Configuring a Serial Port
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8/28/2008
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, check that the DCB structure is configured correctly.
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 make port settings.
Also, port configuration involves using the COMMTIMEOUTS structure to set the timeout values for read/write operations. When a timeout occurs, the ReadFile or WriteFile function returns the specific number of characters successfully transferred. For additional information on configuring timeouts, see Configuring Timeouts.
To configure a serial port:
Initialize the DCBlength member of the DCB structure to the size of the structure.
This is required before passing the structure as an argument to a function.
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.
Modify DCB members as required.
The following are 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. The CBR_ constants are defined in the winbase.h header file.
fBinary
Specifies if binary mode is enabled.
The Microsoft® Win32® API does not support nonbinary mode transfers, so this member must be TRUE.
Using FALSE does 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 is low.
fAbortOnError
Specifies if read and write operations are terminated when an error occurs.
If this member is TRUE, the driver terminates read and write operations with an error status when an error occurs.
The driver does not accept 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 maximum number of bytes allowed in the queue.
The XON character is sent if the number of bytes in the queue falls below this number.
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.
Call the SetCommState function to set the 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.
dwError = GetLastError ();
MessageBox (hMainWnd, TEXT("Unable to configure the serial port"),
TEXT("Error"), MB_OK);
return FALSE;
}