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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to telephone subscriber loop carrier systems and,
more particularly, to the testing of the various channels in such a system
to establish continuity of the communication path.
2. Description of the Prior Art
It has become increasingly common to apply carrier techniques to subscriber
loops in order to avoid or postpone the placement of telephone cable in
situations where cable placement represents a substantial capital
investment. Obvious situations of this type are for rural subscribers
where loops are excessively long or for congested urban areas where cables
laying is excessively expensive. Two such systems are disclosed in J. L.
Caldwell U.S. Pat. No. 3,963,869, issued June 15, 1976 and T. N. Rao et al
U.S. Pat. No. 4,028,628, issued June 7, 1977.
In such subscriber loop carrier systems, a metallic path from the central
office no longer exists for each subscriber. Normal testing of the
subscriber facility cannot be accomplished by the prior art technique of
establishing the direct current continuity of the loop. Such testing,
however, has become a standard part of the preventive maintenance as well
as the troubleshooting procedures for telephone subscriber loops.
Conventional telephone supervisory signaling, likewise depending upon
direct current signaling over a metallic subscriber loop, similarly cannot
be accomplished in the conventional manner over the frequency-derived
subscriber communication channel. Thus, off-hook supervision, dial
pulsing, ring trip detection and ringing itself must be accomplished
utilizing carrier techniques. Prior art systems, such as that disclosed in
the above-noted Rao et al patent, accomplish ringing by interrupting the
carrier signal from the central office at a ringing signal rate. In
carrier systems operating over repeatered transmission facilities,
however, it is often necessary to maintain the carrier on at all times to
control the repeater gain. It is therefore not possible to transmit
ringing information by interrupting the carrier.
SUMMARY OF THE INVENTION
In accordance with the illustrative embodiment of the present invention,
both ringing signaling and continuity testing are accomplished over
carrier-derived communications channels by superimposing a signaling tone
on a continuous carrier. Ringing is signaled by interrupting the tone at
the ringing frequency rate (e.g., 20 Hz). Continuity testing is
accomplished by superimposing continuous tone which, at the remote
terminal, is detected and the return carrier is transmitted from the
remote terminal back to the central office terminal. The returned carrier
is detected at the central office location and thereby confirms continuity
of the talking path. This is therefore called a loopback continuity test.
A particular advantage of this arrangement for continuity testing is that
the continuity test can be initiated in response to standard direct
current voltages used for normal testing of metallic subscriber loops. The
successful completion of the continuity test, in accordance with the
present invention, can likewise be reported to the central office testing
facility by generating a standard voltage condition at the central office
appearance of the carrier-derived subscriber loop.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 is a general block diagram of a subscriber loop carrier system in
which the supervision and testing arrangements of the present invention
may find use;
FIG. 2 is a block diagram of a central office modem useful in the
realization of the carrier system of FIG. 1;
FIG. 3 i s a block diagram of a remote terminal modem which may likewise
find use in the carrier system of FIG. 1;
FIG. 4 is a more detailed block diagram of the central office common
control circuits of the central office modem shown in FIG. 2;
FIG. 5 is a more detailed block diagram of the remote terminal control
circuits of the remote terminal modem of FIG. 3;
FIG. 6 is a detailed circuit diagram of the loop closure detector circuits
of the central office control circuits shown in FIG. 4;
FIG. 7 is a detailed circuit diagram of the ring detector and continuity
test detector circuits of the central office control circuits of FIG. 4;
and
FIGS. 8, 9, 10 and 11, taken together, comprise a detailed circuit diagram
of the remote terminal control circuits of FIG. 5.
DETAILED DESCRIPTION
Referring more particularly to FIG. 1, there is shown a general block
diagram of a subscriber loop carrier system with which the ringing and
continuity testing arrangements of the present invention might find use.
In FIG. 1 a plurality of subscriber loop appearances 10 through 17 are
each connected to a respective one of central office modems 18, the
outputs of which are connected through a common power supply 21 to a
common transmission facility 19. All of modems 18 are identical in circuit
structure, but each operates at a different carrier frequency. The carrier
frequencies for the direction of transmission from the central office to
the remote terminal form a first group of frequencies while the carrier
frequencies received from the remote terminal by each of modems 18 form a
second group of carrier frequencies. These carrier frequencies are thereby
separated into low and high bands for ease of processing in a plurality of
repeaters 20 spaced along transmission facility 19. Common power supply
21, operated from the central office battery, supplies potentials for
powering the repeated line and the remote terminal and interfaces the
modems with the carrier line.
At the remote end of the transmission facility 19, this transmission
facility is connected to a plurality of remote terminal modems 22, each
identical to the others in circuitry but each operating at different
transmitting and receiving frequencies. Modems 22 are connected to
subscriber station drop loops 23 through 30, themselves connected to
individual subscriber telephone station sets. A common power supply 31 is
operated either from the central office battery, supplied over
transmission facility 19, or from a locally derived power source and is
used to provide operating voltages for the electronic circuitry in modems
22.
The subscriber loop carrier system of FIG. 1 has been shown with eight
frequency-derived channels for the purpose of illustration only. A fewer
or greater number of channels can be accommodated simply by adding a
subtracting modems 18 and 22 and assigning carrier frequencies to
accommodate the number of channels actually used.
Similarly, the remote terminal modems have been illustrated as being
"lumped" in one location for illustrative purposes only. Alternatively,
each modem can be located in a different location convenient to the
subscriber it serves, and each modem provided with a separate power
supply.
The provision of more than one subscriber channel over transmission
facility 19 makes very efficient use of the transmission facility but
prevents the use of direct current signaling on the common transmission
facility 19 for all of the subscriber channels. It is therefore necessary
to accommodate such signaling in the carrier-derived channels themselves
and it is to this end that the present invention is directed.
Referring then to FIG. 2, there is shown a more detailed block diagram of
the central office modem 18 of FIG. 1. The central office modem is
connected to the central office by means of tip conductor 40 and ring
conductor 41 and comprises a hybrid transformer 42 to which conductors 40
and 41 are connected. A resistor 43 and normally-open relay contacts 44
are connected in series with the primary winding of transformer 42. A
capacitor 45 is connected in shunt with resistor 43 and contacts 44 to
provide an alternating current bypass across resistor 43 for audio
transmission. The secondary side of hybrid transformer 42 has a resistor
46 connected from its center tap to ground as a balancing impedance.
Audio voltages from tip conductor 40 and ring conductor 41 are developed
across the voltage divider comprising resistors 47 and 48 and are applied
to a compressor circuit 49. Compressor 49 compresses the amplitude range
of the audio signals supplied to its input so as to increase the
signal-to-noise ratio in the frequency-derived carrier channels. The
compressor 49, as well as the complementary expander 50, may be of the
form shown in R. Toumani U.S. Pat. No. 3,919,654, issued Nov. 11, 1975.
The output of compressor 49 is applied to a transmitter 51 to which a
carrier signal from oscillator 52 is also applied. The audio signals from
compressor 49 are amplitude modulated on the carrier frequency from
oscillator 52 and launched on transmission facility 19 via lead 33 and the
power supply 21 as shown in FIG. 1.
Audio modulated carrier signals from the remote terminal arrive on
transmission facility 19 and are supplied through the power supply 21 over
lead 32 to a receiver 53 which detects the audio signals and supplies the
audio signals to expander 50. Expander 50 restores the full amplitude
range of the audio signal and supplies the expanded signal to hybrid
transformer 42. These voice signals are then sent through conductors 40
and 41 to the central office facilities.
Central office control circuits 54 provide several control functions for
the central office modem of FIG. 2. Control circuits 54, for example,
monitor the signal on the central office side of hybrid transformer 42 by
way of lead 55 to detect 20 Hz ringing signals as well as to detect a test
voltage applied to these conductors, e.g., a large positive voltage,
exceeding the normal central office battery supply. This high voltage is
normally supplied to metallic subscriber loops in order to perform a
leakage test of the loop. Control circuits 54 respond to these central
office signals by impressing a supervisory tone of a preselected frequency
on the carrier signal in transmitter 51 via lead 80. In accordance with
one aspect of the illustrative embodiment of the present invention,
ringing signals are relayed to the remote terminal by interrupting the
supervisory tone at a ringing signal rate. The test voltage, on the other
hand, is relayed to the remote terminal by turning the supervisory tone on
and leaving it on for a period exceeding the 50-milliseconds ringing
period.
Control circuits 54 are also connected to receiver 53 via lead 82 to detect
the presence of the carrier signal from the remote terminal of the system.
In the presence of such a carrier signal, a relay is operated to close
relay contacts 44 and provide line closure information to the central
office. At the same time, control circuits 54 provide operating voltage to
compressor 49 and expander 50 to enable these units to respond to voice
signals in the channel. Control circuits 54 will be more fully described
in connection with FIG. 4.
In FIG. 3 there is shown a more detailed block diagram of the remote
terminal modems 22 (FIG. 1) which are connected to transmission facility
19 at a location remote from the central office. Transmission facility 19
is connected to a receiver 60 and a transmitter 61. Receiver 60 responds
to signals from transmitter 51 in FIG. 2, detecting the audio signal
modulated on the assigned carrier frequency and supplying these audio
signals to expander circuit 62. Expander 62 may be identical to expander
50 in FIG. 2 and restores the full amplitude range of the compressed audio
signals supplied to hybrid transformer 63. These audio signals can then be
supplied by tip conductor 64 and ring conductor 65 to the local telephone
subscriber station set.
Audio signals from the subscriber are supplied by way of hybrid transformer
63 to compressor circuit 66 which may be identical to compressor 49 in
FIG. 2. Again, compressor 66 compresses the amplitude range of the applied
audio signals and supplies this compressed audio signal to transmitter 61.
Transmitter 61 modulates the compressed audio signals onto a carrier
supplied by carrier oscillator 67 and launches the modulated signal on
common transmission facility 19.
Remote terminal control circuit 68 provides the necessary supervisory
functions at the remote terminal. First, in response to the supervisory
tone modulated on the received carrier and detected by receiver 60,
control circuit 68 discriminates between ringing signals (tone interrupted
at a 20 Hz rate) and a test signal (a supervisory tone continuously on).
In response to ringing signals, control circuit 68 supplies a locally
generated ringing signal to the ring conductor 65 of the local subscriber
drop. In response to the test signal, control circuit 68 enables
transmitter 61 to transmit the carrier signal back to the central office
location. This carrier signal can then be detected to indicate the
complete continuity of the derived subscriber channel provided by the
associated modems and the repeatered line.
Control circuit 68 also includes a line feed circuit for supplying talking
current to the local subscriber loop. That is, a current is supplied
through diode 69, ring conductor 65, the telephone set, tip conductor 64
and balancing impedance 70 to ground. Control circuit 68 also provides
switchhook detection by monitoring the current in the local subscriber
drop and detecting the presence of a direct current when the subscriber
goes off-hook. Control circuit 68 also provides ring trip detection by
recognizing the drop in ringing voltage that occurs on ring conductor 65
when the telephone set goes off-hook during ringing. This ring trip signal
is used to interrupt the locally generated ringing signals. Both the
switchhook detection signal and the ring trip detection signal are also
used to enable transmitter 61 to transmit a carrier signal back to the
central office terminal. This carrier signal indicates that the subscriber
has gone off-hook. Control circuit 68, by means of its switchhook
detector, is also responsive to dial pulses and likewise enables the
transmission of a carrier signal in synchronism with the dial pulses. As
noted in connection with FIG. 2, carrier signals received at the central
office modem results in the closure of relay contacts to complete a direct
current path between the tip and ring conductors of the central office
appearance. This relay then operates in synchronism with the switchhook or
the dial pulses of the telephone set. Remote terminal control circuit 68
is more fully described in connection with FIG. 5.
Referring to FIG. 4, there is shown a detailed block diagram of the central
office control circuits shown in block 54 in FIG. 2. The control circuit
of FIG. 4 comprises a ringing signal detector 75 and a heat voltage
detector 76 both of which are connected by way of lead 55 to the central
office side of the hybrid transformer 42. The outputs of detectors 75 and
76 are supplied to the inputs of OR gate 77 which, when enabled, operates
tone gate 78. Tone gate 78, when thus enabled, connects a tone source 79
to lead 80 connected to transmitter 51 in FIG. 2. As previously mentioned,
this tone is used to modulate the carrier frequency in the transmitter and
thereby transmit the supervisory tone to the remote terminal location. It
will be noted that tone source 79 (2 kHz in the illustrative embodiment)
can be used to supply supervisory signaling for all of the other central
office modems 18 shown in FIG. 1. The tone from source 79 is, of course,
modulated on different carrier frequencies in the various central office
modems.
The central office control circuits of FIG. 4 also include a loop closure
detector 81 which responds to detected carrier signals on lead 82 from
receiver 53 in FIG. 2 to detect the presence or absence of a received
carrier signal. As previously noted, this carrier signal is used for
supervisory signaling from the remote terminal. The output of detector 81
is applied to line closure relay 83 which, in turn, operates normally open
contacts 44 (FIG. 2) to provide the line closure information to the
central office. It should be noted that this line closure can be
continuous (indicating an off-hook condition) or it can be discontinuous
(indicating dial pulsing). It should also be noted that multifrequency
signaling tones from the remote subscribers are transmitted directly over
the carrier frequency channel, demodulated and transmitted to the central
office by way of hybrid transformer 42.
The output of loop closure detector 81 is also supplied to a power supply
gate 84 which connects a negative power supply 85 to lead 86 in order to
provide operating voltages to compressor 49 and expander 50 in FIG. 2. It
can thus be seen that transmitter 51 and receiver 53 in FIG. 2 are
continuously powered to permit supervisory signaling to the remote
terminal and automatic gain control in the intervening repeaters.
Compressor 49 and expander 50, however, are powered only when the
subscriber channel is actively in use for voice transmission.
In FIG. 5 there is shown a detailed block diagram of remote terminal
control circuits 68 of FIG. 3. For convenience, hybrid transformer 63 has
also been shown in FIG. 5 together with diode 69 and ring conductor 65.
Line feed circuit 90 provides a constant talking current (when the
telephone set is off-hook) to ring conductor 65 through diode 69 from
negative voltage source 91. This talking current is monitored by loop
current detector 92 and, then loop current is detected, detector 92
enables a power switch 96 by way of OR gate 94 and AND gate 95. When
operated, power switch 96 supplies operating voltage from source 97 to
transmitter 61 to enable the transmission of carrier signals to the
central office terminal. As previously discussed, this carrier signal
indicates a line closure at the remote subscriber location.
The output of receiver 60 in FIG. 3 is supplied to a 2 kHz tone detector
100 in FIG. 5 which detects the envelope of the 2 kHz tone signal
modulated on the received carrier frequency. This envelope, indicating
ringing when interrupted at a 20 Hz rate, and indicating a test condition
when continuous, is supplied to integrating circuit 101. Integrating
circuit 101 is utilized to delay recognition of ringing, ring trip, loop
current detection, and the loopback test in order to insure that they are
not implemented in response to spurious transient signals.
When the envelope of the 2 kHz signaling tone, as provided by 2 kHz
detector 100, is high, the output of circuit 101 slowly rises. For a fifty
percent duty cycle envelope, the output of circuit 101 rises to a level
higher than the threshold of ringing detector 106 but lower than the
threshold of loopback test detector 107. Hence, when ringing is being
signaled, the ringing detector 106 is activated after a relay due to the
integrating action of circuit 101. For a continuous output of the 2 kHz
detector 100, the threshold of the loopback test detector 107 is exceeded
and both the loopback test detector and the ringing detector 106 are
activated. Voltage limiter circuit 120 prevents the output of integrating
circuit 101 from reaching the loopback test detector threshold during
ringing. Hence, the loopback test cannot be falsely triggered during
ringing due to irregularities in the 20 Hz duty cycle.
Input signals at inputs D.sub.1 and D.sub.2 to integrating circuit 101
cause the output to decay. An input signal at D.sub.1 will cause a
relatively rapid decay of the output while an input at D.sub.2 will cause
a slower decay. The fast decay is implemented in response to a ring trip
signal provided by ring trip detector 102 and AND gate 103 when a ring
trip condition is sensed and at a time when the high voltage ringing
switch 111 is activated. The slower decay is implemented in response to a
loop current detection or a loopback test detection as indicated at the
output of OR gate 94 over lead 104.
The ringing detector 106 is a comparator with hysteresis. That is, when
activated by a voltage exceeding its threshold voltage, the threshold at
which detector 106 will turn off drops well below the turn-on threshold.
Hence its input must decay significantly before the ringing detector 106
turns off. This is the mechanism by which the ring trip delay and the loop
current detection during ringing delay are implemented.
The output of the ringing detector 106, in conjunction with the fifty
percent duty cycle envelope appearing at the output of the 2 kHz detector
100, form, through AND gate 109, the excitation for the low voltage
ringing switch 112. Circuit 112 provides the low voltage interval for the
ringing voltage appearing at the ring lead 65. The high voltage interval
of ringing voltage is provided by switch 111 switching to the ring voltage
110 in response to the output of AND gate 108. The 2 kHz envelope from
circuit 100 is differentiated by circuit 113. Positive transitions at the
input of differentiating circuit 113 cause positive pulses at the input to
pulse former 114 to which the pulse former responds. The output of the
pulse former 114, in conjunction with the output of the ringing detector
106, forms the inputs to the AND gate 108. In this manner, the high
voltage half-cycle of ringing is excited only on positive transitions of
the 2 kHz envelope which appear after the ringing detector 106 turn-on
delay. Hence, the high ringing voltage cannot be turned on during the
continuous 2 kHz signal appearing during the loopback test.
Ringing signals, then, are supplied to the local subscriber loop on lead 65
by gating a high negative voltage source 110 through high voltage ringing
switch 111 at a 20 Hz rate. A low voltage ringing switch 112 provides a
ground return path for the opposite half-cycles of ringing current and
thus connects ring conductor 65 to ground potential during the time that
ringing switch 111 is off. The rise and fall times of the ringing voltage
are controlled such that the audio components of the ringing signal on the
subscriber loop have sufficiently small frequency components in the audio
range that crosstalk on the pairs in the same cable is minimized.
Loopback test detector 107 turns on in the presence of a continuous
supervisory tone, indicating the requirement for conducting a continuity
test. Loopback test detector 107 responds to the high voltage level
appearing at the output of integrating circuit 101 in the presence of
uninterrupted supervisory tone to set a loopback flip-flop 116. Flip-flop
116 is reset following the termination of the continuous loopback tone
signal from detector 100. When in the set condition, flip-flop 116 enables
OR gate 94. OR gate 94, in turn, partially enables AND gate 95. AND gate
95, however, is not fully enabled since the inverted output of flip-flop
116 is supplied to the other input of AND gate 95. Power switch 96 is
therefore not enabled at this time. However, OR gate 94 being enabled
raises the D.sub.2 input to integrating circuit 101 to a high level via
lead 104. This causes the output of circuit 101 to decay below the
turn-off threshold of ringing detector 106. Hence, the ringing detector
turns off, preventing ringing from occurring during a loopback test.
The inverted output of flip-flop 116 is also supplied to a two-second timer
circuit 119 over lead 225. When the continuous 2 kHz tone disappears
during the performance of the loopback test, the output of the envelope
detector 100 goes low, causing the loopback flip-flop 116 to reset. The
inverting output of flip-flop 116 is thus switched high, activating the
two-second timer 119. The power switch 96 is then activated via OR gate 94
and AND gate 95 for the two-second interval. When activated, power switch
96 connects operating voltage to the transmitter 61 in FIG. 3. Transmitter
61 therefore transmits a carrier signal to the central office terminals
which is detected there and used to operate the loop closure relay
contacts 44. In this way, a successful completion of the continuity test
is indicated at the central office location by a loop closure for two
seconds following the continuity test request. This test request is
indicated by the sequence of a high voltage applied to and then removed
from the central office appearance of that subscriber channel.
In accordance with standard testing procedures for metallic loops, a
testing person at the local test desk in the central office can therefore
apply a high test voltage (higher than the normal office battery voltage
and of opposite polarity) to the subscriber appearance at the central
office. Also in accordance with standard metallic loop testing procedures,
the high voltage is removed and loop closure is monitored on the derived
subscriber channel in order to detect the return of a loop closure signal
to the central office appearance. This closure indicates the completion of
a successful continuity test. Thus, from the point of view of the testing
person at the local test desk, the testing of derived subscriber channels
in a carrier system in accordance with the present invention uses
procedures identical to the testing procedures for standard metallic
loops. This makes it unnecessary to provide special testing procedures for
the carrier-derived channels in the system of the present invention.
Referring more particularly to FIG. 6, there is shown a detailed circuit
diagram of the loop closure detector portion of the central office control
circuits shown in FIG. 4. The detection of a carrier signal in the
received channel for that subscriber is indicated by a signal on lead 82.
This carrier signal is envelope-detected by means of a filter comprising
capacitor 130 and resistors 131 and 132. This carrier envelope is supplied
to an amplifier comprising transistors 133 and 134. The output of
transistor 134 is applied to a delay circuit comprising resistors 135 and
136 and capacitor 137. Transistor 138 therefore does not respond to the
detected envelope until after a brief delay required to charge capacitor
137. This prevents the detector of FIG. 6 from inadvertently responding to
short noise bursts on the transmission line.
When transistor 138 does operate, transistor 139 is enabled through
resistor 140. Transistor 139 completes the operate path for K relay 141
through current limiting resistor 142. A diode 143 is connected across
relay 141 and resistor 142 to prevent the voltage transient occurring when
transistor 139 turns off from damaging the transistor.
The output of transistor 138 is also supplied through resistor 146 to the
base of transistor 144 to enable transistor 144 to connect the negative
voltage supply at its emitter to lead 145. Lead 145 supplies operating
voltage to the compressor 49 and expander 50 of FIG. 2, thus fully
enabling a talking path through the central office modem.
In FIG. 7 there is shown a detailed circuit diagram of the ringing and
loopback test control circuits shown in block form in FIG. 4. Thus lead 55
from the central office side of hybrid transformer 42 is connected through
resistor 150 and capacitor 151 to a 20 Hz ringing detector 75. This
ringing signal is supplied across a voltage divider comprising resistors
153 and 154, the midpoint of which is connected to the base of transistor
155. Diode 156 shunts the negative half-cycles of the ringing signal to
protect transistor 155 during the negative half-cycles. When operated,
transistor 155 discharges capacitor 157 (previously charged through
resistor 158). Thus, capacitor 157 is charged and discharged at the
2-second on, 4-second off ringing envelope rate. The voltage on capacitor
157 is supplied through resistor 159 to the base of transistor 160 to
disable transistor 160 at this ringing envelope rate.
The ringing signal is also supplied through capacitor 151 and resistor 162
to the base of transistor 161. Again, the negative half-cycles of this
ringing signal are shunted across the base emitter path of transistor 161
by way of diode 166. Transistor 161, when enabled by transistor 160 being
off, therefore operates at the 20 Hz ringing rate and serves to enable the
tone gate 78 at the ringing frequency rate, as will be described
hereinafter.
Signals on lead 55 are also connected through resistor 150 to a voltage
divider comprising diode 170, zener diode 171 and resistors 172 and 173. A
test voltage on lead 55, of the proper polarity to pass diode 170 and of
sufficient magnitude to break down zener diode 171, is impressed across
resistors 172 and 173. This voltage charges capacitor 174 and, when
capacitor 174 is sufficiently charged, operates transistor 175. Diode 176
protects the base-emitter path of transistor 175 against reverse voltage.
The output of transistor 175, developed across resistor 177, is supplied
to the base of transistor 178. Thus, when operated, the output of
transistor 178 indicates the presence of a loopback test voltage at the
central office appearance of sufficient duration to eliminate inadvertent
noise spikes. The output of transistor 178 is also used to operate tone
gate 78.
When either transistor 161 or transistor 178 is enabled, a voltage is
developed across the voltage divider comprising resistors 179 and 180. The
midpoint of these resistors is applied to the base of transistor 181 which
connects a positive voltage source 182 through resistor 183, tone gate 78,
resistor 184 and through the collector-emitter path of either transistor
161 or transistor 178 to negative potential source 185. Current flowing in
this path forward biases all of the diodes of tone gate 78, thus turning
tone gate 78 on and connecting tone source 79 to output lead 80 and thence
to transmitter 51. As previously noted, this tone is thereby modulated
onto the carrier for transmission to the remote terminal location.
In FIG. 8 there is shown a detailed circuit diagram of a portion of the
remote terminal control circuits shown as block 68 in FIG. 3 and in block
diagram form in FIG. 5. The control circuits of FIG. 8 include a line feed
circuit 90 which serves to provide a constant talking current to the local
subscriber loop through diode 69. The line feed circuit 90 comprises the
subject matter of a patent application of the present applicant, Ser. No.
974,386, filed of even data herewith, and will not be described in detail.
Line feed circuit 90 provides on lead 200 to loop current detector 92 a
signal proportional to the current that is being supplied to the local
subscriber drop. Lead 200 is connected through diode 201 to the base of
transistor 202 and turns transistor 202 on in the presence of loop
current. The collector of transistor 202 is connected to one input of a
long-tailed pair comprising transistors 203 and 204 having their emitter
electrodes connected together through a common emitter resistor 205. The
other input to the long-tailed pair at the base of transistor 204 is
connected to ground potential.
Transistor 202 remains in the OFF condition when the circuit is in its
quiescent state. The presence of loop current raises the voltage on lead
200 which raises the anode and cathode voltages of diode 201. Hence, the
base of transistor 202 rises with loop current. As it rises, a current is
developed in its collector-emitter path which is proportional to the loop
current. When the loop current reaches a threshold value, the collector
voltage of transistor 202 reaches ground potential and switches the
long-tailed pair. This produces the loop current detection indication at
the output of the loop current detector.
The voltage at the collector of transistor 204 is applied to OR gate 94
and, in particular, to the base of transistor 207. When transistor 204
turns on in response to loop current detection, transistor 207 is
activated. The voltage at the collector of transistor 207 operates
transistor 211, providing a positive voltage through resistor 212 to AND
gate 95 and then to power switch 96. When transistor 211 is operated,
transistor 220 of AND gate 95 is operated unless the input from pulse
former 116 to transistor 223 is low or the input from ring detector 106 is
high. Thus AND gate 95 prevents the power switch from turning on while the
loopback test is in progress or in response to spurious loop current
detections during ringing.
The output of AND gate 95 is applied to power switch 96 comprising
transistor 221. When operated, switch 96 connects negative voltage source
97 to output lead 222. As discussed in connection with FIG. 5, the voltage
on lead 222 comprises the powering source for transmitter 61 in FIG. 3.
In FIG. 9 there is shown detailed circuit diagrams of yet other portions of
remote terminal control circuits of FIG. 5. Thus, signals received from
the central office by receiver 60 (FIG. 3) are detected and supplied on
lead 230. These signals are applied to tone detector 100 which operates to
detect the envelope of the received signal. To this end, the signals on
lead 230 are applied to the base of transistor 231, the emitter of which
is connected to the base of transistor 232. The collector circuit of
transistor 232 includes an inductor 233 and a capacitor 234 tuned to the 2
kHz tone signal used for supervisory signals. In the presence of this
supervisory tone, transistor 235 is enabled to provide a charging path for
capacitor 236 from positive source 237. When the voltage on capacitor 236
builds up to a sufficient level, transistor 238 is enabled by way of the
voltage divider comprising resistors 239 and 240. Resistors 239 and 240
and capacitor 236 filter out the 2 kHz signal so that transistor 238 is
provided a constant activation during the time that the 2 kHz tone is on.
The collector of transistor 238 is connected to the base of transistor 244.
When operated, transistor 238 activates transistor 244 which acts as a
voltage supply gate. This serves as a voltage source to energize the
loopback detector 107 by way of lead 245, as the input to the
differentiator 113 by way of lead 246 and as one input to AND gate 109 by
way of lead 247.
The collector of transistor 238 is also connected by way of lead 249 to the
base of transistor 250 in integrating circuit 101. When thus energized,
transistor 250 provides a voltage across the voltage divider comprising
resistors 251, 252, 253 and 254. A capacitor 255 is connected across
resistor 254 and is charged by the voltage appearing across this resistor.
The signal on capacitor 255 serves as the input to loopback detector 107
and ringing detector 106 by way of lead 256 (see FIG. 5).
The charge on capacitor 255 can be removed and hence the signal removed
from lead 256 by several alternate discharging paths. In the first
instance, the termination of a tone signal from tone detector 100 on lead
249 disables transistor 250 and allows capacitor 255 to discharge through
resistor 254. This discharge is sufficiently slow to maintain a reasonably
constant voltage during the 20 Hz interruptions in the supervisory tone.
However, it is rapid enough to allow capacitor 235 to discharge completely
during the four-second silent interval of ringing.
A second discharge path for capacitor 255 is provided by way of transistor
257 through resistor 253. Transistor 257 is enabled by a signal on lead
216 from AND gate 103 (FIG. 11), indicating that the subscriber being rung
has gone off-hook. Resistor 253 is considerably smaller in value than
resistor 254 and therefore permits a very rapid discharge of capacitor 255
to terminate ringing. Ringing can therefore be rapidly terminated when the
called subscriber goes off-hook (ring trip).
Yet another discharge path for capacitor 255 is supplied by way of
transistor 258 along with resistors 252 and 253. Transistor 258 is enabled
by a signal on lead 104 from OR gate 94. This slower discharge path is
implemented when the subscriber goes off-hook during the silent interval
of ringing or during the low voltage intervals of the 20 Hz cycle. The
delay thus implemented prevents premature termination of ringing due to
ringing transients. However, after a suitable delay, ringing is terminated
due to an off-hook condition.
The voltage limiter circuit 120 prevents a loopback test from being
initiated once ringing has b | | |
