Black Box MD1641A Manuale Utente

The Series II Modem can be in one of two functional states (see Figure
5-1

). These are "Command mode" and "On-line mode". (There is also an in-

between state, "Wait-for-Carrier", where the modem is out of Command
mode but not yet really On-Line.

When the modem is initially powered up, it is in Command mode and 
is ready to accept commands from your keyboard or software. The
Modem enters On-line mode after dialing, making a connection with
another modem, and then detecting a valid carrier signal. If no carrier
is detected within a certain time frame, the modem abandons the call
and re-enters Command mode.

Once on line, the modem exits On-line mode if one of two conditions is
met. If the carrier is lost or intentionally dropped, the modem hangs up
and re-enters Command mode. Also, if the Modem recognizes the
Escape sequence for which it is configured (i.e., either the Escape
characters in the serial data stream, the Remote Escape characters in
the modulated data stream, or the Break signal), the Modem will exit
On-line mode, retain the datacomm link, and enter Command mode.

You can force the Series II Modem into On-line mode without dialing
by sending the 

or 

command to the modem.