Synchronous Mode 1: Sync/Async &Q1
Synchronous mode 1 supports terminals that communicate both synchronously
and asynchronously over the same V.24/RS-232 port. In synchronous mode 1,
a call may be completed using asynchronous methods from the command state
with the D or A command. The modem automatically switches to the synchronous
online state when the CONNECT XXXXX result code is delivered to the terminal.
In this mode call establishment starts from the asynchronous command state;
therefore, the modem must be attached to a computer port with both synchronous
and asynchronous capabilities.
If the modem receives a character while dialing the telephone number, it
aborts the call and sends the OK result code; if a character is received
after dialing but prior to the completion of the connection, it aborts the
call and sends the NO CARRIER result code.
The data terminal must apply an ON condition to DTR before online data transfer
can begin. Once the CONNECT XXXXX result code is received, the modem delays
a period of time determined by S25 before examining the condition of the
DTR signal. If DTR is ON, the modem enters the synchronous online state.
If DTR is OFF, the modem hangs up and returns to the asynchronous command
state. For the responses associated with transitions of the DTR signal for
this mode, refer to the &D command.
The modem exits the synchronous online state and returns to the asynchronous
command state when a loss-of-carrier exceeding the time interval stored
in S10 is detected.
If DTR is OFF and &D2 is selected, the auto-answer feature is disabled regardless
of the value of register S0.
The following diagram depicts modem operation in synchronous mode 1:
