US2367792A - Signaling system - Google Patents

Description

Jam. Z39 3945,. H. M. PRUDEN SIGNALING SYSTEM 9 sheets-sheet 1 Filed Aug. 19, 1945 Im 239 w45. H, M, UDEN 367,792

SIGNALING SYSTEM Filed Aug. 19, 1943 9 Sheets-Sheet 2 /A/ l/E/VTR H M.' PRUDEN A .TTOPNEV JM. 23? @945. H, M PRUDEN v 2,367,792

SIGNALING SYSTEM Filed Aug. 19, 1943 9 Sheets-Sheet 3 I :lc/VAL 1mm-MITTE@ v /NVENTOR H M PRUDEN By @MM ATTORNEY Jan. 239 E945., H. M. PRUDEN SIGNALING SYSTEM Filed Aug. 19, 1943 9 Sheets-Sheet 4 laHl' ATTORNEY Jan 23, E945. H. M. .PRUDEN SIGNALING SYSTEM Filed Aug. '19, 1943 9 Sl'leets--Sheetl 5 /NVENTOR H M PRI/DEN Wd@ WM A-'TRNEY 'www2 Jam., 23, 3945. H. M. PRUDEN SIGNALING SYSTEM Filed Aug. 19, 1945 9 Sheets-Sheet 5 /NVE/vroR H M PRUDEN ATTORNEY 5mm 23, E945.

H. M. PRUDEN SIGNALING SYSTEM Filed Aug. 19; 1943 9 Sheets-Sheet 7 /A/VE/VTOR HMP/PUDEN ATTORNEY Jam. 23, E945. H. M. PRUDEN SIGNALING SYSTEM Fled Aug. 19, 1943 9 Sheets-Sheet 8 mmm /Nl/E/v Tof? H. M PRUDE N Arron/VEV Jan. 23, 1945. H. M. PRUDEN SIGNALING SYSTEM 9 sheets-'sheet 9 Filed Aug'.

/N VEN TUR H. M. PRUDEN Qi? da Patented Jan. 23, 1945 anni UNITED STATES PATENT OFFICE SIGNALING SYSTEM Harold M. Pruden, Maplewood, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application August 1'9, 1943, Serial No. 499,201

9 Claims. (Cl. 179-27) This invention relates to signaling systems and particularly to systems in which voice frequency currents are used for the transmission of signals.

Objects of the invention are the provision of more reliable signal receiving means in systems employing voice frequency signals and particularly the receiving of signals of a character which minimize the possibility of false operations in response to speech or other interference currents.

This invention is a signal receiving means adapted to respond to signals each of which consists of a plurality of current impulses in succession, each current impulse ci a signal being of a different one of three signaling frequencies.

Seizure, selective, ringing and disconnect signals are .received on calls incoming over the trunk or line with which the signal receiving means is associated. Supervisory off-hook and oli-hook signals and disconnect acknowledgement signals are received on calls outgoing over the trunk or line with which the signal receiving means is associated. A seizure signal received on an incoming Call and an off-hook supervisory signal received on an outgoing call consist of an impulse of current of one of the signaling frequencies followed immediately by an impulse of current of another of the signaling frequencies. Selective signals corresponding to a train of dial impulses received on an incoming call consist of an impulse of current of the third of the signaling frequencies for the duration of each dial impulse and an impulse of current of said other of the frequencies :for the duration of the interval between successive dial impulses of a train, the current impulses representing the first dial impulse being immediately preceded by a preparatory or enablement signal which is similar to a seizure signal. A disconnect signal received on an incoming call consists cf the enablement signal followed by an impulse of current of the third of the signaling frequencies for a predetermined interval; and, if disconnect is not effected during receipt of the current of the third frequency, this impulse is followed by alternate impulses of current of the irst and second of the signal frequencies. An on-hook supervisory signal re ceived on an outgoing call -consists of an impulse of current of each of the rst, second and third of the signaling frequencies in succession. A disconnect acknowledgement signal received on an incoming call consists of alternate impulses of current cf the first and second of the signaling frequencies.

A clear and complete explanation of the invention will be facilitated by considering a system in which the invention and its various features are embodied, one such system being represented schematically in the drawings whichform apart of this specification. The invention is not limited in its application to the particular system rdisclosed in the drawing but is generally applicable to -any signaling system in which it is desirable to employ voice frequency currents for the transmission of signals.

Referring to the drawings:

Figs. 1 to 4 show, in a first toll cnice, an operators position OP, an outgoing trunk circuit O'Il, a two-way trunk circuit TWTI, an incoming trunk circuit ITI, a toll Aline circuit TLC i, a signal transmitter STI and a signal receiver SR!y all individually associated with an intertoll trunk line TLl2 extending to a second toll o-fce;

Figs. 5 to 9 show, in the second toll ofce, a toll line circuit TLC2, signal receiver SR2, signal transmitter ST2, two-way trunk circuit TWT2, outgoing trunk circuit 0T2, anda toll route selector circuit TS2, all individually associated with the inter-toll trunk line TLIZ;

Fig. 9 further shows an incoming trunk circuit IT2 terminating at an operators position, and shows an outgoing trunk circuit OTS, two-way trunk circuit TWTS, toll route selector TSB and toll line circuit TLC3 associated with another intertoll trunk line TL23; and

Fig. 10 shows the relative position in which Figs. 1 to 9 are to be placed to form an operative arrangement.

The system represented in the drawings ncludes a plurality of toll oiiices each of which comprises a toll board with operators positions and cords for answering calls incoming from local manual and dial oiices, from toll subscribers lines, and from other toll oilices and for extending incoming calls to switching trunks leading to called local manual and dial oices, to intertoll trunks and to called toll subscribers lines. Outgoing jacks are directly connected to trunks to local oiiices, to toll subscribers lines and to intertoll trunks. Answering jacks are directly connected to recording trunks, to toll subscribers lines, to intertoll trunks andto incoming trunks terminating in the banks of toll route selectors. Toll route selectors are provided for use on incoming calls to establish connections with incoming trunk circuits and with trunks to other toll oces and to called local oices.

Reference may be had to the patent to R. E. King et al., No. 2,209,777 granted July 30, i940, for a detailed description of the cord `and operators position circuits provided in each of the toll offices for interconnecting calling and called lines and trunks and controlling the completion of toll calls. The outgoing trunk circuits OTI, T2 and OT3, incoming trunk circuits ITI and IT2, two-way trunk circuits TWTI, TWTZ and TWT3 and the toll route selector circuits TS2 and TS3 are al1 similar to corresponding circuits of the aforementioned King et al. patent. The toll line circuits TLCI and TL2 and the associated signal receiving circuits SRI and SR2 are arranged for voice frequency signaling in accordance with applicants invention and the associated signal transmitting circuits STI and ST2 are arranged for voice frequency signaling according to the invention described in the applica-tion of J. G. Walsh, Serial No. 499,200, filed on even date herewith. The toll line circuit TLC3 andthe associated signal transmitting and signal receiving circuits may be arranged for composite sigt naling as shown in the aforementioned King et al. patentor may be arranged for voice frequency signaling.

The toll line circuit TLC I comprises a repeating coil RCI, a directionally selective coil 200, a cut-off relay 204 and an outgoing signal transformer 209. The cut-ofi relay 204 is normally operated to close a talking connection between the repeating coil RCI and the talking conductors of the outgoing and two-way trunk circuits OTI and TWTI. When released, relay 204 connects the outgoing signal transformer 209, through windings of coil 200, to the repeating coil RCI; and connects the filter unit 490 of the associated signal receiver SRI tothe input transformer 400 of signal receiver SRI. The toll line circuit TLCZ is similar to the toll line circuit TLCI, the reference characters applied to like elements having the same tens and units digits.

The signal transmitter STI comprises a signal receiving relay 300 controlled by relay |09 of outgoing trunk circuit OTI on outgoing calls and controlled by relay |29 of two-way trunk circuit TWTI on incoming calls. The transmitter STI further comprises sources GI, G2 and G3 of signaling currents of frequencies FI, F2 and F3 respectively, and a plurality of control relays of desired time characteristics to effect the transmission of the required signaling currents. The signal transmitter ST2 is similar to the transmitter STI, except that the three sources of .signaling current G4, G and G6 are of frequencies F4, F5 and F6 respectively. The reference characters applied to corresponding elements of signal transmitters STI and ST2 have the same tens and units digits.

The signal receiver SR2 comprises an incoming signal transformer 500, ampliers 50| and 508, filters 5|0, 520 and 530 tuned to pass current of frequencies FI, F2 and F3 respectively, rectifiers 5| I, 52| and 53| and relays 5|4, 524 and 534 for response to signal impulses of frequencies FI, F2 and F3 respectively. The signal receiver SR2 further comprises auxiliary signal receiving relays SIB, 526, 550, 555 and 560 for repeating the received signals in a direct current signaling circuit. The signal receiver SR2 further comprises a filter network 590 consisting of three sections 59|, 592 and 593 respectively, tuned to the frequencies F4, F5 and F6, to prevent outgoing signals from interfering with the response of relays 5I4, l524 and 534 to incoming signals. The signal receiver SRI is similar to the receiver SR2, except that it is arranged to respond to signal vcurrents of frequencies F4, F5 and FE and is arranged to prevent outgoing signals of frequencies FI, F2 and F3 from interfering with response to incoming signals. The reference characters applied to corresponding elements of signal re ceivers SRI and SR2 have the same tens and units digits.

The three frequencies FI, F2 and F3 used for signaling in one direction over toll line TL|2 differ from the three frequencies F4, F5 and F6 used forsignaling in the other direction over toll line TLIZ. For instance the sources GI, G2 and G3 associated with signal transmitter STI may transmit current of 600 cycles, M300-cycles and 1400 cycles, respectively; in which case the filters 5|0, 520 and 530 of signal receiver SR2 are tuned t0 pass currents of 600, 1600 and 1400 cycles, respectively. And the sources G4, G5 and G6 associated with signal transmitter ST2 may transmit current of 1900 cycles, 900 cycles and 1100 cycles, respectively; in which case the filters 4|0, 420 and 430 of signal receiver SRI are tuned to pass currents of 1900, 900 and 1100 cycles, respectively.

Assume now that the operator at position OP in the iirst toll oftice has answered an incoming call with the plug API of cord CDI and that the plug CPI is inserted in jack J l to extend the call over toll line TLI2 to the second toil office, relays |04, |06 and |09 of outgoing trunk circuit OTI being thereby operated in the manner described in the aforementioned King et al. patent. The operation of relay |09 closes a circuit through conductor ||0 for operating relay 300 of signal transmitter STI; and the operation of relay |06 connects ground to conductor |08. The operation of relay 300 opens the normally closed circuit for operating the cut-off relay 204 of line circuit TLCI, this circuit being traced from battery through the inner lower baci: contact of relay 300, back contacts of relays 3I0, 220 and 250, winding of relay 204, conductor 203, to ground at the back contact of relay 426 of signal receiver SRI. The release of relay 204 connects the right winding of signal transformer 209 to the left windings of repeating coil RCI and connects filter unit 490 to input transformer 400. 'The operation of relay 300 also closes a circuit including the inner upper back contact oi relay 300 for operating relay 315 and closes a circuit including the inner upper back contact of relay 360 and the upper back contact of relay 3.50 for operating relay 350. The operation of relay 300 also closes a circuit from source GI of signaling current of frequency FI through the lowermost back contact of relay 3I0, inner upper front contact of relay 300, left contact of relay 325, inner upper back contact of relay 2|0, and left winding of transformer 209 to ground; and as soon as the cut-off relay 204 has released, signal current of frequency FI :Ps transmitted through transformer 209, back contacts of relay 204, directionally selective coils 200 and repeating coil RCI, over toll line TLIZ to the signal receiver SR2 in the second toll oiiice. The aforementioned operation of relay 300 also opens the normally closed circuit through the lower winding of relay 320, the deenergization of this winding and actuation of the contacts of relay 320 being delayed for about .020 second by current charging condenser 32|. The operation of relay 320 opens the normally closed circuit through the upper winding of relay 330, the deenergization of this winding and actuation of the contacts of relay 330 being delayed for about .020 Second by current charging condenser 322. The

operation of relay 320 also closes a circuit through the lower winding of relay 325 to effect the immediate operation of relay 325. The operation of relay 325 disconnects source GI from, and connects source G2 to, the left winding of transformer 209 thereby'ending the transmission of current of frequency F| and initiating the transmission of current of frequency F2 over the toll line TL|2 to the signal receiver SR2. When relay 330 operates, it closes a circuit including a back contact of relay 350 for operating relay 3|0. Relay 3|0 locks through its inner lower front contact, conductor 233, upper back contact of relay 235, and back contact of relay 2M to ground at a back contact of relay 290.

The aforementioned operation of relay 315 causes the successive operation of relays 210, 21| and 22D. Relay 210 is slow in operating so that the operation of relay 220 occurs about .050 second after the operation of relay 300 and after the operation of relay 3h0. The operation of each of relays 2li! and 220 connects ground through conductor 36| and the inner upper back contact of relay 360 to conductor 30| in parallel with the connection of ground to conductor 30| through the outer lower front contact of relay 300. The operation of relay 220 also closes a circuit from ground through a back contact of relay 240, conductor 24|, uppermost contact of relay 300, conductor 302, uppermost front contact of relay 220, inner lower back contact of relay 240, conductor 242, a front contact of relay 3|0 and the lower winding of relay 320. The energization of the lower winding of relay 320 restores the contacts of relay 320 to normal, thereby causing the release of relays 325 and 330, the release of relay 330 being delayed by current charging condenser 322. The release of relay 325 disconnects the signaling current source G2 from the left winding of transformer 205 to end the current of frequency F2. The aforementioned operation of relay 220 also closes a circuit for operating relay 350; and relay 3530 locks in parallel with relay 380 under the control of relays 230, 29| and 250. The operation of relay 350 opens the operating circuit of relay 3|0; disconnects conductor 35| from conductor 3M, connects ground to the armature of relay 305; and closes a circuit for reoperating the cut-off relay 200, this circuit being traced through conductor 205, upper back contact of relay 250, lowi ermost front contact of relay 220, inner upper front contact of relay 3 l0, inner lower front contact of relay 350 and the inner lower front contact of relay 300. also connects ground to conductor 352 thereby causing the energization of both windings 305 and the energization of the lower winding of relay 390. With both windings energized, the contacts of relay 385 remain in normal position, as shown in the drawings. The energization of the lower winding of relay 390 causes the operation of relay 330. The aforementioned connection of ground to conductor 302 also causes energizaton of the winding of relay 340 but relay 390 opens the circuit for operating relay 340 to prevent its operation at this time. Thus a seizure signal consisting of current of an impulse of current of frequency Fl for about .020 second followed immediately by an impulse of current of frequency F2 for about .030 second is transmitted respon-- sive to the seizure of outgoing trunk circuit OT! to effect the operation `of the signal receiver SR2 as hereinafter described.

The seizure signal thus transmitted over toll The operation of relay350 line TLI'2 responsive to seizure of trunk'circuit OTI is further transmitted through repeating coil RC2, directionally selective coils 600, conductors 60| and 602, transformer 500, amplier 50|, transformer 505, volume limiting amplifier 508 and transformer 509 to the input side of each of lters 5|0, 520 and 530. The impulse of current of frequency .liI is further transmitted through filter 5|0 and rectier 5|| to effect the operative energization of the upper winding of relay 5|4. The operationy of relay 5l'4 opens the normally closed short-circuit across the upper winding of relay 550 and closes a circuit for discharging the normally charged condenser 5|5 thereby to energize the lower, operating winding of relay 5|6 in the same direction as its biasing winding. When the impulse of current of frequency FI ends, relay 5|4 releases and condenser 5|5 is: again charged in series With the lower Winding o-f relay SI5, the direction of the charging current being such that relay 5|6 is operated for a predetermined interval of time. The operation of relay 5|6 opens the circuit through resistor 5H thereby to decrease the cul'- rent through the lower biasing winding of relay 524 to render relay 524 operatively responsive to current of frequency F2. The operation of relay 5|6 connects battery through resistor 5|'| to conductor 553; .but neither of relays 550 and 555 vis operatively affected since the upper winding of relay 550 is short-circuited through the back contact of relay 5|4 and the winding of relay 555 is short-circuited through the back contact of relay 550. The impulse of current of frequency F2 immediately following the impulse of current of frequency FI elfects the operation of relay 524. The operation of relay 524 opens the short-circuit across the middle winding of relay 526, to cause the operation of relay 526, and connects ground to conductor 525 to operate relay 823 of two-way trunk circuit TWT-2. The operation of relay 526 causes the release of cutoff relay 604 of line circuit TLC2 and closes a circuit through the middle winding of relay 5|6 to hold relay 5|6 operated. The drop in potential through resistor 566 is thereby increased so that the current through resistor 566 and the biasing windings of relays 534 and 550 is decreased. 'Ihe aforementioned circuit for operating relay 823 is traced from conductor 525 through a back contact of relay 550, conductor 565, a back contact of relay 90B of outgoing trunk circuit 0T2, conductor 007, back contact of relay 824 of two-way trunk circuit TWT2, back contact of relay 821 and the winding of relay 823. The operation of relay 823 connects ground to conductor 003 to prevent seizure of the trunk circuit TWTZ by any of the selectors such as TSB having access thereto. The operation of relay 823 also closes a circuit for operating relay 825 in series with the line relay 35'! of selector TS2; this circuit is traced from ground at the front contact of relay 524, through conductor 525, back contact of relay 560, conductor 555, back contact of relay 906, conductor 907, back contact of relay 824, winding of rel-ay 825, a front contact of relay 823 and conductor SI5, then in simplex through both windings of retard coil 9|4, conductors 9|| and 9|2, back contacts o-f relay 920, conductors 92| and S22, front contacts of relay 823, conductors and 852, back contacts of relay 853 of selector TS2, resistors 055 and 856, another back contact of relay 853 and through the winding of line relay 857. The operation of relay- 825 closes a circuit for operating the slowto-release relay 821 and closes a circuit through resistor 826 for holding relay 823 after relay 821 operates. The operation of relay 821 opens the operating circuit .and closes a holding circuit through resistor 826 for relay 823. The operation of relay 821 connects ground to conductor 830 to operatively energize the lower winding of relay 690 of signal transmitter ST2 and to hold the connection which is extended through the selector TS2; and this ground is further connected through a back contact of relay 905 of outgoing trunk circuit T2 and conductor 56| to the winding of relay 560 of signal receiver SR2 to effect the operation of relay 560. The operation of relay 550 disconnects conductor 564 from ground at the left contact of relay 534, connects this ground to conductor 555 to hold relays 825 and 851 operated, connects ground from the front contact of relay 524 to conductor 563 to energize the lower winding of relay 526, and connects ground to conductor 562. When the impulse of current oi' frequency F2 ends, relay 524 releases; but relay 56S is held operated until a disconnect signal causes the disconnection `of ground. from conductor 830. as hereinafter described. The release of relay 524 causes the delayed release of relay 525, the delay being introduced by current charging condenser 521; and the release of relay 525` causes the. release of relay 5|5. The

aforementioned operation of relay 851 of selector TS2 closes a circuit for operating the slow-torelease relay 858. Relay 858 connects ground to an alarm circuit to effect the operation of an alarm in case the selective operations of selector TS2 are not completed within a predetermined interval of time. Relay 850. also connects ground to conductor 830 in parallel with the ground connected thereto at the lower front contact of relay 821. The aforementioned operation of relay 690 prepares a locking circuit for relay 620, closes a circuit for Ioperating relay 69 I, and closes a circuit for operating relay 550. The circuit for operating relay 650 is traced from battery at the outer upper front contact of relay 690, winding of relay 650, a back contact of relay 640, inner lower back contact of relay 6304, conductor 562, to ground at a front contact of relay 550. The operation of relay 650 causes the operation of rela-y 680. With relays A59|), 69| and 650 Operated,

the locking ground for relays 1|0 and 160 is supplied at the outer lower front contact of relair 550 instead of through back contacts of relays 690 and 69 When the calling operator dials the digits of the toll route code, and the local oflce code and called subscribers number in case the called subscribers line terminates in a dial oilice, relay |09 of outgoing trunk circuit OTI is alternately released and reoperated in response. to each dial impulse to effect a corresponding release and reoperation of relay 300 of signal transmitter STI. The releasev of relay 300 responsive to the first dial impulse effects the transmission of an enablement signal, which is similar to a seizure signal, followed by an impulse of current of frequency F3 which impulse represents the dial impulse. The enablement signal is effective to prepare the signal receiver SR2 at the other end of the line TLIZ for response to the train of dial impulses corresponding to the digit dialed. The release of relay 300 opens the circuit through the winding of the cut-off relay 204 to-cause the release of relay 204, and connects the signal current source GI to transformer 209 toV transmit current of frequency FI over toll line TL|2, this connection being traced from source GI, through conductor 3|5, lowermost front contact of relay 3|0, upper back contact of realy 300, left contact of relay 325, conductor 2| I, back contact of relay 2 I 0 and the left winding of transformer 209. The release of relay 300 also opens the above-described circuit through the lower winding of relay 320, the operation of relay 320 being delayed for about .020 second by current charging condenser 32|. When relay 320 operates, it causes the immediate operation of relay 325 and the delayed operation of relay 330. The operation of relay 325 disconnects source Gl from, and connects source G2 to, conductor 2|| and the left winding of transformer 209 thereby to end the transmission of current of frequency FI and initiate the transmission of current of frequency F2 over toll line TLI2. "I'he release of relay 300 also disconnects ground from conductor 30| thereby to open a short-circuit across the winding of relay 380 and thus cause the operation of relay 380 in a circuit traced from battery through the winding of relay 315, resistor 316, lower front contact of relay 315, winding of relay 380, back contact of relay 385, to ground at a front contact of relay 360. Relay 315 is held operated by the current in this circuit. The circuit path through the winding of relay 315, resistor 316 and resistor 31| aids in holding relay 315. The operation of relay 380 opens the connection between conductor 30| and resistor 316, closes a circuit for operating relay 260, closes a circuit for energizing the upper winding of relay 390 to cause the release of relay 390, closes a circuit for operating relay 310, and

opens the circuit through the upper winding ofl relay 385. The operation of relay 260 prevents the reoperation of relay 204 while a train of dial impulses is being transmitted. The release of relay 390 closes a circuit for operating relay 340. The operation of relay 310 opens the circuit path through resistor 31|, and connects the Winding of relay 350 to conductor 36| so as to hold relay 350 under the joint control of relays 310 and 380 after the operation of relay 340.

The energization of the upper winding of relay 385 is maintained by current charging condenser 313 for about .040 second after relay 385 operates, the energization of the lower winding of relay 385 thereupon becoming effective to operate relay 385. The operation of relay 385 causes the release of relays 315 and 380. The release of relay 315 short-circuits resistor 316 and the release of relay 380 reconnects the winding of relay 315 to conductor 30| awaiting the reoperation of relay 300. The release of relay 380 also opens the circuit through the upper winding of relay 390 and causes the release of relay 310. Being slow in releasing, relay 260 remains operated until all of the impulses in the train have been received. The energization of the upper winding of relay 380 is maintained after relay 380 releases by current charging condenser 393; and therefore relay 390 does not operate during dialing unless the dial speed is low. If relay 390 reoperates while relay 315 is released, relay 340 is held operated in a circuit traced through the lowermost front contact of relay 350, the front contact of relay 340, upper back contact of relay 315 and conductor 362 to ground at a front contact of relay 350. With relays 310 and 360 released and relay 340 operated, the circuit for operating relay 350 is opened and relay 350 releases. The release of relay 350 disconnects source G2 from, and connects source G3 to, the left winding of transformer 209 thereby ending the transmission of current of frequency F2 and initiating the transmission of current of frequency F3 over toll line TLI2. The connection between source G3 and transformer 209 is traced through the uppermost back contact of relay 240, a front contact of relay 220, conductor 352, lower back contact of relay 350, front contact of relay 325, conductor 2l I, and back contact of relay 2I0. The release of relay 310 closes the circuit for energizing the upper winding of relay 385, thereby causing the release of relay 385. Relay 390 reoperates in case the circuit through its upper winding is not again closed before the current charging condenser 393 decays below the Value required for maintaining the energization of this winding. When relay 300 reoperates at the end of the first dial impulse, it again connects ground to conductor 30| thereby causing the reoperation of relay 315. 'Ihe reoperation of relay 300 also recloses the circuit for energizing the lower winding of relay 320 to effect the immediate release of relay 320. The release of relay 320 closes the circuit for energizing the upper winding of relay 330 thereby causing the immediate release of relay 330. The release of relay 320 also opens the circuit through the lower winding of relay 325 but energization of this winding is maintained by current discharging condenser 354, the discharge path including the inner upper back contact of relay 350 and a front contact of relay 3I0, this current being effective to hold relay 325 operated for about .100 second.

The aforementioned operation of relay 315 opens the locking circuit for relay 340 but relay 340 is held operated through the back Contact of relay 390 unless the dial impulse rate is low; in which case the operation of relay 390 causes the release of relay 340 and the release of relay 340 closes the circuit for operating relay 350, thereby to end the transmission of current of frequency F3 by disconnecting source G3 from transformer 209 and to initiate the transmission of frequency F2 by connecting source G2 to transformer 209. If, however, the dial is not a slow dial, relay 390 does not reoperate since the release of relay 300 responsive to the next dial impulse of the train causes the reoperation of relay 380; and relay 380 recloses the circuit through the upper winding of relay 390 to maintain the energization of this winding and thereby prevent the reoperation of relay 39.0. The reoperation of relay 380 also causes the reoperation of relay 350 thereby disconnecting source G3 from transformer 209 to end the transmission of current of frequency F3 and reconnecting source G2 to transformer 209 to initiate the transmission of current of frequency F2. The reoperation of relay 350 closes a circuit including the lower winding of relay 325 and condenser 353 whereby the operative energization of this winding is maintained by current discharging condenser 353. The release of vrelay 300 responsive to the second dial impulse of the train also opens the circuit through the lower winding of relay 320, the operation of relay 320 being delayed for about .020 second by current charging condenser 32|. The operation of` relay 320 again connects ground to conductor' 326 to maintain the operative energization of the-'lower winding of relay 325. The operation of relay320 also opens the circuit through the upper winding of relay 330, the operation of relay 330 being delayed by current 'charging condenser 322. The reoperation of relay 380 responsive to the second dial impulse received by relay 300 also causes the reoperation of .relay 310; and the reoperation of relay 310 opens the circuit through the upper winding of relay 385, the deenergization of the upper winding and operation of relay 385 being delayed by current charging condenser 313. The operation of relay 385 causes the release of relays 315, 380, 310 and 350. The release of relay 350 disconnects source G2 from, and connectsl source G3 to, transformer 209 thereby to end the transmission of current of frequency F2 and initiate the transmission of current of frequency F3. When relay 300 reoperates at the end of the second dial impulse, relay 315 is reoperated, relays 320 and 330 are released, the transmission of current of frequency F3 is ended, and the transmission of current of frequency F2 is initiated in the manner above set forth as resulting from the reoperation of relay 300 at the end of the first dial impulse. Thus an impulse of current of frequency F3 is transmitted for an interval equal to the length of each dial impulse as indicated by the release of relay 300 and an impulse of frequency F2 is transmitted between succeeding dial impulses except that in the case of slow dials the transmission of current of frequency F3 is ended and transmission of current of frequency F2 is initiated by the reoperation of relay 350 when relay 340 releases due to the operation of relay 390. When relay 300 operates at the end of the last impulse of a train, relay 315 is reoperated, relay 390 operates, relay 340 releases and relay 350 reoperates; and current of frequency F2 is transmitted until relay 325 releases, the release of relay 325 occurring about .1 second after the reoperation of relay 350, at which time the discharge current from condenser 353 is no longer effective to maintain the operative energization of the lower winding of relay 325. Since relay 380 remains normal, relay 260 releases thereby reclosing the circuit for operating the cut-off relay 204.

When a train of dial impulses is transmitted over toll line TLI 2 to the signal receiver SR2, relay 5I4 is operated responsive to the impulse of current of frequency FI relay 524 is operated responsive to the impulse of current of frequency F2, and then relays 534 and 524 are alternately operated responsive to the alternate impulse of current of frequency F3 and F2 representing each dial impulse of the train. The operation of relay 5I4 responsive to the impulse of current of frequency FI causes the energization of the lower winding of relay 5I6 by current discharging condenser 5I5, this current being in the biasing direction. The release of relay 5I4 at the end oi the impulse of current of frequency FI causes the operative energization of the lower winding of relay 5I6 by current charging condenser 5I5. The operation of relay 524 responsive to the impulse of current of frequency F2 opens the short-circuit across the middle winding of relay 526 and connects ground through conductor 525, a front contact of relay 560, to conductor 563 to operatively energize the lower Winding of relay 526. The operation of relay 526 closes a circuit for energizing the middle winding of relay 5I6 to hold relay 5I6 operated after the current charging condenser 5 I 5 is insucient to maintain the energization of the lower winding of relay The operation of relay 526 disconnects ground from conductor 603 to cause the release of 4 operates, it disconnects ground from conductor 565, thereby causing the release of relay 825 of two-way trunk circuit and the release of line relay 851 of selector TS2. The operation of relay 534 connects ground to conductor 563 to maintain the operative energization of the lower winding of relay 526. When relay 534 releases at the end of the rst impulse of frequency F3, it connects ground to conductor 565 to reoperate relays 825 and 851 and disconnects ground from conductor 563, the energization of the lower winding of relay 526 being maintained by current charging condenser 521. When relay 524 reoperates responsive to the impulse of current frequency F2 following the impulse of current of frequency F3, it opens the short-circuit across the middle winding of relay 526 and connects ground to conductor 563 to maintain the operation of relay 526. The alternate operation of relays 534 and 524, responsive to alternate impulses of current of frequency F3 and F2 representing succeeding dial impulses of the train, effects the alternate disconnection and reconnection of ground to conductor 565 to alternately release and reoperate relays 825 and 851 and maintains the operative energization of relay 526. The release of relay 524, at the end of the impulse of current of frequency F2 following the impulse of current of frequency F3 representing the last dial impulse of the train, closes the short-circuit across the middle winding of relay 526 and disconnects .ground from conductor 563 the release of relay 526 being delayed by current charging condenser 521. When relay 526 releases it reconnects ground to conductor 603 to effect the reoperation of cutoi relay 604.

Each release of relay 825 opens the circuit through the winding of relay 821 but relay 821 is slow in releasing and remains operated during the response of relay 825 to dial impulses. The release of relay 851, in response to the rst dial impulse, closes a circuit for operating the vertical stepping magnet 86| and relay 863 in series. The operation of stepping magnet 86| raises the brushes 81|, 812, 813 and 814 up to the rst level of the bank. VON are actuated as soon as the shaft on which the brushes are mounted is moved out of normal position. The operation of relay 863 and actuation of springs VON close a circuit for operating relay 865, this circuit including the lower contact of springs VON, the front contact of relay 863 and the outer lower front contact of relay 858. Relay 865 closes a locking circuit which includes the back Contact of the rotary stepping magnet 862, the upper front contact of relay 865 and the lowermost back contact of relay 853. When relay 851 reoperates at the end of the rst. impulse, the stepping magnet 86| releases, but relay 863 is slow in releasing and remains operated until all of the impulses in the train have been received by relay 851. Relay 858 is also slow in releasing and remains operated during the response of relay 851 to dial impulse. Each succeeding release of relay 851 eiiects the reoperation of stepping magnet 86| thereby stepping the brushes up to the level corresponding to the rst digit of the toll route code. When relay 851 remains operated at the end of the last impulse of the train, relay 863 re- The vertical off-normal springs leases closing a circuit through the lower front contact of relay 865 and back contact of relay 863, for operating the rotary stepping magnet 862. The operation of stepping magnet 862 advances the brushes into engagement with the first set of terminals in the selected level and causes the release of relay 865. The release of relay 865 causes the release of stepping magnet 862. If the trunk connected to the first set of terminals is busy, the terminal engaged by test brush 813 is marked by a ground potential connected thereto; and this ground potential is connected through brush 813, a back contact of relay 853, back contact of stepping magnet 862, lower contact of springs VON, to the winding of relay 865. Relay 865 is thereby reoperated to again close the circuit for operating stepping magnet 862 so as to advance the brushes to the next set of terminals. When the brushes engage a set of terminals connected to an idle trunk, the terminal engaged by brush 813 is not marked by ground potential and relay 853 is operated by the current in a circuit traced from battery through the winding of relay 865, lower contact of springs VON, back contact of stepping magnet 862, winding of relay 853, lower contact of the eleventh-rotary-step springs 859, to grounded conductor 830. Being marginal. relay 865 does not reoperate in series with relay 853. The operation of relay 853 disconnects conductors and 852 from the winding of relay 851 and connects these conductors through front contacts of relay 853 and brushes 81| and 812 to the line conductors of the selected trunk or selected auxiliary selector circuit in case more than one stage of toll route selectors are provided. The operation of relay 853 causes the successive release of relays 851 and 858, the operation of relay 853 being maintained as long as relay 821 of twoway trunk circuit TWT2 connects ground to conductor 830. Additional trains of impulses received by the signal receiver SR2 are repeated over conductor 565 to relay 825 of two-wayv trunk circuit TWT2 and to the line relay of any trunk or selector to which the connection is extended. Assume that the call in question is one to be completed by an operator in the second toll oflice. In this case the selector TS2 selects a set of terminals such as the terminals 816 which are connected by conductors |00I, |002, |003 and |004 to an incoming trunk circuit IT2, The operation of relay 853 extends the connection from conductors 85| and 852 through brushes 81| and 812, conductors |00l and |002 to effect the operation of the line relay |005 of trunk circuit IT2 thereby to light an answering lamp in usual man ner. When the plug of a cord is inserted in jack J I0 to answer the calll the answering lamp is extinguished and arelay |034 is operated in the manner set forth in the aforementioned King et al. patent. Relay |034 connects ground to conductor |004, through brush 814, a front contact of relay 853 of selector TS2, a back contact of eleventh-rotary-step springs 858, to the winding of relay 828 of two-way trunk circuit TWT2. Relay 828 is thereby operated to close a circuit for operating relay of signal transmitter ST2. This circuit includes the front contact of relay 829, back contact of relay 828, conductor 908, back contact of relay 909, conductor 9|0, and winding of relay 100. The operation of relay 100 causes the transmission of an answering or of!- hook supervisory signal consisting of an impulse of current of frequency F4 followed by an impulse of current of frequency F5, the various operations of the signal transmitter ST2 being similar to those of .the signal transmitter STI in sending a seizure signal as hereinbefore described. The signal receiver SRS responds to the olf-hook supervisory signal in a similar manner to that in which the signal receiver SR2 responds to a seizure signal as hereinbefore described. The operation of relay 32d is effective to connect ground through conductor l2-5, a back contact of relay A69, conductor 365, lower front contact of relay m6 of outgoing trunk circuit OTI to the winding of supervisory relay H35 and the winding of relay l i9. Relay I i9 operates and locks to ground on conductor fc3. The operation of relay H35 disconnects ground from the lower, low resistance winding of relay it thereby to extinguish the supervisory lamp SL associated With plug CPI t indicate that the call has been answered. The operation of relay m connects ground to conductor ll to operate relay 460 of signal receiver SR2. Relay 45B disconnects ground from conductor 454, disconnects conductor 425 fromconductor 465, connects conductor 425 to conductor 463, and connects conductor 665 to ground at the back contact of relay 334.

lf the answering operator disconnects from jack 5l@ before the calling operator releases the connection, relay lett is released as described in the aforementioned King et al. patent. The release of relay ill causes the release of relay 82S; and the release of relay 329 causes the release of relay 'lull of signal transmitter ST2. At the time that relay I releases, relays (i-, 526, 65d, elfi, G'Sl, ill, teil, till, lli), 75l), 769, H5 and 'i90 are operated. The release of relay Fll causes the transmission of an on-hook supervisory signal consisting of an impulse of current of frequency F-'l followed by an impulse of current of frequency F5 followed by an impulse of current of frequency F5 in the same manner that the first impulse of a train of dial impulses is transmitted by the signal transmitter ST5 as hereinbefore described. The transmission of current of frequency Ft is continued, since relay S25) is locked under the control of relay 690, until relay Sill? is operated as hereinafter described. The signal receiver SRI responds to the on-hook signal in the same manner that the signal receiver SR2 responds to the first dial impulse of a train, the operation of relay E34 being effective to disconnect ground from conductor it to release relay 65 and thereby effect the lighting of the supervisory lamp SL. Relays IIS and 46%) remain operated until the connection is released as hereinafter described. The operation of rela'y .1355 also connects ground to conductor 463 to maintain the operative energization of the lower winding of relay 126. As long as relay #12S is held operated, the cut-off relay 2M cannot reoperate and relay SI5 is held operated by its middle winding.

If the calling operator wishes to recall the answering operator, relay i953 is momentarily released responsive to the momentary operation of the ringing key of cord CDI in the manner described in the aforementioned King et al. patent. The momentary release of relay i139 causes a like release of relay 3M whereby a rering signal is transmitted. The rerng signal is transmitted in similar manner to that in which the signal corresponding to the digit i is transmitted; and it consists of an impulse of current of frequency Fl followed by an impulse of current of frequency F2 followed by an impulse of current of frequency F3 followed by an impulse of current of frequency F2. ,In responseto the rering sig-.-

operated.

nal, the signal receiver SR2 momentarily disconnects ground from conductor 565 thereby causing the momentary release of relays 825 and H305 to recall the answering operator in the manner described in detail in the aforementioned King et al. patent.

When the calling operator disconnects' plug CPI from jack J I, the outgoing trunk circuit is restored to normal in the manner described in the aforementioned King et al. patent except that relay IIS remains operated while the disconnect signal is being transmitted. The release of relay H39 causes the release of relay 380 of signal transmitter SI. The release of relay Sell effects the transmission of an impulse of current of frequency Fi followed by an impulse of current of frequency F2, followed by an impulse of current of frequency F3 in the same manner as upon release of relay 363 in response to the iirst impulse of a train of dial impulses as hereinbefore described. But relay Sfl does not reoperate as it does at the end of a dial impulse; and therefore relay 375 is not reoperated, relay 34H3 remains operated, and relay 353 is not re- The contained release of relay 375 causes the successive release of relays 210, 21| and 220, the transmission of current of frequency F3 being continued until relay 220 releases, about l second after the release of relay 350. If relay 230 is not operated responsive to a disconnect acknowledgment signal incoming over line rZfLI2 as hereinafter described before relay 22S releases, the release of relay 229 disconnects source G3 from transformer 29S to end the transmission of current of frequency F3 and connects source G2 to transformer 29d to initiate the transmission of current of frequency F2. The release of relay 22H also closes a circuit for energizing the lower winding of relay 325], this circuit being traced through a front contact of relay Sill, conductor 242, inner lower back contact of relay 2453, uppei-most back contact of relay 220, inner upper front contact of relay 36D, to ground at the front contact of relay 33D. Relay 32() is thereby rei leased, causing the release of relay 325 whereby source G2 is disconnected from transformer 29 to end the transmission of current of frequency F2; and source GI is connected to transformer 209 to initiate the transmission of current of frequency Fl over line TLIZ, this connection being traced through conductor 3i5, lowermost front contact of relay 3M), upper back contact of relay 3&0, back contact of relay 325, conductor ZI I, and a back contact of relay Zl to transformer 2l9. The release of relay 329 also causes the energization of the upper winding of relay 330, thereby to effect the immediate release of relay 339. The release of relay 33) causes the deenergization of the lower Winding and reoperation of relay 329 after an interval of about 0.20 second. The operation of relay 32B is followed by the reoperation of relay 325 and the reoperation of relay 33B at the end of a further interval of about .020 second. The reoperation of relay 325 is effective to disconnect source GI and connect source G2 to transformer 209. Relays 32, 325 and 332i are in this manner alternately released and reoperated to alternately transmit impulses of current of frequencies F2 and FI over toll line TLI2 until a disconnect acknowledgment signal is received by the signal receiver SRI as hereinafter described.

This disconnect signal transmitted by the signal transmitter STI effects the successive operations of relays 5M, 524 and 534 responsiveto the impulses of current of frequencies FI, F2 and F3; and effects the alternate operation of relays 524 and 5I4 responsive to the succeeding alternate impulses of frequencies F2 and FI. The release of relay 5I4 at the end of the first impulse of frequency FI causes the operation of relay5I6; the operation of relay 524, responsive to the first impulse of frequency F2, causes the operation of relay 526; and the operation of relay 534, responsive to the impulseof frequency F3, causes relays 5I6 and 526 to be held operated and disconnects ground from conductor 565, thereby causing the release of relays 825 and |085. The release of relay H165 causes a disconnect signal to be given to the answering operator if the answering cord has not been disconnected from jack JIU. If there are no interference currents, the impulse of current of frequency F3 maintains the energization of relay 534 and the release of relay 825 for `a, long enough interval to cause the release of relay 821. The release of relay 821 causes the release of relay 823 and disconnects ground from conductor 830 thereby to cause the release of relay 853 of selector TSZ. The release of relay 853 causes the operation of release magnet 869 to effect the return of selector TS2 to normal in usual and well known manner. The release of relay I005 and the disconnection of the answering cord from jack J I8 restore the incoming trunk circuit to normal in the manner described in the aforementioned King et al. patent. The aforementioned disconnection of ground from conductor 830 due to the release of relay 821 causes the release of relay 560.

If at the time that relay 560 releases, the call has not yet been answered or has been answered and the answering operator has disconnected, relays 829 and 100 are normal; but if the call has been answered and the answering operator has not disconnected, relays 829 and 160 are operated and in this case the aforementioned release of relay 821 causes the successive release of relays 828 and 100. Assume first that the call is one which has not been answered and that the release of relay 560 results from the impulse of current of frequency F3 of the aforementioned disconnect signal. In this case relays 690, 69|, 650 and 680 are operated, but all other relays of the signal transmitter ST2 are normal at the time that relay 560 releases. The release of relay 566 disconnects ground from conductor 562; but, since relay 534 is operated, the release of relay 550 does not connect ground to conductor 564 so that the winding of relay 640 is no longer short-circuited and this winding is operatively energized in series with the winding of relay 658. The operation of relay 648 opens the circuit for operating relay 680. Relay 580 is slow in releasing and remains operated for about .040 second after relay 640 operates. The aforementioned disconnection of ground from conductor 908 causes the deenergization of the lower winding of relay 690; but relay 690 is slow in releasing and the operation of relay 640 closes a locking circuit through the upper winding of relay 680 to hold relay 698 operated. Relay 640 closes a circuit for operating relay 160, closes a .circuit including the front contact of relay 680 for operating relay-1I8, and connects source G4 through conductor 643, back contact of relay 125, conductor 6II and inner back contact of relay GID, to the right winding of transformer 669, thereby initiating the transmission of current of frequency F4 over line TLI2 to the calling toll oiiice. Relays 160 and 1I0 lock through conductor 633 and a back contact of relay 630 to ground at a front contact of relay B50. The operation of relay 'IIQ opens the normally closed circuit for energizing the lower winding of relay 120, the operation of relay 126 being delayed for about .020 second by current charging condenser 12i. The operation of relay 120 closes a circuit for operatively energizing the lower winding of relay 125. The operation of relay disconnects transformer 609 from source G4 to end the transmission of current of frequency F4 and connects transformer 609 to source G5 to initiate the transmission of current of frequency F5. The connection between transformer 609 and source G5 includes the inner back contact of relay Gl, conductor 6I I, front contact of relay 125, lower back contact of relay 150, conductor 152, a back contact of relay 620, and another back contact of relay SIU. The aforementioned operation of relay 120 opens the normally closed circuit through the upper winding of relay 130, the resulting operation of relay 130 being delayed for about .020 second by current charging condenser 122. The operationvof relay 130 closes a circuit for energizing the lower winding of relay 120, this circuit being traced through the middle lower front contact of relay 1I0, conductor 642, inner lower front contact of relay 640, conductor 163, inner upper front contact of relay and the front contact of relay 130. Relay 120 is thereby released, causing the release of relays 125 and 13u. The release of relay 125 disconnects source G5 from transformer 609 ending the transmission of current of frequency F5 and reconnects source G4 to transformer 608 to initiate the transmission of current of frequency F4. Relays 126, 125 and 130 are thus cyclically operated and released to effect the transmission of alternate impulses of current of frequencies F4 and F5 as a disconnect acknowledgement signal. When relay 534 releases at the end of the impulse of current of frequency F3 of the disconnect signal transmitted by the signal transmitter STI, ground is reconnected to conductor t 564 thereby short-circuiting the winding and causing the release of relay 648. Since ground has been disconnected from conductor 830 due to the release of relay 821 of two-way trunk circuit TWTZ, the release of relay 640 causes the release of relay 690. The release of relay 680 causes the release of relays 69| and 650. The release of relay 650 prevents the operation of relay 586 and, since relay 68| is slow in releasing, causes the release of relays 110 and 160, relay 'i60 being slow in releasing. With relay 646 normal, the release of relay 1I0 disconnects source G4 from transformer 608; and, with both of relays Illl and 'IIO normal, relays 120 and 125 are normal so that source G5 is disconnected `from transformer 609; and, therefore, the transmission of the disconnect acknowledgement signal is ended. The release of relay 1I!) closes the normally closed circuit for operating the cut-off relay 604.

Assume now that the call is one which has been answered, that the answering operator has not disconnected and that relay 560 is released and selector TS2 restored to normal in response to the impulse of current of frequency F3 of the aforementioned disconnect signal. At the time that relay 568 releases, relays 620, 650, 810, 61|, 68ll,69D,69I,1Il8,1I0, 150, 160, and 115 of signal transmitter ST2 are operated and the other relays of this transmitter are normal. The release of relay-853 of selector :TS-2 when-:relayg321 of two-'way trunk circuit TWTZ releases, 'causes the successive release of relay 829 of trunk circuit .TWT2 and relay 'l of signal transmitter ST2. The release of relay 56) disconnects ground from conductor 552 thereby causing the operativeenergization of the winding of relay 640 in series With the winding of relay 550. Although the release of relay 821 of two-way trunk circuit TWT2 effects the deenergization of the lower winding of relay 555. relay 69] is slow in releasing and the operation of relay 649 closes a locking circuit through the upper winding of relay 590fto hold relay 695 operated. The release of relay 15D causes the release of relay 'V15y operation of relay 180, release of relay 195, operation of relay 110, operation of relay 140 and release of relay 150 in the same manner as above described when relay 180 releases to effect the transmission of an onhook signal to the calling operator. The release of relay T55 causes the release of relays 610 and @il but relay 520 is held operated by relay 690. The operation of relay 640 immediately connects source Gil to the back contact of relay 125 thereby initiating the transmission of current of frequency F4 over line TLIZ to the calling office. The release of relay 100 and operation of relay 640 open the circuit for energizing the lower 'winding of relay 120, thereby causing the delayed operation of relay 120. When relay 120 operates, it causes the immediate operation of relay 125 and delayed operation of relay 130. The operation of relay 125 ends the transmission of Acurrent of frequency F4 and initiates the transmission of current of frequency F5, source G being connected through the uppermost front Contact of relay B40, inner upper fr ont contact of relay 620, conductor 152, and lower back contact of relay 150 to the front contact of relay 125. Relays 120, 125 and 130 are cyclically operated and released to elect the transmission of alternate impulses of frequencies F4 and F5 as a disconneet acknowledgment signal. When-the impulse of current of frequency F3 incoming over line 'I'Ll2 ends, the release of relay 534 connects ground to conductor 564 to release relay 640. The release of relay 640 causes the release of relay 690 and the release of relay 690 causes the release of relays 62D, 69| and 650. The release of relay 650 causes the release of relays 1|0 and 160, thereby ending the transmission of the disconnect vacknowledgment signal. The-release of relay 110 also causes the reoperation of cut-off relay 604.

Assume next that the call is one-which has been answered, that the answering operator has disconnected and that relay 560 releases responsive to the impulse of current of frequency F3 of the aforementioned disconnect signal. In this case relays 525, 65D, 680, 590, 69|, 1H), 120, 125, 130, 140 and 160 of signal transmitter ST2 are operated at the time that relay 560 releases. Relays G'l, 51| and 665 will also be operated in case current of frequency F5 is being transmitted by signal transmitter ST2 as part of an on-hook signal at the time that relay 560 releases. IThe cut-off relay E04 will be released while an onhook signal is being transmitted by signal transmitter ST2 and will be released due to the operation of relay 525 during receipt of the disconnect signal by signal receiver SR2. The release of relay 550 disconnects ground from conductor 562 to cause the operation of relay B40. The operation of relay 640 disconnects source GS from the front contact of relay 125, connects source G5 to the front contact'of relay 125 and connects source G4 to the back contact of relay 125. Thus the operation of relay 64E) ends the transmissionof current of frequency F6 and initiates the trans'- mission of current of frequency F5. The operation of relay 640 also closes the circuit for energizing the lower winding of relay 12!) to cause the release of relay 120. The release of-re1ay'12 causes the release of relays and 139; and these three relays are cyclically operated andre- -leased to transmit alternate impulses of frequencies F4 and F5 over line TLI2 to the calling officev as adisconnect acknowledgmentsig'- nal until relay 640 releases responsive to the ending of the impulse vof current of frequency'F incoming over line TLI2 from the calling office.'

Assume now that the impulse of current of frequency F3 incoming over line' TLI2 as a disconnect signal is ineffective to cause the operation of relay 534 for a long enough interval to effect l the release of relay 821 of two-way trunk circuit TWTZ. In this case relay 56B is not released and relays 524 and 5|4 are alternately operated vresponsive to the alternate impulses of frequencies F2 and FI transmitted by signal transmitter STI as hereinbefore described. Relays 525 and 5|6 are held operated during receipt of these impulses. The operation of relay 5M by the first impulse of frequency FI, following the impulse of frequency F3, opens the short-circuit 'across the upper winding of relay 550 and this winding is energized in series with resistor '552,"cof1ducto'r '553'and the front contact of 'relay 5 l5; Theoperation of relay 550 opens the short-circuit" across the winding of relay 555, thereby causing theop'er'- ation of relay 555. The operation of relay e555co'nnects ground to conductor- 556 to operate relay 630 of signal transmitter ST2. When relay "5M releases, it again short-circuits the `winding of relay 550, and relay 550'releases; butrelay 555 remains operated as long asv relay 5 l 6 is held operated responsive vto incoming signal 4current impulses. The operation of're'lay 635,0pens s the short-circuit across the winding of relay 54.5 to cause the operation of relay 64I) in servies with relay 650. The operationof relays 64D and 535 connect the locking winding of relay 595 through conductor 562 to ground at a front contact of relay 560. The operation of relay 640 causes the' transmission of a disconnect acknowledgment signal in the manner described above.

When the alternate impulses of current of frequencies F4 and F5, which are transmitted. by the signal transmitter ST2, as .a disconnect acknowledgement signal are received bysignal receiver SRI at the calling end oftoll line TLIZ, relays 4|4 and 424 are alternately operated. The operation of relays 4|4 and -424 causes corresponding operation of relays 4|6 and 42E in the same manner that relays 5l6 and 52'@` of signal receiver SR2 are operated responsive to incoming signals as hereinbefore described. The operation of relay 426 holds relay 4| 6 and the next succeeding operation of relay 4|4 opens the shortcircuit across the upper winding of relay 45|)` to cause the operation of relay 450. The operation of relay 450 opens the short-circuit across the winding of relay 455 so that relay 455 operates;-

When relay 4l4 releases, relay 450` releases due to the short-circuiting of its upper winding but relay 455 remains operated since the releaseofi. relay 45D does not again close the short-circumacross the winding of relay 455. The operation of relay 455 connects ground4 to conductor 455,to

operate relayA 230. The operation -of relay 230 opens the locking circuit for relays 3H] and 360. Relay 3|0 releases but relay 360 remains operated if the operation of relay 230 occurs before relay 220 has released. The release of relay 3|!) closes the normally closed circuit for energizing the lower winding of relay 320 thereby effecting therelease of relay 320. The release of relay 320 causes the release of relays 325 and 330. The release of relay 325 disconnects source VG3 from transformer 209 ending the transmission of current of frequency F3. When relay 220 releases, the cut-off relay `204 is reoperated.

If the disconnect acknowledgment signal is not received by signal receiver ASRI before relay 220 of signal transmitter STI has released, the release of relay 220 ends the transmission of current of frequency F3 and initiates the transmission of current of frequency F2; and impulses of current of frequencies F2 and Fi are alternately transmitted .as above described until the disconnect acknowledgment signal is received. When relay 231)v is operated, responsive to the disconnect acknowledgment signal, it closes a circuit for operating relay 2|0; this circuit is traced through the outer lower back contact of relay 240,t inner upper back contact of relay 220, upper front contact of relay 230, Vand, back contacts of relays 29| and 290. The operation of relay 2|0 endsv the transmission of current of either of frequencies FI and F2 and connects source G3 to transformer 209 to .effect thetransmissien of; en impulse 0f current of: Y.frequency F3 over toll line TML The operation of relay 2|0 also connects ground to conductorl |08 to mark` the z associated trunk circuits busy until, disconnection is completed. The aforementioned operation of relayy 230 disconnects ground from conductor 233 to release relay 3H) and thereby end the cyclic .operation .of relays 320, 325 and 330. The transmission of current of frequency F3 continues as long as the disconnect acknowledgment signal is being received from the signal transmitter ST2.

The current of frequency F3, transmitted by signal transmitter STI immediately following the transmission of current of frequency Fl or VF2 responsive to the disconnect acknowledgment signal received by signal receiver SRI causes the.

operation of relay 534of signal receiver SR2 while the signal transmitter ST2 is transmitting alternate impulses. of current of frequencies F4 and E5'. The operation of relay 5,34 .disconnects ground from conductor 56,5 to cause therelease of the connection in the same manner that it is i effected responsive to the first impulse of current of frequency E3 transmitted as a result of the release of the connection by the calling operator as hereinbefore described. The release of relay 560 disconnects ground from conductor 562 thereby opening the circuit through the locking winding of relay 680. The release ofv relay 690 causes the release of-relaysBSl, 650. 640-and 620. The release of relay 640 causes the release of relays 110i and 160 thereby to endgthe transmission of the disconnect acknowledgment signal as hereinbefore described. When the disconnect-acknowledgement signal is ended, relays 415 and 426 of Vsignal receiver SR-I release. The release of relay 4-l6 causes the successive release of relays 455, 230 and 2M). The release of relay l2I0 ends the transmission of current of vfrequency F3. The release of relay 426 causes the reoperation of cut-off relay 204.

Whatl is claimed is:

1. In a signaling system in which the transmission of a signal is effected by an impulse of current of a first signaling frequency followed by an impulse of current of a second signaling frequency followed by an impulse of current of a third signaling frequency, a signal receiver for responding to said signal, said receiver comprising three relays, means rendering a first one of said relays responsive only to current of the first signaling frequency, means rendering a second one of said relays responsive only to current of the second signaling frequency, means rendering the third of said relays responsive only to current of the third signaling frequency, means rendering said second relay operative for a predeterminedv interval 0f time following the operation and release of said first relay, means rendering said third relay operative for a predetermined interval of time following the operation and release of said second relay, and signal means controlled by said second relay.

2. In a signaling system in which the transmission of a signal is effected by an impulse of current of a first signaling frequency followed by an impulse of current of a second signaling frequency followed by an impulse of current of a third signaling frequency, a signal receiver ior responding to said signal, said receiver comprising three relays, means rendering a first one of said relays responsive only to current of the first. signaling frequency, means rendering a second one ofsaid relays responsive only to current of the second signaling frequency, means rendering the third .of said relays responsive only to current of the third signaling frequency. means rendering said second relay operative for a predetermined inteiyal of time following the operation and release of said first relay. means rendering said third relay operative for a predeterminedv interval of time ,following the operation and release of said second relay, and signal means controlled by said third relay.

3. In a signaling system in which the transmission of a' signal is effected by an impulse of current of a first signaling frequency followed by. an impulse of current of a second signaling frequency followed by an impulse of current of a third signaling frequency, a signal receiver foi` responding to said signal, said receiver comprising three relays, means rendering a first one of said relays responsive only to current of the rst signaling frequency, means rendering a second one of sai'd relays responsive only to current of the second signaling frequency, means rendering the third of said relays responsive only to current of the third signaling frequency, means rendering said second relay operative for apredetermined interval of time following the operation and release of said first relay, means rendering said third relay operative for a predetermined interval of time 'following' the operation and release of said second relay, and signal means controlled by said second and third relays.

4. In a signaling system in which the transmission of each signal is effected by a plurality of impulses of alternating current in succession, each impulse consisting of current of only one of three different signaling frequencies, a signal receiver and signal means controlled by said signal receiver', said signal receiver comprising a rst relay, means rendering said first relay responsive only to current of a first signaling frequency, a second relay, means rendering said second relay responsive only to current of a sec- Vond signaling frequency, a third relay, means `a predetermined interval of time responsive to the operation and release of said rst relay, means normally biasing said second relay to prevent its operation, means for decreasing the bias of said second relay to render said second relay operative While said fourth relay is operated, a fth relay, means for operating 'said fifth relay responsive to the operation of said second relay, means normally biasing said third relay to prevent its operation, means for operatively energizing said fourth relay and for decreasing the bias of said third relay to render said third relay operative while said fth relay is operated, means for operating said signal means responsive to the operation of said second relay, and means for maintaining the operation of said signal means under the control of said third relay.

5. In a signaling system in Which the transmission of each signal is effected by a plurality of impulses of alternating current in succession, each impulse consisting of current of only one of three different signaling frequencies, a signal receiver and signal means controlled by said signal receiver, said signal receiver comprising a rst relay, means rendering said first relay responsive only to current of a first signaling frequency, a second relay, means rendering said second relay responsive only to current of a second signaling frequency, a third relay, means rendering said third relay responsive only to current of the third signaling frequency, a fourth relay, means for operating said fourth relay for a predetermined interval of time responsive to the operation and release of said rst relay, means normally biasing said second relay to prevent its operation, means for decreasing the bias of said second relay to render said second relay operative While said fourth relay is operated, a fifth relay, means for operating said fth relay responsive to the operation of said second relay, means effective to maintain the operaf-ive energization of said fifth relay for a predetermined interval of time after said second relay releases, means normally biasing said third relay to prevent its operation, means for operatively energizing said fourth relay and for decreasing the bias of said third relay to render said third relay operative While said fth relay is operated, means for operating said signal means responsive to the operation of said 4second relay, and means for maintaining the operation of said signal means under the control of said third relay.

6. In a signaling system in which the transmission of each signal is effected by a plurality of impulses of alternating current in succession, each impulse consisting of current of only one of three different signaling frequencies, a signal receiver and signal means controlled by said signal receiver, said signal receiver comprising a first relay, means rendering said rst relay responsive only to current of a first signaling frequency, a second relay, means rendering said second relay responsive only to current of a second signaling frequency. a third relay, means rendering said third relay responsive only tc current of the third signaling frequency, a fourth relay, means for operating said fourth relay for a predetermined interval of time responsive to the operation and release of said first relay, means normally biasing said second relay to prevent its operation', means for decreasing the'bias of said second relay to render said second relay operative While said fourth relay is operated, a fifth relay, means for operating said fifth relay responsive to the operation of said second relay, means normally biasing said third relay to prevent its operation, means for operatively energizing said fourth relay and for decreasing the bias of said third relay to render said third relay operative While said fth relay is operated, means for operating said signal means responsive to the operation of said second relay, means for maintaining the operation of said signal means under the control of said third relay, and means for operatively energizing said fifth relay while said third relay is operated and for maintaining the energization of said fifth relay for a predetermined .interval of time after the release of said third relay.

7. In a signaling system in Which the transmission of each signal is effected by a plurality of impulses of alternating current in succession, each impulse consisting of current of only one of three different signaling frequencies, a signal receiver and signal means controlled by said signal receiver, said signal receiver comprising a first relay, means rendering said rst relay responsive only to current of a rst signaling frequency, a second relay, means rendering said second relay responsive only to current of a second signaling frequency, a third relay, means rendering said third relay responsive only to current of the third signaling frequency, a fourth relay, means for operating said fourth relay for a predetermined interval of time responsive to the operation and release of said first relay, means normally biasing said second relay to prevent its operation, means for decreasing the bias of said second relay to render said second relay operative While said fourth relay is operated, a fth relay, means for operating said fifth relay responsive to the operation of said second relay, means effective to maintain the operative energization of said fifth relay for a predetermined interval of time after said second relay releases, means normally biasing said third relay to prevent its operation, means for operatively energizing said fourth relay and for decreasing the bias of said third relay to render said third relay operative while said fifth relay is operated, means for operating said signal means responsive to the operation of said second relay, means for maintaining the operation of said signal means under the control of said third relay, and means for operatively energizing said fifth relay While said third relay is operated and for maintaining the energization of said fifth relay for a predetermined interval of time after the release of said third relay.

8. In a signaling system in which the transmission of each signal is effectedV by a plurality of impulses of alternating current in succession, each impulse consisting of current of only one of three different signaling frequencies, a signal receiver and signal means controlled by said signal receiver, said signal receiver comprising a first relay, means rendering said rst relay responsive only to current of a first signaling frequency, a second relay, means rendering said second relay responsive only to current of a second signaling frequency, a third relay, means rendering Asaid third relay responsive only to current of the third signaling frequency, a fourth relay, means for operating said fourth relay for a predetermined interval of time responsive to the operation and release of said first relay, means normally bias-

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