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).