AT&T AT&T manual

coder/decoder (codec), hybrid circuit, overvoltage surge protection circuit.

Ringing Application Circuit: This circuit receives ringing voltage from the power supply. It monitors ringing voltage and current, generates signals to the on-board microprocessor indicating zero ringing voltage and current, and detects a terminal user lifting the receiver during ringing. This prevents the application of ringing to the port circuit when a terminal user lifts the receiver during the ringing phase. Maintenance circuitry is also included. The

maintenance	circuitry detects when	a terminal is connected to the port circuitry and checks
for faults in	the ringing application	circuitry.

Port I/O Circuit: This circuit consists of bus expanders connecting the on-board microprocessor a n d t h e p o r t c i r c u i t s . It receives c o m m a n d s f r o m 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 electronic battery feed circuit, ring relay, and

The codec is a 4-wire circuit that converts the analog signal from a voice terminal to a PCM data signal. It converts an incoming PCM data signal from the NPEs to an analog signal. 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. Filtered power is provided for the codec and hybrid circuits.

The electronic battery feed circuit provides talking battery to the voice terminal. It also produces a controlled de battery feed for short and long loops, detects tvhen a receiver is lifted, and provides the message waiting signal by periodically turning off the feed voltage.

The ring relay provides the interface between the ringing application circuit and the port circuit. It causes ringing turn on and turn off.

The overvoltage surge protection circuit provides lightning surge and power line cross protection for the circuit pack.

Note: The TN742 can be used instead of the ZTN78 Tip Ring CP. The TN742 supports up to five bridged single-line voice terminals, however, only two can be off hook at one time. The ZTN78 CP does not support bridged terminals. In addition, the TN742 supports out-of-building, extended, and off-premises stations, the ZTN78 does not.

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