AT&T AT&T manual Did Trunk TN753

Each port circuit consists of a codec,

DID Trunk (TN753)

The DID Trunk Circuit Pack interfaces eight central office trunks arranged for Direct Inwrd Dialing ((DID) and the TDM bus. Figure 3-15shows the following DID Trunk unique circuitry:

●Port Input/output (1/0) 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.

balance network, trunk interface unit, and loop termination circuit.

The codec is a 4-wire circuit that converts the NPEs output to an analog signal. Likewise, it converts the analog signal from the Central Office (CO) to a PCM signal to the NPE.

The trunk interface unit contains a hybrid, a 2-wire interface circuit, and control circuitry. The hybrid circuit converts the 4-wire analog signal from the codec to a 2-wire analog signal that is connected to the analog line by the 2-wire interface circuit. The control circuitry controls loop current, internal signal gain, terminating resistance, battery feed shutdown, and battery reversal. The circuit pack accepts both dial pulse and touch-tone signaling.

The loop termination circuit provides a fixed impedance to the DID trunk.

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