US2367518A - Signaling system - Google Patents

Description

Bam H6, i945. N. D. NEWBY SIGNALING SYSTEM Filed Aug. 19, 1943 5 Sheets-Sheet l /NVE/VTOR By N o. Nen/5y JWM ATTORNEY N. D. NEWBY 2,367,518

SIGNALING SYSTEM Filed Aug. 19, 194s s sheets-sheet 2 .Hm 116, i945.

/NI/ENTOR By N, a van/Br @60. M

ATTORNFV M, @945g N. D. NEWBY SIGNALING SYSTEM 3 Sheets-Sheet 34 Filed Aug. 19, 1943 Patented Jan. 16, 1945 UNTTED STATES PATENT OFFICE:

SIGNALING SYSTEM Neal DL Newby, Leonla, N. J., assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation oi' New York Application August 19, 1943, Serial No. 499,187

11 Claims. (Cl. 179-271 This invention relates to signaling systems and particularly to telephone systems arranged for voice frequency signaling.

Objects of the invention are the improvement of signaling systems in which signals consist of current impulses of each-of a plurality' of freyquencies in succession and the provision of a simple, efficient and dependable signal transmitting arrangement for such systems.

The invention is a signaling system in which signals consist of a plurality of alternating current impulses in succession, each impulse of a dilerent frequency, and in which an enablement or 'gate-opener signall consisting of an impulse of v current of one frequency followed byan impulse of current of another frequency is transmitted to enable response to selective or supervisory signals consisting of current cfa third frequency.- A feature of the invention is the provision of two delay networks in the, signal transmitter of such 'a system to control the transmission of the two impulses of current which constitute an enablement of signal, one. of the networks being eiective to measure the interval for the rst impulse of the enablement signal and the other network lbeing effective to measure the interval for the second impulse of the enablement signal.

A clear and complete description of the invention will be facilitated by considering a system arranged in accordance with the invention and its various features, one such system being shown in thdrawings which forms a part of this speciilcation. The invention is, however, not limited in its application to the system shown in the drawings but is generally applicable to signaling systems in which signals are transmitted by current of each of a plurality of frequencies in succession.

Referring to the drawings:

- Figs. 1 and 2 represent in a toll oflice of atelc-l phone system, an operators position OP, an outgoing trunk circuit OTI, a two-way trunk circuit.

oilices, from toll subscriberslines, -from community dial oiilces, and from other toll ofllces; and for extending incoming calls tolswitching trunks leading 'to called manual and "dial local oilices, to trunks to community d ial oiiices, to intertoll trunks, and to toll subscribers lines.

' Answering jacks are directly connected to recording trunks, toll subscribers lines, trimks from community dialI offices and V`intertoll trunks. Outgoing' jacks are connected to switching trunks,

l toll lines, trunks to community dial oilices and intertoll trunks. Toll route selectors and auxil-V iary toll route selectors are provided in one ou' more 'of the omce's for use on calls incoming over intertoll trunks to establish connections with called toll, local, or commmity Ydial ciices'fhe-` operators' positions are provided with dials for use in controlling the operation of incoming and auxiliary toll route selectors in other toll oiiices and for 'controlling the operation of selector and connector/switches in local or community dial oiilces. Reference may be had lto the patent to R. E. King et al., No. 2,209,777, granted July 30, 1940, for a disclosure of such a system in which composite signaling circuits are provided for the transmission of signals over the intertoll trunks Abetween the various toll ofiices.

The operators position equipment including the telephone, dial and cords, theoutgoing trunk circuit OTI, two-way circuit TWTI, toll route selector TSI, and incoming tnmk circuit ITI may be similar to like circuits disclosed inthe aforementioned patents to R. E. King etal. The signal receiver SRI may be similar to that disclosed in the copending application of N. D. Newby et al., Ser. No. 499,186, led Aug. i9, 1943. The toll line circuit TLCI comprises a repeat-- ing coil 2I2,'a directionally selective coil 2l I, and a cut-off relay 2N. for disconnecting the talking conductors of the "outgoing trunk 'circuit OTI from, and connecting the associated signal transmitter STI to, the intertoll trunk line TLI. The directionally selective coils 2| Itransmit signaling current incoming over toll line TLI through conductors 2 I3 and 2| l to the signal receiver SRI without substantial attenuation and transmit voice. currents' and signaling current from signal transmitter STI to the repeating coil 2I2 vwithout substantial attenuation. But the coils 2II prevent-any substantial portion of the signal energy transmitted by the transmitter STI from passing into the signal receiver SRI ,thereby to prevent false operations and to prevent interference with the response 'of the signal receiver SRI to incoming'sinals.

'Ihe signal transmitter STI comprises signal relays 230 and 23| operatively controlled by relay of the outgoing trunk circuit OTI, relays 232 and 234 controlled by relay 23|, relays 240, 24|, D and 25| controlled by relay 230, relay 260 jointly controlled by relays 234, 24| and 25|, two delay networks NI and N2, and a transformer 236 through which impulses of current of each of three signaling frequencies are transmitted under the direct control of relays'232, 240, 250 and 260. The upper winding of relay 232 is normally connected in series with condenser 233; and, when relay 23| operates, condenser 233 charges through the lower winding of relay 232 to effect the operation ofv relay 232 for a desired interval of time. The network NI delays the response of relays 240 and 24% to each operation and each release of relay 230 for a predetermined desired interval of time, and the network N2 further delays the response of relays 25|) and 25| for a desired predetermined interval of time, thereby to measure the intervals during which impulses of current of each of the Various frequencies are transmitted. The value of such delay networks is in the independence of the delay interval from variations of battery potentials and variations in pulsing speed. These are low pass lters and the pulsing frequency is less than the cut-off frequencies of 'the lters. With this type of delay circuit the delay may be greater than the time between pulses if required. Relays 230, 240 and 250 are polarized relays each having a biasing winding and one or two operating windings. The contacts are shown in normal position, and each of these relays is referred to herein as being released when the contacts are in thisrposition; Relaysi232' and'260 control the circuit for operating the cut-10E relay 2 I0 of the toll line circuit. Each of the sources of signaling current GI, G2 and G3 may be a vacuum tube oscillator or other known source of current of desired frequency. l y f The signal transmitter `STI is arranged to transmit seizure, selective, ringing and disconnect signals on calls outgoing over trunk line TLI, and to transmit off-hook, all-trunks busy, called-linebusy, on-hook and disconnect-acknowledgement signals on incoming calls. A seizure signal consists of an impulse of current of frequency fl followed by an impulse of currentof frequency f2. A train of dial impulse signals is preceded by an enablement signal consisting of an impulse of current of frequency f2 followed by an impulse of current of frequency fl. Each dial impulse of the train is transmitted by current of frequency f3; and current of frequency ,fl is transmitted at the end of each dial impulse until the next dial impulse begins or for `a predetermined interval following the last dial impulse of a train. A ringing signal consists of the enablement signal followed successively by an impulse of current of frequency ,f3 and an impulse of current of frequency fl, the same as for a single dial impulse. A disconnect signal consists of the enablement signal followed by a relatively long impulse of current of frequency f3 and differs from a single dial impulse signal in that the current of frequency f3 is not followed by current of frequency fl. An offhook signal, transmitted when an incoming call is answered, is-similar to a seizure signal on an outgoing call. ,An on-hook signal is similar to a disconnect signal. An all-trunks-busy gr calledline-busy condition effects the alternate transmission of off-hook and on-hook signals.

Assume now that relay of trunk circuit OTI has operated responsive to insertion of the calling plug CPI of a cord CDI into jack J I in the manner described in the aforementioned King et al. patent. The operation of relay IIO closes a circuit through conductor I|5 for operating relays 230 and 23| of signal transmitter STI. 'I'he operation of relay 23| closes a circuit for charging condenser 233 and operatively energizing the lower winding of relay 232. The operation of relay 232 causes the operation of relay 234 and the release of cutofi relay 2 l0 of line circuit TLCI, thereby to connect the transformer 236 to the toll line TLI. The operation of relay 232 closes a circuit from signaling current source GI, through the left contacts of relays 240 and 25D, left winding of transformer 236, to groundat a front'contact of relay 232, thereby transmitting currentof frequency fl through transformer 236 and coils 2| I and 2I2 to toll line TLI. The operation of relay 230 opens the normally closed circuit through the lower windings of relays 24|) and 250 and closes the circuit through the middle winding of relay 245, winding of relay 24|, middle winding of relay 250 and winding of relay 25|, The delay network Ni delays the response of relays 240 and 24| to the operation'of relay 230 for about .025 second after the operation of relay 232, the operation of relay 240 being effective to connect the signaling current source G2 to the lower winding of transformer 236 in pla-ce of source GI, thus ending the transmission of current of frequency ,fl and initiating the transmission of current of frequency f2. The upper winding of relay 250 is normally energized under the control of relay 280; and as long as this winding is energized, the contacts of relay 25|! are held in normal position.' 'I'he networks NI and N2 delay the response of relay 25| tc the operation of relay 230 by the seizure signal and delays the Vresponse of both of relays 250 andrrZIa/I to operations of relay 230 ocqurringvafter the upper winding of relay 250 is deenergized as hereinafter described. The operation of relay 25| connects ground to one side of the lower winding of relay 260, but this winding .is not energized until relay 24| releases as hereinafter described. Relay 232 releases when the current charging condenser 233 falls below the operating level, this being about .100 second after relay 232 operates. 'Ihe release of relay 232 opens the circuit through the left winding of `transformer 236 to terminate the transmission of current of frequency f2, the impulse of current of frequency f2 being about .050 second in length. Thus the seizure of trunk circuit O Tl effects the transmission of a seizure signal consisting of current of frequency fl for about .025 second followed by current of frequency f2 for about .050 second.

When relay is alternately released and reoperated responsive to Ytrains of dial impulses created by the dialing of the digits of the toll route code and local office code, and also the called subscribers number on calls to subscribers in dial offices, relays 23| and 23| are alternately released and reoperated. The release of relay 230 in response to the rst dial impulse of each train opens the circuit for energizing the middle windings of relays 240 and 23| and the windings of relays 24| and 25| and closes the circuit for energizing the lower windings of relays 24| and 25|, the release of relays 24| and 24| being delayed by network Nl and the release of relay 2li being delayed vby both of networks N| and N2. The release of relay 23| closes a circuit for discharging condenser 233 through the lower windlng of relay 232, thereby to operate relay 232 for about .100 second. The operation of relay 232 lease.

causes the operation of relay 234, causes'the release of cut-ofi relay 2 I 0, and closes a circuit from l ground at the inner front contact of relay 232,

4 ate the transmission of current of frequency f| Since rethrough transformer 236 and line TLI. lay 25| is operated at the time that relay 24| releases, the lower winding of relay 280 is thereallowable dial impulse plus the delay introduced by both of the delay circuits Nl and N 2, thereby insuring that an impulse of frequency fl follows the current of frequency ,f3 representing the lastd dial impulse ofthe train. The release of relay 263 closes the circuit for operating the cut-olf relay 2II opens the circuit energizing the upper winding of relay 240 and closes the circuit through the upper winding of relay 250. At this time, relays 230 and 23| are operated, relays 232, 234 and 260 are released, relays 240, 24| and 25| are operated, relay 263 is released and relay 2|! is operated. Thus the two networks N I and N2 are effective to delay the response of relays 240 by operatively energized. Relay 26|! closes a locking circuit through its upper winding under the control of relay 234, opens the circuit through the upper winding of relay 250 to render this relay responsive to each release and reoperation of relay 230, and closes a circuit for energizing the upper winding of relay 240 thereby to prevent the reoperation of relay 240 While relay-230 is re spending to the succeeding dial impulses of a train. The cut-on relay 2li) cannot reoperate until both of relays 232 and 260 releases; and the signal transmitting circuit through the left winding of transformer 2 36 remains closed as long as either of relays 232 and 260 remains operated. About .025 vsecond after relays 240 and 24| release, relay 250'operates dye to the deenergization of its middle winding and energization-of its lower winding. At the same time relay 25| releases opening the operating circuit through the lower winding of relay 260, relay 280. being heldoperatedainder the control of relay 234. The

operation of relay 250 connects the signaling curr rent .source G3 to the lower winding of, transand 250 to each release and reoperation of relay 230 for an interval suicient to permit the sending of an enablement or gate-opener signal consisting of current of frequency f2 for about .025

second followed by current of frequency fl for about .025 second preceding the current of fre- Y quency f3 which represents the first dial impulse of a train. .y

To transmit a ringing or rering signal, relay IIIl of outgoing trunk circuit is momentarilyV released due to the operation of a ringing key by` the calling operator in the manner set forth 'in the aforementionedKing et al. patent, thereby effecting the momentary release and reoper'ation of relays 230 and 23|. STI is thus rendered eil'ective to transmit a ringing signal which consists of the enablement signal 4followed by current of frequency f3 for the duration of' the release of relay 230 followed by current of frequency fI until relay 260 releases. apparent that the transmission of a ringing signal is effected inthe same manner as that of a dial impulse train consisting of a single dial impulse.

When the. calling operator removes the plug i CPI from'jack J I, relay |I0 is released in the former236 in place of source GI thereby terminating the transmission of current of frequency fi and initiating the transmission of current of frequency f3 through transformer 236 and toll line TLI. Since the charging and discharging of condenser 233 is effective to maintain theoperative energization of the one or the other of the manner -set forth in the aforementioned King et al. patent, thereby causing the release of relays 230 and 23|. Th'e release of relays 230'and 23| causes the transmission of the enablement signal consisting of currentof frequency f2 for about 1025 second followed by current of frequency fI for about .025 second and causes the transmis-A sion of current of-.frequency f3 immediately folwindings of relay 232 while relay 23| is responding to a train of dial impulses, relay 232 remains f in response to each release of relay 230. When relay 250 releases, about .050 second after the reoperation of relay 232 at the end of each dial impulse, it ends the transmission of current of frequency f3 and initiates the transmission of current of frequencyfI. ates, about .050 second after the reoperation of When relay 250 reoperflowing the enablement signal. The current' of frequency .f3 is not followed by current of'frequency fI, as in the case of the' transmission of a single dial impulse, because relays 230 andv 23| do not reoperate: and when relay 260 releases, it ends the transmissionA of current of frequency f3. While the signal transmitter STI is not so arranged, it may readily be modified to effect the .repeated transmission of the disconnect signal vuntil a disconnect-acknowledgment signal is received by the signal receiver SRI, in which case the signal transmitter is also arranged to transmit a disconnect-acknowledgment signal on in- -oming callsresponsive to a disconnect signal being received by the signal receiver SRI,

relay 230 at the beginning of each succeeding dial and 230 are slow in releasing so that the interval elapsing between the reoperation of relay 23| and the release of relay 230, at the end of the last impulse of a train, is greater than the longest ,I A#0n an incoming call, the relay 220 of signal receiver SRI is operated by a seizure signal transmitted over line TLI fromfa signal transmitter at its 'distant end. Reference may be had to the aforementioned copending application of N. D.

.Newby et al. for a'description of the manner `in which relay 223 is operated responsive to an incoming seizure signal. The operation of relay 22Il4 connects ground to conductor I I4 thereby causing the operation of relays |20, I2I and I 22 of, twoway trunk circuit TWTI in the manner described in the aforementioned King et al. patent. Relay |22 connects groundto conductor |33to hold the 'Ihe signal transmitterA It isI connection thereafter established. Relay 220 repeats dial impulses over conductor I4 to operate selector TSI inthe manner set forth in the aforementioned King et al. patent. Assuming the call is completed through selector TSI to an incoming trunk circuit ITI, relay |6| is operated, in series with relay |2| of two-way trunk circuit TWTI, to light the answering lamp AL. When an operator answers the call by inserting the answering 'plug of a cord in jack AJ, relay |64 is operated in the manner described in the aforementioned King et al. patent to connect ground to conductor |54,

thence through brush |44 of selector TSI, and

conductor |34, to operate relay |24 of two-way trunk circuit TWT|. The operation of relay |24 closes a circuit for operating relays 236 and 23| of signal transmitter ST| thereby to eect the transmission of .an off-hook supervisory signal over line TLI in the same manner that a seizure signal is transmitted on an outgoing call as hereinbefore described. If the answering operator disconnects, relay |64 releases and an on-hool: supervisory signal is transmitted by signal transmitter STI in the same manner that a disconnect signal is transmitted on an outgoing call as hereinbefore i described. If the selector TSI encounters an alltrunks-busy condition, or if the called subscribers line is busy on' a call to a subscriber in a dial office, relay |24 is alternately operated and released in the manner set forth in the aforementioned King et al. patent, thereby to cause the transmitter STI to send a succession of off-hook and on-hook signals over line TLI.

Assume now that the toll line circuit TLC2. signal transmitter ST2 and signal receiver SR2, shown in Fig. 3, are associated with the trunk circuit OTI of Fig. l in place of the corresponding circuits shown in Fig. 2. 'I'he signal transmitter` ST2 uses rectifier arrangements in place of relay contacts for initiating and ending the transmission of impulses of current of each of the three signal frequencies. The sources GI, G2 and G3 of frequencies Il, f2, and ,f3 are connected by transformers 340, 350 and 360, pairs of rectiilers 343 and 344', 355 and 356, and 365 and 366, and transformers 349, 359 and 369 to contacts of the cut-off relay 3 0 of line circuit TLC2. 'I'he transmission of current from each of these sources to toll line TL2 depends upon whether the associated pair of rectifiers has -a high or a low impedance, the impedance of each pair of rectifers being high when there is substantially no direct current therethrough and low when the direct current therethrough is sufficiently large to effect such a result. The impedance of the three pairs of rectiers is controlled by relays 335, 336 and 331 as hereinafter described. The lowest Windings of relays 335 and 331 are not required unless it is necessary to bias the dial impulse signals to correct for distortion.

When relay H0 operates responsive to seizure of outgoing trunk circuit OTI, it closes a circuit for operating relays 333 and 33| of signal transmitter ST2. The operation 'of relay 333 opens the normally closed circuit through the middle windings of relays 335 and 331 and closes a circuit for energizing the upper windings of these relays, -the response of relay 335 to each operation and release of relay 333 being delayed by network NI 'and the response of relay 331 being delayed'by both of networks NI and N2.. The operation of relay 33| connects condenser 333 to the lower winding of relayl332 whereby relay 332 is operated by the current charging condenser 333 for about .100 second. The operation of relay 332 causes the operation of slow-to-release relay 334 and the release of cut-off relay 3|IJ of line circuit TLC2. vWith relay 334 operated and relay 3|0 released, the transformers 349, 358 and 363 are connected through coils 3H and 3|2 to toll line TL2. At this time a direct current circuit exists from ground through the back contact of relay 335, conductor 336, resistors 34| and 342, rectiers 343 and 344, resistors 341 and 348, and through resistor 345 to negative battery. The current in this circuit is of sufficient amplitude and has the proper polarity to reduce the impedance of' rectifiers 343 and 344 whereby current of frequency ,fl is transmitted from s'ource GI, through transformer 34D, rectifiers 343 and 344 and transformer 349 to toll line TL2. At this time there is a direct current circuit through resistor 353 and rectifier 354 and the current therein reduces the impedance of rectifier 354 so that it constitutes a substantial short circuit across thc rectiers 355 and 356; and the high impedance of rectiers 355 and 356 prevents the transmission of current of frequency f2 from source G2 through transformer 359 to line TL2. At this same time, negative battery is connected through resistors 363, 362 and 36| to one side of these rectiiiers and a like potential is applied through resistor 345, rectifier 346 and resistors 361v and 368 to the other side of these rectiilers; and since there is no direct current through rectifier 365 and 366, the high impedance of rectiflers 365 and 366 prevents the transmission of current of frequency f3 through transformer 369 to line TL2. When relay 335 operates, about .025 second after 332 operates, it opens the direct current circuit through rectiers 343 and 344 and the impedance of these rectiflers increases to end the transmission of current of frequency f| to line TLI. The operation of relay 335 also opens the direct current circuit through rectifier 354, whereby the impedance of rectifiers 355 and 353 is reduced by direct current in a circuit from battery through resistors 353, 352 and 35|, rectiflers 355 and 356, and resistors 351 and 353 to ground. The reduction of the impedance of rectiiiers 355 and 35i effects the transmission of current of frequency f2 from source G2 through transformers 350 and 35S-to toll line TL2. When relay 332 releases, about .10Q second after its operation, it closes the circuit for reoperating the cut-oft relay 3|0, thereby ending the transmission of current of frequency f2 to line TL2. yThe release of relay 332 also causes the release of relay 334. The operation of relay 331 about .025 second after the operation of relay 335 is ineffective to cause the operation of relay 336 while relay 335 remains operated. Thus a seizure signal, lconsisting of an impulse of current of frequency fl for .025 second followed by an impulse of current of frequency f2 for about .050 second, is transmitted over line TL2.

When a train of dial impulses is repeated by relay ||3 of outgoing trunk circuit OTI, relays 333 and 33| are alternately released and reoperated. Each release and reoperation of relay 330 causes a delayed releaseand reoperation of relays 335 and 331. The iirst release of relay l33| causes the-operation of relay 332 and each succeeding reoperation and release o! relay 33| causes the alternate energization of the two windings of relay 332 so that relay 332 remains operated during the response of relay 33| to dial 'impulses The operation of relay 332 causes the operation of relay 334 and the release of cut-oft relay 3III; and relay 334 connects the right windings of transformers 346, 363 and 366 `to the line VTL2. whereby current of frequency 12 is transmitted from source G2 through transformer 353,

line TL2. When relay 335 releases ln response to the first -dial impulse about .025 second after relay 332 operates, ground is reconnected to conductor 338 thereby ending the transmission of current of frequency f2 and initiating the `transmission of current of frequency fl through rectiiiers 343 and 344l andtransformer 346 to-line TL2. Since relay 331 remains operated for about .025 second after relay 335 releases, the release of relay 335 opens the short circuit across the lower winding of relay 336 thereby eifecting the operation of -relay 336. Relay 336locks through its upper winding under control of relay 334, connects ground to'conductor 333 and connects conductor 336l to the back contact of relay 331. When relay 331 releases, .025 second after relay 335 releases in lresponse to the first dial impulse,

nals on outgoing calls until a disconnect-acknowledgment signal is received by the associated sig--A nal receiver and to transmit a disconnectacknowledgment signal on incoming calls in response to a disconnect signal incoming to the associated signal receiver. These and other modifications are within the scope of the invention.'

What is claimed is:

1. In afvoice frequency signaling system; a signal transmitter for converting direct current signais into alternating current signals, said transground is connected to conductor 3,39; whereby direct current through rectifier 346 reduces the impedance of rectifier 346 so that rectiiiers 343 and 344 are short-circuitedand their impedance becomes high to end the transmission of current of frequencyfl. The connection ofground to 'conductor 339 also results in a direct current through rectiers 365 and 366; whereby the impedance of these rectiners is reduced to initiate the transmission of current of frequency l3`from source G3 through transformer 366. rectiflers 365 and 366 and transformer 363 to toll line TL2. The

current of frequency f3 continues until relay 331 reoperates responsive to the reoperation of relay 330 at the end of the rst dial impulse. Since relay 336 maintains the connection of groundto conductor 338 until all the dial impulses in the 'train have been received by relays 330 and 33| and transmitted over toll line TL2, the operation and release of relay 335 in response to succeeding dial impulses of the train are without eect. When relay 331 reoperates responsive to the release of relay 330 at the end of the first dial impulse, conductor 339 is disconnected from ground so that transmission of current of frequency f3 isended and transmission of' current operated so that the current of frequency fl continues until '-.relay 334 releases about .150 second after the reoperation of relay 331. The release 4 of relay 334 causes the release of relay '336 and the reoperation of relay 310.'

A ringing signal is transmitted. in the same manner that the transmission of one dial impulse is effected responsive to the dialing of the digit 1. A disconnect signal is transmitted in the same manner that the transmission of one dial impulserepresenting the digit 1 -is`eected, except that thecurrent of frequency ,f3 is not followedby current of frequency fl; since relay 331 remains released, the currentl of. frequency f3 continues imtil relay -334 releases.

the last impulse of the train,.re1ay 331 remains While the signal transmitter shown. mmHg-3 is not so arranged, it may readily be modied to repeat the transmission of the 'disconnectsigsaid second relay, a battery for use in operatingl said third and fourth relays, a delay network connected in series with said battery and the contacts of said second relay and the windings of said third and fourth relays to delay for -a predetermined interval of time the response of said third and fourth relays to operations of said second relay, another delay'network connected in series with said battery and the windings of said third and fourth relays to further delay for a predetermined interval ef time the response of said fourth relay to operations of said second relay, means including said first relay for transmitting an impulse of current of a ilrst one of the 'three frequencies, vmeanscomprising said third relay for terminating the impulse of current of said first` frequency and fortransmitting an impulse of current of a second one of the three frequencles, and means comprising said fourth relay for terminating the impulse of current of said secv ond frequency and for transmitting an impulse of current of the third frequency.'

2. In a voice frequency signaling system, a signal transmitter comprising relay means responsive to trains of impulses corresponding to numerical digits, flrst and second relays controlled by said relay means, a first delay network delaying the response of said first relay to the operation of said relay means in response to the first lmpulse ofa. numerical train, asecond` delay network, said irst and second delay networks delaying the response of said second relay to the opera- .tion of said relay means in response to the irst impulse of a numerical train, means controlled by the operation of said relay means in response to the first impulse of a numerical train for initiating the transmission of an impulse of current of a iirst one of three signaling frequencies, means effective upon operation of said first relay to ter-A minate the impulse of current of said first frequency and to initiate an impulse of current of a second one of said three frequencies, means effeci tive upon operation of said second relay to terminate the impulse of current of said second frequency and to initiate an impulse of current of the third of said frequencies, means comprising said second relay for-terminating the transmission of the impulse of said third frequency after an4 interval of time equal fto the length of the first vnumerical impulse of the train -received by said relay means and for thereafter alternately eilect.. ing the transmission of impulses of current of said second and third frequencies for intervals o f time respectively equal'to the duration of the intervals a between numerical impulses vin'a train and the duration of thenumerical impulses and for transmitting an impulse of current of said second frequency of predetermined duration following. the 'impulse of current of said third frequency corresponding to the last numerical impulse of a train received by said relay means.

3. In combination, signal receiving means comprising a first relay and a second relay controlled by direct current signals, thre`e sources of alternating current, each source of a different frequency, means comprising a third relay and a fourth relay for transmitting alternating current signals, a battery, a first plurality of retard coils and condensers, one winding of each of said retard coils being connected inseries with said battery and a front contact of said first relay and one winding of said third relay, another winding of each of said retard coils being connected in series with said battery and a back contact of said iirst relay and another winding of said third relay, and each of said condensers being connected across the windings of said retard coils to form with said retard coils a delay network for delaying for a predetermined interval of time the response of said third and fourth relays to each operation and release of said rst relay, a second plurality of retard coils and condensers, one winding of each of said' second plurality of retard coils being connected in series between said one winding of said third relay and one winding of said fourth relay, another winding of each of said second plurality of retard coils being connected in series between said other winding of said first relay and another winding of said fourth relay, and each of said second plurality of condensers being connected across the windings of said second pluralitY of retard coils to form with said retard coils a second delay network for delaying the response ofsaid fourth relay to each operation and release of said first relay for a lpredetermined interval of time after the response of said third relay, means rendered effective by the release of said second relay to initiate the transmission of cur rent from a first one of said sources, means rensaid other winding of said ilrst relay and anotherwinding of said fourth relay, and each of said second plurality of condensers being connected across the windings of said second plurality of retard coils to form with said retard coils a second delay network for delaying the response of said fourth relay to each operation and release of said first relay for a predetermined interval of time after the response of said third relay, means rendered effective by the release of said second relay to initiate the transmission of current fromv a rst one ci said sources, means rendered effective by the response of said third relay to the release of said rst relay to end the transmission of current from said first source and to initiate the transmission of current from a second one of said sources, means rendered effective by the response of said fourth relay to the release of said first relay to end the transmission of current from said second source and initiate the transmission of current from the third one of said sources, and means rendered effective by the continued release of said second relay to end the transmission Iof current from said third source.

5. In combination, signal receiving means comprising a first relay and a second relay controlled by direct current signals, three sources of alternating current, each source of a different frequency, means comprising a third relay and a fourth relay for transmitting alternating current signals, a first plurality of retard coils and dered effective by the response of said third relay to the release of said -iirst relay to end the transmission of current from said first source and to initiate the transmission of current from a second one of said sources, and means rendered effective by the response of said fourth relay to the release of said iirst relay vto end the transmission of current from said second source. and to transmit current from the third one of said sources.

4. In combination, signal receiving means comprising a first relay and a second relay controlled by direct current signals, three sources of alternating current, each source of a different frequency, means comprising a third relay and a\ fourth relay for transmitting alternating current signals, a first plurality of retard coils and condensers, one winding of each of said retard coils being connected in series between a front contact of said first relay and one winding of said third relay, another winding of each of said retard coils being connected in series betweena back contact of said ilrst relay and another winding of said third relay, and each ofsaid condensers being connected across the windings of said retard coils to form with said retard coils a delay network for delaying for a predetermined interval of time the response of said third and'fourth relays to each operation and release of said first relay, a second plurality of retard coils and condensers, one winding of each of said second plurailty of retard coils being connected in series between said one winding of said third relay and one winding of said fourth relay, another winding of each ofisaid second plurality of retard coils being connected in series between condensers, one winding'of 'each of said retard coils being connected in series between a front contact of said first relay and one winding of said third` relay, another winding of each of said retard coils being connected in series between a back contact of said first relay and another winding of said third relay, and each of said condensers being connected across the windings of said retard coils to form with retard coils a delay network for delaying for a predetermined interval of time the response of said third relay to each operation ad release of said ilrst'relay, a second plurality of retard coils and condensers, one winding of each of said second plurality of retard coils being connected in series between said one winding of said third relay and one winding of said fourth relay, another winding of each of said second plurality of retard coils being connected in series between said other winding of said ilrst relay and another winding of said fourth relay, and each of said second plurality of condensers being connected across the windings of said second plurality of retard coils to form with said retard coils a second delay network for delaying the response of said fourth relay to each operation and release of said first relay for a predetermined interval of time after the response of said third relay, means responsive to the operation of said second relay for initiating the transmission of current from a first one of said sources, means rendered effective by the response of said third relay to the operation of said first relay to end the transmission oi' current from said ilrst source and initiate the transmission of current from a second one of said sours, means effective a predetermined interval of time after the operation of said second relay for ending the transmission of current from-said second source, means responsive to the release of said second relay for initiating the transmission of current from said second source, means rendered elective by the response of said third relay to the release o! said '1. In combination, signal receiving means comrst relay to end the'transmission of current from said second source and initiate the transrendered elective by the response of said fourth relay to the release of said first relay to end the transmission of current from said second source and initiate the transmissio of current from the third of said sources, and means responsive to the continued release of said second relay for ending the transmission of currentl from said third source.

6. In combination, signal receiving means comprising a first relay and a second relay controlled by direct current signals, three sources of alternating current, each source of a different frequency, means comprising a thirdrelayand a fourth relay for transmitting alternating current signals, a iirst plurality of retard coils and condensers, one winding of each of said retard coils being connected in series between a front conoperation and release of said first relay, a second plurality of retard coils and condensers, one winding of each of said second plurality of retard coils being connected in series between said one winding of said third relay and one winding of said fourth relay, another winding of each of said second plurality of retard coils being connected in series between said other winding of said first relay and another winding of said fourth relay, and

cach of said second plurality of condensers being connected across the windings of said second plurality of retard coils to form with said retardJ coils a second delay network for delaying the response of said fourth relay to each operation and release of said first relay for a predetermined interval of time after the response Xof said third relay, means responsive to the operation o f said second relay for initiating the transmissionof current from a iirst one of said sources, means rendered effective by the'response of said third relay to the operation of said firstl relay to end the transmission of current from said first source and initiate the transmission of current from a second one of said sources, means eiective a predetermined interval of time after theA operation of said second relay for ending the transmission of current from said second source, means responsive to the release of vsaid seconli relay for initiating the transmission of current from said second source, means rendered effective by the response of said third relay to the release of said first relay to end the transmission of current from said seco d source and initiate the transmission of curre t from said rst source, means rendered effective by the response of said fourth relay to the release of said first relay to end the transmission of current from said second source and initiate the transmission of current from the third of said sources, means responsive to the reoperation of said fourth relay within a predetermined interval of time after its release for ending the transmission of current from said third source and for initiating the transmission of current from said iirst-source,` and means responsiveto,

the continued operation of said second relay for source.

'mission of current from said rst source, means prising a rst relay and a second relay controlled by direct current signals, three sources of alternating current, each' source of a different frequency, means comprising a third relay and a fourth relay controlled by said first and second relays for transmitting alternatingcurrent signais, a first plurality of retard coils and condensers, one winding of each of said retard coils being connected in series between a front contact of said rst relay and one winding of said third relay, another winding of each of said retard coils being connected in series between a back coning of each of said second plurality of retard coils being connected in series between said one winding of` said third relay and one winding of said fourth relay, another winding of each of said second plurality of retard coils being connected in series rbetween said other winding of said-.mst relay and another. winding of said fourth relay,

and each of said second plurality of condensers being connected across the winding of said second plurality of retard coils to form with said retard coils a second delay network for delaying the re sponse of said fourth relay to each operation and release of said iirst relay for a predetermined interval of timeafter the response of said third relay, means responsive to the operation of said a second relay for initiating the transmission of current from a first one of said sources, means rendered eiectlve by the response of said third relay to the operation of said first relay to` end thel transmission of current -from said first source and initiate the transmission of currentfrom asecond one of said sources, means effective a pre-` determined interval of time after the operation of said second relay for ending the transmission of current from said second source, means responsive to the release of said second relay for initiating the transmission of current from said second source, means rendered effective by the response of said third relay to the release of said iirst relay to end the transmission of current from said second source'and initiate thev transmission of current from said first source, means rendered eifective by theI response of' said fourth'relay to the release of said iirstrelay to end the transi mission of current from said iirst source and inr itiate the transmission of current from the third of said sources, means responsive to the reoperation of said fourth relay within a predetermined interval of time after its release for ending the 'transmission' of current from said third source and for initiating the transmission of current from said first source, means responsive to the continued operation of said second relay for ending the transmission of current from said iirst source, and means responsive to the continued'release of said second relay-f for endingthe transmission of current from said third source.

8. In a voice frequency signaling system, a signal transmitter for converting dlrectcurrent sigv nals into alternating current signals, said translmitter comprising first and second relays rel,sponsive to the closing and opening .of a direct current signaling circuit, three sources of alternating current, each source of a different frequency, means including third and fourth relays controlled by said second relay, a battery for use in operating said third and fourth rel'ays, a delay network connected in series with said battery and the contacts of said second relay and the windings of said third and fourth relays to delay for a predetermined interval of time the response of said third and fourth relays to operations of said second relay, another delay network connected in series with said battery and the wind`l to the operation and responsive to the release of fourth relay for transmitting alternating current signals, a battery, a first plurality of retard coils and condensers, one winding of each of said retard coils being connected in series with said bat tery and a front contact of said rst relay and one winding of said third relay, another Winding of each of said retard coils being'connected in series with said battery and a back contact of said first relay and another winding of said third relay, and eachv of said condensers being connected across the windings of said retard coils to Y form with said retard coils a delay network for delaying for a predetermined interval of time the response of said. third and fourth relays to each operation and release of said first relay, a second plurality of retard coils and condensers, one winding of each of said second plurality of retard coils being connected in series between said one winding of said third relay and one winding-of said fourth relay, another winding of each of said said first relay, a line, means including a sixth relay operated responsive to the operation of said fifth relay for initiating the transmission of current of a first one of three signaling frequencies over said line, means responsive to the operation of said third relay for terminating the transmission of current of said first frequency and for initiating the transmission over said line of current of a second one of said signaling frequencies, means controlled by said -fifth relay for ending the transmission of current of said second frequency a predetermined interval of time after the operation of said first relay and for initiating the transmission of'current of said second frequency responsive to the release of said first relay, means including a seventh relay operated responsive to the release of said third relay while said fourth relay is operated, the release of said third relay a predetermined interval of' time after the release of said second relay being effective to end the transmission of current of said second frequency and initiate the transmission of current of said first frequency, means including said sixth relay for holding said seventh relay independent of said third and fourth relays, and means responsive to each release of s aid fourth relay while said seventh relay is operated for ending the transmission of current of said first frequency and for initiating the transmission of current of said third frequency and responsive Y' to each operation of said fourth relay while said second plurality of retard coils being connected Y in series between said other winding of said rst relay and another winding of said fourth relay, and each of said second plurality of condensers being connected across the windings of'said second plurality of retard coils to form with said retard coils a second delay network' for delaying the response of said fourth relay to each operation and release of said first relay for a predetermined interval of time after the response of said third relay, said network being so arranged and connected that said delay intervals are independent of variations in the voltage of said battery.

[10. In combination, signal receiving means comprising a first relay and a second relay, both controlled by direct current signals, third and fourth relays controlled by lsaid secondrelay, first and second delaynetworks, each consisting of a plurality of retard coils and condensers bridged across the-windings of the retard coils, said first delay network connected between the contacts of said second relay and the windings of said third relay and said second delay network connected between said windings of said third relay, and windings of said fourth relay', whereby the response of said third' relay tothe operation-and release of 'said second relay is delayed for a predetermined interval of time and the response of said fourth relay to the operation and release of said second relay is delayed for a further predetermined interval of time, a fifth relay, means seventh relay is operated for ending the transmission of said thirdl frequency and initiating the,

between said windings of said third relay and windings of said fourth relay, whereby the response of said third relay to the .operation and release of said second relay is delayed for a predetermined interval of time and the response of said fourth relay to the operation and release of said second relay is delayed for a further predetermined interval of time, a fifth relay, means including a condenser and contacts of said first relay for operatively energizing said fifth relay for a predetermined interval of time responsive to the operation and responsive to the release of said first relay, a line, a source of current of a first frequency, a source of current of a second frequency, a source of current of a third frequency, means including variable impedance rectifiers individual to each of said sources for connecting said sources to said line, means includingv a sixth relay operatively controlled by said fifth relay for completing the connection between said Asources and said line, means for reducing the creasing the impedance of the rectifiers associated with said second source when said third relay operates to end the transmission of current of said rst frequency and initiate the transmission of current of said second frequency over said line, a seventh relay, means rendered effective by the release of said iifth relaywhile said seventh realy is normal for ending the transmission of current of said second frequency without changing the impedance of the rectiers associated with said second source and rendered eiective by the reoperation of said fifth relay responsive to the release of said rst relay for again initiating the transmission over said line of current of said second frequency, the release of said third relay being effective to increase the impedance of the rectiers associated ,with said second source to end the transmission of current of said second frequency and to decrease the impedance of the rectiers associated with said rst source to initiate the transmission of current of said first frequency over said line, means for operating said seventh relay when said third relay releases while CTI initiate the transmission said fourth relay is operated, means for locking said seventh relay under control of said sixth r'elay, and means effective while said seventh relay is operated to prevent the reducing of the imp edance of the rectiers associated with said second frequency, toy increase the impedance of the rectiers associated with said rst source and reduce the impedance of the rectiilers associated with said third source each time said fourth relay releases to end the transmission of current of the first frequency and initiate the transmission of current of the third frequency, andto increase the impedance of the rectifiers associated with said third source and decrease the impedance of the rectiers associated with said first source each time said fourth relay operates to end the transmission of current of the third frequency and of current of the rst frequency, the release of said sixth relay being effective to end the transmission of current of either said rst or said third frequencies.

NEAL D. NEWBY.

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