Asynchronous Serial Communications Programmer's Reference Manual (32022-90052)

106 Chapter4
Using FDEVICECONTROL
Examples
Controlling Host and Device XON/XOFF
Processing
Another host and device control feature provided by the
FDEVICECONTROL intrinsic is the ability to disable (and
subsequently reenable) XON/XOFF flow control between device and the
DTC. See Figure 4-7 for enabling host XON/XOFF processing.
XON/XOFF protocol provides the main method of flow control used by
MPE/iX systems. The protocol controls the flow of data transmitted
between DTC and the device to protect against data overruns.
Device XON/XOFF flow control allows the device receiving a
transmission to stop the flow of data coming in should the device
become temporarily unable to receive the data. It does this by sending
an XOFF character to the DTC. XOFF is the ASCII DC3 character
(
[CTRL]S).
Host XON/XOFF flow control allows the DTC receiving a transmission
to stop the flow of data coming in should the DTC become temporarily
unable to receive the data. It does this by sending an XOFF character to
the device. XOFF is the ASCII DC3 character.
When the device is once again able to receive data, it transmits an XON
character to signal that the DTC should resume data transmission.
XON is the ASCII DC1 character (
[CTRL]Q)
When the DTC is once again able to receive data, it transmits an XON
character to signal that the device should resume data transmission.
XON is the ASCII DC1 character (
[CNTRL]Q).
By using FDEVICECONTROL with a control directive of 76, it is
possible to disable XON/XOFF flow control between device and the
DTC.
The functionality for device flow control (FDEVICECONTROL 192,26)
is maintained for backward compatibility, even though that can be done
through this FDEVICECONTROL.