ATT System 25 Maintenance Manual

							SYSTEM HARDWARE
MET Line (7N735)
The MET Line circuit pack interfaces four Multibutton Electronic Telephone
(MET) lines and the TDM bus. The MET Line unique circuitry consists of four
port circuits as shown in Figure 3-16.
Port Circuits: The four port circuits are identical. Each port circuit consists
of an analog port, a digital port, and an electronic power feed device.
The analog port circuit consists of a codec, a hybrid circuit, an electronic
battery feed, and a power filter. The codec, hybrid circuit, and power filter
perform the same function as in the Analog Line circuit pack (TN742). The
electronic battery feed provides talking battery to the MET set. The
electronic battery feed produces a controlled dc battery feed current for short
and long loops and detects when a MET set user lifts a receiver.
The digital port circuit provides a full-duplex channel over two 2-wire pairs.
All outgoing lamp (LT, LR) and incoming button depression (BT, BR)
information is carried on these channels. Ringing and switchhook information
is also sent over these channels.
The electronic power feed device provides phantomed -48 V dc power for
the MET terminals over the data channels. The electronic power feed device
is a smart circuit breaker. When it senses an overcurrent condition, it
indicates the condition on an output lead and goes into thermal shutdown if
not turned off by the on-board microprocessor. When the overcurrent
condition disappears, the circuit breaker can be turned on by the on-board
microprocessor.
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Figure 3-16. Unique MET Line (TN735) Circuitry
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Analog Line (TN742)
The Analog Line circuit pack interfaces eight analog voice terminal lines and
the TDM bus. Figure 3-17 shows the following Analog Line unique circuitry:
l Ringing application circuit
l Port Input/Output (I/O) circuit
l Eight port circuits.
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 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, electronic battery feed circuit, ring
relay, and overvoltage surge protection circuit.
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 dc battery feed for short and long
loops, detects when 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.
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Note:The TN742 may be used instead of the ZTN78 Tip Ring circuit pack.
The TN742 supports up to five bridged single-line voice terminals;
however, only two may be off-hook at one time. The ZTN78 circuit
pack does not support bridged terminals. In addition, the TN742
supports out-of-building, extended, and off-premises stations; the
ZTN78 does not.
Figure 3-17. Unique Analog Line (TN742) Circuitry
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DID Trunk (TN753)
The DID Trunk circuit pack interfaces eight central office trunks arranged for
Direct Inward Dialing (DID) and the TDM bus. Figure 3-18 shows the
following DID 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, balance network, trunk interface unit, and loop termination circuit.
The codec is a 4-wire circuit that converts the NPE output to an analog
signal. Likewise, it converts the analog signal from the central office (CO) to
a PCM signal to the NPE.
The trunk interface unit contains a hybrid circuit, a 2-wire interface circuit,
and control circuitry. 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
by the 2-wire interface circuit. The control circuitry controls loop current,
internal signal gain, terminating resistance, battery feed shutdown, and
battery reversal. The circuit pack accepts both dial pulse and touch-tone
signaling.
The loop termination circuit provides a fixed impedance to the DID trunk.
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Figure 3-18. Unique DID Trunk (TN753) Circuitry
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Tie Trunk (TN760B)
The Tie Trunk circuit pack interfaces four 6-wire tie trunks and the TDM bus.
Two tip and ring pairs form a 4-wire analog transmission line. An E and M
pair is used for signaling. The T and R pair transmits analog signals from the
circuit pack. The T1 and R1 pair receives analog signals from the tie trunk.
The E and M pair are dc signaling leads used for call setup handshaking.
The E lead receives signals from the tie trunk and the M lead provides
signals from the circuit pack. The TN760Bs four port circuits support Type I,
Type I Compatible, or Type V signaling. Incoming and outgoing trunks may
be either automatic, immediate start, wink start, or delay dial. Figure 3-19
shows the following Tie Trunk unique circuitry:
l Ground detector circuit
l Port Input/Output (I/O) circuit
l Four port circuits.
Ground Detector Circuit: This circuit determines if a ground has been
applied to the E lead. Ground detector inputs come from the port circuits as
an analog current to the -48 V dc supply. Its 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 port circuits are identical, except for port 3 where part of
the E-lead maintenance circuit is located. Each port circuit consists of a
codec with associated input and output line transformers, analog operational
amplifiers, a power filter, loop-around transistors, port control comparators, a
relay driver, an electronic power feed device, an E-lead test maintenance
circuit, and surge protection circuits.
The codec converts the incoming 4-wire analog signal from the tie trunk to a
PCM data dignal. The codec converts the incoming PCM data signal from
the NPE to an analog signal. Outgoing and incoming line transformers
provide dc isolation to the tip and ring leads. Analog operational amplifiers
provide amplification and buffering for the codec and network and loop-
around gain compensation. Filtered power is provided to the codec and
amplifiers.
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The loop-around transistors are under control of the port control comparators
and provide a loop-around path for the signal for testing purposes. 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 relays and
electronic power feed device control the M-lead circuitry to provide the
proper signaling handshake for call progress tones and dial pulse dialing.
The electronic feed device provides a -48 V dc current to the M-lead circuits.
It also tests the M-lead circuits for opens or shorts and prevents uncontrolled
operation during powerup. The E-lead test circuit provides a ground to the
ground detector circuit for testing purposes. The surge protection circuitry
provides lightning surge and power cross protection for the circuit pack.
Figure 3-19. Unique Tie Trunk (TN760B) Circuitry
Various signaling formats (consists of a mode and a type) are available with
the TN760B. The mode designates the electrical interface and the type
designates the logical signaling used. For each port circuit, the mode is
selected by option switch settings on the CP. Table 3-B lists the preferred
signaling formats for likely-to-be encountered installation situations and
option switch settings. The option switches are shown in Figure 3-20.
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Table 3-B. TN760B Option Switch Settings and Administration
System 25
Installation
Situation
Circumstance
Co-Located
Inter-Bldg.
Co-Located
Inter-Bldg.
Co-Located
Inter-Bldg.
Co-Located
Inter-Bldg.
Co-Located
To
S25/S75
S25/S75
S85
S85
DIMENSION
PBX
DIMENSION
PBX
Other
Other
Net
Int.
Preferred
Signaling Format
System 25
Simplex
Type 5
Simplex
Type 5
Simplex
Type 5
Simplex
Type 5
E&M
Type 1
Compatible
Prot.
Type 1
Compatible
E&M
Type 1
Compatible
Prot.
Type 1
Compatible
E&M
Type 1
Standard
Far-End
Simplex
Type 5
Simplex
Type 5
Simplex
Type 5
Simplex
Type 5
E&M
Type 1
Standard
Prot.
Type 1
Standard
E&M
Type 1
Standard
Prot.
Type 1
Std. Plus
Protection
Unit
Dont
Care
Set
E&M/
SMPLX
Option
Switch
SMPLX
SMPLX
SMPLX
SMPLX
E&M
E&M
E&M
E&M
E&M
Set
Prot/
Unprot
Option
Switch
Either
Either
Either
Either
Unprot
Prot
Unprot
Prot
Unprot
Administer
the Port
Type 5
Type 5
Type 5
Type 5
Type 1
Compatible
Type 1
Compatible
Type 1
Compatible
Type 1
Compatible
Type 1
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Figure 3-20. Tie Trunk (TN760B) Circuit Pack Option Switches
Table 3-C summarizes the conditions present as the transmit and receive
control signals for each signaling type.
Table 3-C. Signaling Type Summary
SIGNALINGTRANSMITRECEIVE
TYPEOn-HookOff-HookOn-HookOff-Hook
I Std.grdbatopen/bat (*)grd
I Compat.
open/bat (*)grdgrd
open/bat (*)
Vopengrd
open
 grd
* An open circuit is preferred over voltage.
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