Black Box MD1641A, MD1640A manual Call Termination

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.

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