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MET Line (TN735)
The MET Line Circuit Pack interfaces four Multibutton Electronic Telephone (MET) lines
and the TDM bus. The MET Line unique circuitry consists of four port circuits as shown in
Figure 3-13.
Port Circuits: The four port circuits are identical.
Each port circuit consists of an analog
port, a digital port, and an electronic power feed device.
The analog port circuit consists of a codec, a hybrid circuit, an electronic battery feed, and a
power filter.
The codec, hybrid circuit, and power filter perform the same function as in the
Analog Line Circuit Pack (TN742). The electronic battery feed provides talking battery to
the MET set. The electronic battery feed produces a controlled de battery feed current for
short and long loops and detects when a MET set user lifts a receiver.
The digital port circuit provides a full duplex channel over two 2-wire pairs. All outgoing
lamp (LT, I,R) and incoming button depression (BT, BR) information is carried on these
channels. Ringing and switchhook information is also sent over these channels.
The electronic power feed device provides phantomed -48 volt dc power for the MET
terminals over the data channels.
The electronic power feed device is a “smart” circuit
breaker. When it senses an overcurrent condition, it indicates the condition on an output
lead and goes into thermal shutdown if not turned off by the on-board microprocessor.
When the overcurrent condition disappears, the circuit breaker can be turned on by the on-
board microprocessor.
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