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The SetCommState function configures a communications device according to the specifications in a device-control block (a DCB structure). The function reinitializes all hardware and control settings, but it does not empty output or input queues.

BOOL SetCommState(

HANDLE hFile, // handle of communications device
LPDCB lpDCB // address of device-control block structure
);


Parameters

hFile

Identifies the communications device. The CreateFile function returns this handle.

lpDCB

Points to a DCB structure containing the configuration information for the specified communications device.



Return Values

If the function succeeds, the return value is nonzero.
If the function fails, the return value is zero. To get extended error information, call GetLastError.

Remarks

The SetCommState function uses a DCB structure to specify the desired configuration. The GetCommState function returns the current configuration.
To set only a few members of the DCB structure, you should modify a DCB structure that has been filled in by a call to GetCommState. This ensures that the other members of the DCB structure have appropriate values.
The SetCommState function fails if the XonChar member of the DCB structure is equal to the XoffChar member.

When SetCommState is used to configure the 8250, the following restrictions apply to the values for the DCB structure's ByteSize and StopBits members:

· The number of data bits must be 5 to 8 bits.
· The use of 5 data bits with 2 stop bits is an invalid combination, as are 6, 7, or 8 data bits with 1.5 stop bits.

typedef struct _DCB { // dcb
DWORD DCBlength; // sizeof(DCB)
DWORD BaudRate; // current baud rate
DWORD fBinary: 1; // binary mode, no EOF check
DWORD fParity: 1; // enable parity checking
DWORD fOutxCtsFlow:1; // CTS output flow control
DWORD fOutxDsrFlow:1; // DSR output flow control
DWORD fDtrControl:2; // DTR flow control type
DWORD fDsrSensitivity:1; // DSR sensitivity

DWORD fTXContinueOnXoff:1; // XOFF continues Tx
DWORD fOutX: 1; // XON/XOFF out flow control
DWORD fInX: 1; // XON/XOFF in flow control
DWORD fErrorChar: 1; // enable error replacement
DWORD fNull: 1; // enable null stripping
DWORD fRtsControl:2; // RTS flow control
DWORD fAbortOnError:1; // abort reads/writes on error
DWORD fDummy2:17; // reserved
WORD wReserved; // not currently used

WORD XonLim; // transmit XON threshold
WORD XoffLim; // transmit XOFF threshold
BYTE ByteSize; // number of bits/byte, 4-8
BYTE Parity; // 0-4=no,odd,even,mark,space
BYTE StopBits; // 0,1,2 = 1, 1.5, 2
char XonChar; // Tx and Rx XON character
char XoffChar; // Tx and Rx XOFF character
char ErrorChar; // error replacement character

char EofChar; // end of input character
char EvtChar; // received event character
WORD wReserved1; // reserved; do not use
} DCB;


Members

DCBlength

Specifies the length, in bytes, of the DCB structure.

BaudRate

Specifies the baud rate at which the communications device operates. This member can be an actual baud rate value, or one of the following baud rate indexes:

CBR_110 CBR_19200
CBR_300 CBR_38400
CBR_600 CBR_56000
CBR_1200 CBR_57600
CBR_2400 CBR_115200
CBR_4800 CBR_128000
CBR_9600 CBR_256000
CBR_14400


fBinary

Specifies whether binary mode is enabled. The Win32 API does not support nonbinary mode transfers, so this member should be TRUE. Trying to use FALSE will not work.

Under Windows 3.1, if this member is FALSE, nonbinary mode is enabled, and the character specified by the EofChar member is recognized on input and remembered as the end of data.

fParity

Specifies whether parity checking is enabled. If this member is TRUE, parity checking is performed and errors are reported.

fOutxCtsFlow

Specifies whether the CTS (clear-to-send) signal is monitored for output flow control. If this member is TRUE and CTS is turned off, output is suspended until CTS is sent again.

fOutxDsrFlow

Specifies whether the DSR (data-set-ready) signal is monitored for output flow control. If this member is TRUE and DSR is turned off, output is suspended until DSR is sent again.

fDtrControl

Specifies the DTR (data-terminal-ready) flow control. This member can be one of the following values:

Value Meaning
DTR_CONTROL_DISABLE Disables the DTR line when the device is opened and leaves it disabled.
DTR_CONTROL_ENABLE Enables the DTR line when the device is opened and leaves it on.
DTR_CONTROL_HANDSHAKE Enables DTR handshaking. If handshaking is enabled, it is an error for the application to adjust the line by using the EscapeCommFunction function.


fDsrSensitivity

Specifies whether 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 whether 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 whether 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 whether 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 whether 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 whether null bytes are discarded. If this member is TRUE, null bytes are discarded when received.

fRtsControl

Specifies the RTS (request-to-send) flow control. If this value is zero, the default is RTS_CONTROL_HANDSHAKE. This member can be one of the following values:

Value Meaning
RTS_CONTROL_DISABLE Disables the RTS line when the device is opened and leaves it disabled.
RTS_CONTROL_ENABLE Enables the RTS line when the device is opened and leaves it on.
RTS_CONTROL_HANDSHAKE Enables RTS handshaking. The driver raises the RTS line when the "type-ahead" (input) buffer is less than one-half full and lowers the RTS line when the buffer is more than three-quarters full. If handshaking is enabled, it is an error for the application to adjust the line by using the EscapeCommFunction function.
RTS_CONTROL_TOGGLE Specifies that the RTS line will be high if bytes are available for transmission. After all buffered bytes have been sent, the RTS line will be low.


fAbortOnError

Specifies whether 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; must be set to zero.

XonLim

Specifies the minimum number of bytes allowed in the input buffer before the XON character is sent.

XoffLim

Specifies the maximum number of bytes allowed in the input buffer before the XOFF character is sent. The maximum number of bytes allowed is calculated by subtracting this value from the size, in bytes, of the input buffer.

ByteSize

Specifies the number of bits in the bytes transmitted and received.

Parity

Specifies the parity scheme to be used. This member can be one of the following values:

Value Meaning
EVENPARITY Even
MARKPARITY Mark
NOPARITY No parity
ODDPARITY Odd


StopBits

Specifies the number of stop bits to be used. This member can be one of the following values:

Value Meaning
ONESTOPBIT 1 stop bit
ONE5STOPBITS 1.5 stop bits
TWOSTOPBITS 2 stop bits


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.



Remarks

When a DCB structure is used to configure the 8250, the following restrictions apply to the values specified for the ByteSize and StopBits members:

· The number of data bits must be 5 to 8 bits.
· The use of 5 data bits with 2 stop bits is an invalid combination, as is 6, 7, or 8 data bits with 1.5 stop bits.

The EscapeCommFunction function directs a specified communications device to perform an extended function.

BOOL EscapeCommFunction(

HANDLE hFile, // handle to communications device
DWORD dwFunc // extended function to perform
);


Parameters

hFile

Identifies the communications device. The CreateFile function returns this handle.

dwFunc

Specifies the code of the extended function to perform. This parameter can be one of the following values:

Value Meaning
CLRDTR Clears the DTR (data-terminal-ready) signal.
CLRRTS Clears the RTS (request-to-send) signal.
SETDTR Sends the DTR (data-terminal-ready) signal.
SETRTS Sends the RTS (request-to-send) signal.
SETXOFF Causes transmission to act as if an XOFF character has been received.
SETXON Causes transmission to act as if an XON character has been received.
SETBREAK Suspends character transmission and places the transmission line in a break state until the ClearCommBreak function is called (or EscapeCommFunction is called with the CLRBREAK extended function code). The SETBREAK extended function code is identical to the SetCommBreak function. Note that this extended function does not flush data that has not been transmitted.
CLRBREAK Restores character transmission and places the transmission line in a nonbreak state. The CLRBREAK extended function code is identical to the ClearCommBreak function.


Return Values

If the function succeeds, the return value is nonzero.
If the function fails, the return value is zero. To get extended error information, call GetLastError.