You can use the _VCOMM_EscapeCommFunction service to direct the port driver to carry out an extended function. Constants defined in VCOMM.INC identify several extended functions that can be used - for example, to get or set the close property, set or clear hardware handshaking lines, or reset an LPT port. Individual port drivers may support only a subset of the extended functions defined in VCOMM.INC.
To expose unique features of a communications device, port drivers can define driver-specific extended functions not in VCOMM.INC.
_VCOMM_EscapeCommFunction