3Com 56K manual Data Set Ready, Signal

7-2CHAPTER 7: CONTROLLING EIA-232 SIGNALING

Data Set Ready	Under normal conditions, the Business Modem sends a Data Set Ready
	(DSR) signal to your computer when it is ready to send and receive data.
	CAUTION: Do not change the default setting of &S0 unless you know
	that your installation requires a different setting. Few Windows
	communications programs, if any, will require the Business Modem to
	control DSR (&S1).
	Use the following command to control how the modem sends the DSR
	signal.

For you modem	Command
To send the DSR signal at all times.	AT&S0 (Default)
When originating a call, to send the DSR signal after dialing	AT&S1
when the Business Modem detects the remote analog device’s
answer tone
When answering a call, to send DSR after the Business Modem	AT&S1
sends its answer tone.
After sending Carrier Detect (CD), to send a pulsed DSR signal,	AT&S2
followed by a Clear to Send (CTS) signal. Use this option for
specialized equipment such as automatic callback units.
After sending Carrier Detect (CD), to send a pulsed DSR signal.	AT&S3
To send a DSR signal to your computer at the same time the	AT&S4
Business Modem sends the CD signal.
To send DSR normally (with CTS) after sending CD.	AT&S5
In order to change the DSR pulse time (in 20-second
increments), set S-Register 24. (See Appendix B,Alphabetic
Command Summary).

Example: Issuing AT&S3 configures the Business Modem to send a pulsed DSR signal after sending the Carrier Detect (CD) signal.