AT&T AT&T manual Ground Start Trunk ZTN76

Each port circuit consists of a

Ground Start Trunk (ZTN76)

The Ground Start Trunk Circuit Pack interfaces eight central office trunks and the TDM bus. Figure 3-8shows the following Ground Start Trunk unique circuitry:

●Ground detector circuit

●Port Input/Output (I/O) circuit

●Eight port circuits.

Ground Detector Circuit: The ground detector circuit determines if ground has been

applied to the tip lead for incoming seizure. It also senses tip

ground on outgoing seizure

indicating

dial

tone is

present.

One ground sensor is used for

each

port circuit. Input for

the

ground

sensor

comes

from the

port circuit as

an analog

current

to

the -48 volt dc supply.

The

output

of

the

ground sensor

is

a port control

point to

the port

1/0

circuit.

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.

coder/decoder (codec), hybrid circuit, line transformer, relay driver, and surge protection circuit.

The codec is a 4-wire circuit that converts the NPEs digital output to an analog signal. Likewise, it converts the analog signal from a central office trunk to a Pulse Code Modulated (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 ground start trunks. The trunks support touch-tone dialing. The surge protection circuit provides overvoltage lightning surge protection for the circuit pack.

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