Instruction manual

FUNCTIONAL DESCRIPTION
Loop Start Trunk (ZTN77)
The Loop Start Trunk Circuit Pack interfaces eight central office loop start trunks and the
TDM bus. Figure 3-13 shows the Loop Start Trunk unique circuitry.
NPE 0
NPE 1
ON-BOARD
MICROPROCESSOR
Figure 3-13. Loop Start Trunk (ZTN77) Unique Circuitry
● Port I/O Circuit
TO
CENTRAL
OFFICE
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 I/O circuit so that a logic high input
3-24