US Robotics 56K Series Manual page 90

Register Default
S25 20
S26 0
S27 0
84
Function
Sets the duration, in hundredths of a second, that DTR must be
dropped so that the modem does not interpret a random glitch as a
DTR loss
Most users will want to use the default
This register is useful for setting compatibility with older systems
running under older operating software
Reserved
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
1 2 Enables unencoded (non-trellis coded) modulation in
V.32 mode
2 4 Disables V.32 modulation