You send a break through the serial device by passing an IOExtSer to the device with SDCMD_BREAK set in io_Command. The break may be immediate or queued. The choice is determined by the state of flag SERF_QUEUEDBRK in io_SerFlags. SerialIO->IOSer.io_Command = SDCMD_BREAK; /* send break */ SendIO((struct IORequest *)SerialIO); The duration of the break (in microseconds) can be set in io_BrkTime. The default is 250,000 microseconds (.25 seconds).