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mode).
If this signal is false the low frequency band is selected for transmitting data (calling mode).
This signal must be available if the modem is used both for incoming and for outgoing calls.
If the modem does not support this signal, the modem pool must be assigned a call
direction: incoming calls only or outgoing calls only.
If a modem does not support this signal, the MLU cannot tell the modem in what
frequency band to transmit and to receive. The modem cannot be used in bothway
operations, it must be used in one direction only: incoming or outgoing and the modem
must be strapped accordingly.
The centre frequency in the channels are 1200 Hz for the low channel and 2400 Hz for
the high channel.
-
CT140 (RIL) remote interface loop:
The MLU can send a request to the opposite modem to force a remote loop back for test
purposes.
-
CT141 (LL) local loop:
The MLU can send a request to the local modem to force a local loop back for test
purposes. Only if the modem is able to connect CT103 and CT104 can the test mode be
entered;
-
CT142 (TI) test indicator:
The modem indicates to the MLU that the test mode is entered after a loop request.
Note: The MLU does not support the RI - ring indicator signal (CT125).
B.1.
V.25 CALLING AND ANSWERING TONES
If a modem is equipped with an Automatic Calling Unit, the connected DTE can send a
destination number to this ACU. The modem now transfers the digits to the network. When
the opposite modem answers the call, calling and/or answering tones can be used for the
synchronisation between the modems and for echo suppression in the network. The DTE can
communicate with the modem by means of the V.25 commands and indications. With V.25
the CT200-series signals are available for this communication. With V.25 bis the
communication is performed via the normal RXD and TXD (CT103 and CT104) lines.
In this part we are not interested in this DTE-DCE communication but what happens on the
line: the calling and answering tones. The calling tone is repetition of a tone burst of 1300 Hz
during 0.5 - 0.7 sec. followed by silence for 1.5 - 2.0 sec. The answering tone is a continuous
tone of 2100 Hz during a maximum of 3.3 - 4.0 sec.
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