USRobotics V.34 - Manual (Page 171)

COURIER HIGH SPEED MODEMS
F-6 Synchronous Operations
Recommended Settings
When the connection is made and the data rate is determined,
host/modem rates may change dramatically to match the
connection rate (when in online synchronous mode, the modem
is transparent on the line). To avoid this dramatic rate
switching (which can be hard on some computers), we
recommend that the modem be set with a fixed rate between the
computer or terminal and modem (%N
n) and that the
connection rate (&N n) be set to the same rate.
Result Codes (Xn)
The Courier displays normal or extended synchronous result
codes, depending on the setting of the X
n command. By
default, the modem is set to X1 for extended result codes. To
change to normal result codes, set the modem to X0.
The normal result codes return messages such as VAL or INV
(VALID or INVALID), whereas the extended codes offer
explanations
 INVPS (INVALID Parameter Syntax Error).
Automatic Answering (S0=1)
When the Courier is operating in V.25 bis mode, it ignores the
DIP switch 5 setting, which controls Auto Answer. To set the
modem to automatically answer incoming calls, set the modem
to S0=1, so it answers on the first ring. You can substitute a
higher value. See the S-Register summary in Appendix B.
Suppressing Auto Answer
To disable Auto Answer, set the modem to answer on zero
rings, S0=0.
Choosing a Synchronous Protocol (&Mn)
Once the &X n, %Nn, &Nn, Xn, and S0 commands are
configured, you must use the &M n command to choose the
HDLC or character-oriented link protocol so that your
synchronous software can properly format its commands.
&M6 Use the character-oriented protocol for synchronous
communications.