Modification of Communications Resource Settings
When the
CreateFile function opens a handle to a serial communications resource, the system
initializes and configures the resource according to the values set up the last time
the resource was opened. Preserving the previous settings enables the user to
retain the settings specified through an MS-DOS®
mode command when the device is reopened. The values inherited from the previous
open operation include the configuration settings of the device control block (a
DCB structure) and the time-out values used in I/O operations. If the device has
never been opened, it is configured with the system defaults.
To determine the initial configuration of a serial communications resource, a
process calls the
GetCommState function, which fills in a serial port
DCB structure with the current configuration settings. To modify this
configuration, a process specifies a
DCB structure in a call to the
SetCommState function.
Members of the
DCB structure specify the configuration settings such as the baud rate, the
number of data bits per byte, and the number of stop bits per byte. Other
DCB members specify special characters and enable parity checking and flow
control. When a process needs to modify only a few of these configuration settings,
it should first call
GetCommState to fill in a
DCB structure with the current configuration. Then the process can adjust the
important values in the
DCB structure and reconfigure the device by calling
SetCommState and specifying the modified
DCB structure. This procedure ensures that the unmodified members of the
DCB structure contain appropriate values. For example, a common error is to
configure a device with a
DCB structure in which the structure's
XonChar member is equal to the
XoffChar member. Some members of the
DCB structure are different from those in previous versions of Microsoft Windows®. In particular, the flags for controlling RTS (request-to-send) and DTR
(data-terminal-ready) flow control have changed.
The
BuildCommDCB function provides another way to modify a
DCB structure.
BuildCommDCB uses a string with the same form as the command-line arguments of the
mode command to specify the baud rate, parity scheme, number of stop bits, and
number of data bits. The remaining members of
DCB are not changed by this function, except that the appropriate members are set
to disable XON/XOFF and hardware flow control.
BuildCommDCB only modifies a
DCB structure; it does not reconfigure the device.
A process can reconfigure a communications resource by using the
GetCommProperties function to get information from a device driver about the configuration
settings that it supports. The process can use this information to avoid specifying
a configuration that is not supported.
The
SetCommState function reconfigures the communications resource, but it does not affect the
internal output and input buffers of the specified driver. The buffers are not
flushed, and pending read and write operations are not terminated prematurely.
A process reinitializes a communications resource by using the
SetupComm function, which performs the following tasks:
- Terminates pending read and write operations, even if they have not been
completed.
- Discards unread characters and frees the internal output and input buffers of
the driver associated with the specified resource.
- Reallocates the internal output and input buffers.
A process is not required to call
SetupComm. If it does not, the resource's driver initializes the device with the
default settings the first time that the communications resource handle is used.
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