Advanced Modem Concepts
E-6
Issue 2 May 1995
If Modem C’s buffer begins to fill because of data sent from
Computer D, Modem C stops data flow either by issuing an
XOFF or by lowering its CTS signal to Computer D. This is
illustrated in link C—D of computer-to-modem flow control in
Figure E-2. Computer D then stops sending data to Modem C
resulting in a halt of all data transfer between Computer A and
Computer D.
Computer-to-Computer Flow Control
Flow control can be handled by the two computers. In this case,
if Computer A’s buffer begins to fill, it can stop data flow by using
an XOFF character to Computer D (see computer-to-computer
flow control in Figure E-2). In this situation, the modems are
configured to ignore and pass through any XON/XOFF
characters they may receive. To resume data flow, Computer A
issues an XON character.
For this type of flow control, the computers are normally
configured for XON/XOFF flow control, and the modems are
configured for XON/XOFF flow control (\Q command) with
XON/XOFF Passthrough enabled (refer to the \X command).
Modem-to-Modem Flow Control
Finally, flow control can occur between two modems. If
Modem B’s buffer begins to fill, and error control is disabled, it
can issue an XOFF to Modem C (see modem-to-modem flow
control in Figure E-2). In turn, Modem C will tell Computer D to
stop sending data. Data flow can be resumed by Modem B
issuing an XON to Modem C.
In this case, the modems are normally configured for
XON/XOFF flow control and have modem-to-modem flow
control enabled (see the \G command on page 4-9).
