Flow Control Commands

Chapter 4—MODEM AT Commands, S-Registers, Result Codes

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.

&Mn Communications Mode

n = 0 Default: &M0

The &M command enables asynchronous communications mode. This is the default.

&Kn 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.

\Qn 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

Modem

MultiModemISI Hybrid Series, ISIHI-2S

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Multi Tech Equipment ISIHI-2S manual Flow Control Commands, Mn Communications Mode

Models: ISIHI-2S