation occurs.

  5. Specify the write timeout multiplier with the WriteTotalTimeoutMultipliermember.

    For each write operation, this number is multiplied by the number of bytes that the write operation expects to receive.

  6. Specify the write timeout constant with the WriteTotalTimeoutConstantmember.

    This member is the number of milliseconds added to the result of multiplying the total number of bytes to write by WriteTotalTimeoutMultiplier.

    The result is the number of milliseconds that must elapse before a timeout for the write operation occurs.

  7. Call the SetCommTimeoutsfunction to activate port timeout settings.

    Note:
    Because there are two timeouts, interval timeout and (total timeout constant + total timeout multiplier * number of bytes), an actual timeout is whichever timeout occurs first.

To assist with multitasking, it is common to configure COMMTIMEOUTso ReadFileimmediately returns with all characters in the input buffer. To do this, set ReadIntervalTimeoutto MAXWORD and set both ReadTotalTimeoutMultiplierand ReadTotalTimeoutConstantto zero.

The following code example shows how to configure timeouts for a serial port.

Copy Code 
// Retrieve the timeout parameters for all read and write
operations
// on the port. 
COMMTIMEOUTS CommTimeouts;
GetCommTimeouts (hPort, &CommTimeouts);

// Change the COMMTIMEOUTS structure settings.
CommTimeouts.ReadIntervalTimeout = MAXDWORD;  
CommTimeouts.ReadTotalTimeoutMultiplier = 0;  
CommTimeouts.ReadTotalTimeoutConstant = 0; 
CommTimeouts.WriteTotalTimeoutMultiplier = 10;  
CommTimeouts.WriteTotalTimeoutConstant = 1000; 

// Set the timeout parameters for all read and write operations
// on the port. 
if (!SetCommTimeouts (hPort, &CommTimeouts))
{
  // Could not set the timeout parameters.
  MessageBox (hMainWnd, TEXT("Unable to set the timeout
parameters"), 
			TEXT("Error"), MB_OK);
  dwError = GetLastError ();
  return FALSE;
}

See Also