Synchronous Mode 4: Hayes AutoSync &Q4
With communication software incorporating the Hayes Synchronous Interface
(HSI), the modem can communicate synchronously without a synchronous interface
adapter card. In AutoSync mode, the modem places the call asynchronously
then automatically switches to synchronous operation once the telephone
connection is fully established.
If the modem receives a character while dialing the telephone number, the
modem aborts the call and sends the OK result code; if a character is received
after dialing but prior to the completion of the connection, then it aborts
the call and sends the NO CARRIER result code.
The data terminal must apply an ON condition to DTR before data transfer
can begin. Once the CONNECT XXXXX result code is received by the DTE, the
modem delays for a period of time determined by S25 before examining the
condition of the DTR signal. If DTR is ON, the modem enters the synchronous
operation state. If DTR is OFF, the modem hangs up and returns to the asynchronous
command state.
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. For the responses associated with transitions of the
DTR signal for this mode, see the &D command.
The diagram depicts modem operation in synchronous mode 4:
