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CHAPTER 5: Command Mode
Xon/Xoff Flow Control &E5
Xon/Xoff is the most commonly used method of flow control. Under
this method, control characters known as “Xon” and “Xoff” are inserted
by the modem into the data to start and stop the flow of data from the
computer or terminal to which the modem is attached. Xoff, which is a
Control-S, stops the flow of data, and Xon, which is a Control-Q, restarts
it. With regard to Binary Data, Xon/Xoff flow control is not recom-
mended because an Xoff character may be part of the data and would
trigger an Xoff of the modem or software package, which would halt
data flow.
Xon/Xoff Pass-Through &E7
So far, you have had three choices to make regarding pacing:
1. You can set the modem to respond to Xon/Xoff pacing.
2. You can set the modem to respond to RTS pacing.
3. You can set the modem to ignore pacing completely.
Well, we’re not done with pacing yet. There’s another choice you can
make (which actually can apply to both pacing and modem-initiated
flow control, although it applies mainly to pacing) and that is
something called “Xon/Xoff Pass-Through.”
This means that if your modem is set to respond to Xon/Xoff com-
mands, you can have the modem do one of the following:
1. The modem responds to the Xon and Xoff pacing commands
while at the same time allowing these commands to pass through
the modem and on to the remote location. We call this “Respond,
Pass-Through”.
2. The modem responds to Xon/Xoff pacing, but does not allow the
pacing signals to pass through the modem and on to the remote
location. We call this “Respond, No Pass-Through”.
When Xon and Xoff commands are allowed to pass through the
modem, the computer or terminal at the remote site will receive these
commands, and depending on how it is configured, the computer or
terminal may respond to them also.