Multi-Tech MultiModem ISIHI-2S, Manual

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MultiModemISI Hybrid Series, ISIHI-2S 79
Chapter 4—MODEM AT Commands, S-Registers, Result Codes
Modem
Flow Control Commands
Flow control refers to techniques used by data terminal equipment and the modem to
pause and resume the flow of information between them. It prevents a device from
accepting more data than it can handle. The modem implements flow control in both
directions. When the modem halts the flow of data, it is called flow control. When the
computer halts the flow, it is called pacing.
&M
n
Communications Mode
n = 0
Default: &M0
The &M command enables asynchronous communications mode. This is the default.
&K n Local Flow Control Selection
n = 0, 3, or 4
Default: &K3
The &K command allows you disable flow control and enable hardware or software flow
control.
&K0 completely disables data flow control initiated by the modem. (This is the same as
\Q0.)
&K3 enables modem’s use of Clear to Send (CTS) signal on the RS-232/V.24 interface
to regulate data flow. When CTS drops, data flow is suspended until the signal goes
high
(on) again. This method of flow control works in conjunction with pacing (i.e., computer-
initiated flow control), which uses the Request to Send (RTS) signal on the RS-232/V.24
interface. Hardware flow control cannot be enabled unless an active error correction
protocol is selected. This is the factory default setting. (This is the same as \Q3.)
&K4 enables XON/XOFF software flow control. XON/XOFF flow control is an in-band
method of data flow regulation. In-band data regulation means that the XON (^Q) and
XOFF (^S) characters are inserted into the stream of data rather than using separate
control lines. When an XOFF character is detected, the data stream is suspended until
an XON character is detected. If you issue the &K4 command to the modem, it will
respond to XON/XOFF pacing, and use XON/XOFF characters as its own method of flow
control to the computer. (This is the same as \Q1.)
The drawback to using this method of pacing is that some files may contain these
characters as part of the file data. If such a file is transferred using a modem with XON/
XOFF flow control enabled, the file transfer could fail due to indefinite suspension.
\Q n Local Flow Control Selection
n = 0, 1, or 3
Default: \Q3
The \Q command allows you disable flow control and enable hardware or software flow
control.
\Q0 completely disables data flow control initiated by the modem. (This is the same as
&K0.)
\Q1 enables XON/XOFF software flow control. XON/XOFF flow control is an in-band
method of data flow regulation. In-band data regulation means that the XON (^Q) and
XOFF (^S) characters are inserted into the stream of data rather than using separate