Note: Descriptions are shown in the official language in which they were submitted.
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01 BACKGROUND OF THE INVENTION
02
03 Field of the Invention
04 The present invention relates to telephone test
05 sets.
06
07 Brief Description of the Drawings
08 Fig. 1 illustrates a prior art telephone test set.
09 Fig. 2 illustrates a ~lock diagram of a telephone
test set in accordance with the present invention.
11 Fig. 3 is a circuit schematic of the switchir.g
12 circuit of Fig. 2.
13 Fig. 4 is a circuit schematic of the polarity
14 indicating circuit of Fig. 2.
Fig. S i5 a circuit schematic of the high voltage
16 protection circuit of Fig. 2.
17 Fig. 6 illustrates an electrical schematic of the
18 preferred embodiment of a test set in accordance with the
19 present invention.
21 Description of the Prior Art
22 Fig. 1 illustrates a prior art telephone test set
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01 which has been manufactured by t'he assignee of the present
02 invention as the DRACON~ Model TS 21-080 test set~
03 A telephone test set is commonly used in two
04 distinct modes of operation. The first mode of operation
05 involves the operation on normal telephone lines which
06 involve the establishment of talking circuits between the
07 test set and the telephone central office which typically have
08 5 volts or more voltage drop within the test set. The second
09 mode of operation involves the so-called "dry loop" wherein a
pair of test sets are connected to the opposite ends of a pair
11 of wires to enable the line persons to identify the ends of a
12 single pair of wires. This dry loop mode of operation does
13 not receive battery voltage from the central office and
14 functions with the pair of test sets sharing 3 volts or less
provided by a small battery minus line losses. There is
16 considerable resistance by the various telephone operating
17 companies to the purchase of test sets which can only operate
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in a single one of the two modes of operation described,
6upra.
Prior art telephone test ~ets which can perform
in both of the above-described operating mode~ typically
include a two-position ~witch which i8 commonly known as a
"keypad-in/out" 6witch. The 6witch permits choice between the
mode of operation requiring signaling capability and the mode
which does not require ~ignaling capability. The prior art
test 6et of Fig. l has a three pole two po~ition switch,
having contacts SlA, SlB and SlC, ueed to determine the mode
of operation under which the test set operates. In the
po6ition labelled "out", the speech network iB connected
directly to the tip and ring lines. The contact SlC prevents
the lineper60n from being subjected to l~ud bur~ts caused by
inadvertent contact with the keypad of the dial pulse 6witch.
The contact S2 i~ used in it6 closed position for conducting a
polarity test of the tip and ring lines to check the direction
of current flow between the tip and ring lines by the
activation of the oppositely poled light emitting diodes DSl
or DS2 which respectively emit light of a different color when
current flows through each diode to inform the lineperson of
the direction of current flow.
With the prior art ctrcu~t of Fig. 1 being operated
in the "keypad in" mode, it i6 necessary to connect the 6peech
mute 6witch in series with the speech network. In thi6
configuration, the mute 6witch mu6t function regardle6s of the
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direction of current passing through the test set and 60
polarity guard BR i6 necessary. Unfortunately, the non-linear
voltage drop of the polarity guard BR can cause the ~peech
network to become inoperative at extremely low tip to ring
voltages which are present when the telephone test 6et i6
used for conducting line tests with dry loops which are
not powered by a central office battery. Moreover, voltage
drops in the dial pul6e switch can al~o æeverely affect the
operation of the speech network in low voltage conditions.
Finally, with low voltage condition~ aggravated by the series
connection of the dial pulse switch, ~peech mute 6witch and
6peech network with the polarlty guard bridge, the total
voltage drop within the tesk Bet i5 such that the insertion of
any additional 6eries connected elements would result in
excessive total voltage drop; and so a continuously indicating
series connected polarity indicating mean6 is not feasible in
this prior art. Thus, the polarity indicating circuit i6
6slectively coupled across the tip and ring lines by the
closure of switch S2 to permit only intermittent polarity
~testing.
The three pole two position ~witches Sl and S2 are
Realed 6witches which are expensive and difficult to obtain,
are a leading cau~e of test ~et in the field failures, maXe
the operation of the test set cumbersome to the lineper60ns
and make it nece66ary for the decign of the hou6ing for the
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te6t set to have a number of openings where moisture ~ay
enter.
United States Patent 3,708,634 disclo~es telephone
test ~et in which a voice receiver unit ~5 partially coupled
across the tip and ring lines. The voice transmitter unit is
connected in 6eries with a polarity guarding bridge. A SCR
is connected in 6erie6 with the polarity guarding bridge and
the voice transmitting unit. The SCR i6 biased into
conduction by the manual closing of a momentary contact switch
which then provides a conduction path for the operation of the
voice transmitter unit. The SC~ is turned on when the voice
transmitter unit and voice receiving unit are operational.
The system disclo6ed in United States
Patent 3,708,634 is not well ~uited for operation on dry
loops which have low operating potential because of the fact
that the voice transmitting unit i6 connected in 6eries with
the polarity guard bridge.
The u6e of a thermi6tor in eeries with the lead of a
telephone teet set which i8 ~dapted to be connected to the tip
or ring lines of a 6ub~criber'6 telephone line i6 known to
protect the telephone test against high current conditions on
the 6ubscriber'6 telephone line. Moreover, the u6e of a
~ilicon controlled rectifier ae a protective mechanism to a
hold circuit in a telephone te6t Bet iB known.
A telephone test oet in accordance with the present
invention hae advantages which are not ~chieved by the prior
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art telephone test sete. The telephone test 6et of the
pre6ent invention may be u6ed to activate the voice
transmitting and receiving circuits when connected to a
dry loop which does not receive operating potential from
the central telephone office. The tran6mitting snd receiving
voice un1t6 are connected directly acro6s the lead6 which are
adapted to be connected to the tip and ring lines which
removee the effect of the voltage drop in the polarity guard
from influencing the operation of the transmitting and
receiving units. The elimination of the voltage drop caused
by the polarity guard makes it possible to connect an
indicator for the direction of current flow between the tip
and ring line~ directly in serie6 with the voice transmitting
and receiving unite to permit a continuous indication of the
direction of current flow which i6 u6eful to the lineper~on.
A telephone test 6et in accordance with the pre6ent
invention includes fir6t and second leads which are
respectively adapted to be connected to a tip line and
a ring line of a telephone line; a full wave rectifier
for providing a rectified output of an input 6ignal and having
a-pair of input terminals which are re6pectively connected to
the ~ir6t lead and the second lead and a pair of ou~put
terminal6 which re6pectively output a voltage of a first
polarity and ~ ~econd polarity; a ~hunt circuit coupled
between the pair of input terminals of the full wave rectifier
which i~ comprised of a serie6 connection of a tran6mitter and
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receiver of voice communication6 and ~ ~witch having an open
circuit condition when a control ~ignal of a fir~t magnitude
iE applied to a control terminal ~nd having a closed circuit
condition when the control 6ignal i6 applied to the control
terminal of a second magnitude to permit current to flow
within the shunt circuit; a signaling circuit for placing a
telephone call over the tip and ring lines, coupled between
the pair of bridge rectifier output termlnals, for producing
~ignals to place a telephone call over the tip and ring lines
to a central office when in an activated ~tate and for not
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D producing signals when in an inactive ~tate;and a control
circuit re~ponsive to the signaling circuit for producing the
control signal having the first magnitude when the signaling
circuit for placing a telephone call is in the active state
and for producing the control ~ignal of the second magnitude
when the signaling circuit for placing a telephone call i6 in
the inactive 6tate.
The shunt circuit may include an indicator for
continuously indicating the direct~on of current flow between
the tip and ring lines of the sub~criber's line which i6
; connected in ~eries with the transmitter and receiver of voice
communications and the switch. The switch may be compri6ed of
a pair of field effect tran6istor6 each having connected
fiource6 located within the path of current flow ln the 6hunt
circuit with ~he gate6 of each of the field effect tran6i6tors
coupled to the signaling circuit. The control of the field
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effect transi6tors may be made by ~n optical i601ator which is
activated by the signaling circuit which causes the field
effect tran~i6tor~ to be turned off when the signaling
circuit iB activated.
The indicator for continuously indicating the
direction of current flow between the tip and ring line6
compri6es a pair of oppositely poled light emitting diodes
which respectively emit light of a different color. An audio
frequency bypass, which 6hunts the oppositely poled light
emitting diodes, permits audio ~requsncy co~munications ~o be
coupled to the sub6cribers' telephone lines without
~ttenuation by the diodes. A resistance is provided, which
6hunt~ the oppo~itely poled light emitting diodes, to permit
conduction o~ current in the ~hunt circuit when a Yoltage
across the light emitting diode6 is insufficient to bias one
of the diodes sub6tantially into conduction. A voltage
limiter is provided which shunts the pair of light emitting
diodes to limit the voltage which is applied to the diodes to
a level which will not cau6e their failure. The voltage
limiter may be a plurality of ~erially connected diodes
located within a bridge.
The ~ignaling circuits are protected by ~ voltage
limiter to prevent exce6sive voltages being applied which i6
comprised of a ~econd shunt c$rcuit coupled between the output
terminal8 which Bhunt6 the output terminals of the full wave
rectifier and the signaling circuit6. The ~econd shunt
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circuit includes a variable impedance which ghunt6 the output
terminal6 with an impedance which decrease6 nonlinearly as an
applied potential increa6es in magnitude. Preferably the
variable impedance i6 a metal oxide varistor. Furthermore, a
~witching circuit iB coupled between the output terminals
which is biased into conduction by the ~pplication of a
control signal of a first magnitude to cause the output
terminal6 to be ~hort circuited together and to have an open
circuit condition when the c~ntrol ~ignal i~ below the fir~t
magnitude. The control signal i8 produced by a RC circuit
co~pled to the variable impedance and to one of the output
terminals. Preferably, the switching circuit i6 a SCR having
an anode and a cathode coupled respectively to the output
terminal6 and a gate coupled to the RC circuit. A voltage
limiting circuit may be coupled to the variable impedance and
one of the output terminal6 with the gate of the SCR being
coupled to $he voltage limiting circuit to protect the gate
against excessive voltage.
An indicating circuit within a telephone fiet for
continuously si~naling the direction of current flow between
the tip and ring lines of a telephone line in which the
indicating circuit i6 connected ln 6erie6 with a tran~mitter
and receiver of voice communications within the test set
include6 a pair of oppo6itely poled light emitting diodes for
re6pectively indicating different direction6 of current flow
between the tip and ring line6 by emitting light of a
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different color, the pair of oppositely poled diodes being
coupled between one of the lines of the telephone line
and the transmitter and receiver of voice communication6; an
audio frequency bypass which shunt6 the oppositely poled light
emitting dlodes to permit audio frequencies to bypass the
light emitting diodes; and a resistance which 6hunts the
oppositely poled light emitting diodes to psrmit conduction of
current ln the shunt circuit when a voltage ~cro~ the
oppositely poled light emitting diodes i5 insufficient to bias
one of the light emitting diodes substantially into
conduction. A voltage limiter may be provided which 6huntR
the pair of light emitting d$odes to limit the voltage which
is applied to the diode~. Pre~erably, the voltage limiter is
a plurality of series connected diodes which are configured in
a bridge.
377~S
Description of the Preferred Embodiment
Fig. 2 illustrates a block diagram of a test set in
accordance with the present invention. A telephone test
6et 10 in accordance with the present invention connects the
speech circuit~ for tran~mitting And receiving voice communi-
cations in H ~eries circuit which i6 connected to leads which
are adapted to be connected to the tip and ring l~nes of a
BUbSCriber t 8 telephone line to remove the speech circuit~ ~rom
being deleteriously influenced by low voltage conditions
existing when the ~ubscriber's telephone line extends a long
distance from the telephone company' B central office or when a
pair of telephone test sets are being used at either end of a
pair of telephone wires which ~re not coupled to the telephone
oentral office battery. In the6e conditions, the presence of
a polarity guarding circuit, which iB connected acros6 the
leads that are adapted to be connected to the tip ~nd ring
l~ne5 of a 8ubscriber's telephone line for protecting
~ignalling c1rcuits for placing a telephone call to the
telephone company central office, does not disable the
transmitter ~nd receiver for voice communications a6 in the
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prior art of Fig. l. Furthermore, because the effect of the
voltage drop across the polarity guard on the operation of the
tran6mitter and receiver for voice communications has been
removed, a polarity indicating circuit may be connected in
series with the transmitter and receiver for voice
communication6 to provide a continuous polarity indication
without di6abling the transmitter and receiver for voice
communications because of the voltage drop required to operate
the polarity i~dicating circuit. A ~peech mute 6witch i6
coupled in ~eries with the transmitter and receiver for voice
communications which ~8 normally closed, that iB ~utomatically
opened in response to the lineperson activating the signaling
circuits for placing a call over the sub6criber' E telephone
line to the telephone company central office.
Fig. 2 illu~trate6 a block diagram of a telephone
test ~et lO in accordance with the present invention. A pair
of leads 12 and 14, which are re6pectively labelled ~tip" and
"ring", are adapted to be coupled to a line pair of a
telephone ~ubscriber~ B telephone line. A thermi6tor 16 i8
connected in 6erie6 with the lead 12 which protects the
telephone te~t set ~gain6t sustained high current conditions
by having a high 6erie6 resistance. The thermistor functions
ns a fixed value resi6tor under normal line current
condition6, but when prolonged overcurrent conditions exist,
it heats to a temperature wherein the resistance lncreases
po6itively with increa6ing temperature. The resl6t~nce of the
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thermistor 16 increa~es rapidly to a thermal equilibrium
producing a high resistance when a ~ustained overcurrent
condition exists. A rectifying bridge 18 having four
diodes 20 has a first input terminal 22 connected to line 12
and a ~econd input terminal 24 to line 14. A fir~t output
terminal 26 i6 coupled to ground. A 6econd output terminal 28
i~ coupled to a high voltage protection circuit 30. The
preferred form of the high voltage protection circuit 30 1
discussed, infra, with regard to Fig. 5. The high voltage
protection circuit 30 16 coupled to the signaling circuits 32 of
conventional design which produce signals for making a call
over the leads 12 ~nd 14 through a ~ubscriber' 8 line to a
central telephone of~ice. The signaling circuits will
produce DTMF tones or loop di6connect pul6es for placing a
call. Thelinput terminals 22 and 24 are coupled to a shunt
circuit 34 which is compri6ed of a 6eries combination of a
polarity indicating circuit 36, which i~ de~cribed in detail,
Lne~, in conjunction with Fig. 4, a speech mute 6witch 38,
which i6 de6cribed in detail, lnfra, in con~unction with
Fig. 3, ~nd ~peech circuits 40 which are comprised of a
conventional transmitting and receiving unit. With the
configuration illustrated in Fig. 2, the speech circuits 40
~re not rendered inoperatlve by low voltage line condition~ ~s
a con~equence of a voltage drop across the rectlfying
bridge 18 a~ with the prior art in Fig. 1. Moreover, the
polarity indicating circuit 36 may be connected in ~eries with
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the spQech circuits to produce a continuous indicat~on of the
direction of current flow between the tip and ring line6 as a
consequence of the 6peech circuits not being rendered
inoperative by the voltage drop acro6s the full wave
rectifying bridge 18 as with the prior art of Fig. 1 during
low voltage condition6.
Fig. 3 illustrate6 the preferred form of 6peech mute
~witch 38 in ~ccordance with the pre6ent invention. An
optical ~601ator 40 having a light e~itting diode 42, which is
activated by the operation of the signaling circuits 32, is
optically coupled to a phototransi6tor 44. The
phototransistor 44 shunt6 a serie6 combination of a
resistor 46 ~nd a battery 48. A series combination of two
field effect tran6istor6 50 and 52, with their source6 coupled
together, functions as a high impedance when the signaling
circuits 32 are in the active 6tate and as a low impedance
when the signaling circuits are in the inactive 6tate. The
activation of the field effect tra~sistors 50 and 52 into the
low impedance condition is produced when the signaling
circuits are inactive which provides the 6hunt circuit 34 with
a path for coupling the received and transmitted voice
communications directly with the lines 12 and 14. The
re~istance 46 provlde6 a high impedance which limits the
current drawn by the phototran~istor 44 when it i8
conductive. The connection of the source6 together of the
~ield effect tr~nsistor~ 50 and 52 provide6 ~n extremely high
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impedance against the application of signals of either
polarity to the le~d6 12 and 14.
Fig. 4 illustrate~ the preferred embodiment of the
polarity indicating circuit 36 o~ Fig. 2. The polarity
indicating circuit 36 ha6 a pair o~ oppositely poled light
emitting diodes 60 ~nd 62, which respectively emit green and
red light continuously, to ~ignal the direction of current
flow between the leads 12 and 14 when forward bi~ed. A
re~istance 64 i6 coupled in ~erie6 with the oppositely poled
parallel combination o~ light emitting diodes 60 ~nd 62. ~he
re61stance 64 limits the ~mount of current which may be drawn
by the light emitting diodes. A capacitance 66 ohunt~ the
oppo~itely poled light emitting diodes 60 and 62 to provide an
audio frequency bypa6s of low impedance for communications
tran6mitted and received by the speech circuits 40. A
resi~tance 68 shunts the oppositely poled light emitting
diodes 60 and 62 which permit6 the speech circuit~ 40 to
continue to operate even when the line potential is too low to
6upport substantial conduction of either the light emitting
diode 60 or 62. A voltage limiter 70, which is compri~ed of a
plurality of diodes 72 connected in ~eries in ~ bridge, l~mits
the amount o~ voltage which can be applied to the diodes 60
and 62.
Fig. 5 lllu~trates the high voltage protection
circuit 30 o~ Fig. 2. Metal oxide varistor 80 iB connected
betwe~n terminal 28 ~nd the cathode of ~ Zener diode 82 having
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01 an anode connected to terminal 24. The function of the metal
02 oxide varistor is to provide a nonlinear resistance
03 characteristic which rapidly decreases with an increase in
04 applied potential to limit the voltage drop applied across the
05 terminals 26 and 28 to safeguard the signaling circuits 32 and
06 circuits within the shunt circuit 34. An RC circuit 84
07 consisting of a resistor 86, capacitor 88 and resistor 90 is
08 connected between the cathode of Zener diode 82 and the gate of
09 the voltage limiting SCR 92. The RC circuit 84 has a time
constant whch integrates the voltage applied across resistor 86
11 by conduction of the metal oxide varistor 80 to cause a short
12 circuit between the terminals 26 and 28 when the high voltage
1 condition has existed for a time sufficient to bias the gate of
14 the SCR 92 sufficiently to turn the SCR on. Once the SCR 92 is
turned on, the potential applied to the signaling circuts 32
16 and to the shunt circuit 34 is limited to the forward drop
17 across the SCR. When the terminal 28 is effectively grounded
18 by conduction of SCR 92, the input terminals 22 and 24, which
19 are respectively coupled to the tip and ring lines, appear
virtually shorted together (two diode drops) which limits the
21 potential applied to shunt circuit 34 to a safe level.
22 Resistance 94 is connected between the RC circuit 84 and the
23 gate of the SCR 92, to isolate the gate of SCR 92 from the
24 transient loading effect of capacitor 88.
Fig. 6 illustrates a preferred embodiment of a
26 telephone test set in accordance with the present invention.
27 Idential parts are identified with the same reference numerals
28 as in the preceding figures. Semiconductor chips are
29 identified by their manufacturer's part number.
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While the invention has been de6cribed in terms of
its preferred e~bodiment, it 6hould be understood that
numerous modification6 may be made thereto without departing
from the ~pirit and scope of the invention 8S defined in the
appended claims. It i6 intended that ~11 6uch modifications
fall within the ~cope of the ~ppended claim6.
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