3Com 56k User's Manual And Reference page 70

Register Default
S22 17
S23 19
S24 0
S25 20
S26 0
S27 0
66
Technical Reference
Function
Stores the ASCII decimal code for the Xon character
Stores the ASCII decimal code for the Xoff character
Reserved
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 for UK calls
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.32 bis mode