ATT System 25 Maintenance Manual

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Ground Start Trunk (ZTN76)
The Ground Start Trunk circuit pack interfaces eight central office trunks and
the TDM bus. Figure 3-11 shows the following Ground Start Trunk unique
circuitry:
l Ground detector circuit
l Port Input/Output (I/O) circuit
l Eight port circuits.
Ground Detector Circuit: The ground detector circuit determines if ground
has been applied to the tip lead for incoming seizure. It also senses tip
ground on outgoing seizure indicating dial tone is present. One ground
sensor is used for each port circuit. Input for the ground sensor comes from
the port circuit as an analog current to the -48 V dc supply. The ground
sensors output is a port control point to the port I/O circuit.
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. Each port circuit consists
of a coder/decoder (codec), hybrid circuit, line transformer, relay driver, and
surge protection circuit.
The codec is a 4-wire circuit that converts the NPEs digital output to an
analog signal. Likewise, it converts the analog signal from a central office
trunk to a Pulse Code Modulation (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 operates the appropriate relay. The relay
control circuitry provides the proper signaling for ground start trunks. The
trunks support touch-tone dialing. The surge protection circuit provides
overvoltage lightning surge protection for the circuit pack.
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Figure 3-11. Unique Ground Start Trunk (ZTN76) Circuitry
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Loop Start Trunk (ZTN77)
The Loop Start Trunk circuit pack interfaces eight central office loop start
trunks and the TDM bus. Figure 3-12 shows the following Loop Start Trunk
unique circuitry:
l Port Input/Output (I/O) circuit
l 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. 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 operates the appropriate relay. The relay
control circuitry provides the proper signaling for loop start trunks. The
trunks support touch-tone dialing and dial pulse signaling. The surge
protection circuit provides overvoltage lightning surge protection for the
circuit pack.
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Figure 3-12. Unique Loop Start Trunk (ZTN77) Circuitry
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Tip Ring Line (ZTN78)
The Tip Ring Line circuit pack interfaces eight analog tip and ring voice
terminal lines (single-line voice terminals) and the TDM bus. Figure 3-13
shows the following Tip and Ring Line unique circuitry:
l Ringing application circuit
l Port Input/Output (I/O) circuit
l –48 V to -24 V Power Conditioner
l Eight port circuits.
Ringing Application Circuit: This circuit receives ringing voltage from the
power supply. It monitors ringing voltage and current and generates signals
to the on-board microprocessor indicating zero ringing voltage and current. It
also detects when a terminal user has lifted the receiver during ringing,
preventing the application of ringing to the terminals handset receiver.
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.
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.
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Figure 3-13. Unique Tip Ring Line (ZTN78) Circuitry
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ATL Line (ZTN79)
The ATL Line circuit pack interfaces eight hybrid voice terminal (7300H
series) lines and the TDM bus. It terminates three pairs of wires from each
terminal: analog voice pair, digital control pair, and power pair. Figure 3-14
shows the following ATL Line unique circuitry:
l Protocol handler
l Eight port circuits.
Protocol Handler: The 8-bit on-board microprocessor translates the control
information in CCMS message format to the control information message
format used by the 7300H series voice terminals. The protocol handler sends
the messages to the terminals using transceivers located in the port circuits.
Port Circuits: Each port circuit is identical. A port circuit consists of an
analog port, one-half of a transceiver, and an electronic power feed device.
The analog port circuit consists of a codec, a hybrid circuit, an isolation
transformer, and associated power filtering circuitry. The codec and hybrid
circuits perform the same function as the codec and hybrid circuits in the
Analog Line circuit pack (TN742). The output of the hybrid circuit is
connected to the primary of the isolation transformer. The secondary of the
transformer is connected to the analog voice pair.
The transceiver interfaces the voice terminal pair to the protocol handler.
The electronic power feed device provides -48 V dc on the power pair to the
voice terminal. The device is polled by the on-board microprocessor,
periodically and on demand, to test for an overcurrent or no-current
condition.
Each Electronic Power Feed (EPF) circuit supports two ports. If one of the
associated lines become overloaded, the associated pair of lines will also be
out of service. One EPF supports Ports 0 and 1, one supports Ports 2 and 3,
one supports Ports 4 and 5, and one supports Ports 6 and 7. The on/off
state of the device is controlled by the on-board microprocessor.
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Figure 3-14. Unique ATL Line (ZTN79) Circuitry
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Data Line (TN726)
The Data Line circuit pack interfaces eight Asynchronous Data Units (ADUs)
data devices and the TDM bus. The ADUs are typically, in turn, connected to
RS-232C-type devices. Figure 3-15 shows the Data Line unique circuitry that
includes:
l A bit clock
l Bus isolation
l Eight port circuits.
Bit Clock: The bit clock circuitry is used to provide the OATMEALs (Octal
Asynchronous Terminal Mode Two EIA Asynchronous LSIs) with a clock
frequency that is a multiple of each baud rate. In addition, the clock rate is
divided down to 160 kHz. The 160 kHz is then compared to the systems 160
kHz data clock and is phase-locked to the system clock. The phase-locked
circuit is required for low-speed operation.
Bus Isolation: This portion of the circuit pack is used to isolate the
microprocessor bus. Isolation is required because the realized bus load
exceeds the maximum limit specified for this device, due to the large number
of devices controlled by the NPE. The OATMEALs are isolated from the
common bus structure.
Port Circuits: Each of the eight identical port circuits allows the connection
of interface equipment having an RS-232C compatible serial interface to the
switch. The circuit provides an asynchronous full-duplex subset of standard
data speeds from 300 to 19,200 bps. Each port includes an Asynchronous
Data Unit (ADU) to extend the serial communications link length and provide
safe isolation. The ADU terminates to another ADU at the Customer-
Provided Equipment (CPE). The distance between the digital switch and CPE
is inversely proportional to the speed at which the link is run. See Reference
Manual (555-520-200) for details.
Throughout the circuit, various gates are used to provide a means of isolating
devices for automated circuit pack testing. Typically, these devices are
crystal oscillators or memory components attached to the microprocessor
bus.
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Figure 3-15. Unique Data Line (TN726) Circuitry
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