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SERIES II INTELLIGENT DATA/FAX MODEMS
When a call is dialed from the originating modem, the called modem
responds to the ringing by switching into Answer mode and by turning
on its RI (Ring Indicator, RS-232C/V.24 Pin 22) signal as the rings are
detected. (The Modem-MAC does not use DTR.)
Note that in order for the called modem to be able to answer the call, it
must have a high DTR (Data Terminal Ready) signal. This signal comes
from the computer or terminal to which it is attached, on RS232C Pin 20.
When the called modem answers the call, its OH (Off Hook) circuit
comes on and the ringing stops. Two seconds after the call is answered,
the called modem begins transmitting its answerback tone.
The originating modem, which has been off hook and waiting for the
answerback tone, turns on its transmitter when it hears the tone. Then
after a one second delay, the orignating modem's CD (Carrier Detect)
comes on.
The called modem then responds to the originating modem by turning
on its CD (Carrier Detect) signal, and the handshaking is completed.
Note: With the modem in Reliable or Auto-Reliable mode, some
additional handshaking takes place. This is explained in Section 5.4.8.
4.11 Call Termination
There are a number of ways to terminate a call, or simply stated, hang
up. They are:
1) Command Mode Control. To terminate a call by Command
Mode, enter +++ATH then RETURN; or, send a BREAK signal
followed by ATH and a RETURN.
2) DTR Control. If DTR (Data Terminal Ready) is turned off for 50
milliseconds or more, a disconnect occurs. This is probably the
most common method used by computer systems at the automatic
answer end of the line to cause the answering modem to
disconnect after logging off procedures.
3) Loss of Carrier. After a data connection is established, the
modem disconnects if a loss of carrier occurs for 700 milliseconds
(0.7 seconds) or more. Note that one cause of carrier loss would be
if one of the modems disconnect normally.