Platform SDK: Files and I/O

DCB

The DCB structure defines the control setting for a serial communications device. 

typedef struct _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 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 must be TRUE. Using FALSE will not work.
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. 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 flow control is activated to inhibit the sender. Note that the sender may transmit characters after the flow control signal has been activated, so this value should never be zero. This assumes that either XON/XOFF, RTS, or DTR input flow control is specified in fInX, fRtsControl, or fDtrControl.
XoffLim
Specifies the maximum number of bytes allowed in the input buffer before flow control is activated to allow transmission by the sender. This assumes that either XON/XOFF, RTS, or DTR input flow control is specified in fInX, fRtsControl, or fDtrControl. 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
SPACEPARITY Space

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:

Requirements

  Windows NT/2000: Requires Windows NT 3.1 or later.
  Windows 95/98: Requires Windows 95 or later.
  Header: Declared in Winbase.h; include Windows.h.

See Also

Communications Overview, Communication Structures, BuildCommDCB, ClearCommError, EscapeCommFunction, GetCommState, SetCommState

Built on Thursday, February 17, 2000