AT&T AT&T manual Loop Start Trunk ZTN77

Loop Start Trunk (ZTN77)

The Loop Start Trunk Circuit Pack interfaces eight central office loop start trunks and the TDM bus.

Figure 3-9shows the following Loop Start Trunk unique circuitry:

●Port Input/Output (I/O) circuit

●Eight port circuits.

Port I/O Circuit: This, circuit consists of bus expanders				for communication	between	the
on-board microprocessor and		the port circuits.	It receives	commands from	the on-board
microprocessor and	distributes	them to the individual port circuits. It also accesses the				port
circuit scan points	and passes the information to		the on-board microprocessor.

Port Circuits: The eight port circuits are identical. Each port circuit consists of a codec, hybrid circuit, line transformer, relay driver, and surge protection circuit.

The codec is a 4-wire circuit that converts the NPEs output to an analog signal. Likewise, it converts the analog signal from a central office trunk to a PCM data signal to the NPE. The hybrid circuit converts the codec 4-wire analog signal to a 2-wire analog signal that is connected to the central office trunk by the line transformer.

The relay driver buffers and	inverts the relay drive	signals from the port 1/0 circuit so that
a logic high input operates	the appropriate relay.	The relays control circuitry provides the

proper signaling for loop start trunks. The trunks support touch-tone dialing and dial pulse signaling. The surge protection circuit provides overvoltage lightning surge protection for the circuit pack.

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