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PN 88300150
lead on the RS232C interface (Clear to Send - Pin 5). Most terminals and computers support one or both
of these methods.
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, (CTRL-S), stops the flow of data,
and Xon, (CTRL-Q), restarts it. With regards to binary data, Xoff/Xon flow control is not recommended
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.
Hardware Flow Control (&E4)
With Hardware Flow Control, the modem uses its RS232C interface to control the flow of data from the
computer or terminal to which it is attached. The CTS (Clear to Send) signal on Pin 5 of the RS232C
interface is brought low to stop the flow of data, and is brought high to restart it.
When you select Hardware Flow Control as your Modem-Initiated Flow Control method, you are also
selecting it for Pacing. The difference between the two, however, is that Modem-Initiated Flow Control
uses the Pin 5 CTS output signal, while Pacing uses the Pin 4 RTS input signal. (Refer to Section 5.9.)
Modem commands are used to select the method of flow control used by the MT2834MR when its error
correction capabilities are used. These commands are covered in Section 5.13. If neither method is
selected, the modem will default to no flow control.
5.9
Terminal/Computer-Initiated Pacing (&E13)
As mentioned earlier, the MT2834MR can initiate flow control by issuing Xon/Xoff commands or by
toggling the CTS signal on the RS232C interface. The modem can also be configured to react to similar
commands and signals from the computer or terminal to which it is attached via the RS232C interface.
We refer to computer or terminal initiated flow control as "Pacing". When the modem is set for Pacing On,
the modem responds to the terminal or computer pacing. When the modem is set for Pacing Off, it will
ignore pacing.
In order for the MT2834MR to be set for Pacing On, a modem-initiated method of flow control must be
previously selected. Once this is done, the MT2834MR will respond to either Xon/Xoff commands, or to
the toggling of the RTS (Request to Send) signal on Pin 4 of the RS232C interface, depending on what
you selected earlier as your Modem-Initiated Flow Control method.
If you use RTS pacing, be sure that the RTS DIP-Switch is set so that RTS is not forced on (see Section
7.3).
5.10 Xon/Xoff Pass-Through (&E7)
So far, you have had three choices to make regarding pacing. You can set the modem to respond to
Xon/Xoff pacing, or to respond to RTS pacing, or you can set the modem to ignore pacing completely.
Another choice you can make (which actually can apply to both pacing and modem-initiated flow control,
although it applies mainly to pacing) is called "Xon/Xoff Pass-Through". This means that if your modem is
set to respond to Xon/Xoff commands, 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".
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