Toshiba V.90 - Manual (Page 64)

S-Registers
S-Register values
45
S38 K56flex Dial line rate
S38 enables and disables K56flex mode and sets the maximum
K56flex downstream speed that the modem attempts to connect.
NOTE: Not all modems support K56flex. If this command
returns an ERROR, then K56flex is not supported.
S38 56K Downstream Rate
To force a particular 56K downstream rate, use S-register S38.
S38=0 disables 56K, and may allow a more reliable V.34
connection. S38=1 (default) allows the modem to select the
downstream rate automatically. Other values of S38 force the
downstream rate, with fallback to V.34 if unsuccessful at the
configured rate:
S37 = 13 19200 bps
S37 = 14 21600 bps
S37 = 15 24000 bps
S37 = 16 26400 bps
S37 = 17 28800 bps
S37 = 18 31200 bps
S37 = 19 33600 bps
S38=0 56K disabled (default)
S38=1 56K enabled: automatic speed selection of
maximum modem speed
S38=0 Disable 56K
S38=1 Automatic rate selection (default)
S38=2 32 kbps