Note: Descriptions are shown in the official language in which they were submitted.
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RING TRIP DETECTION SYSTEM
I:)ESCRI PTION
The present invention relates ~o telephony, and
particularly to a ring trip detection system which con-
trols the application of ringing signals to the ringers
-~ of telephone sets, causing the disconnection of the
ringing signals when the telephone set to which ringing
, is applièd goes off-hook.
A ring trip detection system provided by the
invention is especially adapted for use in the
ground/loop start line interface of a telephone system
where a telephone set or sets is connected to a line
circuit via the ring and tip leads of the loop.
Every automatic telephone system has a ring
trip detection circuit which causes the ringing signal
generator to be disconne~ted from the telephone set when
the set goes off~hook~ There are circuits which respond
to the change in duty cycle of the ringing current which
use a differential relay. The relay makes and breaks
contacts over different intervals when the telephone set
is on~hook during ringing and when it goes off-hook. A
problem in ring trip detection which is prevalent with
known circuits is that they are application dependent in
that they are designed for a specific application, such
as where a specific ringing frequency or specific
voltage levels are used. Ringing frequencies may range
from fifteen to seventy Hertz (Hz). In party line
systems several ringing frequencies may be usedt each
: for a different party on the line. Filters have be~n
used to accommodate different ringing frequencies and
some frequency variation. The use of such filters makes
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the ring trip detection circuit in which the ~ilters are
included application dependent. Further disadvantage
may arise in that the filter~ increase the time required
for a ring trip to bq detected.
In practical telephone systems the voltages
which are applied to the loop (e.g., thq battery feed
-~ voltage) may vary at the point where such voltages are
tapped for ring trip detection purposes. The voltages
vary with the leng~h of the loop9 the num~er of tele-
p~one sets in the loop as well as with environmental
effects, Both the battery feed and the ringing signal
voltages thus may vary in different applications. These
voltages also vary with transients which may be picked
up on the loop. In prior ring trip detection systems
which depend on the change in duty cycle of the ringing
signal, such voltage variations~ especially when
variable with the loop length and telephone sets con-
nected in the loop -- load dependence -- may result in
erroneous ring trip detection. Erroneous ring trip
detection may also be caused by leakage resistance
between the tip and ring leads. In some prior art
systems the problem is exacerbated by the use of
voltages for ring trip detection purposes tapped from
. tn~ return path (tip lead), after voltage drop in the
loop has occurred. The above mentioned ring trip system
also could not be used in certain party-line applica-
tions using ground connected ringers which would bypass
the detection means. Changes in ringing frequency may
also cause erroneous ring trips.
It is a feature of th~ present invention to
provide an improved ring trip detection system which is
adapted for use in many telephone systems and is not
application dependent. The detection system provided by
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the invention has a further feature of being responsive
to ring trip conditions regardless of the frequency of
the rin~ing signal, whether the ringing signal be 15 ~z,
70 Hz or inbetween these frequencies. Another feature
of the invention is to provide reoperation of the ring-
ing xelay to reconnect the ringing signal to the tele-
phone sets even if a ring trip is detected should there
not be a true off-hook condition. In the event of a
ringin~ circuit failure9 the ringing generator may short
to ground ~a ring short condition)~ It is a feature of
the ring trip detection system provided by the invention
to disconnect the ringing generator from ground, if a
ring short condition occurs; thus reducing the possibil-
ity of permanent damage to an expensive telephone system
component. In a ring trip detection system provided by
the invention, the ringing supply (ring lead) is tapped
for sensing voltages for ring trip detection purposes,
rather than the return path (tip lead) where the voltage
sensed may be load dependent.
It is the object of the present invention to
provide an improved ring trip detection system in which
~; the aforementioned deficiencies of prior ring trip
detection techniques are substan ially obviated and the
features stated above are obtainedc
Briefly described, a ring trip detection system
embodying the invention is operative to detect a ring
trip condition when the telephone set having a ringer to
which ringing signals are applied goes off-hook. The
system uses an up-down counter. Means responsive to the
duty cycle of the ringing signals with respect to a
reference voltage level condition the counter into an
underflow state when the duty cycle does not exceed
50%. The duty cycle of the signal is the ratio of the
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duration of the signal when it is of one polarity with
respect to the reference voltage level to the period of
an entire cycle of the signal. A 50% duty cycle thus
- defines a signal having portions of equal duration dur-
ing a cycle above and below the reference level.
Preferably, a system embodying the invention u~ilizes a
comparator to which the ringing signal which is super-
imposed on battery feed vol age is applied to one input
and a reference voltage to the o~her input. Inasmuch as
the ba tery feed voltage is attenuated upon occurrence
of an off-hook condition, the duty cycle of the ringing
signal with respect to the reference voltage is always
equal to or less than 50% when the telephone set is
on-hook (there being no drops in voltage due to the
resistance of ~he receiver and transmitter of the tele-
phon~ set)~ When the telephone set goes off-hook the
drops shift the d.c. voltage level of the ringing
signal, and therefore the duty cycle of the ringing
signal with respect to th~ reference voltage is always
greater than 50%. The output of the comparator has a
~i duty cycle corresponding to the duty cycle of the ring-
ing signal wi h respect to the reference voltage and may
be used to control up and down counting in the counter.
By resetting the counter at the beginning of each cycle
of the ringing signal, an underflow condition must occur
when the duty cycle is equal to or less than 50%, but
cannot occur if the duty cycle is greater than 50~.
Means responsive to the absence of the underflow state
of the counter provides the ring trip detection output.
This output may be used to control the connection of the
ring generator to the telephone set ringers, for
example, by deactuation of a ringing relay.
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The foregoing and other objects, features and
advantages of the invention as well as a presently pre-
ferred embodiment thereof, will become more apparent
from a reading of the following description in connec-
tion with the accompanying drawings in which:
- FIG. 1 is a schematic diagram of the portion of
the ring trip detection sys~em provided in accordance
with a presently preferred embodiment of the invention
which senses the duty cycle of the ringing signal with
respect to a fixed reference voltage and provides an
output corresponding thereto;
FIG. 2 is a block diagram illustrating the por-
tion of the system which responds to the output provided
by the system portion illustrated in FIG. 1 and detects
ring trip conditions to enable disconnection of a ring-
ing generator from the loop;
FIG. 3 is a wave form diagram which is explana-
tory of the operation of the portion of the system shown
in FIG. l; and
FIG. 4 is a wave form diagram explanatory of
: the portion of the system shown in FIG. 2.
Referring to FIGS. 1 and 3 there is shown a
.~. line circuit 10 with its ground/loop start line inter-
face to a telephone set (TS) having a receiver and
transmitter 12 and a ringer (RNG) 14. A hook switch 16
is shown open in ~he on-hook condition of ~he telephone
set. The ringer is connected with the telephone set and
its hook switch through a capacitor 18 across the ring
(ringing supply) and tip (return) leads of t'ne loop.
3 Battery feed voltage (-48 volts) is applied to the ring
lead via the line circuit 10. A ringing generator 20 is
connected to the ring lead by way of a resistor ~.
C-form contacts R~-l and RR-2 connect the ringing
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generator 20 and ground (GND) to the ring and tip leads,
respectively. These contacts also connect the ring and
tip leads to the line circuit. The contacts to the line
circuit 10 are normally made or closed when the ringing
relay is not actuated. When the ringing relay is
actuated, the normally open or broken leads connect th~
ringing generator 20 and ground to the riny and tip
leads. The winding of the ringiny relay (RR WDG) 22 is
operated depending upon ring trip detection by the
portion o~ the system shown in FIG. 2.
The line circuit includes a calling bridge
which provides an output (CB) ~o the switching logic o~
the telephone system in the presence of direct current
flow through the loop, as occurs when the telephone set
12 is off-hook; when the normally closed contacts of the
ringing relay connect the tip and ring leads to the line
circuit. This calling bridge output is also used to
prevent reopera~ion of the ringing relay in the event
t.hat a ring trip is detected, unless the telepho set
12 is on-hook. The volta~e across the resistor ~ wi~h
the telephone set 12 on-hook is shown in FIG. 3~ wave
form (a). The ringing signal is essentially sinusoidal
with its base line at the battery feed voltage, -48
volts. When the telephone set 12 go~sJoff-hook the drop
in the voltage across the resistance ~ presented by the
transmitter and receiver to the direct current there-
through, which does not pass through the ringer circuit
due to the capacitor 18~increases ~closer to ground).
Depending upon the loop resistance, which may vary, for
3 ~xample, from 100 to 2400 O~MS, the base line of the
ringing signal, which is also attenuated due to the
load, may, for example, drop from -4B to -24 volts, as
shown in wave form ~b) of FIG. 3.
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~ his ringiny signal is applied via resistors 24
and 26 to an input o~ a comparator 2~. A zener diode 30
is connected in parallel with ~he ringing generator and
biased at the battery feed voltage (-~8 volts). Suit-
ably the breakdown voltage of this zener diode is 27volts so as to protect the input circuits of the com-
parator 28. In effect, the zener diode 30 clamps the
ringing signal within a 27 volt range as indicated by
the dash horizontal lines 32 and 34 in wave forms (a)
and ~b) of FIG. 3. The comparator responds to the
crossings of the ringing signal at the reference voltage
,f level (VR~F), Accordingly, the clamping of the input
signal to the comparator by the zener diode 30 does not
affect response of the comparator to the duty cycle of
the ringing sig~al with respect to the reference voltage
level.
The reference voltage is derived from the -48
volt supply by a voltage divider network including three
resistors 36, 38 and 40 connected between -48 volts and
ground. A source ~ollower ~ield effect transistor 42,
connected between the junction of the divider resistors
38 and 40 and to ground via another resistor 41, pro-
vid s operating voltage for the comparator from the -48
; volts supply. The reference voltage and the supply
voltage to the comparator thus vary with supply voltage
and compensate for supply voltage variations~
The output of the comparator 28 is coupled
through an optical coupler 44 to provide the ring detect
(~D) output. The current flows through the light emit
ting diode of the optical coupler when the comparator
output is low, and the RD output is taken across the
collector r~sistor of the phototransistor in the optical
coupler. Accordingly, the RD output is a square wave
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form having the duty cycle o the ringing si~nal with
~spect to the reference voltage. Th~ ~D wav~ form is
shown in FIG. 4.
It will be noted ~hat the reference voltage is
closer to ground than the battery feed voltage as shown
in wave forms ~a) and (b) of FIG. 3. When the ringing
signal crosses the reference voltage base line in a
positive direction, ~he comparator output voltagq goes
high. Conversely, upon crossing the re~erenc~ voltage
base line in the negative direction, the comparator
~r~ output goes low. Inasmuch as the battery feed voltage
~,9 ,~`' as detected across the resistor ~ drops when the tele-
phone set 12 goes off-hook ~the hook switch 16 then
closing), the duty cycle of the ringing signal with
respect to the referènce voltage level is equal to or
less than 50% during on-hook conditions and must be
~reater than 50% when the telephone set goes off-hook.
The ring detect wave form for less than 50% duty cycle
in the on-hook case shown in wave form (a) of FIG. 3 is
illustrated in wave form (c) of FIG. 3, while the
greater than 50~ duty cycle for the off-hook case shown
~ wave form (b) is illustrated in wave form (d) of
;~ ~IG. 3.
It will be noted that the sensing circuit pro-
vided by the comparator is connected directly to the
ring lead so that it is not adversely affected by dif-
~feren~ loadings due to di~ferent loop leng~hs. The
reference voltage is selected considering any effects
caused by leakage resistance across the loop. Accord-
ingly, it should be as close to battery feed voltage aspracticable to detect the duty cycle variations. The
resistors 36, 38 and 40 in the reference voltage supply
circuit are also desirably precision resistors.
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Referring more particularly to FIG5. 2 and 4,
there is shown a clock generator 46 which may be an
oscillator providing a train of pulses ~CG)~ In a
system suitable for ring trip detection, with ringing
signals from 15 ~z to 70 Hz, the CG pulses may be at a
14 KHz rate. These pulses are divided by two in a
flip-flop 48 to provide complementary clock pulse trains
; (CLK) and (CLK). The CLK pulses are used to sample and
reclock the RD output in a D-type flip-flop 50. This
reclocking synchronizes operation of the system to the
CLK pulses. The output of the flip-flop is the up-down
control level for an up-down counter 52. The counter
may be implemented by a pair of 4-bit counters. The
`' ripple carry output of the first counter is connected to
the enable input of the second such that when the first
counter overflows the second counter begins counting in
the direction determined by the level of the reclocked
RD output which is applied to its U/D input. The
up-down counter 52 is shown as a single counter for
; 20 purposes of simplifying the illustration. The up-down
counter 52 counts the CLK pulses which will be at a rate
of 7 KHz when the clock generator pulses are at 14 KHz.
'! The use of two counters provides an 8-bit Scounler which
is capable of counting 512 bits per~cyc~e. This capac-
~, 25 ity is selected giving consideration for the resolution
of timing to ~ffect the ring trip detection operation
and also for the lowest ringing frequency which is
expected to be encountered. An 8-bit counter in con-
junction with a clock frequency of 7 KHz has been found
3 suitable for use, even with low ringing frequencies of
15 Hz~ The CG, CLK and CLK pulses, the RD level~ the
U/D level from the reclocking flip-flop 50 and the
counting periods in the up and down direction (count
U/D) are all illustrated in FIG. 4.
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The duty cycle of ~he ringing signal wi~h
respect to the reference as represented by the RD output
causes the counter 52 to overflow in the negativP direc-
tion if the duty cycle is equal ~o or less than 50%.
Then the ripple carry (RC~ output of the counter and the
lower order bit, QA, thereof will be low representing
the presence of an overflow condition in the negative
direction or a net negative count. This condition is
detected by logic including a count out gate 54. This
gate 54 is an OR gate illustrated as a negative logic
J AND gate to denote i~s function. The CO output will be
low at the end of the ringing cycle and will go high
when the next ~ pulse reaches the input of the
counter S2; the U/D input already having gone high upon
the next cycle of the ringiny signal. The RC output
goes high immediately upon the up/down going high at the
onset of the next ringing signal cycle. To prevent race
conditions in the system, another D flip-flop 56 is
included in the logic to delay the CO output 1/2 cycle
~0 of the CG clock rate.
The delayed CO output is indicated at HO. This
;; output is stored in a ring trip latch flip-flop 58 which
is also a D flip-flop. This flip-flop is clocked by an
AND Gate 60, illustrated as a negative logic OR gate to
denote its function. On the occurrence of the positive
going edges of the U/D and RC outputs, the ring trip
latch 58 will be clocked. The HO output is then low
(see FIG. 4) Accordingly, a low output is applied to
an OR gate 62, illustrated as a negative logic AND gate
to denote is function. Inasmuch as ringing is still
commanded by the telephone system, the RNG signal which
is applied to an input of that OR gate 62 is also low.
An optical coupler 64 then works in its illuminated
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condition~since current flow through the light emitting
diode thereof is permitted. Current then flows through
the emitter follower connected transistors of the
coupler 64 through the ringing relay winding 22~ The
ringing relay then remains actuated; there being no ring
trip detection~ Ring trip i5 detected when there ls a
net positive count tno negative overflow) in the up-down
counter 52. Then the delay flip-flop 56 HO OlltpUt will
be high. At the onset of the next ringing cycle, the
ring trip latch flip-flop 58 will be clocked to place a
high on its Q output~ The output of the OR gate 62 is
then high and the light emitting diode of the opto
coupler 64 is cut off, causing current flow through the
ringing relay winding 22 to be cut off. Upon
de-actuation of ringing relay contacts RR-l and RR-2,
the tip and ring leads are returned to the line circuit
10~ An off-hook condition on tip and ring causes the
calling bridge output of the line circuit (~~) to go to
the off-hook state, causing ~he ring trip catch 58 to
remain in the tripped condition. Iat~h
In ~he event that the telephone set 12 has
~ remained on-hook the calling bridge output from the line
.r circuit 10 (FI~. 1), indicated as CB, will allow the
ring trip latch flip-flop 58 to be reset upon the start
! 25 of the succeeding ringing signal cycle. Then the output
of the AND gate 62 returns to the low state causing
operating current to again flow through the ring relay
operating winding 22 to reoperate the ring relay7 When
the telephone system turns off ringing, the RNG command
to the OR gate 62 will go high. The telephone system
thus controls ringing and the disconnection of ringing
in accordance with its normal operation to signal the
telephone set 12 of incoming callsO
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Should the count overflow in the positive
direction for any reason, such as a ring short ~o
groundt a net positive count will occur. This positive
`- count will be clocked into the ring trip latch 58 by the
high edge of RC, be treated just like a ring trip detec-
tion and cause current flow through the ringinq relay
winding 22 to be cut off. Accordingly, damage to the
ring generator is obviated. The ringing relay winding
is bridged by a diode 66 which protects the winding
against reverse currents.
From the foregoing description it will be
appar~nt that there has been provided an improved ring
trip detectîon system. A preferred embodiment of the
system has been described herein to illustrate the
invention. Variations and modifications in the herein
described embodiment, within the scope of the invention,
will undoubtedly suggest themselves to those skilled in
the artO Accordingly, the foregoing description should
be taken as illustrative and not in a limiting sense.
..1,.
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