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The purpose of the timer for the Answer modem is to determine when it will accept a dial-up call. When
both timers have expired and the leased line has not been established the dialback procedure will start.
The number dialed is the one stored in location N9 of the originate modem’s phone number memory. In
preparation for proper dialback operation, you must enter the proper number in the N9 location using the
commands detailed in Chapter 3. Keep in mind that the number also can be a V.25bis command for
synchronous operation.
2.5
Dial-up Automatic Answering
With your MT2834MR in the dial-up mode (12-Position DIP Switch #10 in the UP position), the
MT2834MR can be used as an automatic answering modem.
The MT2834MR, when in its idle state, will be set up with originate mode frequencies. An incoming ring
signal will automatically switch the modem into answer mode.
You can program the modem to answer a call after a certain number of rings by using the modem’s S-
Register S0. (See Chapter 4). Refer to Section 2.6 for the handshaking procedures used by the
MT2834MR in an automatic answering application.
2.6
Dial-up Handshaking Details
With your MT2834MR in the dial-up mode (12-Position DIP Switch #10 in the UP position), the following
briefly explains what happens between two modems in a normal call. We are assuming that there are
MT2834MR’s at both the originating and at the answering end of the telephone line. (If other brands of
modems are used with our modems, they will still communicate, but each manufacturer tends to use
slightly different delay timings and sequences, and many of the brands vary in their LED designations.)
Before V.34 negotiation takes place, remote and local modems transfer their functional capabilities using
modulated calling and answering tones. First, the Calling Menu (CM), a data sequence using V.21 low-
band modulation (a handshaking technique originally developed for 300 bps modems), is sent from the
originating V.34 modem to the answering V.34 modem. This describes the range of functional
capabilities the originating modem supports. The answering V.34 modem responds to the CM with a
Joint Menu, or JM (using V.21 high-band modulation), indicating the common capabilities of the modem
at each end.
Once this is done, a probing signal is passed between the modems to identify impairments in the
telephone channel. After receiving the results of the probing signal, the modem receivers and
transmitters will start with the lowest speed and move up until reaching the highest speed at which they
can exchange data. All this occurs in about five seconds.
Note:
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, or it can be forced on by a switch in the modem.
Note:
If you are using the Reliable or Auto-Reliable mode, some additional handshaking takes place.
This is explained in Chapter 5, in the “Auto-Reliable Mode” section.
2.7
Call Termination
With your MT2834MR in the dial-up mode (12-Position DIP Switch #10 in the UP position), there are a
number of methods by which you can terminate a call, or simply stated, hang up. They are:
1.
Command Mode Control. It is possible to enter Command Mode while remaining on-line by entering
the Escape Sequence (+++AT<CR> in most cases). After you have entered Command Mode, you may
return to the On-Line state by typing ATO, or you may terminate the call with the H command. Typing
ATH will hang up the line (bring it on-hook), bringing the modem out of the On-Line state. So the
command to hang up a call is three plus signs and then ATH.
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