US2422700A - Reciprocal signaling trunk telephone system - Google Patents

Description

June '24, 1947. N. A. NEWELL RECIPROCAL SIGNALING TRUNK TELEPHONE SYSTEM Filed June 15, 1945 4 Sheets-Sheet 1 lNl/ENTOR N. A; NE WE LL ATTORNE Y June 24, 1947. I NEWELL I 2,422,700

RECIPROCAL- SIGNALING TRUNK TELEPHONE SYSTEM Filed June 15, 1945 4 Sheets-Sheet 2 INVENT OR M A. NE WELL A TTORNEV June 24, 1947.

RECIPROCAL SIGNALING TRUNK TELEPHONE SYSTEM N. A. NEWELL Filed June 15, 1945 lNVENTOR N. A. NE WELL ATTORNE Y June 24 1947. E L 2,422,700

RECIPROCAL SIGNALING TRUNK TELEPHONE SYSTEM- Filed June 15, 1945 4 Sheets-Sheet 4 INVENTOR N. A. NEWELL ATTORNEY Patented June 24, 1947 RECIPROCAL SIGNALING TRUNK TELEPHONE SYSTEM Norman A. Newell, Millburn, N. J assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application June 15,1945, Serial No, 599,537

17 Claims. 1

Objects of the invention are the provision of 5 more reliable signal transmitting and receiving means in systems employing voice frequency signaling, the provision of such signaling means for the transmission of signals over four-wire lines,

and the prevention of false operation in response lo to voicecurrents or other interfering currents.

This invention is a signaling system in which the termination of current of a particular frequency normally transmitted over a line constitutes a first signal, in which an impulse of ticular frequency are transmitted over a line, in

current .of said particular frequency constitutes a second signal, and in which the continued transmission of current of said particular frequency constitutes a third signal.

A feature of the invention is a signaling system in which the termination of current of a particular frequency normally transmitted over a line constitutes a first signal,-in which the transmission, of an impulse of current of said particular frequency and of a particular limited duration constitutes a second signal, in which the continued transmission of current of said particular frequency constitute a third signal, and in which the current of said particular frequency is reduced in amplitude when continued beyond a predetermined interval of time.

Another feature of the invention is a signaling system in which the termination of current of a particular frequency normally transmitted over a line constitutes a first signal, in which means normally efiective to prevent response to current of other frequencies or to current of the signaling frequency in combination with current of other frequencies is disabled by the response to said first signal, in which the transmission of an impulse of current of said particular frequency constitutes a second signal, and in which the continued transmission of current of said particular frequency constitutes a third signal.

Another feature of the invention is a voice freparticular frequency constitutes a seizure or supervisory signal, in which short impulses of current of this particular frequency constitute selective or supervisory signals, and in which the continued transmission of current of said particular frequency constitutes a disconnect or supervisory 5 signal. 7

Another feature of the invention is a telephone and voice frequency signaling system in which the transmission of voice and signaling current is effected over four conductor lines,one pair of conductors being used for transmission in one direction and the other pair of conductors for transmission in the other direction, current of a particular frequency being normally transmitted over each pair as long as the line is idle, the termination of the normally transmitted current constituting a seizure or supervisory signal, the transmission of impulses of current of said particular frequency constituting selective or supervisory signals, and the continued transmission of current of said particular frequency constituting a disconnect or supervisory signal.

Another feature of the invention is a telephone and voice frequency signaling system in which voice current and signaling current of a parwhich the talking circuit over the line is normally closed and signaling current of said particular frequency is transmitted over the line while idle, and in which the talking circuit is opened for a predetermined interval of time each time the transmission over said line of current of said particular frequency is initiated or terminated.

Another feature of the invention is a voice frequency signaling system in which voice current and signaling current of a particular frequency are transmitted over a line, in which the talking circuit over the line is normally closed and signalin current of saidparticular frequency is transmitted over the line while idle, in which termination of the transmission of current of said particular frequency constitutes a seizure or supervisory signal, in which the transmission of an impulse of current of said particular frequency constitutes a selective or supervisory signal, and in which the continued transmission of current of said particular frequency constitutes a disconnect or supervisory signal, the amplitude of the signaling current transmitted over the line while idle being smaller than the amplitude of the signaling current transmitted as selective, supervisory or disconnect signals.

Another feature of the invention is the control of an amplifier by two networks energized by the output of the amplifier, the voltage across the two networks being rectified and applied to the control grid to effect operation of a signal relay responsive to voice frequency signaling current, to prevent operation of the relay responsive to voice current, and to effect release of the relay responsive to termination of the signaling current.

A complete understanding of the invention will be facilitated by considering a system embodying theinvention and its features," one such system being represented schematically in the drawings which form'a part of this specification. The inventior'ris: not, limited in its application to the particular system and circuit arrangements shown in the drawings, but is applicable generally to any voice frequency signaling system.

Referring to the drawings:

Figs. 1 and 2 represent a first toll office com prising an operators position OPI, an outgoing trunk circuit OTI, a two-way trunk circuit TWT l, a selector TSI, an incoming trunk circuit ITI, a signal transmitting circuit STI, and a signal receiving circuit SRI;

Figs. 3 and 4 represent another toll oiiice comprising an operators position P2, an outgoing trunk circuit 0T2, a two-way trunk circuit TWTZ, a selector T52, an incoming trunk circuit 1T2, a signal transmitting circuit ST2 and a signal receiving circuit SR2; and

Fig. 5 shows the relative positions of Figs. 1 to 4 inclusive, to form an operative arrangement.

The system represented in the drawings includesa plurality of toll ofiices each of which comprises a toll board with operators positions and "cords for answering calls incoming from manual or dial local offices, from toll subscribers lines; from community dial oftices, or from other toll offices; and for connecting-calling intertoll trunks with switching trunks leading to called manual or dial local offices, to called toll subscribers lines, to trunks to community dial offices, and to intertoll trunks to other toll oflices. Outgoing jacks are connected to switching trunks, toll subscribers lines, trunks to community dial offices and intertoll trunks. Answering jacks are connected to recording trunks, toll subscribers lines, trunks from community dial offices and intertoll trunks. Intertoll firstselectors, and intertoll second selectors, if required, are provided for use on calls incoming over intertoll trunks to establish connections with other-toll, local or community dial ofiices. The operators"positions are provided with dials for use in controlling the operation of intertoll selectors in other toll offices and for controlling the operation of selector and connector switches in local or community dial offices.

Reference may be had to the patent to R. E. King-and-Ohnier R. Miller No.2,209fl 7-7, granted July- 30, 1940, and to the patent to J. G. Walsh No.

2306336, granted December 22, 1942, for a complete disclosure of the cord and operators position circuits providedin each of the two toll ofiices for interconnecting calling andcalledlines and trunks and controlling the completion of toll calls. The outgoing trunk circuits O-T| and GT2, incoming trunk circuits ITI and 1T2, two-way trunk circuits TWTI and TWIZ; and the toll route selector circuits TS I- and TS2 are all similar to corresponding circuits of the aforementioned King-Miller and Walsh" patents.

The two toll ofiices shown in the drawings are interconnected by four-wire lines, one being shown in the drawings as consisting of two twooffice and to receive and transmit voice current incoming over toll line TLI to these trunk circuits. Balancing networks BNI and BN2 are connected to hybrid coils HCI and HCZ in usual and wellknown manner.

The signal transmitter STI comprises a generator 230 of alternating current of a particular voice frequency, for instance 3,200 cycles per secend, a relay 200 responsive to seizure, selective, supervisory and disconnect signals which are to be transmitted over the toll line TLI, an auxiliaryrelay 220 and a cut-off relay 2H] for disconnecting the toll line TLI from the hybrid coil HCI during the transmission of signals outgoing over toll line TLI. The generator 230 is normally connected through resistors 21 and 233, resistors 232 and 23, back contacts of relay 200, conductors 2M and 242- and repeating coil 24!] to the con ductors of toll line TLI, the continued presence of a tone of 3,200 cycles on the toll line indicating that the line is idle. A one-way amplifier Al, connecting toll line TLZ to the hybrid coil HCI, prevents the transmission of signaling current from signal transmitter ST! over toll line TL2.

The signal transmitter ST2 associated with toll line Th2 in the second toll oflice is similar to the signal transmitter STl, corresponding elements having reference characters with the same tens and units digits. Thus signaling current from generator 330 is normally-transmitted over toll line TLZ; and. the amplifier A2 is efiective to prevent the transmission of signaling current from generator 330 over toll line TLI.

The signal receiver SRI- comprises an input transformer 253 which is connected by conductors 25H. and 2 52 to the repeating coil 2511 of toll line TLZ in parallel with the input side of amplifier A! through which voice currents are transmitted to the hybrid coil HCI. The signal receiver SRl' further comprises a thermionic amplifier 260, an output transformer 2M, a detector 2'10, an output transformer 21 l two tuned circuits connected in series across. the output of transformer 2-H, a signalrelay 290 connected in the plate circuit of tube 210 and an auxiliary relay 285. The amplifier 260 is arranged for negative feedbackfrom the plate circuit through condenser 254 and varistor 25 5 so as to limit the level of signal applied through transformer 26-! to the detector- 2 1i]; (-)ne of the aforementioned tuned circuits consists of an inductance coil 214 connected parallel with condenser 2-13 and the other tuned circuit consists of'the'cond'enser 217 connected in series with the inductance coil 21 8. Both of'these tuned circuits are tuned to the signali-ngfrequencyso that the parallel tuned circuit has a very high impedance at the-signaling frewire lines TLI and TL2 associated with trunk I circuits OTI and TWTI in the first toll 'ofiice and with trunk circuits 0T2 and TWTZ in the second toll oifice. The line TLI is used to, transmit voice and voice frequency signaling current from the first toll ofiice t the second and the line TL2 is used to transmit voice and voice frequency signaling current from the second toll office 'to the first, A hybrid coil HCI individual to trunk circuits OTI and TWT! is arranged to transmit voice current from these trunk circuits overt'oll line TLI to the second tollofiice and to receive and transmit voice current incoming over toll line TL2 to these trunk circuits. A. like hybrid coil HC2 individual to trunk circuits 0T2, and TWT2 is arranged to transmit voice current from these trunk circuits over toll line TLZ to the first toll quency and a low impedance at "other frequencies whereas the series tuned circuit has a low im pedance at the signaling frequency "and a very high impedance at other frequencies. The detector 2:10 is biased substantially to cut-oifand the alternating component in itsoutput circuit is applied through transformer 211 across. the two tuned circuits. The voltage thus developed across the; parallel tuned circuit. is, rectified. by rectifier 212 and applied through, resistor 216 to the grid of tube 2. 20 Produce a positive reaction. The lta e d' l e ioped. across the. series tuned circuit is rectified by rectifier 282 and applied'through resistor 28;! to, the grid of tube 210 to produce a negative. reaction. Since. the volta e across. the parallel resonant circuitis a maximum. at signal.- ing frequencies, theplate current of tube 2-10 is increased by the positive reaction in response to signaling'current to effect the operation of relay 290; and since the voltage across the series resonant circuitris a maximum at frequencies other than the signaling frequency, the negative reaction guards against the operation of relay 290 in case voice or interference current incoming over line TL2 has a component of the signaling frequency. Relay 290 requires about .010 second for operation' thereby affording additional protection against false operation in response to a signaling frequency component in speech. When relay 290 operates, it closes a circuitfor operating auxiliary ating and this condenser is discharged through resistor 286 when relay 285 operates. Relay 290 controls the connection of ground to signaling conductor I10.

The signal receiver SR2 is similar to the signal receiver SRI in that it responds to the same signals but constitutes an alternative arrangement. Both of the signal receivers connected to lines TLI and TL2 may be arranged the same as receiver SRI in Fig. 2 or both may be arranged the same as receiver SR2 in Fig. 3. Signal receiver SR2 comprises an input transformer 353 which is connected by conductors 35I and 352 to the repeating coil 350 of toll line TLI in parallel with the input side of amplifier A2 through which voice currents are transmitted to the hybrid coil H02. The signal receiver SR2 further comprises a thermionic volume limiting amplifier 355, an out put transformer 359, a thermionic detector 310, a signal relay 314, an auxiliary relay 315, a voltage dividing network and two tuned networks normally connected in series across the output of transformer 359 to control the bias of the grid of detector 310. Resistor 354 included in the grid circuit of amplifier 355 limits the level of signal transmitted through transformer 359; and the varistor 360 connected across one side of this transformer also limits the voltage applied to the voltage dividing network and tuned networks aforementioned. The voltage dividing network comprises rectifier 363, resistor 364, condensers 365 and 366, resistor 361 and rectifier 368. One of the tuned networks consists of condenser 38I and induction coil 382 in parallel and the other consists of condenser 383 and coil 384 connected in series. The voltages developed across the tuned circuits are applied through the voltage dividing network and conductor 369 to the grid of detector 310, which is normally biased by the voltage drop in resistor 312. The rectifiers 363 and 368 are poled so that the grid of tube 310 becomes more positive when signaling current is being received and becomes more negative when current of a frequency other than the signaling frequency is being received over line TLI, the impedance of the parallel tuned circuit being a maximum at the signaling frequency and the impedance of the series tuned circuit being a maximum at other frequencies. Condenser 31I delays a change in the grid bias of tube 310 thereby giving further protection against response to voice current having a signaling frequency component. Resistor 385 connected in parallel with the series tuned circuit limits the voltage developed thereacross,

Condenser 316 delays the operation of relay 315 andresistor 311 discharges condenser 316 when relay 315 operates. Relay'314 controls the connection of ground to signaling conductor 410; and the auxiliary relay 315, when operated, shortcircuits the series tuned circuit so that voice or interference current is ineffective to cause the release of relay 314 on calls routed to an intercept operator.

Assume now that the plug API of the cord CDI at the position OPI in the first toll office is inserted in an answering jack to answer a call incoming over a recording trunk or an intertoll trunk or a, trunk from a community dial olfice, that the operator has determined the destination of the call and that the plug CPI of this cord is inserted in a jack associated with an outgoing trunk circuit over which the call may be extended, and that the trunk 0TI is associated with the jack J I into which the plug CPI is inserted. The insertion of plug CPI in jack J I closes a circuit for operating relay I04, thereby causing the operation of relay IIO as described in the aforementioned King et al. patent, and closes a circuit including conductor I02 for operatively energizing the upper winding of signaling relay I49. The operation of relay IIO disconnects the signaling conductor I10 from conductor I69 thereby to prevent the operation of relay I003 of two-way trunk circuit TWTI when ground is connected to signaling conductor I10, connects answering supervisory relay I05 of the trunk circuit OTI to conductor I10 and connects ground to conductor I68 to operate relay I004 of twoway trunk circuit TWTI. Relay I004 locks to conductor I68 independent of relay I003 and further opens the connection between the winding of relay I003 and the signaling conductor I10. The operation of relay I 40 connects signaling battery to conductor I64 to operate the signal relay 200 of the signal transmitter STI. The operation of relay 200 of signal transmitter STI terminates the transmission of signaling current of 3200 cycles per second fromthe generator 230 over toll line TLI thereby causing the release of the normally operated relay 314 of signal receiver SR2. Relay 200 also closes a circuit through its inner lower front contact for operating cut-off relay 2"). Relay 2I0 disconnects the repeating coil 240 of toll line TLI from conductors 20I and 202 leading to the hybrid coil HC I, connects the repeater termination comprising resistor 2I3 in series with condenser 2I4 across the repeating coil 249, connects ground to conductor I68 to hold relay I004 operated independent of relay H0, and closes a circuit for operating relay 220. Relay 220 locks under the sole control of relay 200, opens the operating circuit of relay 2I0 and opens a shunt circuit through resistor 235 across the generator 230 thereby to increase the voltage applied to conductors MI and 242 when relay 200 releases as hereinafter described. The release of relay 2I0 disconnects the repeater termination and closes the talking connection between coil 240 and hybrid coil HCI.

The termination of the signaling current normally transmitted over toll line I'LI effects the release of relay 314 of signal receiver SRI, therevby connecting ground to signaling conductor 410 to cause the operation of relay 503 of trunk circuit TWTZ. The circuit for operating relay 503 is traced from ground at the back contact of relay 314, through conductor 410, lower back contact of relay 4I0 of trunk circuit 0T2, conductor 469, back contact of relay 504 of trunk circuit TWT2,

'7 back contact of relay 50! and the winding of relay 503. Relay 503 connects ground to conductor 460 to guard the outgoing trunk circuit T2 from being seized on an outgoing call. Relay 503 closes a circuit for operating relay 505, this circuit being traced from ground at the back contact of relay 314, through conductor 410, lower back contact of relay 4! 0 of outgoing trunk circuit 0T2, conductor 469, back contact of relay 504 of two-way trunk circuit TWTZ, winding of relay 505, uppermost front contact of relay 503, conductor 461, thence in simplex through resistors 42! and 422, backcontacts of relay 420 of outgoing trunk circuit 0T2, windings of retard coil 465, conductors 45! and 462, inner back contacts of relay 430, conductors 50! and 502, front contacts of relay 503, backcontacts of relay 560 of selector TS2, resistors 56!, 562 "and 563, lowermost back contact of relay 560 and the winding of line relay 551. The'line relay 551 of toll selector T82 is thereby operated and so also is relay 505 of the two-way trunk circuit TWT2. Relay 505 closes a circuit through resistor 506 for holding relay 503 operlated'and closes a circuit for operating the slow-to-release relay 501. Relay 501 also closes a circuit through resistor 506 for holding relay 503 operated, opens the operating circuit of relay 503 and connects ground to conductor 553 of selector TSZ to hold the selector T82 and any additional selector through which the connection is extended in the second toll office until the connection is released at the first toll oflice'.

Whenthe calling operator at the first toll office operates the dialing key (not shown) the upper winding of relay 140 is controlled by the impulse contacts of the dial at position OPI. Each release and reoperation of relay I40 in response to the dialing of the first digit of the called number opens and closes the circuit through conductor I64 and'the winding of relay 200, causing a corresponding release and reoperation of relay 200. When relay 200 releases responsive to the first impulse of a digit, generator 230 is reconnected through resistors 23L, 232, 233 and 234, back contacts of relay 2-00 and conductors 24! and 242 tothe repeating coil 2'40, whereby an impulse of 3200-cycle current is transmitted over toll line TL! during the interval that relay 200 is released. This impulse of current is amplified by tube 355 of signal receiver SR2 and effects the operation of relay'314. When relay 200 reoperates at the end of'the first impulse, generator 230 is disconnected from conductors 2-4! and 242 thereby terminating the impulse of current of the signaling frequency to effectthe release of relay 314. Each release and reoperation of relay I40 in response to the dialing of the first digit of the called number opens and closes the circuit through conductor I64 to cause a corresponding release and reoperation of relay 200, whereby a series of impulses of current of signaling frequency is transmitted over toll line TLI-, each impulse causing the momentary operation of relay 314. Accordingly, relay 314 momentarily disconnects ground from conductor 410 as many times as there are impulses in the series received over toll line TLI, and relay 505 of trunk circuit TWT2 and line relay 551 of'selector TS2 are thereby alternately released and reoperated a like number of times. Since relay 501' is slow in releasing, the response of relay 505 to dial impulses is ineffective to cause the release of relay 501. Each release of relay 551 causes the operation of the vertical stepping magnet (not shown) of selector TSZ' whereby the brushes 51!, 512, 513 and 514 are tepped up to the level corresponding to the digit dialed. At the end of this train of impulses, the brushes are advanced step by step in the selected level until a set of terminals connected to an idle trunk or succeeding selector are encountered; whereupon relay 560 is operated to extend the connection to the selected switch or trunk, all in usual and well-known manner.

Each succeeding train of dial impulses created by operation of the calling operators dial is repeated by relay I40 to relay 200 and a corresponding train of impulses of voice frequency signaling current is transmitted over toll line TL! to operate relay 314; and the operation of relay 314 effects the transmission of corresponding trains of impulses, through brushes 51! and 512 and the selected terminals of selector T82, to operate suc ceeding switches through which the connection is extended or to operate digit registers. When dialing is completed, the dial key (not shown) is restored to normal and relay R30 is operated in the manner described in the aforementioned King-Miller and Walsh patents to complete the talking connection between cord CD! and hybrid coil HC'I.

Assume that the call in question is one which is routed through selector T82 and a selected idle incoming trunk circuit to a toll operator position in the second toll onice and that the selected trunk circuit is the trunk circuit 112 terminating at jack J8. In this case the operation of relay 560 of selector TSZ closes a circuit for operating the line relay 6 05 of trunk circuit IT2. This cir cuit is traced from groundat the .back contact of relay 314, through conductor 410, lower back contact of relay 4I0 of outgoing trunk circuit 0T2, conductor 460', back contact of relay 504, winding of relay 505, outer upper front contact of relay 503, conductor 461, thence in simplex through resistors 42! and 422, back contacts of relay 420, winding of retard 0011 460, conductors 46! and 462, back contacts of relay 430, conductors 501 and 502, front contacts of relays 503 and 560, brushes 51! and 512 of selector T82, conductors 60! and 602 of trunk circuit 1T2, windings of retard coil 600, resistors 615, M6 and 6H, and the winding of line relay 605. vRelay 505 is operated by the current in this circuit, thereby to hold relay 501 and thus maintain the connection of ground to conductor 553. Relay 655 close a circuit for lighting lamp L8. When the plug of a cord CD2 at a position 0P2 is inserted in jack J8 to answer the call, the lamp L8 is extinguished and a talking connection is completed through condensers 650 and 65!. Relay 634 is operated in the manner described in the aforementioned P te t to J. G. Walsh and closes a circuit from ground through conductor 604, brush 514 of selector TS2, a, front contact of relay 560, and the winding of relay 5!! of two-way trunk circuit I'WTZ. The operation of relay 5|! closes a, circuit for operating relay 300 of signal transmitter ST2. This circuit is traced from battery through conductor 466, front contact of relay 5, back contact of relay 5l0, conductor 455, back contact of relay 440, conductor 464 and the winding of relay 300. The operation of relay 300 disconnects generator 3.30 from conductors 34! and 342 to terminate the transmission of signaling current over toll line TL2 and thereby cause the release of relay 290 of signal receiver SRI. The operation of relay 300 also closes a circuit for operating cut-ofi relay 3l'0. Relay 31-0 opens the talking connection between repeating coil 340 and hybrid coil H02, connects the repeater termination comprising resistor 3I3 and condenser 3I4 across coil 340, connects ground to conductor 468 to maintain a busy indication for trunk circuits T2 and TWT2 while a signal is being transmitted over line TL2, and closes a circuit for op erating relay 320. The operation of relay 320 opens the operating circuit of relay 3 I 0'and opens the normally closed shunt circuit through resistor 335 across generator 330, thereby to increase the voltage applied across conductors 3M and 342 when relay 300 release to transmit a signal, for instance an on-hook or busy signal. The release of relay 3I0 transfers coil 340 from the repeater termination to coil H02 and opens the operating circuit of relay 320; and rela 320 i then held operated under the sole control of relay 300.

The release of relay 290 responsive to the termination of signaling current over toll line TL2 causes the release of relay 285 and connects ground to conductor I10, thereby operating supervisory relay I of trunk circuit 0T2. Relay I05 disconnects ground from the low resistance winding of sleeve relay I04, thereby causing the calling supervisory lamp CS to be extinguished to indicate that the call has been answered in the called toll ofiice. When the answering operator disconnects from jack J8, relay 634 releases and disconnects ground from conductor 604, causing the successive release of relay 5 of trunk circuit TWT2 and relay 300 of signal transmitter ST2. The release of relay 300 reconnects generator 330 to coil 340, thereby transmitting signaling current over line TL2 to cause the reoperation of relay 290 of signal receiver SRI. The operation of relay 290 disconnects ground from conductor I'I0, releasing relay I05 and thereby causing the lighting of lamp CS to give th calling operator anon-hook signal. The aforementioned release of relay 300 also causes the operation of relay 3I0 and the release of relay 320. The release of relay 320 causes the release of relay 3 I 0 and again closes the shunt circuit through resistor 335 across generator 330, whereby the voltage applied across conductors MI and 342 is reduced and the signaling current transmitted over line TL2 is reduced to the normal, idle condition. I

If a busy condition is encountered in extending the connection, for instance if selector TS2 finds all trunks busy in the selected level, relay 5| I of two-way trunk circuit TWT2 is alternately operated and released under the control of an interrupter (not shown) thereby causing the alternate operation and release of relay 300 of signal transmitter STZ. Relay 300 alternately disconnects and reconnects generator 330 to coil 340, whereby signaling current is intermittently transmitted over line TL2 to cause the alternate release and reoperation of relay 290 of signal receiver SRI. The alternate release and reoperation of relay 300 causes the alternate operation and release of supervisory relay I05 of trunk circuit OTI whereby the supervisory lamp CS of cord CDI is intermittently extinguished and lighted, thus giving the calling operator a busy signal.

When the calling operator disconnects plug CPI from jack J I, relay I 04 releases; but if relay I05 is operated at this time, relays H0 and I 40 are held operated. With relay I 05 released, or as soon after the release of relay I04 as relay I05 releases, relays H0 and I40 are also released.

The release of relay I40 causes the release of relay 200 whereby generator 230 is reconnected across conductors 24I and 242 thereby to effect the transmission of signaling current over line TLI to effect the reoperation of relay 314 of signal receiver SR2. The release of rela 200 also causes the operation of relay 2 I 0 and the release of relay 220. The operation of relay 220 causes the release of relay 2I0 and closes the shunt circuit through resistor 235 across generator 230 whereby the voltage applied to conductors MI and 242 is reduced to the normal level. The operation of relay 314 disconnects ground from conductor 410 causing the release of relay 505 of trunk circuit TWT2 and line relay 605 of trunk circuit 1T2. The continued release of relay 505 causes the release of relays 501 and 503 of trunk circuit TWT2 and the release of relay 560 of selector TS2 whereby selector TS2 is restored to' normal in usual and. well-known manner. The release of relay 503 also disconnects ground from conducto 46B, thereby terminating the busy marking of trunk circuits 0T2 and TWT2.

What is claimed is:

1. In a signaling system, a line, a first means normally effective to transmit current of a particular frequency and amplitude over said line, means for terminating the transmission of said current as a first signal; means for transmitting an impulse of current of said particular frequency and of greater amplitude than that normally transmitted as a second signal, and means for transmitting current of said particular frequency for a predetermined interval of time and of said greater amplitude as a third signal.

2. In a signaling system, a line, a first means normally effectiveto transmit current of a particular frequency and amplitude over said line;- means for terminating the transmission of said current as a first signal, means for transmitting an impulse of current of said particular frequency and of greater amplitude than that normally transmitted as a second signal, and means for transmitting current of said particular frequencyfor a predetermined interval of time and of said greater amplitude as a third signal and for continuing the transmission of current of said particular frequency and normal amplitude.

3. In a signaling system, a line, a first means operated responsive to current of a particular frequency normally transmitted over said line, means normally eifective to prevent the operation of said first means responsive to current of frequencies other than said particular frequencyor to current of said particular frequency and other frequencies, means operated responsive to th release of said first means upon the termination of current of said particular frequency and efiective to disable said preventing means, means responsive to the reoperation of said first means by an impulse of current of'said particular frequency, and means responsive to the operation of said first means by current of said particular frequency for more than a predetermined interval of time.

4. In a signaling system, a line,signa1 transmitting means at one end of said line comprising a generator of alternating voltage of a particular frequency for use in transmitting signals over said line, a trunk circuit comprising talking conductors over which voice current is transmitted, means normally connecting said line to. said talking, conductors and to said generator,

means effective to disconnect said generator from said line as a seizure signal, and signal receiving means at the other end of said line comprising a relay operated by'current of said particular frequency, means for preventing the operation of said relay responsive toscurrent other than signaling current transmitted over said line which includes a component of said particular frequency, means rendered effective by the operation of said relay to disable said preventing means, and other means controlled by said relay.

5. In a signaling system comprising a line for transmitting voice current and signaling current of a particular frequency in the voice frequency range, signal receiving means connected to said line, said signal receiving means comprising a relay, means for operating said relay responsive to current of said particular frequency, means re sponsive to current of frequencies other than said particular frequency for preventing the operation of said relay when voice or interference current which has a component of said particular frequency is transmitted over said line and means rendered efie'ctive by the operation of said relay for disabling said preventing means, and signal means controlled by said relay.

6. In a signaling system, a line, a trunk circuit comprising talking conductors normally connected to one end of'saidline, a generator of alternating voltages of a particular frequency for use in transmitting signals over said line, means normally connecting said. generator to said one end of said line, signal receiving means connected to the other end of said'line comprising a signal relay, means responsive to current of said particular frequency for operating said relay, means responsive to current of frequencies other than said particular frequency for preventing the operation of said relay in response to voice or interfering current which includes a component of said. particular frequency, means rendered effective by the operation of said relay to disable said preventing means, and means responsive to seizure of said trunk circuit for momentarily disconnecting said talking conductors from said line and for disconnecting said generator from said line to cause the release of said signal relay.

'7. In a, signaling system, a line, a trunk circuit comprising talking conductors normally connected to one end of said line, a generator of alternating voltage of a particular frequency for use in transmitting signals over said line, means comprising contacts of a signal transmittingrelay connectingsaid generator to said one end of said line, signal receiving means connected to the other end of said line comprising a signal receiving relay, means responsive to current of said particular frequency for operating said signal receiving relay, means responsive to current of frequencies other than said particular frequency'for preventing the operation of said relay in response a to voice or interfering current which includes a component of said particular frequency, means rendered efiective by the operation of said relay to disable said preventing means, means responsive to seizure of said trunk circuit for operating said signal transmitting relay to disconnect-said generator from said line thereby to effect the release of said signal receiving relay, means for momentarily releasing said signal transmitting relay to effect the operation and release of said signal receiving relay, means for releasing said signal transmitting relay to effect only the operation of said signal receiving relay, and means controlled by said signal receiving relay,

8. In a signaling system, a line, a trunk circuit comprising talking conductors normally connected to one end of said line, a generator of alternating voltage of a particular frequency for use in transmitting signals over said line, means comprising contacts of asignal transmitting relay connectingssaid generator to said one end of said line, signal receiving means connected to the other end of said line comprising a signal receiving relay, means responsive to current of said particular frequency for operating said signal receiving re lay, means responsive to current of frequencies other than said particular frequency for preventing the operation of said relay in response to voice or interfering current which includes a component of said particular frequency, means rendered effective by the operation of said relay to disable said preventing means, means for operating said signal transmitting relay to disconnect said generator from said line thereby to effect the release of said signal receiving relay, means for momentarily releasing said signal transmitting relay to eifect the operation and release of said signal receiving relay, means for releasing said signal transmitting relay to efiect only the operation of said signal receiving relay, means rendered effective by each operation and each release of said signal transmitting relay to open the connection between the talking conductors of said trunk circuit and said line, and means controlled by said signal receiving relay. 9, In a signaling system, a line, a trunk circuit comprising talking conductors normally connected to one end of said line, a generator of alternating Voltage of a particular frequency for use in transmitting signals over said line, means comprising contacts of a signal transmitting relay connecting said generator to said one end of said line, signal receiving means connected to the other end of said line comprising a signal receiving relay, means responsive to current of said particular frequency for operating said signal receiving relay, means responsive to current of frequencies other than said particular frequency for preventing the operation of said relay in response to voice or interfering current which includes a component of said particular frequency, means rendered effective by the operation of'said relay to disable said preventing means, switching means, means responsive to seizure of said trunk circuit for operating said signal transmitting re lay to disconnect said generatorfroni said line thereby to effect the release of said signal receiving relay to prepare said switching means for op eratio-n, means for momentarily releasing said signal transmit-ting relay to effect the operation and release of said signal receiving relay to selectively operate said switchingmeans, means for releasing said signal transmitting relay to eilect only the operation of said signal receiving relay to release said switching means, and means controlled by said signal receiving relay. i

10. In a signalin system, a line, a trunk circuit comprising talking conductors normally connected to one end of said line, a generator of a1- ternating voltage of a particular frequency for use in transmitting signals over said line, means comprising contacts of a signal transmitting relay connecting said generator to said one end of said line, means for operating said signal transmitting relay to terminate the transmission of current of said particular frequency over saidline, means for momentarily releasing said signal transmit ting relay to effect the transmission of impulses of current of said particular frequency over said line, means for releasing said signal transmitting relay thereby to continuously transmit current of said particular frequency over said line, and

means rendered effective by each operation and each release of said signal transmitting relay tolay connecting said generator to said one end of W said line, means for operating said signal transmitting relay to terminate the transmission of current of said particular frequency over said line, and means rendered effective by the opera- 7 tion of said signaltransmitting relay to momentarily open the connection between the talking conductors of said trunk circuit and said line.

12. In a signaling system, a line for transmitting, voice current and signaling current of a par ticular frequency in the voice frequency band and signal'receiving means connected to said line, said means comprising a thermionic amplifier, first and second tuned networks coupled to the output of said amplifier, said first network parallel resonant at the signal frequency and said second network series resonant at the signal frequency, means comprising said first network and a unidirectional device for applying a positive bias to the grid of said'amplifier responsive to current of the signal frequency incoming over said line, means comprising said second network and a unidirectional device for applying a negative bias to the grid of said amplifier responsive to current of frequencies other than the signal frequency incoming over said line, and a signal relay operatively controlled by said amplifier.

13. In a signaling system, a four-conductor line, one pair of the conductors for transmitting voice and signaling current in one direction and the other pair of conductors for transmitting voice and signaling current in the other direction, a trunk circuit at one end of said line havmg a pair of talking conductors, for transmitting voice current in both directions, means comprising a hybrid coil connecting said talking conductors to both pairs of conductors of said line, signal transmitting means associated with the pair of conductors of said line used to transmit voice current from the talking conductors of said trunk circuit and signal receiving means associated with the pair of conductors of said line used to transmit voice current to the talking conductors of said trunk circuit, said signal transmitting means comprising a generator of alternating voltage of a particular frequency used for signaling, means normally connecting the generator to the pair of conductors used to transmit voice currents from the talking conductors of said trunk circuit and a signal transmitting relay controlling the connection between said generator and pair of conductors, the operation of said signal transmitting relay being effective to terminate the transmission of current of said particular frequenc as a signal of a first character, the momentary release of said signal transmitting relay effecting the transmission of an impulse of current of said particular frequency as a signal of a second character, and the continued release of said signal transmitting relay effecting the transmission of current of said particular frequency as a signal of a third character, said signal receiving means comprising a signal receiving relay, means responsive to current of said particular frequency for effecting the operation of said signal receiving relay, means responsive to current of other frequencies for preventing the operation of said sig- 14 nal receiving relay in response to voice or interfering current which has a component of said particular-frequency and means rendered effective by the operation of said signal receiving relay to disable said preventing means.

14. In a signaling system, a four-conductor line, one pair of the conductors for transmitting voice and signaling current in one direction and the other pair of conductors for transmitting voice and signaling current in the other direction, current of the same particular frequency being used for transmitting signals in both directions, trunk circuits, one at each end of said line, each of said trunk circuits having a pair of talking conductors for transmitting voice current in both directions, means at each end of said line comprising a hybrid coil connecting the talking conductors of the associated trunk circuit to both pairs of conductors of said line, and signal transmitting means and signal receiving means at each end of said line, the signal transmitting means at each end being associated with the pair of conductors of said line which transmits voice and signaling current to the other end and the signal receiving means at each end being associated with the pair of conductors of said line which receives Voice and signaling current from the other end,'each of said signal transmitting means comprising a generator of alternating voltage of a particular frequency for use in transmittin signals over the associated pair of conductors, means normally connecting the generator to the associated pair of conductors of said line and a signal transmitting relay controlling the connection between said generator and pair of conductors, the operation of said signal transmitting relay being effective to terminate the transmission of current of said particular frequency as a seizure or supervisory signal, the momentary release of said signal transmitting relay efiecting the transmission of an impulse of current of said particular frequency as a selective or supervisory signal, and the continued release of said signal transmitting relay effecting the transmission of current of said particular frequency as a disconnect and idle signal, each of. said signal receiving means comprisin a signal receiving relay, means responsive to current of said particular frequency for effecting the operation of said signal receiving relay, means responsive to current of other frequencies for preventing the operation of said signal receiving relay in response to voice or interfering current which has a component of said particular frequency and means rendered effective by the operation of said signal receiving relay to disable said preventing means.

15. In a signalin system comprising a line, a generator of alternating voltage of a particular frequency for use in transmitting signals over said line, a signal relay for controlling the transmission of signals over said line, means including contacts of said relay normally connecting said generator to said line thereby to transmit current of said articular frequency over said I line while idle, means for limiting the amplitude transmission of current of said particular frequency as a disconnect signal, and means for disabling said amplitude limiting means during the transmission of selective, supervisory and disconnect signals. 7

16. In a signaling system comprising a line, a generator of alternating voltage of a particular frequency for use in transmitting signals over said line, a signal relay controlling the transmission of signals over said line, means including contacts of said relay normally connecting said generator to said line thereby to transmit current of said particular frequency over said line while idle, the operation of said signal relay being effective to terminate the transmission of current of said particular frequency as a seizure or supervisory signal, each momentary release of said relay being effective to transmit an impulse of current of said particular frequency as a selective or supervisory signal and the continued release of said relay being effective to initiate the continued transmission of current of said particular frequency as a disconnect sisnal, and means efiective to increase the amplitude of the current of said particular frequency nect signals as compared with the current normally transmitted, over the trunk when idle.

1'7, In a. signaling system, a trunk line, agenerator of alternating voltage of a particular signaling frequency used in transmitting signals over said line, means normally connecting said generator to said line thereby to transmit current of said signaling frequency over said line 16 while idle, means comprising a signal relay controlling the connection of said generator to said line, the operation of said relay being eifective to terminate the transmission of signaling current over said line as a seizure or supervisory signal, each momentary release of said relay eil'ecting the transmission of an impulse of signaling current as a selective or supervisory signal, and the continued release of said relay effecting the transmission of signaling current as a disconnect signal, means for limiting the amplitude of the signaling current transmitted over the line while idle, means for increasing the amplitude of the signaling current transmitted as selective, supervisory and disconnect signals and means effective a predetermined interval of time after the release of said relay to reduce the amplitude of the signaling current transmitted as a disconnect signal over said line to that of the signaling current transmitted over said line while idle.

25 transmitted as selective, supervisory and dlscon- Y NORMAN A. NEWELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,449,644 White Mar. 27, 1923 1,512,907 Conway Oct. 28, 1924 2,005,836 Almquist June 25, 1935 2,387,525 Murray Oct. 23, 1945

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