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QPC452 Basic 500/2500 Analog Line Card
553-3001-105
Standard 5.00
June 1999
Line interface units
The basic 500/2500 analog line card contains eight identical and individually
configurable line interface units (also referred to as circuits). Each unit
provides 600 ohm impedance matching and a balancing network in a signal
transformer/analog hybrid circuit. This balancing network also contains
circuitry that allows conversion from a 2-wire to a 4-wire transmission path.
Relays are provided in each unit to apply ringing onto the line, and the timing
of the application of ringing current is controlled to prevent switching during
current peaks. These relays are disabled during the power-up cycle to prevent
false ringing. The line interface units also contains signal detection circuits
that monitor on hook/off hook signaling.
Codecs
Each line interface unit contains a filter and a coder/decoder (codec). Audio
signals received from the telephone line are passed through a low-pass
monolithic A/D filter that limits the frequency spread of the input signal to a
nominal 200–3400 Hz bandwidth. The audio signal is then applied to the
input of the codec. Audio signals coming from the codec are passed through
a low-pass D/A monolithic filter that integrates the amplitude modulated
pulses coming from the codec and then filters and amplifies the result. The
codec performs A/D and D/A conversion of the line analog voiceband signal
to a digital PCM signal.
Multiplex control
The multiplex control logic is common to all eight channels. This logic
interfaces the individual line circuits to the peripheral bus signaling channel.
Circuits are provided in the multiplex control logic to retime the digital
signals received from the peripheral bus. Circuits are also provided to decode
the address information contained on the peripheral bus to enable the
individual line circuits during their selected time slots. Further, logic is
provided to enable or disable the front panel LED to indicate the service state
of the card.
Summary of Contents for NT1R20
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