MSDN Library

Configuring a Serial Port

Windows CE 3.0

The most critical phase in serial communications programming is configuring the port settings with the DCB structure. Erroneously initializing the DCB structure is a common problem. When a serial communications function does not produce the expected results, the DCB structure may be in err.

A call to the CreateFile function opens a serial port with default port settings. Usually, the application needs to change the defaults. Use the GetCommState function to retrieve the default settings and use the SetCommState function to set new port settings.

Also, port configuration involves using the COMMTIMEOUTS structure to set the time-out values for read/write operations. When a time-out occurs, the ReadFile or WriteFile function returns the specific number of characters successfully transferred.

To configure a serial port

  1. Initialize the DCBlength member of the DCB structure to the size of the structure. This initialization is required before passing this member as a variable to any function. The DCB structure must be complete, including hardware and software flow control, before continuing.
  2. Call the GetCommState function to retrieve the default settings for the port opened with the CreateFile function. To identify the port, specify in the hPort parameter the handle that CreateFile returns.
  3. Modify DCB members as required. The following table shows the DCB structure members.
    Member Description
    DCBlength Before calling the GetCommState function, set this member to the length of the DCB structure. Neglecting to do this can cause a failure or return erroneous data.
    BaudRate Specifies the device communication rate. Assigns an actual baud rate or an index by specifying a CBR_ constant.
    fBinary Specifies if binary mode is enabled. The Win32 API does not support nonbinary mode transfers, so this member must be TRUE. Using FALSE will not work.
    fParity Specifies if parity checking is enabled. If this member is TRUE, parity checking is performed and errors are reported
    fOutxCtsFlow Turns the CTS flow control on and off. To use RTS/CTS flow control, specify TRUE for this member and RTS_CONTROL_HANDSHAKE for the fRtsControl member.
    fOutxDsrFlow Turns the DSR flow control on and off. DSR flow control is rarely used. A typical port configuration is to set this member to FALSE, while enabling the DTR line.
    fDtrControl Specifies the DTR flow control. DTR_CONTROL_ENABLE turns on the DTR line during the connection. DTR_CONTROL_HANDSHAKE turns on DTR handshaking. DTR_CONTROL_DISABLE turns off the DTR line.
    fDsrSensitivity Specifies if the communications driver is sensitive to the state of the DSR signal. If this member is TRUE, the driver ignores any bytes received, unless the DSR modem input line is high.
    fTXContinueOnXoff Specifies if transmission stops when the input buffer is full and the driver has transmitted the XoffChar character. If this member is TRUE, transmission continues after the input buffer has come within XoffLim bytes of being full and the driver has transmitted the XoffChar character to stop receiving bytes. If this member is FALSE, transmission does not continue until the input buffer is within XonLim bytes of being empty and the driver has transmitted the XonChar character to resume reception.
    fOutX Specifies if XON/XOFF flow control is used during transmission. If this member is TRUE, transmission stops when the XoffChar character is received and starts again when the XonChar character is received.
    fInX Specifies if XON/XOFF flow control is used during reception. If this member is TRUE, the XoffChar character is sent when the input buffer comes within XoffLim bytes of being full, and the XonChar character is sent when the input buffer comes within XonLim bytes of being empty.
    fErrorChar Specifies if bytes received with parity errors are replaced with the character specified by the ErrorChar member. If this member is TRUE and the fParity member is TRUE, replacement occurs.
    fNull Specifies if null bytes are discarded. If this member is TRUE, null bytes are discarded when received.
    fRtsControl Turns the RTS flow control on and off. RTS_CONTROL_ENABLE turns on the RTS line during the connection. RTS_CONTROL_HANDSHAKE turns on RTS handshaking. RTS_CONTROL_DISABLE turns off the RTS line. RTS_CONTROL_TOGGLE Specifies that the RTS line is high if bytes are available for transmission. After all buffered bytes have been sent, the RTS line will be low.
    fAbortOnError Specifies if read and write operations are terminated if an error occurs. If this member is TRUE, the driver terminates all read and write operations with an error status if an error occurs. The driver will not accept any further communications operations until the application has acknowledged the error by calling the ClearCommError function.
    fDummy2 Reserved, do not use.
    wReserved Not used, set to zero.
    XonLim Specifies the minimum number of bytes accepted in the input buffer before the XON character is sent.
    XoffLim Specifies the maximum number of bytes accepted in the input buffer before the XOFF character is sent. The maximum number of bytes accepted is calculated by subtracting this value from the size, in bytes, of the input buffer.
    ByteSize Specifies the bits per byte transmitted and received.
    Parity Specifies the parity scheme. Do not confuse this member with the fParity member, which turns parity on or off. Because parity is rarely used, this member can usually be NOPARITY.
    StopBits Specifies the number of stop bits. ONESTOPBIT is the most common setting.
    XonChar Specifies the value of the XON character for both transmission and reception.
    XoffChar Specifies the value of the XOFF character for both transmission and reception.
    ErrorChar Specifies the value of the character used to replace bytes received with a parity error.
    EofChar Specifies the value of the character used to signal the end of data.
    EvtChar Specifies the value of the character used to signal an event.
    WReserved1 Reserved, do not use.
  4. Call the SetCommState function to set the new port settings.

The following code example shows how to use the GetCommState and SetCommState functions to configure a serial port.

DCB PortDCB;

// Initialize the DCBlength member. 
PortDCB.DCBlength = sizeof (DCB); 

// Get the default port setting information.
GetCommState (hPort, &PortDCB);

// Change the DCB structure settings.
PortDCB.BaudRate = 9600;              // Current baud 
PortDCB.fBinary = TRUE;               // Binary mode; no EOF check 
PortDCB.fParity = TRUE;               // Enable parity checking 
PortDCB.fOutxCtsFlow = FALSE;         // No CTS output flow control 
PortDCB.fOutxDsrFlow = FALSE;         // No DSR output flow control 
PortDCB.fDtrControl = DTR_CONTROL_ENABLE; 
                                      // DTR flow control type 
PortDCB.fDsrSensitivity = FALSE;      // DSR sensitivity 
PortDCB.fTXContinueOnXoff = TRUE;     // XOFF continues Tx 
PortDCB.fOutX = FALSE;                // No XON/XOFF out flow control 
PortDCB.fInX = FALSE;                 // No XON/XOFF in flow control 
PortDCB.fErrorChar = FALSE;           // Disable error replacement 
PortDCB.fNull = FALSE;                // Disable null stripping 
PortDCB.fRtsControl = RTS_CONTROL_ENABLE; 
                                      // RTS flow control 
PortDCB.fAbortOnError = FALSE;        // Do not abort reads/writes on 
                                      // error
PortDCB.ByteSize = 8;                 // Number of bits/byte, 4-8 
PortDCB.Parity = NOPARITY;            // 0-4=no,odd,even,mark,space 
PortDCB.StopBits = ONESTOPBIT;        // 0,1,2 = 1, 1.5, 2 

// Configure the port according to the specifications of the DCB 
// structure.
if (!SetCommState (hPort, &PortDCB))
{
  // Could not configure the serial port.
  MessageBox (hMainWnd, TEXT("Unable to configure the serial port"), 
              TEXT("Error"), MB_OK);
  dwError = GetLastError ();
  return FALSE;
}

 Last updated on Friday, April 02, 2004

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