If an XOFF has been received from the host,
data will not be sent to the host until the host sends an XON.
During the XOFF phase, the host is free to carry on other chores
and accept data from other devices.Used only with RS-232.
Definition:
This option enables the host to send the XON
and XOFF command as a single byte transmission command of start
(^Q) or stop (^S).
