3Com 56K manual Remote digital loopback, Line, and at the timeout the Business Modem hangs up

I-4APPENDIX I: S-REGISTERS

Register	Default	Function
S16	0	Bit-mapped register. (See Chapter 1, Using the AT
		Command Set, for instructions on setting bit-mapped
		registers.) For testing, see Chapter 14, Testing.

		Bit	Value	Result
		2	4	Test pattern.
		3	8	Remote digital loopback.
S18	0	Test timer for software-initiated loopback testing (&Tn);
		disabled when S18 is set to 0. Used to set the duration of
		testing, in seconds, before the Business Modem
		automatically times out and terminates the test.
S19	0	Sets the duration, in minutes, for the Inactivity Timer. The
		timer activates when there is no data activity on the phone
		line, and at the timeout the Business Modem hangs up.
		S19=0 disables the timer.
S21	10	Sets, in 10-millisecond units, the length of breaks sent from
		the Business Modem to the computer or terminal. Applies
		to ARQ mode only.
S22	17	Stores the ASCII code for the XON character.*
S23	19	Stores the ASCII code for the XOFF character.*
S24	150	Sets the duration, in 20-millisecond units, between pulsed
		DSR signals when the Business Modem is set to &S2 or &S3.
		The default is 3 seconds.
S25	5	Sets DTR recognition time in 10-millisecond units.
S27	0	Bit-mapped register. (See Chapter 1, Using the AT
		Command Set, for instructions about setting bit-mapped
		registers.)

Bit	Value	Result
0	1	Enable ITU-T V.21 modulation at 300
		bps for overseas calls. In V.21 mode,
		the Business Modem answers both
		Bell 103 and V.21 calls, but only
		originates V.21 calls.
1	2	Enable unencoded (non- trellis-coded)
		modulation in V.32 mode; this option
		is part of the ITU-T V.32
		recommendation, but is rarely used.
2	4	Disable V.32 modulation; used for
		testing HST modulation.