US Robotics 005689-03 Manual page 53

Register Default
S27 0
S28 8
54
Function
Bit-mapped register setup. To set the register, see instructions for S13.
Bit Value Result
0 1 Enables ITU-T V.21 modulation at 300 bps for overseas calls;
in V.21 mode, the modem answers both overseas and domestic
(U.S. and Canada) calls, but only originates V.21 calls.
(Default Bell 103)
1 2 Enables unencoded (non-trellis coded) modulation in
V.32 mode.
2 4 Disables V.32 modulation.
3 8 Disables 2100 Hz answer tone to allow two V.42 modems to
connect faster.
4 16 Enables V.23 fallback mode.
5 32 Disables V.32bis mode.
6 64 Disable V.42 selective reject.
7 128 Software compatibility mode. This setting disables the codes
and displays the 9600 code instead. The actual rate of the call
can be viewed on the ATI6 screen. Used for unusual software
incompatibilities. Some software may not accept 7200,
12000, 14400 bps, or greater result codes.
Value
0 Eliminates the V.32 answer tones for a faster connection.
8 Default time, all times are in tenths of seconds.
255 Disables all connections except V.32 at 9600 bps.