Instruction manual

FUNCTIONAL DESCRIPTION
● Port I/O Circuit
This circuit includes bus expanders connecting 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.
● -48 V To -24 V Power Conditioner
This circuit converts -48 V power from the power supply into a conditioned source of
-24 V power for the electronic battery feed circuits.
● Eight Port Circuits
Each port circuit is identical.
A port circuit consists of a coder/decoder (codec),
hybrid circuit, battery feed circuit, and ring relay.
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 battery feed circuit provides talking battery to the voice terminal. It also detects
when a receiver is lifted, and provides the message waiting signal by periodically
reducing the feed voltage to zero.
The ring relay provides the interface between the ringing application circuit and the
port circuit. It causes ringing to turn on and off.
System Resources
The System Resource Circuit Packs (CPS) are the Service Circuit Clock (ZTN131), the Tone
Detector (TN748B), and the Pooled Modem (TN758).
Service Circuit Clock (ZTN131)
The Service Circuit CP (Figure 3-18) provides the clock signals of the system and generates
and receives tones. It provides four touch-tone receivers, generates all tones for the system,
and supplies the system clocks.
The ZTN131 can support up to 75 Dual Tone
Multifrequency (DTMF) dialers depending on call traffic; the TN748Bs might be required in
heavy traffic situations, even with less than 75 DTMF dialers. Each System 25 must contain
one Service Circuit CP.
Power for the circuit pack (+5 volts dc) is provided on the
backplane. The Service Circuit has the following unique circuitry:
3-31