US Robotics 56K Series Manual page 95

Register Default
S38
S39-S40
S41 0
S42 0
Function
Sets an optional delay, in seconds, before a forced hang-up and clearing
of the Transmit buffer when DTR drops during an ARQ call. This
allows time for a remote modem to acknowledge receipt of all
transmitted data before it is disconnected. The modem immediately
hangs up when DTR drops.
This option only applies to connections terminated by dropping DTR. If
the modem receives the ATH command, it ignores S38 and immediately
hangs up.
Reserved
Reserved
Reserved
89