US2842620A - Toll switch trains - Google Patents

Description

July 8,1958 c. E. LOMAX 2,842,620

TOOL SWITCH TRAINS Filed Oct. 4. 1955 4 Sheets-Sheet 1 ss/ ss2 sss ss4 sss ss6 s57 ssa PO55" 4 9 2 38 3 9 5-9 3-8 2 TRANS/:-

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INVENTOR. 6762/6066 5 Lama/r United States TOLL SWITCH TRAINS Application October 4, 1955, Serial No. 538,357 20 Claims. (Cl. 179-27 This invention relates to toll switch trains and particularly to toll switch trains adapted to camp on busy trunk lines.

Heretofore, conventional toll switch trains have included mechanisms operated from an operators position for causing connectors to camp on busy trunk lines. These provided mechanisms have accomplished thepurpose of providing a facility for completing ultimately to a station for which all of the ingoing trunks are busy, a connection when one of the busy trunks becomes idle and without the operator having to reinitiate the call. However, the system has proven inefficient in that the camping condition is set up from the operators posi tion; any number of switch trains can be camped on the same group of busy trunks, and plural camping on the some groups of trunks prevents any one of the camping switch trains from completing a connection to a trunk that may become idle, with the result that often a large amount of switching equipment is uselessly tied up. Additionally, the camping condition is now generally set up only on the last trunk in a busy group so that the camping period is determined solely by the busy period of the last trunk line which period may be prohibitively long.

It is an object of this invention to provide an improved camp-on-busy toll switch train overcoming the above described difficulties.

Another object of the invention is to provide in a toll switch train an improved facility for automatically setting up camping conditions on a trunk line of a busy group.

Another object of the invention is to provide in a toll switch train an improved arrangement for permitting but one switch train to establish camping conditions on a group of busy trunk lines.

Another object of the invention is to provide in a toll switch train an improved facility for permitting the camped-on-busy switch train to engage automatically any one of the trunk lines of a busy group that may become idle.

Another object of the invention is to provide an improved trunk hunting connector for automatically setting up camping conditions on a trunk line of a busy group.

A further object of the invention is to provide an improved trunk hunting connector for automatically set ting up camping conditions on a trunk line of a busy group and for preventing other trunk hunting connectors forsetting up camping conditions on any trunk line in the'same group.

A further object of the invention is to provide an improved trunk hunting connector for automatically setting up camping conditions on a trunk line of a busy group and for automatically engaging any one of the group of busy trunk lines that may become idle.

An additional object of the invention is to provide in a station having a plurality of ingoing trunk lines a pilot trunk line adapted to register a group busy marking atent O when all of the ingoing trunks thereto are busy and adapted to be camped on when all of the trunks are busy.

Another object of the invention is to provide an improved trunk circuit for a pilot trunk line in a group of ingoing trunk lines for applying thereto a marking when all of the trunks to the station are busy in order to cause a trunk hunting switching train to camp on the pilot trunk line and to apply a busy signal when the pilot trunk line has been camped on.

Another object of the invention is to provide an improved trunk circuit for a pilot trunk line in a group of ingoing trunk lines for permitting a toll switch train provided with camp-on-busy facilities to camp thereon without interfering with the operation of other switch trains not provided with camp-on-busy facilities that may engage the pilot trunk line.

A specific object of the invention is to provide an improved toll switch train having access to a P. B. X board including a toll camp-on-busy trunk hunting connector and a pilot trunk line and a trunk circuit associated with the pilot trunk line for automatically setting up camping conditions for but one switch train at a time engaging the trunk lines of the P. B. X board and for automatically causing the camped-on-busy toll switch train to engage any one of the busy trunk lines that may become idle without otherwise interfering with the operating of other toll or local switch trains that may engage the trunk lines incoming to the P. B. X board.

Further features of the invention pertain to the particular arrangement of the elements of the system, whereby the above outlined and additional operating features thereof are attained.

Section ].Tlze general armngement of the telephone system The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be best understood by reference to the following specification taken in connection with the accompanying drawings. Figures 1 through 4, taken together, are a diagram partially in block form illustrating the area served by the telephone system and the details of certain of the apparatus incorporated in the telephone system, which apparatus has embodied therein the features of the invention as briefly outlined above. More particularly, Fig. 1 and a portion of Fig. 2, illustrate the details of one of the toll trunk hunting connector circuits disposed in the toll switch train of the local exchange; another portion of Fig. 2, taken in conjunction with Fig. 3, illustrates the components of the toll switch train and the local switch train in the local exchange; a portion of Fig. 4 illustrates the P. B. X trunk connections including P. B. X trunk circuits in the local exchange; and the other portion of Fig. 4 illustrates the toll switch train terminus in a call originating distant exchange.

Referring now more particularly to Figs. 1 through 4 which, when joined in end-to-end relationship, illustrate the complete telephone system incorporating the invention, the telephone system therein illustrated serves an area comprising a local exchange iii and a distant exchange 20. The local exchange 14) is of the automatic type, serves subscriber stations of any suitable type and comprises a manual dial toll board 2%; and the distant exchange 20 is of any suitable type and comprises a manual dial toll board 430. Specifically, the local exchange 10 serves a plurality of groups of subscriber stations, one of the groups including the subscriber station T300, and a plurality of P. B. X boards including the P. B. X board 420. The subscriber station T300 has a subscriber line 311 extending thereto that is provided with an individually associated line circuit 301 of any suitable type, and the P. B. X board 420 has a group of trunk lines individual thereto including the pilot trunk line 315 and the through trunk lines 316, 317, 318, 319 and 310, which extend respectively to the P. B. X trunk busy control circuit 415 and to the P. B. X trunk circuits 416, 417, 418, 419 and 410. The P. B. X trunk busy control circuit 415 is of unique construction and operation is disclosed in detail hereinafter, while the remainder of the P. B. X trunk circuits are of any suitable type. In addition, the toll board 200 has a trunk line 202 extending to a toll line circuit 260 of any suitable type, and the toll board 430 in the distant exchange 20 is connected by a trunk line 431 to a toll line circuit 440 of a conventional type adapted for simplex dialing. interconn cting the local exchange and the distant exchange 20 is a group of toll lines of the two way type, including the toll line 261, terminated in the toll line circuits 260 and 440 of the local exchange 10 and'the distant exchange 20, respectively.

The local exchange 10 comprises a local switch train including a plurality of finder-connector groups and a toll switch train including a plurality of toll transmission selectors and toll trunk hunting connectors, and the distant exchange 20 includes a toll switch train which is shown only in part. Specifically, the local switch train in the local exchange 10 comprises a group of finderselector links including the link illustrated comprising the finder 330 and the local trunk hunting connector 380, which group is provided with a distributor 320; the finder 330 having access to the subscriber line 311 extending to the subscriber station T300 and the line circuit 301 individually associated with the subscriber line 311 being connected to the distributor 32.0 by the start conductor C302. The toll switch train in the local exchange 10 comprises a plurality of toll transmission selector and toll connector links, including the link illustrated comprising the toll transmission selector 270 and the toll trunk hunting connector 110, which is interconnected by a trunk line 105 and a wiper 271 associated with the toll transmission selector 270. The toll transmission selector 2.70 is accessible from the toll line circuit 260 via the trunk 262 and in turn has access at wiper 271 via the trunk 273 to the inward trunk circuit 280 which has access to the toll board 200 over the trunk 281. The toll board 200 is provided with direct access to a toll trunk hunting connector 350 via a trunk 272. In the distant exchange 20, the toll line circuit 440 has access via trunk 441 to the toll transmission selector 450 comprising a part of a toll switch train, not shown in detail, which is substantially identical to that illustrated in local exchange 10.

Section 2.The apparatus incorporated in the telephone system In the local exchange 10, the toll board 200 is of the conventional manual dial type, including cord circuits, and is identical to the toll board 430 of the distant exchange 20. The toll line circuit 260 and the toll transmission selector 270 are of any conventional type and are identical respectively to the toll line circuit 440 and toll transmission selector.450 of the distant exchange 20. While it is not necessary, it is preferred'for purposes of describing all of the aspects of the invention that the toll transmission selectors 270 and 450 be of a simplex type adapted to repeat therethrough both normal busy'and group busy signals and also a flash busy signal. The inward trunk circuit 280 of local exchange 10 and the line circuit 301 are of any conventional type, and the distributor 320, finder 330 and the local trunk hunting connector 380 are of conventional Strowger types. The P. B. X trunk circuits 416, 417, 418, 419 and 410 in the local exchange are of a conventional type and include therein bridge cutoff relays R476, R477, R473, R479 and R480, respectively, which when operated, complete a chain connection over conductor 421 through contacts 486, 487, 488, 489 and 490, from ground to a l. B. X trunk busy control circuit 415. The P. B. X trunk busy control cir- 4 cuit includes a P. B. X busy relay R460 operable from the chain ground extended thereto and a marking relay R470.

The toll trunk hunting connector 110 includes the line relay R120, hold relay R140, ring cutofi relay R130, control relay R150, test relay R160, trunk test relay R210, dial relay R220 and the camping relay R230. In addition, the toll connector 110 is provided with a sequence switch SS having a plurality of individual switchcs SS1 through S510, which are operable through a plurality of positions, here illustrated as nine positions, by a motor magnet MM250. Also, the toll connector 110 has associated therewith a Strowger mechanism 110A provided with a wiper set having five wipers including a positive and a negative line wiper and C1, C2, and C3 control wipers adapted to engage the terminals of the contact sets in the connector bank 11013, a vertical magnet VM240 for driving the wiper carriage step-by-step in a vertical direction away from its normal vertical position, a rotary magnet RM170 for driving the wiper carriage step-by-step in the rotary direction away from its normal rotary position, and a release magnet RL175 for releasing the wiper carriage and for causing it to be returned to its normal rotary and vertical positions. Also associated with the wiper carriage of the Strowger mechanism 110A is the vertical off-normal switch, including the contacts 176 and 177, which are operated when the wiper carriage is driven one step in the vertical direction away from its normal vertical position.

In addition, the toll trunk hunting connector preferably, though not necessarily, includes a strapping conductor Y connected between the front and back springs of contacts 131A in the individual switch SS1 and a strapping conductor Z connected between the front springs of the individual switches SS3 and SS4 for returning, simul taneously, both tone and flash busy to a toll board position. Removal of the Y conductor causes flash busy to be returned singularly to the toll board and removal of the Z conductor causes tone busy to be returned singularly to the toll board.

The toll trunk hunting connector 350 is identical to the toll trunk hunting connector 110 illustrated in Figs. 1 and 2 and includes a Strowger mechanism 350A having a wiper set adapted to engage the contact sets of the connector bank 350B; and the local trunk hunting connector 380 is conventional and has associated therewith the Strowger mechanism 380A that includes a wiper set having four Wipers adapted to engage the contact sets of the connector bank 380B. Each of the connector banks 110B, 3503 and 38013 is represented in Fig. 3 by a single level of contact sets, though it is understood that each connector bank includes a plurality of levels and the levels illustrated are arbitrarily chosen to represent the first level in each connector bank. As illustrated in Fig. 3, the first level in each of the connector banks serves at the first four contact sets the individual subscriber lines 311, 312, 313 and 314, respectively, and at the remaining six contact sets serves the trunk lines 315, 316, 317, 318, 319 and 310, respectively, of the P. B. X board 420; the contact sets of each level being connected in multiple to the conductors of the corresponding lines at the bracketed terminals thereof. The subscriber lines 311, etc., have access, respectively, to the line circuits 301, etc., individual to the subscriber stations; the pilot trunk line 315 is a dead trunk insofar as through transmission to the P. B. X board is concerned and has access to the P. B. X trunk busy control circuit 415 which is common to and controlled by the P. B. X trunk circuits 416, etc., in a manner as described in detail hereinafter; the trunk lines 316, etc., have access through the P. B. X trunk circuits 416, etc., which may be of any suitable type, to the P. B. X board 420.

A better understanding of the connection and arrangement of the apparatus incorporated in the telephone sys- 5 tern will be facilitated from a consideration of an extension of calls involving the difierent exchanges and switch trains in each of the exchanges.

Section 3.Local calls to the P. B. X board First assuming that a call is initiated at the subscriber station T300 in the local exchange to be extended over the local switch train to the P. B. X board 420 for which the directory number corresponding to the position in the connector banks of the pilot trunk line 315 is 15, the line circuit 301 is controlled via the subscriber line 311 in order to govern the operation of the distributor 320 over the start conductor C302; whereby an idle one of the links such, for example, as that illustrated, is assigned thereto. The finder 330, of the link illustrated, operates to find the subscriber line 311 extending to the calling subscriber station T300; whereby the line circuit 301 is operated in order to mark the subscriber line 311 as busy to the other links having access thereto. Also, the connector 380, of the link illustrated, operates to return dial tone over the subscriber line 311 to the calling subscriber station T300; whereby the subscriber at the calling subscriber station T300 proceeds to dial the directory number of the called P. B. X board, which number is a two-digit number in the present illustration. The first digit dialed at the calling subscriber station T300 operates the local trunk hunting connector 380 to raise the wiper set 380A thereof to the first level of the connector bank 380B. The second digit dialed at the subscriber station T300 operates the local trunk hunting connector 380 to rotate the wiper set thereof into engagement with the fifth contact set in the first level of the connector bank 380B. At the fifth contact set, the local trunk hunting connector 380 is connected via the pilot trunk line 315 to the P. B. X trunk busy control circuit 415. It is appreciated that for purposes of com pleting a call to the P. B. X board, the pilot trunk line 315 is actually dead and is utilized only to provide to the connector an indication as to whether the P. B. X trunks to the P. B. X board 420 are busy or idle.

Assuming that all of the through P. B. X trunk lines 316, etc., are idle, the P. B. X trunk busy control circuit 415 applies to the pilot trunk line 315 permanent ground at the conductor C1 and ground at the conductor C2 via the contacts 472 and 461. In the local trunk hunting connector 380, ground on the C1 terminal of the engaged contact set is extended to the C1 Wiper thereof and ground on the C2 terminal of the contact set is extended to the C2 wiper thereof, whereby the Strowger mechanism 380 is caused to rotate ofi of engagement with the pilot trunk line 315 and through the P. B. X trunk lines to engage the sixth contact set in the bank 380B and is stopped thereon thereby completing a connection to the P. B. X trunk line 316.

Having previously assumed that the P. B. X trunk 316 is idle, the local trunk hunting connector 380 completes a connection thereover through the P. B. X trunk circuit 416 to the P. B. X board 420 in a conventional manner; whereby the BCO relay R476 in the P. B. X trunk circuit 416 is operated closing contacts 486. At the P. B. X board, the call from the subscriber station T300 is completed in a conventional manner and at the conelusion of the completed connection, when the calling and called subscribers disconnect, release of the local trunk hunting connector 380 and of the finder 330 is accomplished in a conventional manner and the subscriber line 311 is rendered idle.

From the foregoing, it can be easily understood that should all of the P. B. X trunk lines 316, 317, 318, 319 and 310 'be rendered busy by calls made to the P. B. X board 420 in the manner described above, the BCO relays R476, R477, R478, R479 and R430 in the respectively corresponding P. B. X trunk circuits 416, etc., will be operated so as to close ground over conductor 421 extending to the P. B. X trunk busy control circuit 415 via contacts 486, 487, 488, 489 and 490. In the P. B. X trunk busy control circuit 415 ground potential on the conductor 421 is extended to negative battery via the winding of the P. B. X busy relay R460 whereby the relay R460 operates interrupting ground potential on conductor C2 at the contacts 461 and applying negative battery to the conductor C3 from the winding of the marking relay R470 via contacts 474 and 463. In this connection, the trunk 315 has ground potential applied to the C1 conductor and negative battery applied to the C3 conductor thereof, thereby marking the pilot trunk line 315 that all of the P. B. X trunks 316, etc., are busy.

Assuming now that all of the P. B. X trunks 316, etc., are busy and that the P. B. X busy circuit 415 is operated in a manner described above, a call initiated at the subscriber station T300 to the P. B. X board 420 will be connected to the P. B. X pilot trunk line 315 via the local trunk hunting connector 380 in the manner previously described. Ground on the C1 conductor operates the busy relay in the local trunk hunting connector 300 and the connector maintains its connection to the trunk 315 and busy tone is returned to the subscriber at station T300. The subscriber proceeds to disconnect whereby the completed connection to the P. B. X pilot trunk line 315 is interrupted, the link circuit is released and the subscriber line 311 is rendered idle.

Section 4.A call from a toll board in a. distant exchange to an individual subscriber in the local exchange routed directly from a transmission selector in the local exchange Assuming that a call is initiated at the toll board 430 in the distant exchange 20 by the operator inserting a front plug of a cord circuit thereat into the jack terminating the trunk 431 extending to the toll line circuit 440 and to the toll line 261, the toll line circuit 440 which is adapted for simplex type operation seizes the toll line 261 extending to the toll line circuit 260 in the local exchange 10 whereby a connection is completed therethrough via the trunk 262 to the toll transmission selector 270 in the conventional manner.

Assuming that the call initiated at the toll board 430 in the distant exchange 20 is to be extended to the called subscriber station T300 in the local exchange 10 directly from the toll transmission selector 270 to a connector, the operator at the toll board 430 dials the proper digits X, l and 1; the first digit indicating the route mentioned and the second and third digits identifying the line terminal before the connector of the subscriber line 311 extending to the called subscriber station T300. More particularly, when the first digit X is dialed at the calling toll board 430 in the distant exchange 20, the Strowger mechanism 271 of the toll transmission selector 270 in the local exchange 10 is operated to a level in the contact bank accessible to a group of toll trunk huntin connectors including the toll trunk hunting connector 110 over a plurality of connector trunk lines 105, etc. The Strowger mechanism 271 of the toll transmission selector 270 in the local exchange 10 is provided with four wipers (not shown), including a positive and a negative line Wiper and a C1 and a C2 control wiper, accessible to the four conductor trunk lines at a corresponding four terminal contact set in the selector bank. The operator at the toll board 430 in the distant exchange 20 causes ground potential to be applied to the C1 and C2 wipers of the Strowger mechanism 271 associated with the toll transmission selector 270 in the local exchange 10. Responsive to the dialing of the first digit, the wipers 271 search across the contact sets of the designated level for a toll trunk hunting connector marked idle by negative battery on the C2 conductor of the connector trunk lines accessible thereto. Assuming that the toll trunk hunting connector 110 applies negative battery to the C2 conductor of the corresponding trunk via resistor 285 and contacts 283, 222 and 145B, and responsive thereto is seized by the toll transmission selector 270 via the Strowger mechanism 271 and the trunk 105, the toll connector 110 is prepared for operation. Inasmuch as the operator at the toll board 430 in the distant exchange has caused the line conductors of the connector trunk 105 to be closed in a loop connection and as the sequence switch SS in the toll connector 110 is in its normal first position, the line relay R120 is operated from ground through the upper winding thereof, the contacts 182A of the individual switch SS1, the positive and negative line conductors, the contacts 183A of the individual switch SS1 and the lower winding of relay R120 to negative battery. Operation of the line relay R120 closes contacts 121 thereby operating the hold relay R140 through the winding thereof. The operation of the relay R140 completes a hold circuit to ground potential on the conductor C2 at the contact 145A and removes battery potential from the conductor C2 at the contacts 1453. The toll hunting connector 110 is now seized and prepared to receive the digits identifying the line terminal of the subscriber line 311 extending to the called subscriber station T300.

Should the operator at the toll board 430 in the distant exchange 20 at this time desire to release the toll trunk hunting connector 110 without completing the calling connection, the line conductor loop of the connector trunk 105 first is caused to be interrupted at the transmission toll selector 270, whereby the line relay R120 is restored and a circuit is completed from ground at contacts 122 for operating the dial relay R220 and the vertical magnet VM240. Specifically, ground is applied to the battery connected relay R220 via contacts 122 and 146 and contacts 281 of the individual switch SS9 and 281 to the winding thereof and to the battery connected to vertical magnet VM240 via the contacts 122 and 146 and contacts 281 and 282 of the individual switches SSO and SS10 to the winding thereof. Thereupon, the relay R220 is operated to close at contacts 221 ground potential to the motor magnet MM250; whereby the mgnet MM250 is operated but the switch is not operated, the switch being of the type that is operated when the magnet is restored. At the same time the vertical magnet VM240 operates to raise the Strowger mechanism 110A one step in the vertical direction causing the ver tical otf-normal switch to close contacts 176 whereby ground potential is extended to the release magnet RL175 via contacts 172 causing the Strowger mechanism to restore to its normal vertical position. Upon restoring the Strowger mechanism operates the vertical offnormal switch to prepare at contacts 177 one point in a circuit including contacts 172, 284 and 251 for operating the motor magnet MM250. Thereafter, the operator at the toll board removes ground potential from the C1 and C2 conductors of the trunk 105 causing the relay R140 to restore and opening contacts 146 to interrupt the above traced path to the magnet VM240 and the relay R220. Shortly thereafter, the dial relay R220, being of the slow to release type, restores to interrupt at contacts 221 the circuit for operating the magnet MM250, whereby the latter restores and the sequence switch SS is rotated to its second position and contacts 234 of the individual switch SS10 are closed to complete the previously traced circuit for operating the motor magnet MM250. The operated magnet MM250 interrupts the operating circuit at contacts 251 causing the magnet MM250 to restore, to step the sequence switch SS, and to recomplete the operating circuit at contacts 251, so that the sequence switch is rotated through position 9 to position 1 and thereat the above traced operating circuit is interrupted at contacts 284. The toll trunk hunting connector 110 is now in its normal idle state having negative battery applied to the conductor C2 of the connector trunk 105 via the resistor 285, and the contacts 283, 222 and 145B.

Now assuming that instead of releasing the seized toll trunk hunting connector, the operator at the toll board 430 in the distant exchange 20 proceeded to dial the second digit 1 corresponding to the first digit of the directory number of the called subscriber station T300, the toll line circuit 440 responds thereto repeating this digit over the toll trunk 261 to the toll transmission selector 270 in the local exchange 10 wherein the line conductor loop of the connector trunk is interrupted and recompleted in accordance with the dial pulsing. Responsive to the interruption of the line conductor loop, the relay R in the toll trunk hunting connector 110 restores and reoperates causing the contacts 122 to close and open; whereby the dial relay R220 and the vertical magnet JR 1220 are energized. Specifically, the dial relay R220 is energized from ground potential via contacts 122 and 146, contacts 281 of the switch SS9 through the winding thereof to negative battery; and the vertical magnet VM240 is energized from ground via contacts 122 and 146 and contacts 281 of switch SS9 and contacts 282 of switch SS10 through the winding of VM240 to negative battery. The dial relay R220 is operated and remains operated during the dialing period and restores shortly thereafter, it being of the slowto-release type. During its period of operation, relay R220 closes ground to the motor magnet MM250 thereby energizing the magnet hut not operating the switch in the manner previously described. The vertical magnet VM240 follows the ground pulses at contacts 122 causing the Strowger mechanism 110A to rise one step vertically into proximity with the contact sets in the first level of the connector bank 110B. After dialing of this digit is completed at the toll board 430, the relay R220 restores interrupting contacts 221 and causing the motor magnet VM250 to operate the sequence switch SS to its second position.

In its second position, the sequence switch SS closes ground potential on the conductor C1 to battery connected at the lower winding of the ring cutoit relay R via the contacts 184 of the individual switch SS2; whereby the relay R130 operates and closes a hold circuit to ground on the conductor C1 at the contacts 136. At the same time, the control relay R150 is operated from ground via contacts 193 of the individual switch SS8 through the winding thereof to negative battery. At this time, the relays R120, R130, R and R are operated and the toll trunk hunting connector 110 is ready to receive the digit corresponding to the second directory number of the called subscriber station T300. The operator at the toll board 430 in the distant exchange 20 may still cause the toll trunk hunting connector 110 to be released by first opening the line conductor loop of the connector trunk 105 and then removing ground potential from C1 and C2 conductors in a manner described previously.

Assuming that the operator at the toll board 430 in the distant exchange 20 proceeds to dial a third digit 1 corresponding to the second digit of the directory number of the called subscriber station T300, the toll line circuit 440 responds thereto repeating this digit over the toll trunk 261 to the toll transmission selector 270 in the local exchange 10 wherein the line conductor loop of the connector trunk 105 is interrupted and recompleted in accordance with the dial pulsing. Responsive to the interruption and recompletion of the line conductor loop, the line relay R120 restores and reoperates whereby ground pulses are applied at contacts 122 to operate the dial relay R220 and the rotary magnet RM170. Specifically, the ground pulse operates the dial relay over a path including contacts 122 and 146 and contacts 231 of the individual switch SS9 through the winding of relay R220 to negative battery; and ground pulses are applied over a path including contacts 122 and 146 and contacts 281 of the individual switch SS9 are contacts magnet RM170, the Strowger mechanism 110A is driven one step in the rotary direction away from its normal rotary position whereby at this time the Strowger mecha nism 110A engages the contacts set in the level 110B of the bank contacts terminating the subscriber line 311.

Additionally, and shortly after the completion of the dialing of this digit, the dial relay R220 restores whereby the motor magnet MM250 is restored and the sequence switch SS is rotated to its third position and the energization path to the relay R150 is interrupted at the individual switch SS8 but the relay R150 is not restored, it being of the slow-to-release type. At this time, the test relay R160 in the toll trunk hunting connector 110 tests the subscriber line 311 for idle or busy condition.

Assuming that the subscriber line 311 is busy, ground potential appears on the control conductor C1 thereof which is applied to the control Wiper C1 of the Strowger mechanism 110A and over a path including the contacts 154 and the winding of the test relay R160 to negative battery, whereby the latter relay is operated. It is noted that at thistime the contacts 154 are completed because the transfer relay R150, although deenergized, has not yet restore-d. Shortly thereafter, the transfer relay R150 restores completing at the make-before-break contacts 155 a circuit from ground potential via contacts 148 and 164 for maintaining the test relay R160 operated and interrupting at contacts 154 ground potential returned to the relay R160 from the wiper C1 of the Str'ow'ger mechanism 110A. Upon restoration of the transfer relay R150, busy tone which comprises a 120 cycle tone interrupted at 60 times a minute is applied to the negative line conductor via contacts 181A of the individual switch SS1 and the multiple strap Y, contacts 161 and 151. This busy signal is repeated over the connector trunk 105 to the toll transmission selector 270 and repeated over the trunk 261 via the toll line circuit 440 to the operator at the toll board 430 in the distant exchange 20. Simultaneously, with the application of the busy tone to the negative line conductor, ground potential is applied to the motor magnet MM250 via contacts 162, 152 and 144, contacts 187 of switch SS4, strapping conductor Z, and contacts 251 whereby the sequence switch SS is operated to its fourth position. In its fourth position, the sequence switch SS, at its contacts 182A and 133A in the individual switch SS1, interrupts the line conductor loop to the relay R120 whereby the latter relay is restored and at the contacts 181B in the individual switch SS1 completes a connection from the flash busy source via resistor 124 and contacts 161 and 151 to the negative line conductor of the connector trunk 105. This flash busy source applied ground potential to the negative line conductor at a rate of 60 impulses per minute which signal is repeated via connector trunk 105, the transmission toll selector 270, the toll trunk 261 and the toll line circuit 440 to the operators position on the toll board 430 in the distant exchange 20 whereby a signal lamp thereat is flashed at the rate of 60 times per minute thereby giving a visual indication of the called line busy. At this time, the operator at the toll board 430 in the distant exchange 20 has both an audio and a visual line busy indication and responsive thereto proceeds to interrupt the connection to the subscriber line 311 in the local exchange 10. The interruption is facilitated by the operator at the toll board 430 withdrawing the plug from the cord circuit thereat causing the line conductor loop in the connector trunk 105 to be interrupted and ground potential to be removed from the C1 and C2 conductors of the connector trunk 105; whereby all the relays in the toll trunk hunting connector 110 are restored, the Strowger mechanism 110A is returnedto its normal rotary and vertical position and the 10 sequence switch SS is rotated through the ninth position to its first position causing battery potential to be applied to the control conductor C2, thereby indicating that the toll trunk hunitng'connector is idle and in a condition to be seized.

Now assuming that the subscriber line 311 associated with the subscriber station T300 is idle and the wiper set of the toll trunk hunting connector 110 is in engagement with the contact set terminating the subscriber line 311, battery potential appearing on the control conductor CS. thereof is returned to the control wiper C1 of the Strowger mechanism 110A thereby preventing the operation of the test relay R160. Thereupon, the sequence switch SS is operated successively from its third position to its ninth position. Specifically, ground is appiied to the motor magnet MM250 over a path including contacts 148, 165 and 156, contacts 186 of the individual switch SS3 and contacts 251 through the winding of magnet MM250 to negative battery. The motor magnet MM250 interrupts and recompletes the energization path at the contacts 251 responsive to the energization and operation thereof in a manner explained above whereby rotation of the sequence switch is achieved. When the sequence switch is rotated to its ninth position, the energizing ground potential is removed from the motor magnet MM250 by the interruption of the contacts 186 at the individual switch SS3.

In order to provide seizure supervision control to the operator at the toll board 430 in the distant exchange 20, the toll trunk hunting conductor 110 provides for the operation of the individual switch SS1 when the sequence switch SS is in its fourth position whereby the .line conductor loop through the windings of relay R are interrupted at the contacts 182A and 183A. The interruption of the line conductor loop is repeated via trunk 105, the toll transmission selector 270, the trunk 261 and the toll line circuit 444) to a lamp at the operators position in the toll board 430 in the distant exchange 20 where a control lamp is lighted indicating to the operator that the called subscriber line has been seized and is idle. Thereupon, the operator at the toll board 430 in the distant exchange 20, in order to set up ringing connections, causes ground potential to be removed from the control conductor C1 at the connector trunk 105 whereby the ring cutoff relay R is restored and contacts 131 and 134- are closed. The removal of ground potential from the control conductor C1 of the connector trunk 105 need be only for an interval sufficient to permit the relay R130 'to restore and thereafter ground potential is reapplied to the control conductor C1. When the sequence switch SS has rotated to its ninth position, a circuit is completed from a battery biased ringing generator via the upper winding of the relay R130, contacts 131 and 142 and the contacts 189 of the individual switch SS5 to the negative line wiper of the Strowger mechanism 110A for transmission through the negative line conductor of the subscriber line 311 and the station apparatus at the subscriber station T300, back through the positive line conductor of the subscriber line 311 and the positive line wiper of the Strowger mechanism 110A through the contacts 190 of the individual switch SS5 and contacts 143 and 134 to ground potential; whereby ringing current is applied to the equipment in the subscriber station T300 and the called subscriber is rung. At the same time, ring back tone from a source is applied via contacts 138 of the individual switch SS5 through the contacts 133 and the contacts 18313 of the individual switch SS1 to the negative line conductor extending through the toll switch train to the toll board 430 in the distant exchange 20 thereby indicating to the opera- .tor thereat that the called subscriber line is being rung.

If after a reasonable time the called party at the subscriber station 300 does not respond to the ringing thereof, the operator at the toll board 430 proceeds to release the connection by causing ground potential to be removed from conductors C1 and C2 in a manner as explained previously.

However, assuming that the party at the subscriber station T300 answers in response to the ringing, a direct current loop is completed at the subscriber station T300 from the battery biased ringing generator through the upper winding of relay R130 over the above described ringing path; whereby the relay R130 is operated. The relay R130 completes at contacts 136 a holding circuit from ground on conductor Cl through the lower winding thereof to negative battery and disconnects at contacts 134 and 131 ground potential and the battery biased ringing generator from the established ringing loop and connects thereto at contacts 132 and 135 the negative and positive line conductors of the connector trunk 105 via the contacts 183B and 182B, respectively, of the individual switch SS1; whereby the toll board 430 in the distant exchange 20 is switched through to the subscriber station T300 in the local exchange 10. Specifically, the switch through occurring at the contacts 132 and 135 is extended over. the connector trunk 105 through the toll transmission selector 270, the toll line circuit 260, the toll line 261, and the toll line circuit 440 to the toll board 430 in the distant exchange 20 causing the lighted supervisory lamp at the toll board 430 to be extinguished, thereby providing the operator thereat with answering supervision and indicating that the called party has answered. The operator at the toll board 430 in distant exchange proceeds to complete the connection thereat from the calling subscriber line. At the conclusion of the established connection when the party at the called subscriber line disconnects, the loop therefrom to the operators position at the toll board 430 in the distant exchange 20 is interrupted, whereby the supervisory lamp thereat is again illuminated to give the operator disconnect supervision. Thereupon, the operator causes the established connection to be released in a manner previously described by withdrawing the plug of the cord circuit at the toll board 430. Thereafter, the toll trunk hunting connector 110 returns to its idle state having negative battery applied to the control conductor C2 of the con nector trunk 105 and the toll transmission selector 270 is idled and prepared to receive another incoming toll call.

Alternatively to completing a connection to an individual subscriber station T300, etc., in the local exchange 10 directly from the toll transmission selector 270 in the manner explained in detail in this section, the operator at the toll board 430 in the distant exchange 20 may also cause the call to be completed through a toll board 200 in the local exchange 10 in a manner as illustrated in the drawings. More particularly, the operator at the toll board 430 in the distant exchange 20 may connect to the operator at the toll board 200 in the local exchange 10 by inserting a front plug of a cord circuit thereat into the jack terminating the trunk 431 extending to the toll line circuit 440 which toll line circuit 440 terminates the toll line 261 extending to the toll transmission selector 270 in the local exchange 10 through the toll line circuit 260. By dialing a proper digit or digits X, the operator at the toll board 430 in the distant exchange may operate the toll transmission selector 270 to cause the Strowger mechanism 271 associated therewith to engage the contact set terminating the trunk 273 extending to the inward trunk circuit 280 having access over the trunk 281 to the toll board 200 in the local exchange 10. The operator at the toll board 200 is thereupon signaled that she is being called and responsive thereto completes a connection for receiving from the operator at the toll board 430 in the distant exchange the information necessary to complete the desired connection in the local exchange 10. Assuming that the operator at the toll board 430 desires to have a connection completed through to the subscriber station T300 in the local exchange 10, this information is relayed to the operator at the toll board 200 in the local exchange and the connection between the two toll boards in the distant exchange 20 and in the local exchange 10 may at this time be interrupted, or should the operators so choose, be maintained awaiting the completion of the call. The operator at the toll board 200 proceeds to complete a connection to one of a group of toll trunk hunting connectors including the toll trunk hunting connector 350 by inserting a plug of a cord circuit thereat into the jack terminating the trunk 272 extending to the toll trunk hunting connector 350. Thereupon, the operator at the toll board 200 proceeds to dial the digits corresponding to the directory number of the called subscriber station whereupon the toll trunk hunting connector 350, which is identical to the toll trunk hunting connector 110, completes a connection to the contact set terminating the called subscriber line. The operation of the toll trunk hunting connector 350 is identical to that as explained previously with reference to the operation of the toll trunk hunting connector 110 except that in this case, control to the connector is provided directly from a toll board in the local exchange and seizure, answering and disconnect supervision is returned directly to the toll board in the local exchange. Upon completion of the connection from the toll board 200 to the called subscriber station in the local exchange via the toll trunk hunting connector 350, the operator at the toll board 200 proceeds to extend the connection therefrom to the toll board 430 in the distant exchange 20 via the trunk 202, the toll line circuit 260, the toll trunk line 261 and the toll line circuit, whereat the final connection between the calling and called subscriber station is completed. At the conclusion of the established connection, the operator at the toll board 200 is given disconnect supervision whereupon the connection to the called subscriber station is to be released and the toll trunk hunting connector 350 is restored to its idle state.

Section 5 .A call from a toll board in a distant exchange to the P. B. X board in the local exchange routed directly from a toll transmission selector in the local exchange Assuming that a call is initiated at the toll board 430 in the distant exchange 20 by the operator inserting a. front plug of a cord circuit thereat into a jack terminating the trunk 431 extending to the toll line circuit 440 which terminates the toll trunk 261, the toll line circuit 440 is controlled to cause the toll line circuit 260 terminating the toll trunk 261 in the local exchange 10 to seize the toll transmission selector 270 over the trunk 262. Assuming further that the present call initiated at the toll board 430 in the distant exchange 20 is to be extended to the P. B. X board 420 in the local exchange 10 directly from the toll transmission selector 270 to a toll trunk hunting connector, the operator at the toll board 430 dials the proper digits comprising the digits X, 1 and 5; the first digit indicating the route mentioned and the second and third digits corresponding to the directory number of the P. B. X board and identifying the contact set of the connector bank terminating the P. B. X pilot trunk line 315 extending to the P. B. X trunk busy circuit 415 which is associated with the P. B. X board 420.

More particularly, when the first digit X is dialed at the toll board 430 in the distant exchange 20, the Strowger mechanism 271 or" the toll transmission selector 270 in the local exchange 10 is operated to complete a connection to engage the contact set terminating the connector trunk 105, for example, extending to the idle toll trunk hunting connector 110. Thereafter, the seized toll trunk hunting connector 110 is operated responsive to the dialing of the second and third digits at the toll board 430 to engage the Strowger mechanism 110A thereof to the contact set of the connector bank 110B terminating the P. B. X pilot trunk line 315, in the same manner as explained previously in Section 4. At this time, the

line relay R120, the ring cutoff relay R130, ,hold relay R140, the control relay R150 and the line test'relay R160 in the toll trunk hunting connector 110 are operated and the sequence switch SS is in its third position; and the trunk test relay R210 tests the P. B. X pilot trunk line 315 to determine if all of the P. B. X trunks having access to the P. B. X board 420 are busy.

Assuming that at least one of the. P. B. X trunks is idle, the P. B. X busy circuit 415 associated therewith has ground applied to the C1 and the C2 control conductors and the control conductor C3 is open as explained in Section 3. These respective potentials are returned via the pilot trunk line 315 and the corresponding terminals of the engaged contact set to the control wipers C1 and C2, respectively, of the Strowger mechanism 110A associated with the connector 110. In the toll trunk hunting connector 110, the line test relay R160 is operated from ground potential on the C1 Wiper via contacts 154 and the winding thereof extending to negative battery. The operation of the relay R160 closes contacts 163 thereby completing an operating circuit for the trunk test relay R210 from ground potential on the C2 wiper of the Strowger mechanism 110A via a path including contacts 234, 224, 163, 153 and 171 through the winding of the relay R210 to negative battery. The operation of the trunk test relay R210 closes ground at contacts 211 to the control relay R150 whereby the deenergized but operated relay R150 is reenergized and maintained operated. At the same time, ground is extended via contacts 211 and the contacts 192 of the individual switch SS7 through the winding of the rotary magnet RM170 to negative battery whereby the rotary magnet is energized interrupting closed contacts 171, restoring the trunk test relay R210, opening contacts 211 and causing the rotary magnet RM170 to restore; whereby the wiper carriage of the Strowger mechanism 110A is rotated on additional step away from its normal rotary position. At this time, the wipers of the Strowger mechanism 110A engage the contact set terminating the first through P. B. X trunk line 316 extending via the trunk 360, the P. B. X trunk circuit 416 to the P. B. X board 420.

Assuming that the P. B. X trunk 316 is busy, ground is returned from the P. B. X trunk circuit 416 over the control conductors C1 and C2 of the P. B. X trunk line 316 via the corresponding terminals of the engaged contact set to the wipers C1 and C2 of the Strowger mechanism 110A. At this time, the relays R160 and R210 have restored but with ground potential again applied to the wipers C1 and C2 of the Strowger mechanism 110A the relays R160 and R210 are operated in a manner as described above, whereby the wiper set thereof is rotated an additional step in the rotary direction to engage the contact set terminating the second P. B. X trunk circuit over P. B. X trunk line 317. Responsive to the rotation of the wiper carriage, the relays R160 and R210 are restored and the control relay R150 is deenergized but the latter relay remains operated inasmuch as it is of the slow-to-release type. At this time, the relays R160 and R210 test to determine the idle or busy condition of the P. B. X trunk line 317.

Assuming that the P. B. X trunk 317 is idle, the C1 and C2 control conductors thereof return battery potential via the corresponding terminals of the associated contact set to the wipers C1 and C2 of the Strowger mechanism 110A. With battery applied to the wipers C1 and C2, neither of the relays R160 or R210 are operated whereby the control relay R150 is permitted to restore a short time thereafter thereby completing a circuit from ground potential via the contacts 186 of the individual switch SS3 to energize the motor magnet MM250 of the sequence switch SS in a manner as described previously in Section 4; whereby the sequence switch SS is rotated from its fourth position to its ninth position.

As explained in Section 4 hereof, the sequence switch SS in its fourth position interrupts the line conductor loop of the connector trunk at the relay R thereby providing seizure supervision to the operator at the toll board 430 in the distant exchange 20, and in position 5 closes the contacts 191 in the individual switch SS6 to apply ground potential via contacts 141, 191 and 149 to the C1 wiper of the Strowger mechanism 110A. Ground potential on the C1 wiper is extended through the contact set to the C1 conductor of the P. B. X trunk line 317 whereby the ECG relay R477 in the P. 3., X trunk circuit 417 is operated and contacts 487 in the busy chain circuit extending to the P. B. X busy circuit 415 via conductor 421 are closed. The operator at the P. B. X board 420 is rung in a manner as previously described in Section 4 and the call completed thereto. At the conclusion of the established connection, the toll trunk hunting connector 110 is restored to its idle position by control of the operator at the toll board 430 in the distant exchange 20 in a manner as described in Section 4 hereof.

Now assuming that all of the through P. B. X trunks 316, 317, 318, 319 and 310 have been busied by other calls extended thereto over local trunk hunting connectors as explained in Section 3 or over toll trunk hunting connectors as explained above, the RC0 relays R476, etc., of the P. B. X trunk circuits 416, etc., are operated whereby ground is extended via the contacts 490, 489, 488, 487 and 486 and conductor 421 through the winding of the busy relay R460 in the P. B. X trunk busy control circuit 415 to negative battery. As ex-' plained previously in Section 3, the operation of the busy relay R460 removes ground potential from the control conductor C2 and applies battery potential to the conductor C3. Assuming, further, that the operator at the toll board 430 in the distant exchange 20 now initiates a call to the P. B. X board 420 in the local exchange 10 via the toll transmission selector 270, the toll trunk hunting connector 110 and the P. B. X pilot trunk line 315 in the local exchange 10 in a manner as described above, the Strowger mechanism 110A finds ground potential on the C1 wiper thereof and battery potential on the C3 wiper thereof. Responsive to ground potential on the C1 wiper, the test relay R is operated in a manner as described above preparing a holding circuit from ground at the contacts 164. At the same time, the camping relay R230 in the toll trunk hunting connector 110 is operated and immediately thereafter, the marking relay R470 in the P. B. X trunk busy control circuit 415 is operated from ground potential extending via contacts 147 and 223, the upper winding of relay R230, the C3 wiper of Strowger mechanism 110A, the C3 conductor of the P. B. X pilot trunk line 315, contacts 463 and 477 and the winding of relay R470 to negative battery. The prior operation of the relay R230 closes ground at contacts 232 for maintaining the control relay R150 operated and for energizing the rotary magnet RM via contacts 192 of the individual switch SS7, causes a group busy signal comprising a 120 cycle tone interrupted at the rate of 120 times a minute to be returned to the negative line conductor of the connector trunk 105 via contacts 231 and 157, and prepares a circuit for maintaining the relay R230 operated via the lower winding thereof by closing contacts 233. The subsequent operation of the relay R470 in the P. B. X trunk busy control circuit 415 closes a hold circuit from ground on conductor 421 at contacts 473 and applies ground at contacts 471 via contacts 462 to the C2 conductor of the P. B. X pilot trunk line 315 and removes at the break-after-m'ake contacts 474, negative battery potential from the conductor C3 of the P. B. X pilot trunk line 315. In the toll trunk hunting connector 110, ground potential applied to the C2 conductor of the P. B. X pilot trunk line 315 is extended via the C2 control wiper, contacts 233 and the lower winding of relay R239 to negative battery, thereby maintaining the relay R230 operated; and the removal of battery from the C3 conductor of the trunk 315 deenergizes the upper winding of the relay R239. Thus, at this time, the toll trunk hunting connector 110 has its relays R120, R130, R140, R150, R169 and R230 operated, the sequence switch SS is dormant in its third position and the wiper carriage of the Strowger mechanism MBA is camped on the contact set associated with the P. B. X pilot trunk line 315. The group busy signal applied to the negative line conductor 195 is repeated via the toll transmission selector Jr), the toll trunk 2B1 and the toll line circuit 446} to the toll board 43% in the distant exchange 2%, whereby the operator thereat is rendered camping supervision, that is, the operator at the toll board 430 is given to know that the P. B. X trunks 315 through 313 are busy, that the toll trunk hunting connector 110 has seized and is camped on the P. B. X pilot trunk line 315, and that the camped on busy toll trunk hunting connector 11% will automatically engage the first one of the P. B. X trunks 315 through 310 to become idle. At this time, the operator at the toll board 430 in the distant exchange may release the established connection in a manner as explained in Section 4, thereby interrupting the connection to the P. B. X busy circuit 415 or the operator may maintain the established connection and wait for a busy one of the P. B. X trunks to become idle.

Assuming that the operator at the toll board 430 in the distant exchange 29 maintains the established connection to the P. B. X trunk 315 and subsequently one of the busy P. B. X trunks, for example, the P. B. X trunk 316, becomes idle, the P. B. X trunk busy control circuit 415 is operated to cause the wiper carriage of the Strowger mechanism 115 A to be rotated into engagement with the contact set corresponding to the idle P. B. X trunk 316. Specificaliy, the interruption of the busying connection to the P. B. X trunk line 3E6 causes the BCO relay R476 in the P. B. X trunk circuit 416 to restore thereby interrupting at contacts 436 the ground potential applied via conductor 421 to the P. B. X trunk busy control circuit 415. Responsive thereto, the relay R460 restores and the relay R470 restores shortly thereafter, it being of the slow to release type, whereupon ground potential to the C2 conductor of the trunk 315 is interrupted for a brief period. The interruption of ground potential on the C2 conductor of the trunk interrupts the circuit extended over the C2 wiper of the toll connector so that relay R230 is restored at the lower winding thereof. The restoring of relay RZEB removes at contacts 232 ground potential from rotary magnet RMY/"B whereby the latter is restored and closes contacts 234 for applying ground potential from the C2 conductor of the trunk for operating relay R216 whereby the Strowger mechanism 110 is caused to rotate to engage the contact set terminating the P. B. X trunk 316 the relays B162) and R211? are restored and prepared to test for the idle or busy condition of the P. B. X trunk 316. Having assumed that the P. B. X trunk 316 has been rendered idle, the C1 and C2 control conductors thereof are at battery potential, which potential is returned to the wipers Cl and C2 of the Strowger mechanism MBA. Responsive thereto, the toll trunk hunting connector 110 is operated to complete the connection through the P. B. X trunk 316 to the P. B. X

board 42% in a manner as described previously. At the same time, the BCO relay R476 in the P. B. X trunk circuit 415 closes contacts 436 and, in the absence of any other idle P. B. X trunks, ground potential is returned via conductor 323. to the relay new in the P. B. X trunk busy control circuit 4E5. Responsive to seizure of the idle P. B. X trunk 316, answering supervision is provided to operator at the toll board 43 1 and responsive to the interruption of the established connection, disconnect supervision is provided to the operator at the toll board 1B 43%) in the distant exchange it in a manner as described in Section 4. l

Assuming now that all of the P. B. X trunk circuits 316, etc., having direct access to the P. B. X board 420 are busy and that a toll trunk hunting connector, such as the toll connector 350, for example, is camping on the pilot trunk line 315 associated with the P. B. X trunk busy control circuit 415, a call made to the P. B. X board 420 over the toll trunk hunting connector 119 will be returned as busy to the calling party. Specifically, assuming that the operator at the toll board 4 34 in the distant exchange 24? now initiates a call to the P. B. X board 420 over a route including the toll transmission selector 270 and the toll trunk hunting connector 110, the C1 and C2 wipers of the Strowger mechanism A engages the contact set terminating the P. B. X pilot trunk line 315 in a manner previously explained and finds ground potential on the conductors C1 and C2 thereof. Inasmuch as at this time the toll connector Ht as previously explained, has the relays RlZt), R139, R1462 and R operated, though the latter has been deenergized, and the sequence switch thereof is in its third position, the line test relay R is operated from ground potential on the control wiper C1 and the trunk test relay R210 is operated from ground potential on the wiper C2 whereby the wiper carriage of the Strowger mechanism MM; is rotated an additional step in the rotary direction to engage the contact set associated with the P. B. X trunk 316, and the line test relay R166 and the trunk test relay R214) in the toll connector 110 are restored. Having assumed that all of the P. B. X trunks 316, etc., are busy, the toll trunk hunting connector 110 at the contact set terminating the P. B. X trunk 316 finds ground potential on the control conductors C1 and C2 thereof whereby the toll connector is operated in a manner explained above to cause the wiper carriage of the Strowger mechanism 110A to rotate an additional step to engage the contact set terminating the P. B. X trunk line 317. This operation of the toll connector 11% is repeated until the wiper carriage of the Strowger mechanism 110A engages the contact set terminating the P; B. X trunk 310. When the Wiper carriage of the Strowger mechanism 110A engages the contact set associated with the P. B. X trunk 310, ground potential is returned over the control conductor C1 only, inasmuch as the C1 and C2 conductors thereof are not strapped, whereby the toll trunk hunting connector lit operates in a manner as described in Section 4 hereof to return busy tone to the negative line conductor of the conductor trunk 105 and to operate the sequence switch to the fourth position whereby both busy tone and flash busy is returned to the negative line conductor of the connector trunk 105. Thus, the operator at the toll board 430 is informed that all of the P; B. X trunks to the P. B. X board 429 are busy and that the camp-on facility of the pilot trunk line 315 at this time is not available for use. Thereupon, the operator at the toll board 430 in the distant exchange 2%) proceeds to release the established connection and restore the toll trunk hunting connector 110 to its idle state in a manner as previously explained.

From the above given explanation it should be clear that any one of the local trunk hunting connectors 380, etc., engaging the contact set terminating the P. B. X pilot trunk line 315 when the camp-on facility thereof is being utilized by any one of the toll trunk hunting connectors 110, 350, etc., will be operated to test through each one of the busy P. B. X trunks 316, 317, 318 and 319 and will make final contact with the busy trunk 310 causing a busy signal to be returned to the calling subscriber.

In. the event that all of the P. B. X trunks 31.6, etc., having direct access to the P. B. X board 420 are busy, causing the P. B. X trunk busy control circuit 415 terminating the P. B. X pilot trunk line 315 to be operated, and in the event that a local trunk hunting connector 380 in attempting to establish a connection to the P. B. X board 420 engages and is busied on the contact set terminating the P. B. X pilot trunk line 315 in a manner as described in Section 3, the operation of a calling toll trunk hunting connector subsequently engaging the contact set terminating the P. B. X pilot trunk line 315, as described above, is in nowise affected. More particularly, responsive to the connection of a local trunk hunting connector, such as for example, the local trunk huntnig connector 336 to the P. B. X pilot trunk line 335, the local trunk hunting connector 354i is locked in its busy state by ground on the Cl conductor and does not affect the P. B. X trunk busy control circuit. The subsequent connection of the P. B. X trunk hunting connector 1th, for example, to the P. B. X pilot line 355 causes the l. B. X trunk busy control circuit 435 to be operated in a manner as described in Section 4 to return ground to the $2 conductor of the P. B. X pilot trunk line 315. Ground returned on the C2 conductor causes the P. B. X trunk hunting connector 11% to be held on the pilot trunk line 315 in a camping condition but does not affect the local trunk hunting connector inasmuch as it is already locked in its busy condition. Therefore, the local trunk hunting connector 38% continues to engage the control set terminating the pilot trunk line 315 and returns a busy signal to a calling sub-- scriber notwithstanding any changes that may take place in the potentials on the conductors of the pilot trunk line 315, and the P. B. X trunk hunting connector 119 is camped on the pilot trunk line prepared to seize any P. .3. X trunk that may become idle and in fact will so operate without any interference from the busied local trunk hunting connector engaging the contact set of the pilot trunk line 315.

it is additionally clear from the explanation given in Section 4 hereof that should the operator at the toll board 534) in the distant exchange 20 find it necessary, or so choose, a call therefrom to the P. B. X board 42%) in the local exchange may be completed through the toll board 260 in the local exchange Ill as explained in Section 4; whereby a connection from the toil board 2th) to the P. B. X board 42'} may be completed substantially as described in this section and that connection extended from the toll board 200 in the local exchange lit to the toll board 43b in the distant exchange for completion of the call.

Section Cir-Conclusions in view of the foregoing, it is apparent that there has been provided an improved toll switch train including a novel toll trunk hunting connector and novel P. B. X station equipment having a pilot trunk line and a trunk busy control circuit which permits but one toll switch train to automatically camp on the pilot trunk line associated with a group of busy trunk lines and permits the camped on busy switch train to automatically complete a connection to the i. B. X station when any one of re trunk lines thereto becomes idle, without interfering with the normal operation of other switching trains that may seek to complete a connection to the B. station during the camped-on-busy period.

While there has been described what is, at present, believed to be the preferred and most useful embodiment of the invention, it is well understood that the invention is not limited to use with P. B. X stations and that the invention may be practiced with any station provided with a group of ingoing trunk lines, and that various modifications may be made in the described embodiment; it being intended that the appended claims should cover both the described embodiment and all such modifications as fall within the true spirit and scope of the invention.

What is claimed is: p

l. In a telephone system, a telephone station, a plurality of automatic switches of the trunk hunting type controllable from said telephone station for extending connections therefrom, a pilot line and a plurality of trunk lines arranged in a group and accessible to each of said automatic switches, said pilot line occupying a position preceding the positions of said trunk lines in said group so that any one of said automatic switches selects and tots said pilot line preceding the trunk hunting operation thereof into said trunk lines said group, a busy circuit terminating said pilot line and controlled by the idle condition of any one of said trunk dues in said group for applying an idle marking to said pilot line and controlled by the busy condition of all of said trunk lines in said group for applying a busy marking to said pilot line, means included in any one of said automatic switches and controlled upon the selection of said pilot line in response to testing of said idle marking thereon for initiating trunk hunting operation thereof into said trunk lines in said group, means included in any one of said automatic switches and controlled upon selection of said pilot line in response to testing of said busy marking thereon for holding said one automatic switch and for converting said busy marking on said pilot line to a camping marking thereon and for returning a camping signal therefrom to said telephone station, said busy circuit also being controlled by the subsequent appearance of an idle trunk line in said group to remove said camping marking from said pilot line, means included in any one of said automatic switches that is holding said pilot line and responsive to the removal of said camping marking therefrom for initiating trunk hunting operations of said one automatic switch into said trunk lines in said group, and means included in any one of said automatic switches and controlled upon the selection of said pilot line in response to testing of said camping marking thereon for returning a busy signal over said one automatic switch to said telephone station.

2. In a telephone system, a telephone station, a plurality of automatic switches of the trunk hunting type controllable from said telephone station for extending connection therefrom, a pilot line and a plurality of trunk lines arranged in a group and accessible to each of said automatic switches, said pilot line occupying a position preceding the positions of said trunk lines in said group so that any one of said automatic switches selects and tests said pilot line preceding the trunk hunting operation thereof into said trunk lines in said group, a busy circuit terminating said pilot line and controlled by the idle condition of any one of said trunk lines in said group for applying an idle marking to said pilot line and con trolled by the busy condition of all of said trunk lines in said group for applying a busy marking to said pilot line, means included in any one of said automatic switches and controlled upon the selection of said pilot line in response to testing of said idle marking thereon for initiating trunk hunting operation thereof into said trunk lines in said group, means included in any one of said automatic switches and controlled upon selection of said pilot line in response to testing of said busy marking thereon for holding said one automatic switch and for converting said busy marking on said pilot line to a camping marking thereon and for returning a camping signal therefrom to said telephone station, said busy circuit also being controll d by the subsequent appearance of an idle trunk line in said group to remove said camping marking from said pilot line, means included in any one of said automatic switches that is holding said pilot line and responsive to the removal of said camping marking therefrom for initiating trunk hunting operations of said one automatic switch into said trunk lines in said group, means included in any one of said automatic switches and controlled upon the selection of said pilot line in response to testing of said camping marking thereon for initiating trunk hunting operation thereof into said trunk lines in said group notwithstanding the busy condition thereof, and means included in any one of said automatic switches and responsive to a. trunk hunting operation thereof into said trunk lines in said group when all of said trunk lines are busy for returning a busy signal over said one automatic switch to said telephone station.

3. in a telephone system, a telephone station, a plurality of automatic switches of the trunk hunting type controllable from said telephone station for extending connection therefrom, a pilot line and a plurality of trunk lines arranged in a group and accessible to each of said automatic switches, said pilot line occupying a position preceding the positions of said trunk lines in said group so that any one of said automatic switches selects and tests said pilot line preceding the trunk hunting operation thereof into said trunk lines in said group, a busy circuit terminating said pilot line and controlled by the idle condition of any one of said trunk lines in said group for applying an idle marking to said pilot line and controlled by the busy condition of all of said trunk lines in said group for applying a busy marking to said pilot line, means included in any one of said automatic switches and controlled upon the selection of said pilot line in response to testing of said idle marking thereon for initiating trunk hunting operation thereof into said trunk lines in said group, means included in any one of said automatic switches and controlled upon the selection of said pilot line in response to testing of said busy marking thereon for stopping said one automatic switch on said pilot line, said busy circuit also being controlled by the busy condition of all of said trunk lines in said group and by the selection of said pilot line by any one of said automatic switches for applying a camping marking to said pilot line and for removing thereafter said busy marking from said pilot line and controlled by the subsequent appearance of an idle trunk line in said group for removing said camping marlc'ng from said pilot line, means included in any one of said automatic switches that is stopped on said pilot line and controlled in response to the camping marking on said pilot line for holding said one automatic switch thereon and for returning a camping signal over said one automatic switch to said telephone station and controlled in response to the removal of said camping marking from said pilot line for initiating trunk hunting operation of said one automatic switch into said trunk lines of said group, and

means included in any one of said automatic switches and controlled upon the selection of said pilot line in response to testing of said camping marking thereon for returning a busy signal over said one automatic switch to said telephone station.

4. In a telephone system, a telephone station, a plurality of automatic switches of the trunk hunting type controllable from said telephone station for extending connections therefrom, a pilot line and a plurality of trunk lines arranged in a group and accessible to each of said automatic switches, said pilot line occupying a position preceding the positions of said trunk lines in said group so that any one of said automatic switches selects and tests said pilot line preceding a trunk hunting operation thereof into said trunk lines in said group, a busy circuit terminating said pilot line and controlled by the idle condition of any one of said trunk lines in said group for applying a group idle marking to said pilot line and controlled by the busy condition of all of said trunk lines in said group for applying a group busy marking to said pilot line, a plurality of trunk circuits correspond ing to said plurality of trunk lines wherein each of said trunk circuits is controlled by the idle condition of the corresponding trunk line for applying thereto a trunk idle marking and wherein said trunk circuit corresponding to the one of said trunk lines in the last position of said trunk hunting operation in said group is controlled by the busy condition of said one trunk line to apply thereto a trunk busy marking and wherein each of the others of said trunk circuits is controlled by the busy condition of the corresponding trunk line for applying thereto a trunk hunting marking, means included in any one of said automatic switches and controlled upon the selection of said pilot line in response to testing of said group idle marking thereon for initiating trunk hunting operation thereof into said trunk lines in saidgroup, means included in any one of said automatic switches and controlled upon the selection of said pilot line in response to testing of said group busy marking thereon for stopping said one automatic switching on said pilot line, said busy circuit also being controlled by the busy condition of all of said trunk lines and by the selection of said pilot line by any one of said automatic switches for applying a camping marking to said pilot line and for removing thereafter said busy marking from said pilot line and controlled by the subsequent appearance of an idle trunk line in said group for removing said camping marking from said pilot line, means included in any one of said automatic switches that is stopped on said pilot line and controlled in response to the camping marking on said pilot line for holding said one automatic switch on said pilot line and for returning a camping signal over said one automatic switch to said telephone station and controlled in response to the removal of said camping marking from said pilot line for initiating trunk hunting operation of said one automatic switch into said trunk lines of said group, means included in any one of said automatic switches responsive to the selection and testing of any one of said trunk lines having said trunk idle marking thereon for completing a connection thereover from said telephone station to said one trunk line, means included in any one of said automatic switches responsive to the selection and testing of any one of said trunk lines having said trunk hunting marking thereon for continuing the trunk hunting operation thereof in said trunk lines in said group, and means include-d in any one of said automatic switches responsive to the selection and testing of said one of said trunk lines in said group having said trunk busy marking thereon for returning a busy signal over said one automatic switch to said telephone station.

5. The telephone system set forth in claim 4, wherein said camping marking is the same as said group idle marking, whereby any one of said automatic switches is controlled upon selection of said pilot line in response to said camping marking thereon for initiating trunk hunting operation thereof into said trunk lines in said group notwithstanding the busy condition of all of said trunk lines in said group and is controlled upon selection of each of said busy trunk lines in response to the testing of said trunk hunting marking thereon to continue the trunk hunting operation therein to said one trunk line in said group having said trunk busy marking thereon and is further controlled upon the selection and testing of said one trunk line having said trunk busy marking thereon to return a busy signal over said one automatic switch to said telephone station.

6. In a telephone system, a plurality of subscriber stations, a plurality of first automatic switches of the trunk hunting type controllable from said subscriber stations for extending connections therefrom, an operator position, a plurality of second automatic switches of the trunk hunting type controllable from said operator position for extending connections therefrom, a pilot line and a plurality of trunk lines arranged in a group and accessible to each of said first and second automatic switches, said pilot line occupying a position preceding the positions of said trunk lines in said group so that any one of said first and second automatic switches selects and tests said pilot line preceding the trunk hunting operating thereof into said trunk lines in said group, a busy circuit terminating said pilot line and controlled by the idle condition of any one of said trunk lines in said group for applying an idle marking to said pilot line and controlled by the busy condition of all of said trunk lines in said group for applying a busy marking to said pilot line, means included in any one of said first and second automatic switches and controlled upon selection of said pilot line in response to testing of said idle marking thereon for initiating trunk hunting operation thereof ammo into said trunk lines in said group, means included in any one of said first automatic switches and controlled upon selection of said pilot line in response to testing of said busy marking thereon for arresting further operation thereof and for returning a busy signal thereover to the connected one of said subscriber stations, means included in any one of said second automatic switches and controlled upon selection of said pilot line in response to testing of said busy marking thereon for holding said one of said first automatic switches and for converting said busy marking on said pilot line to a camping marking thereon and for returning a camping signal therefrom to said telephone station, said busy circuit being controlled by the subsequent appearance of an idle trunk line in said group to remove said camping marking from said pilot line, means included in any one of said first automatic switches that is holding said pilot line and responsive to the removal of said camping marking thereon for initiating trunk hunting operation of said one of said first automatic switches into said trunk lines in said group, and means included in any one of said first and second automatic switches and controlled upon selection of said pilot line in response to testing of said camping marking thereon for returning a busy signal over said one of said first and second automatic switches to the corresponding subscriber station or operator position.

7. In a telephone system including a telephone station, and a pilot line and a plurality of trunk lines arranged in a group for trunk hunting operation, the pilot line having selectively applied thereto a trunk hunting marking and a busy marking and occupying a position preceding the positions of the trunk lines in the group so that the pilot line is selected and tested preceding the trunk hunting operation into the trunk lines in the group; the combination comprising a plurality of automatic switches of the trunk hunting type controllable from the telephone station for extending connections therefrom to the pilot line and to the plurality of trunk lines, anyone of the automatic switches being controlled upon selection of the pilot line in response to a trunk hunting marking thereon for initiating trunk hunting operation thereof into the trunk lines of the group and being controlled upon selection of the pilot line in response to a busy marking thereon for holding the one of the automatic switches on the pilot line and for causing the busy marking on the pilot line to be converted to a trunk hunting marking thereon and being controlled thereafter in response to the removal of the trunk hunting marking frornrthe pilot line for initiating trunk hunting operation of the one automatic switch into the trunk lines of the group, and a busy circuit terminating the pilot line and accessible from the plurality of trunk lines in the group and including means responsive to the idle condition of any one of the plurality of trunk lines for applying to the pilot line a trunk hunting marking and responsive to the busy condition of all of the plurality of trunk lines in the group for applying to the pilot line a busy marking and responsive to the busy condition of all of the trunk lines in the group and to the selection of the pilot line by any one of the automatic switches for converting the busy marking on the pilot line to the trunk hunting marking thereon and responsive to the appearance thereafter of any idle trunk line in the group of trunk lines for removing the trunk hunting marking from the pilot line.

8. The telephone system combination set forth in claim 7, wherein said means includes a first relay and a second relay operated in a first combination responsive to the idle condition of any one of the plurality of trunk lines and operated in a second combination responsive to the busy condition of all the plurality of trunk lines and operated in a third combination'responsive to the busy condition of all of the trunk lines in the group and dam to the selection of the pilot line by any one of the automatic switches.

9. The telephone system combination set forth in claim 8, wherein said first and second relays are restored in said first combination thereof and one of said first and econd relays is operated in said second combination thereof and said first and second relays are operated in said third combination thereof.

10. The telephone system combination set forth in claim 9, wherein said first relay is operated in said second combination.

ll. The telephone system combination set forth in claim 9, wherein said second relay is of the slow to release type, whereby in changing the operating states of said first and second relays from said third combination to said first combination responsive to one of the plurality of busy trunk lines becoming idle said first relay is restored prior to the restoring of said second relay thereby removing the trunk hunting marking from the pilot line and subsequently said second relay is restored thereby reapplying the trunk hunting marking to the pilot line.

12. In a telephone system including a calling station, and a pilot line and a plurality of trunk lines arranged in a group for trunk hunting operation, wherein the pilot line occupies a position preceding the positions of the trunk lines in the group so that the pilot lineis selected and tested preceding the trunk hunting operation into the trunk lines of the group; the combination comprising a busy circuit terminating the pilot' trunk line and controlled by the idle condition of any one of the trunk lines for applying a trunk hunting marking to the pilot line and controlled by the busy condition of all of the trunk lines in the group for applying a busy marking to the pilot line and controlled by the busy condition of all of the trunk lines in the group and by the selection and testing of the pilot line in a trunk hunting operation for applying to the pilot line the trunk hunting marking and for removing thereafter the busy marking and controlled by the appearance of an idle trunk line in the group for subsequently removing and shortly thereafter reapplying the trunk hunting marking to the pilot line, and a plurality of trunk hunting connectors accessible to the calling station and having access to the pilot line and to the plurality of trunk lines; any one of said trunk hunting connectors including switching apparatus provided with a wiper set operated in accordance with dialing pulses from the calling station for selecting one of the lines of the group including the pilot line and the plurality of trunk lines, first means controlled upon selection of the pilot line in response to the testing of the trunk hunting marking thereon for initiating trunk hunting operation of said Wiper set into the trunk lines of the group, second means controlled upon selection of the pilot line in response to the testing of the busy marking thereon for disabling said first means and for returning to the calling station a camping signal and for causing said busy circuit to apply the trunk hunting marking to the pilot line and thereafter to remove the busy marking therefrom, and third means responsive to the operation of said second means and to the conjoint application of the trunk hunting marking to the pilot line for maintaining said first means disabled, whereby said one of the trunk hunting connectors is held on the pilot line and others of said trunk hunting connectors selecting the held pilot line are caused to initiate trunk hunting operations into the trunk lines of the group, and responsive to the subsequent removal of the trunk hunting marking from the pilot line for enabling said disabled first means therein, whereby said one trunk hunting connector is caused to initiate trunk hunting operation into the trunk lines of the group.

13. The telephone system combination set forth in claim 12, wherein said second and third means in any one of said trunk hunting connectors jointly include a relay'having a first winding corresponding to said secestates ond means for operating said relay responsive to the testing of the busy marking on the pilot line and having a second winding corresponding tosaid third meansfor maintaining operated said relay responsive to the application thereafter of the trunk hunting marking to the pilot line.

14. The telephone system combination set forth in claim 12, wherein any one of said trunl: hunting connectors further comprises a sequence switch having a plurality of switch positions, means responsive to ap plication of dialing pulses from the calling station for operating said sequence switch from its normal position to its first position, said sequence switch in its first position completing a circuit for operating said switching apparatus in response to the dialing pulses to select one of the lines of the group including the pilot line and the plurality of trunk lines, means controlled upon selection of any one of the plurality of trunk lines in response to the testing of the busy condition thereof for operating said sequence switch from its first position to its second position, said sequence switch in its second position completing a circuit for returning a busy signal to the calling station, means controlled upon selection of any one of the plurality of trunk lines response to the testing of the idle condition thereof for operating said sequence switch from its first position to its last position, said sequence switch in its last position completing a circuit for returning a seizure signal to the calling station and responsive thereafter to ringing control from the calling station for completing a circuit for applying ringing signals to the idle selected trunl; line and for completing a circuit for applying ring back tone to the calling station, and means controlled upon interruption of the connection through said one trunk hunting connector from the calling station to any one of the trunk lines of the group in response to the restoration of said wiper set of said switching apparatus to its normal position for operating said sequence switch to its normal position.

15. The telephone system combination set forth in claim 14, wherein said sequence switch its first position completes a circuit in response to the testing of a busy condition on any one of the plurality of trunl; li for returning a busy signal of the tone type to the calling station and wherein the busy signal returnec to the call ing station over said circuit completed by said sequence switch in said second position is of the flash type.

16. In a telephone system, a plurality of subscriber stations, a plurality of first automatic switches of the trunk hunting type controllable from said plurality of subscriber stations for extending connections therefrom and each including a first control terminal and a second control terminal, an operator position, plurality of second automatic switches of the trunk hunting type controllable from said operator position for extending connections therefrom and each having a first control terminal and a second control terminal and a third control terminal, a pilot line and a plurality of trunk lines arranged in a group wherein each of said lines in said group includes a first control con .uctor and a second control conductor accessible respectively to said first and scco control terminals of said first nd second auton t switches and said pilot inc further a thi trol conductor accessible to said third control teir t in said second automatic switches, said pilot line occ i ing a position preceding the positions of said tru'. in said group so that any one of said. first and second automatic switches selects and tests said pilot line preceding the trunk hunting operation into the trunk lines in said group, a source of power including a first voltage terminal and a second voltage terminal, a busy circuit terminating said pilot trunk line and including means controlled by the idle condition of any one of said trunk lines in said group for applying to the first and second control conductors of said pilot line said first potential and controlled by the busy condition of all of said trunk 24 lines in said group for applying to the first and third control conductors of said pilot line respectively said first potential and said second potential, a plurality of trunk circuits respectively corresponding to said trunk lines wherein each of said trunk circuits is controlled responsive to theidle condition of the corresponding trunk line to apply to the first control conductor thereof said second potential and wherein the one of said trunk circuits corresponding to the one of said trunk lines in the last position the trunk hunting operation is controlled responsive to the busy condition thereof to apply to the first control conductor thereof said first potential and wherein each of the others of said trunk circuits is controlled responsive to the busy condition of the corresponding trunk ii 2 to apply to the first and second control conductors :of said first potential, first means included in any of said first and second automatic switches and controlled upon selection of any one of said linesin said group in response to the testing of said first potential on the first control conductor thereof for returning over said one of said first and second automatic switches to the correspondingly connected operator station or subscriber station a busysignal, second means included in any one of said first and second automatic switches and controlled upon selection of any one of said lines in said group in response to the testing of said first potential on the first and second control conductors thereof for causing said one of said first and second automatic switches to trunk hunt in the trunk lines of said group, third means cluded in any one of said first and second automatic switches and controlled upon selection of any one of said trunk lines in said group in response to the testing of said second potential on the first control conductor thereof for completing a connection thereover from the corresponding calling subscriber station or operator station to the selected one of said trunk lines, fourth means included in any one of said second automatic switches and controlled upon selection of said pilot line in response to testing of said first potential of the first control conductor thereof and said second potential on the third control conductor thereof for disabling said second means and for returning thereover to said operator station a camping signal, said means included in said busy circuit being further controlled in response to the selection or said pilot line by any one of said second automatic switches when all of the trunk lines in said group are busy for applying to the second control conductor thereof said first potential and for removing thereafter said second potential from the third control conductor thereof and controlled by the subsequent appearance of an idle trunk line in said group for removing and thereafter reapplying said first potential on said second control conductor of said pilot line, fifth means included in each of said second automatic switches responsive to the operation of said fourth means and to the conjoint application of said first potential to the second control conductor of said pilot line for maintaining said second means disabled, whereby said second automatic switch is held on said pilot line and others of said first and second automatic switches upon selection and testing of said held pilot line are caused to initiate trunk hunting operation thereof into the trunk lines of said group, and responsive to the removal of the first potential from the second conductor of said held pilot line for enabling said disabled second means in said second automatic switch, whereby said second automatic switch is caused to initiate trunk hunting operation into the trunk lines of said group.

17. The telephone system set forth in claim 16, and further comprising means in each one of said second automatic switches for applying said first potential to the third control terminal thereof, whereby said means in said busy circuit is further controlled upon selection of said pilot line by any one of said second automatic switches when all of the trunk lines in said group are busy and in response to said first potential on the third 25 control terminal of said one second automatic switch for applying to the second control conductor of said pilot line said first potential and for removing thereafter from the third control conductor thereof said second potential.

18. The telephone system set forth in claim 17, wherein said means included in said busy circuit is provided with a bus connecting said first voltage terminal to said first control conductor of said pilot line and a first relay and a second relay, said first and second relays occupying their restored position responsive to the idle condition of any one of said trunk lines of said group for connecting said first voltage terminal to said second control conductor of said pilot line, said first relay being operated responsive to the busy condition of all of said trunk lines of said group for interrupting the connection between said first voltage terminal and said second control conductor of said pilot line and for completing via the winding of said second relay a connection from said second voltage terminal to the third control conductor of said pilot line, whereby said second relay is operated upon selection of said pilot line by any one of said second automatic switches in response to the first potential on the third control terminal thereof, said operated first and second relays recompleting the connection from said first voltage terminal to said second control conductor of said pilot line and interrupting the connection from said second voltage terminal to said third control conductor of said pilot line and being restored responsive to any one of said busy trunk lines of said group becoming idle.

19. The telephone system set forth in claim 18, wherein said second relay is of the'slow to release type, whereby responsive to the appearance of an idle trunk line when said first and second relays are operated said first relay is restored prior to the restoring of said second relay thereby interrupting the connection from said first voltage terminal to said second control conductor of said pilot line and subsequently said second relay is restored thereby reestablishing the connection from said first voltage terminal to said second control conductor of said pilot line.

20. The telephone system set forth in claim 17, Wherein said fourth and fifth means included in each one of said second automatic switches are jointly provided with a relay having a first and a second winding, whereof said first winding corresponds to said fourth means and completes a connection therethrough from said first voltage terminal of said source of power to the third control terminal of the corresponding second automatic switch, whereby said relay is operated upon selection of said pilot line by said second automatic switch in response to the second potential on said third control conductor thereof, and whereof said second winding corresponds to said fifth means and completes responsive to the operation of said relay a connection therethrough from said second voltage terminal to the corresponding second control terminal, whereby said relay is maintained operated responsive to the application thereafter of said first potential to the second control conductor of said pilot line.

References Cited in the file of this patent UNITED STATES PATENTS 2,097,165 Stokely Oct. 26, 1937 2,151,767 Hovland Mar. 28, 1939 2,609,456 Hall Sept. 2, 1952

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