Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
ROUND START DETECTION IN TELEPHONE LOOP SIGNALLING CIRCUIT
The following commonly assigned patent application is
related to th-is application:
Title: DETECTION OF GPOUND OR LOOP START CONFIGURATION
IN TELEPHONE LOOP SIGNALLING CIRCUIT
Inventors: Puran Singh KASBIA and Yuan-Lu LI
Serial No. 451,894
Filed: in Canada on April 12, 1984
This invention relates to a loop signalling circuit for
replicating signalling functions on a telephone line and more particularly
to a substantially universal circuit which will function on either a loop
start or a ground start configuration o-F telephone equipment connected to
the line, even in the presence of relatively high induced power-line
voltages.
Background of the Invention
Loop signalling circuits have been developed in the past
two increase the loop operating range by providing a replication of
signalling functions particularly for long loop applications such as
when the remote terminal (e.g. a telephone set or a PBX-private branch
exchange) is more than about 5,000 meters from the central office. Such
loop signalling circuits employed numerous option switches to provide for
the difFerent operating arrangements of the telephone equipment. For
example, one switch was used to select either a loop s-tart or a ground
start mode of operation. Another switch was used to change the ringing
detector response to provide for various duration ringing waveforms. Yet
another switch was used to enable the detection of ringing signals from
an ungrounded source at the central office. These loop signalling
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~L2~ 3
circuits were also inserted in series with the telephone lines to
isolate some of the interference signals such as longitudinally induced
power-line signals, and to boost and repeat the wanted signalling functions.
It is therefore desirable to have a sinyle loop signalling
circuit in which the signalling functions on the central office side of
the telephone line are replicated on the remote terminal side and
vice versa, regardless of the operating configuration (e.g. loop or
ground start) of the telephone equipment. Such a circuit must provide
all signalling functions. These include loop closure normally provided
by a hookswitch, pulse dialling provided by a rotary switch, and ringing
detection provided by the ringer at the remote terminal; and in addition,
ring-ground detection, loop current detection, and dial pulse detection
at the central office. In alternate applications such as with emergency
telephones, ground start operation at the remote terminal and detection
at the central office must also be replicated. Voice and data message
signals are generally routed around the signalling circuit via a separate
d-c isolated path.
A problem which is encountered in a circuit that operates
under both loop start and ground start conditions, is the erroneous
detection of a ground start condition in the presence of induced 60Hz
power line voltages. This problem is particularly acute when the
circuit must function with both short and long loops that result in
widely differing steady state current conditions.
Statement of the Invention
It has been discovered that by utilizing two line current
detectors, one driven via a low pass filter, to control a D-type flip-flop
or latch which in turn applies a ground start condition to the line, that
the ground start condition can be accurately replicated.
Thus, in accordance with the present invention, there is
provided a loop signalling circuit capable of operating in the presence of
spurious signals for replica-ting signalling functions on a telephone line
serially connected between a central office and a remote terminal operating
in either a loop start or a ground start con-Figuration. The circuit
comprises a low-pass filter means for passing signals from the remote
terminal below the frequency of said spurious signals; a first detector
means for detecting when the line current on the remote terminal side of
the telephone line exceeds a preselected value, and a second detector
means for detecting signals from the low-pass filter means when the line
current exceeds the preselected value. In addition, the circuit includes
a clock generator which is synchronized on the occurrence that the first
detector means detects current in the line that exceeds said preselected
value, a latch means set to the output state of the second detector means
on each occurrence of a pulse signal from the clock generator, and a
control means responsive to the latch means for applying a ground start
impedance on the central office side of the telephone line.
Brie-F Description of the Drawings
An example embodimen-t of the invention will now be
described with reference to the accompanying drawings in which, combined:
Figures 1 and 2 illustrate a block and schematic circuit
diagram of a loop signalling circuit connected in series with the tip
and ring of a telephone line between a remote terminal and a central ofFice.
Description of the Preferred Embodiment
Referring to Figures 1 and 2, the schematic illustrates
input circuitry at the central office, generally 10, with a standard
ground start configuration. In this configuration9 the tip lead T
remains ungrounded due to the open contacts 11 during an idle state
while the -48 volt battery is applied to the ring lead R via the ringing
transfer contacts 13. The balance of the components at the central
office 10 are well known and will only be referred to as required to
understand the operation of the loop signalling circuit. Llkewise, a
ground start configuration is illustrated at a manually operated remote
telephone terminal, generally 20, which shows the normally open ground
start contacts 21, and the normally open hookswitch contacts 22 which
are in series with the representative impedance 23 of the telephone 20.
In a loop start configuration, a jumper 15 is placed
across contacts 11 in the office 10, while contacts 21 would either be
not present or remain open-circuited at all tires during the operation
of the remote terminal 20. As is well known, the manually actuated
ground start terminal 20 could be replaced by a loop start private line
set or a ground start PBX.
As discussed previously, the present invention is directed
to a loop signalling circuit. voice and data message signals carried by
the telephone line are routed around the circuit via a d-c isolated path.
This is illustrated in simplified form by a two-way amplifier circuit
including d-c isolating capacitors 30 and 31, transformers 32 and 33,
and amplifiers 34 and 35. The gain of the amplifiers 34 and 35 can be
reduced by actuation of the normally open contacts MA-l of the maximum
attenuation relay MA as described hereinafter.
The central office side of the loop signalling circuit
has a high impedance bias source generally 39, comprising a -48 volt
battery feeding the tip and ring leads through current limiting
-- 4 --
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resistors 40 and 41 respectively. A high impedance monitoring network
is also connected from each lead of the line to ground through
isolating resistors 42, 43, 44, and 45. The monitoring network on the
central office side of the line includes a lOOHz low-pass filter 47
feeding a d-c detector 50 (the output oF which goes high whenever the
difference in voltage between the tip and ring leads is greater than
30 volts), a spurious signal (e.g. 60Hz power line) detector 51, and a
ringing (e.g. 20Hz) signal detector 52, each of which monitors the signal
between the junction of the resistors 42, 43~ and the junction of
resistors 44, 45.
GROUND START IDLE STATE
In the ground start idle circuit state (i.e. a floating
tip lead due to the open contacts 11), the d-c voltage on the tip lead
supplied by the bias source 39 is substantially the same as the -48 volt
battery voltage supplied by the central office 10. This condition results
in a low at the output of the >30 volt detector 50 which inhibits the AND
gate 55 and in turn the output o-f AND gate 90 and OR gate 91 so that the
latter's output is in a low state and the tip ground relay TG is unactuated.
Should a strongly induced spurious (e.g. 60Hz power line) signal be
present, the voltage difference between the essentially a-c grounded ring
lead and the floating tip lead Jay periodically exceed 30 volts during
a portion of each 60Hz cycle. As a result the output of the detector 50
will alternate between a high and a low state. During such a condition
the 60Hz detector 51 goes high thereby inhibiting the AND gate 55
through its inverted input. This prevents false operation in a ground
start condition by ensuring the relay TG remains unactuated. As a
result -the normally open contacts TG-l stay open thereby replicating
-- 5 --
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2~
a floating tip condition on the terminal side of the loop signalling
circuit.
ROUND START RINGING STATE
.
During the ground start ringing state, a 20Hz intermittent
ringing generator 14 is connected to the telephone line at the central
office 10 upon actuation of the ringing transfer contacts 13 in a well
known manner. The resultant ringing signal is detected across the tip
and ring by the 20Hz detector 52 which causes the output of AND gate 56
to go high. Concurrently, the ground applied to the tip lead via
transfer contacts 13 is detected by the >30 volt detector 50 which
causes the output of AND gate 55 to go high as well. Chile the
detector 52 is shown as a 20Hz detector which is the standard ringing
signal frequency in North America, it may have a detection range of
16 to 33Hz so as to cover all ringing signal frequencies. Likewise,
while the detector 51 is designated as a 60Hz detector which is the
standard power line frequency in North America and hence the greatest
source of interference on this continent, it preferably detects the
entire range of spurious signal frequencies both below and above those
of the ringing signals. The purpose of this detector 51 is to insure
power-line induced signals, particularly on the open-circuited tip side
of the line, which occur during a ground start condition, are not
erroneously detected by the >30 volt d-c detector as indicating a loop
start condition. In addition the detector 51 controls an inverted
input to AND gate 56, to insure that any leaka~le of a strong 60Hz power
line signal through the 20Hz detector 52 during a ground start idle state,
does not allow the output of the gate 56 to go high.
-- 6 --
If, as explained hereinafter, the inverted input to
AND gate 57 is low and the output of OR gate 91 is high (indicating
a tip ground condition at the central office 10), the output of AND
gate 57 is driven high by the output of AND gate 56 thereby actuating
the ringing control relay RC. This causes the ringing transfer contacts
RC-l to actuate thereby applying a 20Hz ringing signal from a ringing
signal generator 58 to the ring side of the line. Concurrently, the
TG relay operates closing contacts TG-1 thereby applying a relatively
low impedance or virtual ground to the tip side of the line. The ringing
signal generator 58 continues to be periodically applied to the terminal
side of the circuit substantially in unison with the application of ringing
signals from the generator 14 at the central office 10.
GROUND START INITIATION AND ANSWER STATE
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During a ground start, switch contacts 21 at the terminal 20
are momentarily closed which causes a current to flow from the -48 volt
source in the loop signalling circuit through the ring lead to ground.
This is detected by a voltage differential developed across a bridging
circuit comprising high impedance resistors 60, 61, 62, and 63 due to
an imbalance generated across the two line terminating resistors 64 and 65
resulting from the line current flowing therethrough. This voltage
differential is passed through a lOOHz low-pass filter 66, a lOHz low-pass
filter 67, to a >15mA detector 68 which in essence detects the presence
of a d-c voltage resulting from a line current of >15mA through the
terminating resistor 65. The output of the detector 68 goes high in
response to the detected current in the line to set a D-type flip-flop 71
at about 90ms intervals whenever a >15mA signal through the resistor 65
is detected directly by a second detector 70. Initially, a free-running
clock 69 generates a square-wave signal at about a 5.5Hz rate with a
180ms period, as long as the output of the detector 70 is low. If the
output of the detector 70 goes high during the half cycle that the
output of the clock 69 is low, the latter is resynchronized immediately
and its output then goes high for one complete half cycle, i.e. 90ms.
Whenever the clock 69 output goes low during the other half cycle, it
immediately goes to, or returns to a high state if the output of the
detector 70 has become or remains high. This produces a series of short
pulses which continues to clock the flip-flop 71 at the 90ms rate. When
10 the detector 70 output goes low for a sustained period, the clock 69 will
again return to its free-running state. The high output signal from the
D-type ~lip-flop 71 causes the output of AND gate 72 to go high providing
both the output of AND gate 55 and the output oF a 125ms single-shot
rnul tivibrator 73 are both low. This in turn activates the ground return
relay GR which actuates contacts GR-l and places a ground start
resistance 49 in the ring lead of the central office side of the circuit.
The purpose of the filter 67, the detectors 68 and 70,
the clock 69, the D-type flip-flop 71, and the AND gate 72, is to ensure
a ground start condition is not erroneously detected in the presence of
20 relatively large induced power-line voltages or other spurious signals.
When the loop on the remote terminal side if short, the loop current,
under either ground or 1Oop start conditions, will be relatively high
and will be substantially in step with actuation of the switches 21 or 22.
With longer loops, the steady state current will be much lower and there
will be a lag in reaching this state due to the relatively long time
constant encountered in the loop. In addition, during an idle ground
start state, with a floating tip lead, relatively large induced 60Hz
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power line signals can result in the peak loop current exceeding 15mA.
In order to ensure that this does not initiate a yround start condition,
the incoming signal is sirnultaneously coupled directly to the detector 70
and through the low pass filter 67 to the detector 68. The fast-acting
detector 70 resets a 7Hz clock 69 whenever the current initially
exceeds 15mA. Thus, the clock generator 69 provides a controlled
delay of about 150ms in the operation of the D-type flip-flop after
current begins or ceases to flow in the terminal loop. This ensures
that the D-type flip-flop 71 is not set unless the incoming current
exceeds 15mA for a continuous period greater than 150ms, thus effectively
eliminating any false ground-start state.
The function of the AND gate 72 is to ensure that the
ground return relay GR can operate only when the tip lead on the central
office side of the circuit is floating. Whenever ground is not present
on the central office side of the circuit, the output of gate 55 goes
low thereby enabling an inverted input to AND gate 72. Due to the slow
discharge of the terminal side of the loop, this may well result in the
output of the D-type flip-flop 71 momentarily remaining in a high state
so that a spurious ground start condition is indicated by operation of
20 relay GR. This is prevented by delaying the operation of AND gate 72
for an additional 125ms via the multivibrator 73 which drives a second
inverted input to gate 72. The single-shot multivibrator 73 in turn
is triggered by AND gate 74 whenever the output of both OR gate 91
(indicating a ground is being applied at the central office 10), and
detector 70, go high. The fast-acting output from the detector 70
ensures an immediate response whenever current is interrupted on the
terminal loop, such as during dial pulsing. The 125ms delay circuit 73
g
is used to ensure that the maximum attenuation relay MA, the yround
return relay Go, and the build-out resistance relay 30R do not follow
dial pulses which temporarily interrupt current flow in the loop. As
a result, maximum attenuation in the amplifiers 34, 35, is maintained
throughout the dial pulse interval, while the relays GR and BOR are
inhibited by the signal from the delay circuit 73 at the inverted inputs
to the gates 72 and 81.
The output of the AND gate 74 actuates the line pulse
relay which in turn closes the contacts LP-l and places a line termination
resistor 46 across the tip and ring leads on the central office side of
the loop signalling circuit. When the output of AND gate 74 goes high
first one and then, after a 125ms delay, the other input to NAND gate
76 goes high, causing its output to go low which releases the maximum
attenuation relay MA. This in turn opens the contacts MA-l which raises
the gain of the amplifiers 34 and 35. Immediately upon actuation of
dial pulses or when the receiver 20 is returned to an on-hook state,
the output of AND gate 74 goes low which causes the output of NAND gate 76
to go high. This in turn actuates the relay MA and causes the gain of the
amplifiers 34 and 35 to be attenuated. Between dial pulses the output of
AND gate 74 again goes high thereby actuating the line pulse relay LP.
However the delay circuit 73 momentarily prevents the NAND gate 76 from
following it so that the relay MA remains actuated and the gain of the
amplifiers 34 and 35 reduced throughout the dialling interval.
In addition, the output of the AND gate 74 directly
controls an inverted input to AND gate 57 through a 750ms delay circuit 77.
The delay circuit 77 functions only when the loop, on the termination
side of the loop signalling circuit, goes from an on-hook to an
-- 10 --
off-hook state. Its purpose is to prevent the loop signalling circuit
from entering an off-hook re-ring state between the time it senses a
ring-tripped condition at the terminal 20~ and the time the central
office 10 (which is still applying ringing) senses the same condition
resulting from closure of the relay LP which places the resistor 46
across the central office side of the line. During this interval,
which can be anywhere from l60 to 500ms, ringing would continue to be
received at the terminal 20 were it not for the delay circuit 77 which
disables the ringing control relay RC for a period oF 750ms immediately
upon detection of current in the loop.
In addition, the detection of a current >50mA at the
output of the filter 66 is coupled through level detector 92 which sets
a D-type flip-flop 80 upon beiny clocked from a 120Hz clock generator 93
via AND gate 81 in the absence of an output from the delay circuit 73.
This in turn operates the build-out resistor relay BOR which in turn
opens the contacts BOR-l and BOR-2 thereby placing the current limiting
resistors 82 and 83 in series with the terminal side of the telephone loop.
The loop current then drops to between 30 and 36mA and remains in this
region. However should the loop current for any reason drop below 23mA,
it is sensed by the level detector 92 and causes the output of the
D-type flip-flop to return to a low state when next clocked by the 120Hz
clock generator 93. The purpose of the generator 93 is to provide a
delay to prevent the relay BOR from following the current interruptions
caused by dial pulses.
The state of the relay BOR is determined by that of the
flip-flop 80 when clocked at l20Hz intervals. During the dial-pulsing
state, the input to the flip-flop 80 will alternate between high and
low in unison with the pulses. To prevent the relay BOR from changing
state during this interval the clock input is inhibited by the signal
from the delay circuit 73 which drives the inverted input of AND gate 81.
By removing the 120Hz clock the flip-Flop 80 and hence the relay BOR
remains in the correct state until the dial pulsing is completed.
Once the line termination resistor 46 is placed across
the line, the voltage between the tip and ring leads can fall to only
a few volts due to the resistance of the line from the central office.
Hence concurrent with the actuation of relay LP which places resistor 46
across the line, the AND gate 90 is disabled while AND gate 94 enabled.
The voltage across the termination resistor 46 is passed through a lOOHz
low-pass filter 95 and depending upon the normal or reverse polarity oF
the applied current from the central oFfice lO, is detected by one of
the >15mA detectors 96 or 97 respectively. Current detection by either
of these detectors 96, 97, causes their respective outputs to go high
which in turn is coupled through an OR gate 98, AND gate 9~, and
OR gate 91 to maintain a grounded tip condition by continued actuation
of relay TG. Should a reverse battery condition be initiated at the
central office lO by actuation of contacts 12 and be detected by the
detector 97, it causes the reverse battery relay RB to operate through
AND gate 99. This in turn operates the reverse battery contacts RB-l
and RB-2 on the terminal side of the circuit. Reverse battery is applied
from the central office lO to provide answer or toll diversion signalling
to the terminal 20 for a telephone or a PBX installation. AND gate 99
has an inverted input from gate 56 which prevents the reverse battery
relay RB from being erroneously actuated during the ringing cycle when
the tips of thP 20Hz ringing signal can cause a momentary reverse current
to flow in the central office side of the loop.
- 12 -
LOOP START IDLE STATE
In the loop start idle state, the jumper 15 is placed
across contacts 11 at the central office 10 thereby continuously
applying a ground to the tip side of the telephone line. This is
continuously detected by the >30 volt detector 50 so that the output
of AND gate 55 remains high thereby keeping the tip ground relay TG
operated which maintains the TG-l contacts closed at all times. The
low impedance to ground on the tip side keeps any induced power 1ine
voltages at a much lower value than may be experienced in a ground
start installation when the tip is in an open circuit condition during
the idle state. As a result, the output of the 60Hz detector 51 remains
low thereby allowing either or both AND gates 55 and 56 to operate.
LOOP START RINGING STATE
The ringing signal conditions on the telephone line are
very similar in both the loop start and ground start conditions since
in both configurations the tip leac' on both the central office and
terminal side of the signalling circuit is maintained in a ground
condition by the actuation of the ringing transfer contacts 13 and the
continuous operation of the tip ground relay TG.
_OP START INITIATION AND ANSWER STATE
A major difference occurs when a call is initiated
and the remote terminal 20 goes into an off-hook condition thereby
closing contacts 22. This condition is detected by a current flow
through resistors 64 and 65 resulting in the output of the >15mA
detectors 68 and 70 going high. As a result this drives the output of
flip-flop 71 high which indirectly causes the contacts of the line pulse
relay LP to close thereby placing the line termination resistor 46
- 13 -
across the central office side of the circuit. However, because the
output of AND gate 55 is held hiyh, the AND gate 72 is inhibited from
going high and the ground relay GR remains unactuated. This prevents a
ground start condition from being applied to the central office side of
the circuit by actuation of ground contacts GR-l which would then connect
the ground resistance 49 to the ring sicle of the line therehy simulating
the ground start onciditons at the remote terminal 20. Pulse dialling
under loop start conditions is essentially the same as in the ground
start case with the line pulse relay LP following individual dial pulses
from the terminal 20.
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