Actiontec electronic Plug n' Play Voice/Fax Modem V.90 ISA - Manual (Page 66)

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S36 Negotiation Fallback (default 7)
This register specifies the action to take in the event of negotiation failure when error
control is selected.
S36 = 0, 2 Hang up.
S36 = 1, 3 Fall back to an asynchronous connection.
S36 = 4, 6 Attempt MNP. If MNP fails, hang up.
S36 = 5, 7 Attempt MNP. If MNP fails, fall back to asynchronous
connection.
S37 Dial Line Rate (default 0)
S37 = 0 maximum modem speed
S37 = 1 reserved
S37 = 2 1200 bits/s and 75 bits/s
S37 = 3 300 bits/s
S37 = 4 reserved
S37 = 5 1200 bits/s
S37 = 6 2400 bits/s
S37 = 7 4800 bits/s
S37 = 8 7200 bits/s
S37 = 9 9600 bits/s
S37 = 10 12000 bits/s
S37 = 11 14400 bits/s
S37 = 12 16800 bits/s
S37 = 13 19200 bits/s
S37 = 14 21600 bits/s
S37 = 15 24000 bits/s
S37 = 16 26400 bits/s
S37 = 17 28800 bits/s
S37 = 18 31200 bits/s
S37 = 19 33600 bits/s
S38 56K Dial Line Rate (default 1)
There are 2 S-registers which support K56flex connections. S38 sets the maximum
56K downstream speed that the modem attempts to connect. To disable K56flex, set
S38 to 0. The S37 register is used to control the upstream V.34 rate. (ref. V.34 Data/
Fax Document). Use the S109 register to control V.90 connections.
S38 = 0 K56flex disabled - V.34 connection (see S37)
S38 = 1 K56flex autorate - maximum achievable connection