US2286431A

US2286431A - Telephone system

Info

Publication number: US2286431A
Application number: US318398A
Authority: US
Inventors: Molnar Imre; Morris E Griffins
Original assignee: Associated Electric Laboratories Inc
Current assignee: Associated Electric Laboratories Inc
Priority date: 1940-02-12
Filing date: 1940-02-12
Publication date: 1942-06-16
Anticipated expiration: 1959-06-16

Description

June 16, 1942. MOLNAR Erm. 2,236,431

I TELEPHONE SYSTEM Filed Feb. 12, 1940 4 Sheds-Sheet 1 Finder 29 Szaz'fCb 20 Line Circul'z /3 To Connecfor DzlsfribaforZ 6 INVENTORS Imre Molnar B Morris 5. Griffins y WW I TTORNEYS June 1942- l. MOLNAR ETAL 8 31 TELEPHONE SYSTEM 1 Filed Feb. 12, 1940 4 Shts-$heet 3 Connector Suzi tch 25 346 G282 357 INVENTORS Imre Molnar Morn'sE. Griffins g M a By ATTORNEYS June 16, 9 4 1. MOLNAR ETAL, 2,286,431

TELEPHONE SYSTEM Filed- Feb. 12, 1940 4 Sheets-Sheet 4 I T0 Fl'nderSuL Connecfor Switch 25 I I I I #34452 Line Circuit mvzmons Imre Molnar Morris E Griffins RING/N6 I LEADS ATTORNEYS Patented June 16, 1942 TELEPHONE SYSTEM Imre Molnar, Chicago, and.Morris E. Griffins, Riverside,.Ill.-, assignors to Associated Electric hicago, 111., a corporation Laboratories, Inc., C

of Delaware Application February 12,1940, Serial No. 318,398

32 Claims.

The present invention relates to telephone systems and more particularly to improved automatic line switching equipment which is controllable in a simple and reliable manner to provide different classes of service to subscribers paying different rates.

It is an object of the present invention to provide, in a telephone system, an improved arrangement for automatically rendering diflerent classes of service to different subscribers served by the same multiparty line.

It is another object of the invention to provide improved automatic line switching apparatus of the character indicated which is so arranged that any substation of the system may be converted from one service class to another by the simple expedient of substituting a calling device of one character for an installed calling device of another character.

It is a further and more specific object of the invention to provide automatic switching apparatus of the character described which is so arranged that the subscribers of one class and served by multiparty lines are prevented from using certain specified trunks, such, for example,

as central ofiice trunks, while other subscribers served by the same multiparty lines are permitted to use the trunks.

It is a further object of the invention to provide automatic switching apparatus of the character described which is so arranged that local call connections routed therethrough and resulting from calls originating at; substations having only restricted service are automatically released after they have been established for a predetermined time intervaljwhile local call connections resulting from calls originating at substations having unrestricted or semirestricted service are only released under the control of the calling subscribers.

It is still another object of the invention to provire automatic switching apparatus of the character described which serves to provide reverting call signaling service and is so arranged that the switches utilized in setting up a reverting call connection are automatically released a predetermined time interval after they are seized in the event they are not released under the control of the calling or called subscriber before the predetermined time interval expires.

It is a still further object of the invention to provide improved line switching equipment of the character described which includes apparatus operative automatically to release the operated switches involved in partially established connections resulting from line permanents, or the like, and which is so arranged that the switch release apparatus is rendered inactive when the switches involved in a partially established connection are seized over a line arranged for local calls of I have access to one or more groups of trunks which are provided to serve only the substations arranged for unrestricted service and may extend, for example, to one or more distant suburban or central exchanges. The finder-connector links are provided with apparatus for giving service of one class to certain of the subscribers servedby the multiparty lines and service of a different class to other subscribers served by the same multiparty lines. More particularly, this apparatus includes call blocking means for preventing the subscribers in one of the restricted service classes from obtaining access to the interofice trunks mentioned above. The trunk call blocking means is controlled in accordance with the types of calling devices provided at the substations of different class; calling devices of special character being provided at the substations arranged for nonrestricted service which are operative to render the trunk call blocking means of the automatic switching equipment inefiective; and calling devices of conventional character being provided at the substations arranged for restricted service. The latter calling devices are not effective to prevent the call blocking means of the automatic switching apparatus from operating. Each of the finder-connector links also includes a link release relay which is operative under the control of timing apparatus common to the links to release the associated link in the event the link is seized over a line having a calling condition thereon and a talking circuit is not established by way of the link within a predetermined time interval following its seizure. The circuit arrangement of the link is such that the link release relay also functions to release the associated link when occupied with a reverting call initiated on one of the multiparty subscribers lines in the event a loop circuit is not established by way of this line within a predetermined time interval after the link is seize'd. As indicated above, the

subscribers lines served by the finder-connector links are divided into two groups, the lines of the respective groups being arranged for different classes of service. In accordance with another feature of the present invention, timing devices are provided in the links for automatically releasing connections involving calling lines in one of the two groups after the connections have been established for a predetermined time interval and means individual to the lines in the other group are provided for preventing the operation of the timing means. More specifically, each link in cludes a timing device having a start timing relay embodied therein, each timing device being operative under the control of its start timing relay to release a connection routed through the associated link a predetermined time interval after the connection is established. The circuit wiring of the links is such that the start timing and link release relays of any link are prevented from operating to perform their assigned functions when the associated link is seized over a calling line arranged for nonrestricted service.

The novel features believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention, both as to its organization and method of operation, together with further objects and advantages thereof will best be understood by reference to the specification taken in connection with the accompanying drawings in which Figures 1, 2, 3 and 4, when placed side by side in the order named, illustrate an automatic telephone system having embodied therein the features of the invention as briefly outlined above.

Referring to the drawings, the telephone lines and automatic switching apparatus there illustrated may comprise, for example, a small exchange designed to serve a suburban area of limited population which is geographically adjacent a larger city or town served by a central exchange. The subscribers substations served by the automatic switching apparatus illustrated may be arranged for three different classes of service. For example, the line III which serves only the substation A and terminates in the exchange in a line circuit [3 is arranged for unlimited central exchange trunk service and also for local calls of unlimited duration. To-this end, the calling device provided at the substation A is equipped with anextra set of shunt springs 28 which are arranged to connect the positive side of the line H! to ground during each operation of the device, and the multipled extra control contacts of the contact sets, individual to the finder switches and to which the conductors of this line are connected, are grounded in the line circuit l3. All other lines which are to provide unrestricted service of the character just described, are arranged in a similar manner. The two other subscribers lines illustrated in the drawings, namely, the lines H and I! which terminate in the exchange in line circuits I4 and I5, are of the multiparty type, the line H being arranged to serve two substations B and C and the line l2 being arranged to serve two addi tional substations D and E. The substation C is similar to the substation A in that it is equipped with a calling device having an extra pair of shunt springs 3| which are arranged to connect the positive side of the line I l to ground, whereby the subscriber at the substation C is permitted to obtain access to the central exchange trunks for the purpose of setting up connections directly to the central exchang $1.1

scribers without resorting to the expedient of routing such connections through a toll operator's position. The substations B, D and E, on the other hand, are not equipped with calling devices of the character provided at the substations A and C but'with calling devices of conventional construction and, accordingly, users of these three substations can only directly complete local call connections. Moreover, the arrangement of the line circuit I4 is such that all local connections can only be used for a predetermined time interval, after which they are automatically released. Thus, the subscriber at the substation C is restricted as to the duration of local calls but is provided with central exchange toll trunk service. The substations B, D and E, on the other hand, are restricted both as to the period of using local call connections and in the making of central exchange calls. From the foregoing explanation it will be apparent that in order to provide central exchange trunk service to any substation associated with one of the multiparty subscribers lines of the system, it is only necessary to substitute at the substation a calling device of special character for an installed calling device of conventional r character. If all of the substations associated with a particular line are to have central exchange trunk'service, the line circuit terminating this line may be modified for this purpose without changing the calling devices provided at the substations associated with the line. To this end, the extra control conductor of the line circuit terminating the line is connected to the negative terminal of the exchange battery through a resistor of appropriate resistance. This expedient, which is shown by way of illustration as being incorporated in the line circuit [4, may also be used in the line circuits terminating incoming trunks from another suburban exchange or a private branch exchange.

The automatic switching apparatus provided in the exchange for setting up connections between the various lines terminating thereat comprises a plurality of finder-connector links of one hundred-lines capacity, one of which is illustrated as comprising the finder switch 20 and the connector switch 25. For the purpose of assigning these links to the use of calling lines in a definite order there is provided a distributor 26, preferably of the preselecting type and arranged to initiate the operation of the finder portion of a preselected link to search for a calling line in response to the grounding of the distributor start lead 21 in the line circuit associated with the calling line. Since the finder switch 20 and the distributor 26 may be of conventional arrangement, these switching units have been only schematically illustrated. The line circuits l3, l4 and IS, with the exceptions pointed out above,

may also be of conventional arrangement and accordingly the details thereof have not been shown. The connector switch 25 is illustrated in Figs. 2, 3 and 4 as being of the well-known Strowger type having its contact field divided into ten levels of bank contacts, the lowermost levels of which are reserved for the termination of subscribers lines, branch exchange trunk lines and one or more central exchange trunk lines extending to a toll operators switchboard in a distant central exchange. The uppermost levels of the connector switch are used to terminate the regular central exchange trunks over which the subscribers having unrestricted service may route connections to the central oflice subscribers without the intervention of a toll operator. For example, the tenth level of bank contacts in the connector switch 25 may be reserved for the termination of trunks of the last-mentioned roup.

Neglecting for the present the details of the circuit wiring of the connector switch 25 and considering the operation of the system generally, if a subscriber at the substation B, for example, initiates a local call intended for the substation D, the line circuit l4 associated with the calling line H operates in the usual manner to mark the calling line as busy in the bank contacts of the connector switches having access thereto and connects the distributor start conductor 2'! to ground. In response to the latter operation, th apparatus of the distributor 26 initiates the operation of the finder switch embodied in a selected link to seize the calling line.

Assuming, for example, that the link illustrated,

namely, that comprising the finder switch 20 and the connector switch25 is assigned to the use of the calling line I I, the finder. portion of this link upon seizing the calling line extends the calling loop circuit to the connector portion of the link and release the distributor 26. When the connector switch 25 is thus seized, the control apparatus thereof functions to return the usual dial tone signal to the calling subscriber. In order to extend the connection to the desired called line the calling subscriber dials the three 'digits of the directory number identifying the desired substation D. The first digit is utilized for the purpose of causing the connector switch 25 to elevate its wipers to a position opposite the level of bank contacts terminating the group of ten lines in which the called line I2 is included. The second digit is utilized to direct the wipers of the connector switch 25 into engagement with the contacts terminating the conductors of the desired line l2. The third digit is utilized for the purpose of selecting ringing current of the frequency or code designating the desired substation D. In the arrangement illustrated provisions are only made for harmonic ringing, but it will be apparent to those skilled in the art that the ringing arrangement disclosed may be modified to include pfovisionsfor code ringing without departing from the scope of the present invention. The arrangement of the control apparatus embodied in the connector switch 25 is such that the called line is not tested until shortly following the end of the ringing digit. If this line tests busy, the usual busy tone signal is returned to the calling subscriber. On the other hand, if the line is idle ringing current of the selected frequency or code is transmitted over the calling line for the purpose of signaling the subscriberat the substation D that his substation is being called. The ringing operation is terminated and the desired talking circuit is fully established when the call is answered at the called substation. The answering of the call also serves to initiate the operation of a conversation timing device which, in the case under consideration, functions to release the operated switches 20 and 25 in the event the call is not terminated and the connection released under the control of the calling subscriber within a predetermined time interval after it is completed. The normal release of the connection is under the control of the calling subscriber, the operated switches 20 and Z5 and the line circuit M being restored to normal when thecalling loop circuit is interrupted at the calling substation B. Had the call originated at a substation, such, for example, as the substation A, arranged for fully unrestricted service, operation of the above-mentioned conversation timing apparatus is not inimay be routed directly to the central office sub-- ,scribers, the call is blocked in the connector switch and a busy signal is returned to the calling subscriber. As indicated above, the call blocking means provided in the connector switch 25 is rendered inactive under the control of the calling devices provided at the substations equipped for unrestricted central ofiice trunk service. Accordingly, if a central ofllce trunk call originates at the substation A, for example, the call may proceed through the, automatic switching apparatus illustrated and the automatic switching apparatus of the central exchange without being routed through a toll operators position; In this connection it is pointed out that the connector switch 25 is arranged for switching-through operation on central omce trunk calls. Reverting call connections are completed by directory number dialing, the arrangement of the switching apparatus being such that any link occupied with a reverting call is released under the control of the timing apparatus common to the links in the event the busy link is not released in the normal manner within a predetermined time interval after it is seized.

Referring now in greater detail to the connector switch 25, the Strowger switching mechanism of this switch comprises a set of wipers 480 and the usual vertical and rotary magnets 341 and 348, respectively, which are operative in conjunction with their associated ratchet and pawl mechanisms, not shown, to impart vertical and rotary movement to the wiper set. This mechawhen energized, permits the wipers 480 to be returned to vertical and rotary normal in the usual manner, and normal post springs 262, 325,, 325 and 321 which perform additional circuit control operations described with particularity hereinafter and are arranged to be moved off-normal from the normal positions shown in the drawings in response to elevation of the wipers 4.80 to one or more predetermined vertical off-normal positions, The equipment for controlling the ver-' tical, rotary and release magnets of the switching mechanism and for performing the auxiliary control operations required incident to the setting up of the various types of connections referred to above includes a pair of line relays R260 and R320 which are arranged to be energized in series over the two talking conductors of the trunk I00 incoming to the connector switch 25. This equipment further comprises a slow-acting hold relay R300, a slow-acting relay R220 which is a slave to the hold relay, a pair of relays R255 and R2 I0 which are utilized in setting up reverting call connections between two substations associated with any one of the-multi-party lines of the system, a link release relay R230, 2. switching- The arrangement of V through relay R240 and a back bridge relay R250. Additional relays embodied in the connector switch and shown in Fig. 3 of the drawings include a pair of control relays R310 and R330 which are utilized in providing the diflferent classes of service described above; a pulse circuit transfer relay R340 of the slow-acting type; a pair of pulse responsive control relays R350 and R360 also of the slow-acting type and a second pulse circuit transfer relay R310. The relay equipment illustrated in Fig. 4 and also embodied in the connector switch 25 comprises a slow-acting ring cut-oi! relay R410, a relay R400 which is a slave to the ring cut-ofi relay, a busy test relay R420, 8. pair of ringing control relays R430 and-R440 and a slow-to-operate line switching relay R450. Selection of the frequency of ringing current which is to be projected over a called line is accomplished through the provision of a minor switch 410 of the eleven point type which includes three wipers 411, 412 and 413, the usual operating and release magnets 414 and 415, respectively, and off-normal contact springs 416 and 411. This minor switch also functions as a timing device for determining the duration of local call connections established by way of the connector switch 25, operation of the switch 410 for this purpose being initiated under the control of a slow-to-operate start timingrelay R460.

Local calls Referring now more particularly to the operation of the connector switch 25, when the finder switch 20 operates in the manner outlined above to seize a calling line, such, for example, as the line 11, a ground connection extending from the finder switch 20 to the control or release conductor C108 is completed in the usual manner. When this conductor is grounded, the relays R2 I R3 I 0 and R340 are energized and operate. The circuit for energizing the relay R210 extends by way of the grounded conductor C108, RA205 and the winding of R210 to battery. The circuit for energizing the relay R310 extends by way of the grounded conductor C108, the off-normal springs 306, RA312 and the winding ,of R310 to battery. The circuit for energizing the relay R340 extends by way of the grounded conductor C108, the offnormal springs 306 and the upper winding of R340 to battery. The relay R340 upon operating completes a loop circuit which extends through the windings of the two relays R260 and R320 and the finder switch 20 to the calling substation. Assuming that the call was initiated at the substation B, this circuit extends from ground by way of the secondary winding of the transformer 308, RA341, the oil-normal springs 324, the winding of R320, C261, RA252, RA244, RA202, C106, C104, the direct current bridge across C103 and C104 at the calling substation B, C103, C105, RA201, RA242, RA254 and the winding of R260 to battery. When energized over this circuit the two relays R260 and R320 both operate. Further, the dial tone voltage introduced in this circuit through the transformer 308 is reproduced by the receiver provided at the calling substation B to inform the calling subscriber that the dialing operation may be started.

At RA342', the relay R340 prepares a circuit for energizing its lower winding in parallel with the vertical magnet 341 and opens a point in the circuit for energizing the rotary magnet 348 in parallel with the relays R350 and R360. The relay R210, upon operating, opens, at RA215, a point in the circuit for energizing the release magnet 346 and at its other armatures prepares certain operating and holding circuits described more fully hereinafter.

The relay R260 in operating, completes a circuit for energizing the slow-acting hold relay R300, this circuit extending from ground by way of RA245, RA261, C268 and the winding of R304 to battery. At RA261, the relay R260 also opens apoint in the circuit for energizing the relay R340 and the vertical magnet 341 in parallel. The relay R320, upon operating, prepares, at RA321, a holding circuit for the relay R310. The relay R300 in operating, completes the lastmentioned circuit, this circuit extending from ground by way of RA301, RA321, RA311, RA312 and the winding of R310 to battery. At RA302, the relay R300 further prepares the circuit for energizing the lower winding of the relay R340 and the vertical magnet 341 in parallel. At RA303, the relay R300 completes a circuit extending by way of C210 for energizing the slowacting slave relay R220. At RA304, the relay R300 completes a multiple path for impressing ground potential upon the control conductor C108 and in so doing completes locking circuits for the two relays R210 and R340. This path extends from ground by way of RA232, C211 and RA304 to the control conductor C108 and the completion thereof serves to prevent the release of the operated finder switch 20 and the line circuit 14, in the usual manner. At RA305, the relay R300 prepares a circuit for energizing the upper winding of 'the line. switching relay R450. The relay R310 in operating, opens, at RA313, a point in one branch of the automatic cut-in circuit, described hereinafter, for energizing the rotary magnet 348 in parallel with the relay R350. At RA314, the relay R310 prepares a second branch of the automatic cutin circuit.- The relay R220 in operating, completes, at RA221, an obvious path for grounding the timer start lead C201, and at RA222, completes an obvious multiple path for grounding the control or release conductor C108. Following the relay operations just described the connector switch is conditioned to respond to the impulses of the first digit dialed at the calling substation B.

Assuming that the substation D is desired by the calling subscriber, three digits are dialed at the calling substation B, the first two digits being utilized in the manner briefly described above to direct the wipers 480 of the connector switch 25 into engagement with the contacts terminating the conductors of the called line 12 and the third digit being utilized to select the frequency of the ringing current which corresponds to the desired substation D. The line relays R260 and R320 follow the impulses of the three digits and the relay R260 functions to repeat the impulses of the first digit to the vertical magnet 341 and the. relay R340 in parallel over the above-mentioned pulsing circuit. This circuit extends from ground by way of RA245, RA261, C269, RA302 to RA342, where it divides, one branch extending through the lower winding of R340 to battery and the other branch extending through the winding of the magnet 341 to battery. Each time the relay R320 restores, it opens the above-traced holding circuit for the control relay R310, but the latter relay, due to its slow-to-release characteristic, remains operated during pulsing. Each time the vertical magnet 341 is energized it operates to elevate the wipers 480 one step. When these wipers are elevated to the first vertical off-normal position, the off-normal springs 324 are disengaged, the off-normal springs 323 are moved into engagement, the off-normal springs 306 are disengaged and the off-normal springs 351 are moved into engagement. At the offnormal springs 323 and 324 the secondary winding of the dial tone transformer 308 is excluded from the loop circuit extending to the calling substation in an obvious manner. At the offnormal springs 351 the operating circuit for the release magnet 346 is prepared. At the offnormal springs 306 the operating circuits for the relays R3i0 and R300 are interrupted. Due to the sloW-to-release characteristic of the relay R340, this relay remains operated until the first series of impulses is terminated. For the same reason the slow-acting hold relay R300 remains in its operated position during impulsing.

During the interdigit pause between the first and second digits, the relay R300 restores, to open, at RA3M, a further point in the initially completed loop circuit extending to the calling substation and to open, at RA302, a point in the above-traced circuit for energizing its lower winding in parallel with the vertical magnet 301. At RA302, the relay R300 further prepares the above-mentioned circuit for energizing the rotary magnet 300 in parallel with the relays R350 andR3B0. At its other armatures, the relay R300 performs certain additional circuit control operations referred to in detail hereinafter.

During the impulses of the second digit, the relay R260 functions to complete the last-mentioned circuit during the open circuit period of each impulse. This circuit extends from ground by way of RA205, RA26i, C260, RA302, RA342 and C330 to RA055 where it divides, one branch extending by way of RA422, C318 and the winding of R360 to battery, a second branch extending by way of RAOZ3, C301, RA312 and the winding of the magnet 308 to battery, and a third branch extending by way of RA023, C38 I, RA312, RA36i and the winding of R350 to battery. The relay R360 operates when the circuit just traced is initially completed, and due to its slowto-release characteristic, remains operated for the duration of the second series of impulses. In operating, the relay R360 completes, at RA36 i, a holding circuit for therelay R350, which circuit extends from ground by way of RA056, C385, RA36i and the winding of R350 to battery. At RA302, the relay R360 opens a point in the circuit for energizing the busy test relay R020. The relay R350, uponoperating, opens, at RA35I, a point in the circuit for impressing the voltage of the busy tone current source upon the established loop circuit extending to the calling substation. At RA352, the relay R350 completes a path for short-circuiting the winding of the relay R310, this path extending by way of the grounded conductor C385, RA36l, RA352, RA310, the winding of R310, C386 and R'A4l0 back to ground as connected to the control conductor C108. At RA353, the relay R350 prepares a circuit for energizing the trunk hunting relay R440. At RA355, the relay R350 opens a point'in the operating circuit for the line switching relay R050 and prepares the operating circuit for the busy test relay R020. At RA350, the relay R350 preparesa circuit for energizing the relay R240. Each time the rotary magnet 308 is energized in response to an impulse of the second digit it functions to rotate the wipers 480 one step so that at the conclusion of the second digit these wipers are positioned in engagement with the contacts terminating the conductors of the called line l2.

During the interdigit pause between the second and third digit, the relay R360 releases to interrupt, at RA36I, the holding circuit for the relay R350 and the above-traced path short-circuiting the winding of R310. At RA362, the relay R300 further prepares the circuit for energizing the busy test relay R420. When the path short-circuiting the winding of the relay R310 is interrupted this relay is energized in series with the relay R350 over a circuit extending by way of the grounded control conductor' C600, RAOi0, C386, the winding of R310, RA310, RA352 and the winding of R350 to battery. When energized over this circuit the relay R310 operates to complete a holding circuit for itself, which holding circuit extends by way of the grounded conductor C100, RA4I4, C306, the winding of R310, RA310,RA313 and the resistor 315 to battery. After this circuit is completedflhe relay R300 opens its operating circuit as traced above and in so doing deenergizes the relay R350. At RA31I, the relay R310 prepares the operating circuit for the rin ing control relay R030. At RA312, the relay R310 opens the pulsing circuit to the relay R350 and the rotary magnet 30B, and prepares a circuit for energizing the operating magnet 010 of the minor switch 010 in accordance with the impulses of the third digit, thereby to select the frequency of the ringing current to be transmitted over the selected line 12. This circuit is completed during the open circuit'period of each impulse of the third digit and extends from ground by way of RA245, RAZBH, C260, RA302, RA302, C380, RA055, RA3213, C381, RA312, C390 and the winding of the magnet 310 to battery. The branch of this circuit which extends by way of RA022, C318 and the winding of R360 is also completed each time the pulsing relay R260 restores at the beginning of an impulse. Accordingly, the relay R300 is reoperated at the beginning of the first impulse of the third digit and remains operated until the digit is ended Upon reo'perating, the relay R360 recompletes the previously traced circuit for energizing the relay R350 and opens the operating circuit for the busy test relay R020. The relay R350, upon reoperating, performs the circuit control operations previously described.

' Each time the operating magnet 0'50 of the switch 410 is energized over its operating circuit as traced above it functions to advance the wipers 41l to 413,.inclusive, one step so that at the conclusion thereof these wipers stand in engagement with the contacts corresponding to the frequency of the ringing current to which the signal device provided at the desired substation D is tuned to respond. As explained in greater detail hereinafter, operation of the switch 0710 also serves to select the side of the line over which the ringing current is to be projected in the event divided bered contacts of the inner contact set associated with the wiper "I are not included in the operating circuit for the control relay R440, being utilized solely for the purpose of transmitting ringing current over the called line. Accordingly, the relay R440 is not energized in the event a third or ringing digit comprising an odd number of impulses is dialed at the calling substation. When the wipers of the switch 410 are stepped to the first off-normal position in response to the first impulse of the third digit, the oft-normal contact springs 418 are disengaged to open a further point in the operating circuit for the release magnet 345 and the contact springs 411 are engaged to prepare a circuit for energizing the release magnet 415 of the switch 410. When the wiper 412 is moved off-normal in response to the first impulse of the third digit'it engages the first contact of the associated contact multiple, thereby further to prepare the operating circuit for the busy test relay R420.

The relay R380, which is deenergized in the manner previously explained at the end of the third digit, restores shortly thereafter to complete the operating circuit for the busy test relay R420 and to open the circuit for energizing the slow-to-release relay R350. If the selected line I2 is busy, guarding ground potential is impressed upon the private conductor thereof in the usual manner. Accordingly, the busy test relay R420 is energized over its operating circuit which extends by way of the grounded private wiper 483, the wiper 412 and one of its associated multipled contacts, C211, RA362, RA355, RA358, C333, the upp r winding of R420 and RA458' to battery. When energized over this circuit the relay R420 operates, to open, at RA424, a, point in the operating circuit for the line switching relay R450.

At .RA421, the relay R420 prepares a holding circuit for itself, this holding circuit being completed in response to the release of the relay R350 which occurs shortly following operation of the relay R420 and extending by way of the grounded control conductor Cl08, RA421, C384, RA358, C333, the upper winding of R420 and RA458' to battery. At RA422 and RA423, the relay R420 further opens the previously traced operating circuits for the relays R350 and R350, the rotary magnet 348 and the operating magnet 414 of the minor switch 410, thereby to prevent further operation of the calling device provided at the calling substation B from eifecting further operation of the control apparatus embodied in the connector switch 25. At RA425, the relay R420 prepares a circuit for energizing the control relay R440 during an automatic trunk hunting operation on the part of the connector switch 25. At RA42I, the relay R420 completes the circuit for transmitting busy tone current-over the established loop extending to the calling substation. The circuit traversed by this current extends from one terminal of the busy tone current source, not shown, by way of C401, the condenser 405, RA42I, C311, RA35I, C283, the condenser 248, RA242, RAI, C105, CI03, the receiver at the substation B, CI04, Cl08, RA202, RA244, RA252, C281, the winding of R320, the oft-normal springs 323 and RAI to the opposite terminal of the busy tone current source, Energization of the receiver provided at the calling substation by the busy tone current traversing this circuit serves to inform the calling subscriber that the desired connection cannot be obtained. The manner in which the operated switch train is released when the call is abandoned at the calling substation is fully described hereinafter.

m the event the selected line 12' is idle. batswitching relay R450 is completed. This circuitextends from ground by way of RA305, C331, RA424, the upper winding of R450, RA423, C33l.

' RAISE, RA352, C211, the wiper 412, the private wiper 483 and through the winding of the cutofl relay embodied in the line circuit l5 to battery. When energized over this circuit the relay R450 operates to complete, at the "X" contacts associated with RA451, alocking circuit'i'or itself this locking circuit extending from ground by way of RA30I, C318, RA451 and the lower'winding of R450 to battery. After this circuit is completed, the relay R450 connects the private wiper 483 to ground at RA453, thereby to mark the line l2 as busy in the bank contacts of the other connector switches having access thereto and to cause the operation of the cut-oi! relay embodied in the line circuit l5 in the usual manner. At RA451 and RA452, the relay R450 further prepares the talking circuit to the called substation D. At-RA454, the relay R450 completes a circuit extending by way of RA434, C318, RA31I and C319 for energizing the ringing control relay R430. At RA455, the relay R450 opens a point in the common portion of the previously traced operating circuits for the relays R330 and R350, the rotary magnet 348 and the operating magnet 414 of the'minor switch 410, thereby to prevent further operation of the calling device provided at the calling substation from efiecting further operation of the control apparatus embodied in the connector switch 25. At RA455. the relay R450 opens a point in one of the previously traced circuits for energizing the relay R350. At RA451, the relay R450 further prepares th circuit for transmitting ringing current to the signal device associated with the called substation. At RA458, the relay R450 prepares a locking circuit for the relay R440. At RA458', the relay R450 opens a further point in the previously traced operating and holding circuits for the busy test relay R420. Finally, the relay R450 prepares, at RA459, a circuit for energizing the link release relay R230. The relay R430, upon operating, further prepares the ringing circuit at RA432 and completes at .RA433 and RA444 a locking circuit for itself. Thi locking circuit extends from ground at RA30I by way of C315, RA433, RA434, C318, RA31I, C319 and the winding of R430 to battery. At RA43I, the relay R430 completes a path for projecting ringing current by way of the loop circuit extending to the calling substation, whereby the usual ringback tone signal is reproduced by the receiver provided at this substation. The path traversed by the ring-back tone current extends from one terminal of one of the ringing current sources by way of C408, RA43I, RA40I, the condenser 401, C253, the condenser 248, RA242, RA20I, CW5, CW3, the receiver at the calling substation B, CI04, Cl08, RA202, RA244, RA252, C281, the winding of R320, the off-normal springs 323 and RA30| to the other terminal of the ringing current source. Following the operation of the ringing relay R430, ringing current of the selected frequency is transmitted over the called the substation D. Assuming that this signal deline !2 to energize the signal device provided at the called substation D each time the ringing interrupter conventionally embodied in the ringing apparatus, not shown, operates to connect the selected ringing current source to the selected ringing lead. Assuming that the line I: is arranged for bridged ringing and further that the wipers of the switch 410 stand in engagement with their associated first contacts, the path traversed by the ringing current may be traced as extending by way of the No. 1 ringing current lead, the wiper 41! and its associated first contact, RA432, RA442, HA4 2, RA452, the wiper 482, the signal device provided at the substation D and bridged between the line conductors of the line !2, the wipartM, RA45I, RA4! I, RA44I, the lower winding of the ring cut-off relay R4!0 and the exchange battery to ground. Obviously, portions of this ringing current also traverse the signal devices individually provided at th other substations connected to the line 52. Since, however, these other signal devices are tuned to respond to ringing currents having frequencies different from that of the ringing voltage impressed on the first ringing lead associated with the wiper 41! of the switch 410, they do not opcrate. The signal device provided at the substation D responds to the ringing current traversing its windings over the circuit just traced and operates to indicate an incoming call. event the line !2 is arranged for divided ringing and the signal device at the called substation D is connected between the positive conductor of the line !2 and ground, the ringing current is conducted to the windings of the signal device provided at the substation D over a circuit substantially similar to that traced above, but in conductor of the line !2- and ground and 'thefrequency of the ringing current to which this signal device responds is the frequency of the signal voltages impressed on one of the second, fourth, sixth, eighth or tenth ringing leads associated with the wiper 41!, the relay R440 is operated during the first ringing cycle which follows operation of the relay R450. More specifically, the relay R440 is, in this case, energized over a circuit extending from ground through the selected ringing current source over the selected even numbered ringing lead, the wiper 41! and two of its associated even numbered contacts, RAfil, RAMS, C390, the armature 349, C384 and the lower winding of R440 to battery. When energized over this circuit the relay R440 operates to complete a locking circuit for itself, which locking circuit extends by way of the grounded control conductor CI00, the upper winding of R440, RA458, RA446, RA445, C390, th armature 349, 0384 and the lower winding.

of R440 to battery. At its armatures RA44!,'

RA442 and RA443, the relay R440 completes a circuit for projecting ringing current of the selected frequency over the called line I2. In this case the circuit traversed by the ringing current extends by way of one of the even numbered ringing leads, the wiper 41! and two of its associated even numbered contacts, RA45l', RA443, RA432, RAMI, RA4! I, RA45I, the negative line wiper 40! and the negative side of the'line I2 to one terminal of the signal device provided at In the v vice isconnected between the negative conductor of the line !2 and ground, the current after traversing the windingsv of this device, is returned directly to the grounded terminal of the selected ringing current source. On the other hand, if the signal device provided at the substation D is bridged across the conductor of the line !2 in series with a condenser the'ringing current is returned to the grounded terminal of the selected ringing current source over a path including the positive conductor of the line !2, the positive line wiper 482, RA452, RA4I2, RA442, the lower winding of R4! 0 and the exchange battery. From the foregoing explanation it will be apparent that the control relay R440 determines the side of a selected called line over which ringing current is to be projected in accordancewith the odd or even number of the ringing digit dialed at a calling substation.

The desired talking circuit between the calling and called substations is completed when the call is answered at the called substation. More particularly, when the hook switch provided at the called substation D is operated to establish a direct current bridge between the conductors of the line 52, the ring cut-off relay R4!0 is energized over a circuit the character of which depends upon whether the line !2 is arranged for bridged or divided ringing. Assuming the line !2 is arranged for bridged ringing and that an odd numbered ringing lead has been selected, the circuit for energizing the relay R4!0 extends by way of the selected grounded ringing lead, the

wiper 47!, RA432, RA442, RA4I2, RA452, the positive line wiper 402, the positive side of the line 22, the bridge across the conductors of the line !2, the negative side of the line I2, the negative line wiper 48!, RA45!, RA4!!, RAM! and the lower winding of R4!0 to battery. Assuming that an even numbered ringing lead has been selected, the circuit for energizing the relay R4!0 extends by way of the selected grounded ringing lead, the wiper 41!, RA45'I, RA443, RA432, RA44!, RA4! RA45!, the negative line wiper 40!, the negative side of the line I2, the direct current bridge across the conductors of the line !2, the positive side of the line i2, the

' positive line wiper 482, RA452, RA4! 2, RA442 and the lower winding of R4!0 to battery. In the event a call is initiated at one of the substations associated with the line !2 substantially simultaneously with the operation of the line switching relay R450, the ring .cut-ofi relay is energized over a circuit extending from ground by way of RA432, RA442, RA4I2, RA452, the positive line wiper 482, the positive conductor of the line !2, the established bridge across the conductors of this line, the negative side of the line !2, the negative line wiper 48!, RA45I, RA4!!, RA44! and the lower winding of R4!0 to battery. When energized over any one of the circuits traced above, the relay R4!0 first operates to complete a, locking circuit for itself at the X contacts associated with RA4I3, this locking circuit extending-from ground by way of RA30I, C316, RAMS and the upper winding of R4!0 to battery. At RA4! and RA4I2, the relay R4!0 store. At RA4I0, the relay R4I0 completes an obvious circuit for energizing its slave relay R400.

The relay R400, upon operating, opens, at RA40I, the above-traced circuit for transmitting ringing current over the calling and of the connection and prepares a circuit for transmitting a special signal current over the established connection after a predetermined time interval has elapsed. At RA402, the relay R400 completes a circuit for energizing the slow-to-operate timer start relay R460, this circuit extending from ground through the winding of R460 and by way of RA402, C216 and the resistor 246 to battery. At RA403, the relay R400 opens one of the multiple operating circuits for the relay R2I0. At RA405, the relay R400 further opens the operating circuit for the release magnet 346. Finally, at RA404, the relay R400 completes a circuit for energizing the release magnet 415 of the minor switch 410. This circuit extends from ground at RA464 by way of RA404, the oil-normal springs 411 and the winding of the magnet 415 to battery. When energized over this circuit the release magnet operates in the usual manner to restore the wipers "I, 412 and 413 to normal.

When the ring cut-off relay R4l0 operates in the manner explained above, it functions to establish a loop circuit extending from the called substation through the windings of the back bridge relay R250. More particularly, the relay R250 is energized over a circuit extending from ground through its upper winding and by way C265, RA4I2, RA452, the positive line wiper 402, the positive side of the line l2, the bridge across the conductors of this line, the negative side of the line l2, the negative line wiper 40l, RA45I, RA4H, C263 and the lower winding of R250 to battery. When thus energized, the relay R250 operates and at its armatures RA25I to RA254, inclusive, reverses the direction of current flow over the loop circuit extending to the calling substation in the usual manner. This current reversal may be utilized for various supervisory or control purposes with which the present invention is not concerned. At RA255 and RA256, the relay R250 opens points in the operating and holding circuits for the link-release relay R230. Following operation of the relay R250 the connection between the calling and called substations is fully established, talking battery being supplied to the loop extending to the calling substation through the windings of the line relays R250 and R320, and to the loop extending to the called substation through the windings of the back bridge relay R250.

The normal release of the established connection is under the control of the calling subscriber at the substation B. Thus, when the connection is released at the called substation the loop circuit extendingthrough the windings of the back bridge relay R250 is interrupted and this relay restores to again reverse the direction of current flow over the calling loop circuit and to reprepare the operating and holding circuits for the link release relay R230. When the connection is released at' the calling end thereof the two line relays R260 and R320 are deenergized and restore in the usual manner, the first of these relays functioning to open the operating circuit for the hold relay R300 and th second thereof operating to open the holding circuit for the relay R3l0. Thus, the four relays R260, R300, R3l0 and R320 are deenergized and restore. The relay R300 in releasing, opens, at RA30I, the previously traced holding circuits for the relays RAW and R450.

arenas:

At RA303, the relay R200 interrupts the operating circuit for the relay R220. At RA305, the relay R300 opens a point in the previously traced operating circuit for the relay R450. The relay R4l0, upon restoring, opens, at RA4I5, the operating circuit for the slave relay R400, and the relay R460,'in releasing, opens, at RA450, the holding circuit for the relay R440. Thus, the relays R220, R400 and R440 are deenergized and restore, At RA453, the relay R450 disconnects ground from the private wiper 482, thereby to cause the release of the cut-of! relay embodied in the line circuit l5 in the usual manner. As a result, the line I2 is again marked as idle in the bank contacts or the connector switches having access thereto. The relay R400 in restoring, opens, at RA402, the previously traced operating circuit for the relay R460 causing the latter relay to restore. The relay R220, upon restoring in response to the release 0! the hold relay R300, disconnects ground from the control conductor Cl00 at its armature RA222 and in so doing opens the holding circuit for the relay R2l0, thus causing the latter relay to restore. At RA22I, the relay R220 opens one of the multiple paths for impressing ground potential upon the timer start conductor C201. When the control conductor CHO is disconnected from ground, the finder switch 20 and the line circuit 14 are released in the usual manner. Following the release of these two switching units, the calling line H is marked as idle in the bank contacts of the connector switches to which it extends. The relay R2l0 in releasing, opens, at RA2I2, the prepared operating circuit for the link release relay R230 and completes, at RA2I5, one of the operating circuits for the release magnet 415. This circuit extends from ground by way of RA2I5, RA241, C282, the oil-normal springs 411 and the winding of the magnet 415 to battery. When energized over this circuit the release magnet 415 operates, permitting the wipers 41l to 413, inclusive, to be restored to normal in the usual manner. When these wipers are returned to their respective home positions the oil-normal springs 411 are disengaged to deenergize the release magnet 415 and the oil-normal springs 416 are moved into engagement to complete the prepared operating circuit for the release magnet 346. The latter circuit extends from ground by way of RA2I5, RA241, C282, the off-normal springs 416,

RA405, C350, the oil-normal springs 351 and the winding of the magnet 346 to battery. When the release magnet 346 is thus energized, the wipers 400 of the Strowger switching mechanism are restored to normal in a well-known manner. At this time the off-normal springs 351 are disengaged to deenergize the release magnet 346 and the off-

normal springs

306 and 322 are restored to normal. Following the release of the Strowger switching mechanism the connector switch 25 is fully restored to normal.

As indicated previously, the arrangement of the apparatus is such that the relay R460 in cooperation with the minor switch 410 and the I pulse generating equipment, not shown, functions to time a connection which results from a call originating on the line I I, for example, arranged for restricted service, and to release the connection a predetermined time interval after it is established in the event the connection is not released under the control of the calling subscriber before the time interval elapses. More specifically, the relay R460, upon operating, opens at RA46I, a further point in one of the multiple operating circuits for the relay R2l0 and interrupts at RA464 the previously traced operating circuit for the release magnet 415. At RA462 and RA463, the relay R460 further prepares the above-mentioned path for impressing a warning signal voltage on the established connection from a source of tick-tone current, not shown. At RA462, the relay R460 also prepares one of the operating circuits for the line switching relay R200. At RA465, the relay R460 prepares an altemative circuit for energizing the operating magnet 414 of the minor switch 410. Following the operation of the relay R460, this alternative circuit is periodically completed through the application of ground potential to the time pulse lead C461 by a pulse generator, not shown. These ground pulses may be applied to the time pulse lead C461 at intervals of any desired spacing from a. ground pulse generator of conventional construction and arrangement. Each time the lead C461 is grounded, the magnet 414 is energized and operates to advance the wipers of the switch 410 one step. After nine current pulses have been delivered to the magnet 414 such that the wipers of the switch 410 are operated to engage their associated ninth contacts, the above-mentioned path for impressing the voltage of tick-tone current source upon the established connection is completed, this path extending from the ungrounded terminal of the current source by way of C465, the condenser 465, RA463, the wiper 413 and its associated ninth contact, RAM! and the condenser 401 to the negative side of the established loop circuit extending to the called substation. The current flowing over this path is projected over the calling and called ends of the connection to energize the receivers respectively provided at the calling and called substations and thus indicate to the subscribers conversing over the established connection that the connection is about to be forcibly released. 'In the event the calling subscriber does not immediately hang up to initiate the release of the connection in the normal manner prior to operation of the wipers of the switch 410 to their respective tenth oif-normal positions, the relay R200 is energized and operates to interrupt the established talking circuit. More particularly, when the magnet 014 receives the tenth ground pulse from the pulse generating equipment it operates to advance the wiper 013 into engagement with its associated tenth contact, thereby to complete the above-mentioned operating circuit for the relay R200. This circuit extends from ground at RA232 by way of C21i, the wiper 413, RA062, C280 and the winding of R200 to battery. When energized over this circuit, the relay R200 operates to complete a locking circuitfor itself, this locking circuit extending from ground by way of RA232, RA222, Cl08, RA206 and the winding of R200 to battery. At RA203, the relay R200 connects multiple ground to the timer start lead C201. At RA204, the relay R200 completes a multiple holding circuit for the relay R450, this circuit extending from ground by way of RA232, RAZH, RA200, C212 and the lower winding of R450 to battery. At RA205, the relay R200 opens the operating circuit .for the relay R2l0 causing the latter relay to restore after an interval. At RA20i and RA202, the relay R200 opens two points in the established loop circuit extending from the calling substation through the windings of the line relays R260 and R320. When these two line relays restore, the operated relays of the connector switch 25 are caused to release in the manner previously explained. In this case the release of the minor 1 switch 410 and of the Strowger switching mechanism occurs concurrently with the release of the operated relays. Thus, when the relay R2l0 restores shortly following operation of the relay R200, it completes the previously'traced operating circuit for the release magnet 415, whereby the wipers of the switch 410 are permitted to restore to normal. During the homing opera tion of these wipers and when the wiper 413 is disengaged from its associated tenth contact, the above-traced operating circuit for the relay R200 is released. Also, the ofi-normal-springs 411 are disengaged to deenergize the release magnet 415 and the off-normal springs 416 are engaged to complete the operating circuit for the release magnet 346. When the magnet 346 is energized, the wipers 480 of the Strowger switching mechanism are restored to normal, at which time the oil-normal springs 351 are opened to deenergize the release magnet 346. During the movement of the wipers 480 to normal and when these wipers are disengaged from the contacts of the conductors terminating the line l2, the back bridge relay R250 is deenergized and restores.

During the release of the operated relays embodied in the connector switch 25 and when the relay R220 restores to disconnect ground from the control conductor Ci00, the relay R200 is deenergized and restores. The removal of ground potential from the control conductor 0808 also results in the release of the operated finder switch 20 and the line circuit M in the usualmanner.

Aside from the differencespointed out, the sequence in which the operated relays of the connector switch 25 are released in response to operation of the relay R200 is exactly the same as previously described with reference to the normal release of this switch.

As explained in the introductory portion of the specification, on local calls originating on lines having unrestricted local call service and routed by way of the connector switch 25, the timing apparatus comprising the relay R460 and the minor switch 410. is prevented from operating to release the connection a predetermined time interval after it is established. To this end, the extra control contact of each finder switch contact set terminating a line of this character is connected directly to ground, so that a path is completed for short-circuiting the timer start relay R460 when the slave relay R400 operates. Thus, the line I0 is illustrated as being arranged for unrestricted local call service, the extra control contact of the contact set in the finder switch 20 which terminates this line being connected toground in the line circuit l3 over the conductor Clll. With the line circuit l3 wired in this manner if a call which has been routed from the line 10 through the finder switch 20 and the connector switch 25 to the substation D, for example, is answered at this substation to cause the operation of the relays R4I0 and R400 In the event the called line I2 is busy at the time it is selected, only the relays R210, R220, R280, R300, R3I0, R320, R310 and R420 are operated at the time the connection is released at the calling end thereof. During the release of 5 the enumerated relays, the relays R3 0 and R420 are deenergized and restore when the control conductor C100 is disconnected from ground in response to the release of the relay R220. The other enumerated relays of the connector switch 1J0 are released in the exact sequence described above. I

Reverting calls The arrangement of the connector switch 25 is such that in setting up reverting call connections the selective signaling of called sub-- stations is accomplished by directory number dialing. More particularly, if the above-described call originating at the substation B is intended for another substation, such, for example, as the substation C, also connected to the line H, the calling subscriber after initiating. the call in the manner previously described dials the directory number of the desired substation C. The first two digits of this directory number designate or identify the line H, while the third digit is utilized to select ringing current of the particular frequency to which the signal device provided at the.substation C is responsive. .The manner in which the connector switch 25 responds to the three digits is exactly the same as described above with reference to the call routed to the substation D. In this'case since the line selected, namely, the line H, is marked as busy,

due to the operated condition of the line circuit l4, the usual busy tone signal is reproduced by the receiver provided at the calling substation B shortly following the end of the third digit. Upon receiving the busy tone signal, the calling subscriber must hang up in order to initiate the 40 ringing operation necessary to signal the called substation. In response to this release operation performed at the substation B, the calling loop circuit is interrupted to cause the release of the two line relays R200 and R320 followed by the release of the hold relay R300 and the relays R3l0 and R220, In releasing, the relay R300 opens, at RA30I, a point in the previously traced locking circuits for the relays R4l0 and R450 in addition to interrupting the holding circuit for 5 the relay R3l0. At RA305, the relay R300 opens a paint in the previously traced operating circuit for the relay R450. At RA302, the relay R300 interrupts the common portion of the pre-- viously traced pulsing circuits extending to the magnets 30?, 34B and 414. At RA304, the relay R300 opens the initially completed path for impressing ground potential upon the control conductor C|08 and completes an alternative path for maintaining ground potential upon this control conductor and for holding the private wiper 403 grounded. This alternative path extends from ground by way of RA232, C2H, RA304, C303, RA420, C39I, RA355, RA302, C231 and the wiper 412 to the private wiper 403. From this point the ground connection is extended through the line circuit 14 and the finder switch 20 to the control conductor CW8 to hold operated the relays R2l0, R310 and R420. The busy test relay D420 is held operated pending operation of the line switching relay R450. The relay R220, upon releasing in response to restoration of the hold relay R300, completes the operating circuit for the relay R200, this circuit extending by way of the grounded control conductor Cl08, RA222,

RA2I4, C28l, RA428; C280 and the winding of R200 to battery. The relay R200 in operating, completes, at RA206, an obvious locking circuit for itself and opens, at RA205, one of the multiple operating circuits for the relay R2l0. At RA203, the relay R200 completes one of themultiplepaths for grounding the timer start conductor C201. At RAZM and RA202, the relay R200 opens two additional points in the calling loop circuit as initially established through the finder switch 20. At RA204, the relay R200 completes an alternative circuit for energizing the line switching relay R450, this circuit extending from ground by way of RA232, RA2H, RA204, C212 5 and the lower winding of R450 to battery. When energized over this circuit the relay R450 completes the prepared operating circuit for the ringing relay R430 and prepares at RA45I and RA452 the ringing circuit to the line H. At RA453, the relay R450 completes an obvious mul tiple path for grounding the private wiper 483. At RA45'I', the relay R450 prepares the previously traced operating circuit for the relay R440, whereby the latter relay is caused to operate and look if an even numbered ringing current lead has been selected by the minor switch 410. At RA458, the relay R450 interrupts the locking circuit for the busy test relay R420, causing the latter relay to restore. At RA433 and RA434, the relay R430, upon operating, completes an alternative locking circuit for itself, this cir cuit extending from ground at RA232 by way of RA2H, RA204, C212, RA451, RA433, RA434, C318, RA3H, C319 and the winding of R430 to battery. Upon restoring, the relay R420 opens, at RA428, the above-traced operating circuit for the relay R200 and at RA421 a further point in its own holding circuit.

Following operation of the line switching relay "R450, the ringing control relay R430 and possibly the relay R440, ringing current of the selected frequency is transmitted by way of the line" H to energize the signal device provided at the substation C over one of the three ringing current circuits traced previously with reference to the local call routed to the substation D. When the call is answered at the called substation, the previously traced circuit for energizing the ring cut-ofi relay R4l0 is completed- At RA4I3, the relay R4I0 first completes an alternative holding circuit for itself, this alternative circuit extending from ground at RA232 by way of RA2H, RA204, C212, RA451, RA4I3 and the upper winding of R4I0to battery. At RA4I5, the relay R4l0 completes the operating circuit for the slave relay R400. At RA4I4, the relay R4|0 opens the holding circuit for the relay R310, causing the latter relay to release and open, at RA3'H, the locking .circuit for the ringing relay R430. The relay R400 in operating, opens, at RA403, the only completed circuit for energizing the relay R210. The relay R210 now releases to open, at RA2H, the previously traced operating circuit for the relay R450 and the holding circuit for the relay R4l0, thereby to cause the release of the relays R450, R440, if operated, RAW and R400 in the order named. The relay R450 in releasing, disconnects the private wiper 483 from ground at RA453. Since the control conductor C108 is connected through the finder switch 20 and the line circuit I4 to the private wiper 483, this conductor is also disconnected from ground in response to the release of the relay R450. Accordingly, the relay R200 is deenergized and restores. The relay R2I0 in releasing, also com wipers oi the minor switch 410 and the wipers 480 of the Strowger switching mechanism are sequentially restored to normal in the manner previously explained.

When ground potential is removed from the private wiper 483 in response to the release of the line switching relay R450, the cut-oil relay conventionally embodied in the line circuit l4 restores to reconnect the line relayof this line circuit across the conductors 'of the line H. Accordingly, this line relay reoperates to complete a locking circuit for the lookout relay conven-. tionally embodied in the line circuit l4. With the line and lockout relays operated in the line circuit M, the line H is locked out of service,

ground potential is maintained upon the private conductor of this line to mark the line as busy, and the ground marking path to the distributor start conductor 21 is interrupted. Thus, the automatic switching apparatus utilized in setting up the connection is fully released and the connection is guarded against seizure for the duration of the call due to the operated condition of the line and lockout relays embodied in the line circuit l4. It will be understood that the calling subscribed after hanging up his receiver in order to initiate the ringing operation in the manner explained above, waits a reasonable time interval during which the called substation is signaled and then again removes his receiver from its supporting hook or cradle in order to ascertain if the call has been answered. If the call has been answered at the called substation, the desired conversational circuit is immediately established when the calling subscriber removes his receiver from its supporting hook, talking battery being supplied to the two substations over the line I I through the windings of the line relay embodied in the line circuit I4. This line circuit is released when the calling and called subscribers hang up to terminate the connection. More particularly, when the direct current paths bridging the line I! at the calling and called substations B and'C, respectively, are both interrupted, the line relay embodied in the line circuit i4 is deenergized and restores to open the locking circuit for its associated lockout relay. When this lockout relay is deenergized and restores the line circuit I4 is fully released.

Trunk call-Three-diUit trunk selection The connector switch 25 is arranged to have access to groups of trunks which may be selected in response to either single digit or threedigit calling operations at the various substations. Thetrunks in the group or groups which are selected in response to three series of impulses transmitted to the connector switch may comprise, for example, ring-down trunks extending to one or more manual exchanges. For the purpose of providing automatic selection of idle trunks of these groups, each trunk of each group is terminated in the contact field of the connector switching mechanism at a set oi contacts comprising the usual private contact and an extra control contact, and these private and extra control contacts of each contact set except that terminating the last trunk of the group are strapped together.

In order to select an idle trunk of a desire-d group arranged for three-digit selection a calling subscriber, such, for example, as a subscriber nates the level of cbntacts at which the trunks of the desired group are terminated, a second digit which designates thefirst trunk of the group and a third. or ringing digit. The'manner in which the control apparatus of the connector switch'responds to these three digits is exactly I the same as described previously with reference to the local call originating at the substation B. At the end of the second digit the wipers 480 are positioned in engagement withthe contacts-terminating the first trunk of the group. If this trunk is busy at the time it is thus selected, ground potential is present upon the private contact thereof and, due to the strapping between this contact and the extra control contact, is also impressed upon the extra control wiper 484. At "the time the first trunk of the group is thus selected, the relays R2l0, R220, R260, R300, R3I0, R320, R350, R360 and R310 are all operated.-

The third or ringing digit'is utilized in the manner pointed out above to step the wipers of the minor switch 410 to one of its off-normal positions. Shortly following the end of the third digit, the relay R360 restores to open the holding circuit for the relay R350 and to complete, at RA362 the previously traced operating operating circuit for the relay R360. At RA423,

the relay R420 interrupts the pulsing circuit extending to the rotary magnet 348. At RA425, the relay R420 completes a circuit for energizing the lower winding "of R440, this circuit extending by way of the grounded extra control wiper 484, C388, RA353, C389, RA425, RA445, C390, the armature 349, C384 and the lower winding of R440 to battery. Obviously this circuit and the operating circuit for the busy test relay R420 are not energized if the first trunk of the selected group is idle such that negative battery potential is impressed upon the private and. extra control wipers 483 and 484. When energized overthis test circuit the relay R440 operates and completes a locking circuit for itself, this locking circuit extending byway of the grounded control conductor C108, RA344, C392, RA458, RAMS, RA445, C380, the armature 349, C384 and the lower winding of R440 to battery. At RA44 I, the relay R440 completes a circuit for energizing the rotary magnet 348 in parallel with the relay R350, this circuit extending from ground at RAI by way of C316,

at the substation B, dials a first digit which desig- RA45'F, c391, RA345, '0396, RA44'I, case and the normal post springs 325 to RA332 where it divides, one branch extending through the winding of the magnet 348 to battery and the other branch extending by way of 'RA36I and the winding of R350 to battery. When thus energized, the rotary magnet 348 operates to step the wipers 480 one step in the rotary direction, thereby to select the second trunk of the selected group. At its armature 349, the magnet 348 interrupts the above-traced operating and holding circuits for the relay R440. The relay R440 now restores to open its holding circuit and the circuit for energizing the relaygroup is also busy, ground potential is held on the private and extra control wipers 483 and 484, respectively, so that energization of the ,relay R420 is maintained and the relay R440 is again energized. The relay R440 in reoperating, performs the functions described above. The interrelated operation of this relay and the rotary magnet 3 48 continues until an idle trunk of the selected group is found or until all of the trunks of the selected group have been tested and found to be in a busy condition. During this trunk hunting operation, the relay R350, due to its slow-to-release characteristic, remains operated.

Assuming that an idle trunk in the selected group is available, the wipers 480 are operated step by step until the contacts terminating the idle trunk are engaged thereby. As indicated above, an idle trunk is identified by the presence of battery potential upon the private and extra control contacts in the contact set at which it is terminated. Accordingly, when an idle trunk is selected the lower winding of the relay R440 is short-circuited and, hence, does not reoperate. Due to the presence of battery potential upon the private wiper 483, the upper winding of the busy test relay R420 is also deenergized when the idle trunk of the selected group is found. Accordingly, the relay R420 immediately restores to repreparethe operating circuit for the line switching relay R450. Shortly following the release of the relay R420, the relay R350 also restores to complete the operating circuit for the relay R450. From this point on, the manner in which ringing current of the selected frequency is transmitted over the selected idle trunk to signal the operator attending the switchboard at which the distant end of this trunk terminates is exactly the same as described previously with reference to the local call originating at the substation B, Moreover, the operation of the connector switch incident to the answering of the call at the called end of the connection and incident to the release of the connection at the calling-substation are the same as described above with reference to the setting up and releasing of a local call connection.

It Will be understood from the foregoing explanation that if the first trunk of the selected group is idle the relays R420 and R440 are not caused to operate. In such case the relay R350 is deenergize'd and restores to complete the operating circuit for the line switching relay R450 shortly following the end of the third digit. The relay R450 in operating initiates the ringing operation in the manner previously explained.

If all of the trunks of the selected group are busy, the wipers 480 are, through the interrelated operation of the relay R440 and the rotary magnet 348, operated into engagement with the contacts of the contact set terminating the last trunk in the group. In this contact set, the extra control contact is not wired. Hence, the extra control wiper 484 is open-circuited when the last trunk of the group is selected. Accordingly, the relay R440 remains in its restored position and the trunk hunting operation of the connector switch is arrested. Since ground potential is, under the conditions stated, present on the private wiper 483 the busy test relay R420 is not deenergized and remains in its operated position. Shortly following the selection of the last trunk, the relay R350 restores to complete the previously traced holding circuit for the busy test relay R420 and to complete, at RA35I, the previously traced path over which the busy tone current source is connected to the upper side of the talking circuit extending back to the calling subscribers substation. when this path is completed the usual busy tone signal is reproduced by the receiver provided at the calling substation, whereby the calling sub scriber is informed that the connection cannot be obtained. It will be noted that at this time the condition of the connector switch is exactly the same as previously described with reference to the selection of a busy subscriber's line. Accordingly, the manner in which this connector switch, the preceding finder switch 20 and the line circuit terminating the calling line are released when the call is abandoned at the calling substation will be apparent from the previous explanation.

Trunk call-One-dz'git trunk selection on switchinc-through levels In order to provide one-digit automatic trunk selection on switching-through levels, the Strowger switching mechanism of the connector switch 25 is equipped with normal post springs 262, 327 and 418 which are arranged respectively to be operated into engagement in response to the elevation of the wipers 480 to a position opposite a level of bank contacts terminating trunk groups arranged for this type of selection. In this regard, it will be recalledfrom the preceding explanation that the groups of trunks arranged for one-digit selection are terminated in the uppermost levels of the connector switch contact bank, whereby the enumerated normal post springs are only moved off-normal when a trunk in one of these groups is to be selected.

With this arrangement if a call originating at the substation A, for example, is routed by way of the finder switch 20 to the connector switch 25 and the calling subscriber dials a single digit designating one of the group of trunks arranged for one-digit selection, the indicated normal post springs are moved into engagement immediately the wipers 480 are elevated to a position opposite the contact level terminating the desired group of trunks. At the springs 262 the operating circuit for the switching-through relay R240 is prepared. At the springs 418 an obvious path shunting the wiper 412 and its associated contact set is completed, thereby to prepare the operating circuit for the relay R420. At the normal post springs 321 a circuit is completed for energizing the lower winding of R330 in series with the lower winding of the relay R440, this circuit extending by way of the grounded control conductor Cl08, RA344, RA343, the lower winding of R330, the springs .321, the armature 349, G384 and the lower winding of R440 to battery. The relay R440 in operating,

prepares its holding circuit at RA445 and RA446,

and prepares, at RA441, an automatic cut-in circuit for energizing the rotary magnet 348. The other circuit switching operations performed by the relay R440 are without efiect at this time. The relay R330 in operating, prepares a circuit for energizing its upper winding. The purpose of this relay and the circuits controlled thereby is described with particularity in the following portion of the specification dealing with restricted and unrestricted classes of service.

Shortly following the end of the single digit ating circuit for the busy test relay R420. In

dialed at the calling substation A, the relay R340 restores in the manner previously explained to complete, at RA344, the above-mentioned holding circuit for the relay R440, this circuit extending by way of the grounded control conductor Cl08, RA344, C392, RA458, RA446, RA445, C390, the armature 349, C384 and the lower winding of R440 to battery. After this circuit is completed, the relay R340 opens, at RA343 and RA344, the initially completed circuit for energizing the lower winding of R330 in series with the lower winding of R440. Thus, the rev the event the first trunk of the selected group is lay R330 is, in the case under consideration,

deenergized and restores shortly following the end of the first digit. At RA345, the relay R340 in releasing, completes the above-mentioned cutin circuit for energizing the relay R350 in parallel with the rotary magnet 348, this circuit extending from ground by way of RA30i, C316, RA451, C391, RA345, C396, RA441, C398 and the normal post springs 325'to RA332 where it divides, one branch extending through the winding of the rotary magnet 348 to battery and the other branch extending by way of RA30I and the winding of R350 to battery. Another branch of this circuit extends by way of RA3'I2, C38l, RA423, RA422, C318 and the winding of R360 to battery. The relay R360, upon operating,

completes, at RA36I, the holding circuit for the relay R350 and opens a point in the operating circuit for the busy test relay R420. The relay R350 in operating, short-circuits the winding of the relay R310; opens, at RA35l, the abovetraced path for conducting busy tone.current to the calling loop circuit; prepares the operating circuit for the busy test relay R420 and completes the operating circuit for the relay R240, this latter circuit extending by way of the grounded control conductor C408, RA354, C213, the normal post springs 262 and the lower winding of R240 to battery. When energized in parallel with the two relays R350 and R300,

the rotary magnet 348 operates to step the wipers 480 into engagement with the contacts terminating the first trunk of the selected group. When its lower Winding is energized over the above-traced circuit, the relay R240 partially operates to complete. at the X contacts associated with RA246. a path for short-circuiting its upper winding, this path extending from the control conductor C108 by way of RA354, C213, the normal post springs 262, the upper winding of R240, RA246, C218 and RA421 back to the control conductor CIOB. The relay R240 is of the two-step type, being so constructed that it does not complete its-operation until its two windings are energized in series.

When the magnet 348 operates it opens, at its armature 349, the above-traced operating circuit for the relay R440. The relay R440 now restores, to open, at RA441, the cut-in circuit for energizing the rotary magnet 348 in parallel with the two relays R350 and R380. Shortly following the release of the relay R440, the relay R360 restores, to open, at RA364, the path short-circuiting the winding of the relay R310. As a result, the relay R310 is energized in series with the relay R350 and operates to complete its locking circuit; following which, it opens, at RA314, the circuit serially including its own winding and the winding of the relay R350. At RA312, the relay R310 opens the above-traced branch circuit for energizing the relay R360 in parallel with the relay R350 and the rotary magnet 348. The relay R360, upon restoring, further prepares the operbusy such that ground potential is impressedupon the private wiper 483 and the extra control wiper 484 from the trunk circuit associated with this trunk, the previously traced operating cir cuit for the busy test relay R420 is completed in response to the release of the relay R360. The busy test relay, upon operating,'recompletes the previously traced circuit including the grounded wiper 484 for energizing the relay R440. The relay R440 in operating, recompletes the circuit for energizing the relay R350 in parallel with the rotary magnet 348, whereby the wipers 480 are stepped a second step into engagement with the contacts terminating the second trunk 'of the selected group. From this point on, the manner in which the relay R440 interacts with the rotary magnet 348 to step the wipers 480 successively into engagement with the contact sets terminating succeeding trunks of the selected group until an idle trunk is found or all of the trunks have been tested is exactly the same as described previously with reference to three-digit trunk selection. In the event none of the trunks of .the selected group are idle, the wipers are advanced to engage the contacts terminating the last trunk of the group at which time the last is tested, the stepping operation of the wipers 480 is arrested and busy tone current is returned over the calling loop circuit for reproduction by the receiver provided at the calling substation.

Assuming that an idle trunk in the selected group is available, the trunk hunting operating is arrested when the wipers 480 are moved into engagement with the contacts terminating the conductors of the idle trunk. More specifically,

when the wipers 483 and 484 encounter contacts having battery potential thereon indicating an idle trunk, no further circuit is available for energizing the relay R440 and this relay remains in its restored position. In addition, the operating winding of the busy test relay R420 is short-'- circuited, causing this relay to restore. Shortly following the release of the relay R420, the relay R350 restores, to open, at RA355 and RA356, the prepared operating circuit for the busy test relay R420 and to prepare, at RA355, the operating circuit for the line switching relay R450. At RA354, the relay R350 opensthe path short-circuiting the upper winding of the relay R240 permitting the two windings of the latter relay to be energized in series over a circuit which extends by way of the grounded control conductor C808, RA421, C218, RA246 and the two windings of R240 in series to battery. The relay R240 now fully operates to complete, at RA240, a path for short-cir'cuiting the operating winding of the relay R450, this path extending from ground at RA305 by way of C381, RA424, the upper winding of R450, RAMS, C394, RA355, RA362, C241,

RA248, C218 and RA421 to the grounded control conductor Cl08. Thus, the line switching relay R450 is prevented from operating. At RA248, the relay R240 also completes a path for impressing ground potential upon the private wiper 483, this path extending by way of the grounded control conductor CI08, RA421, C210, RA248,=

The relay R240 in operating, opens, .at RA241, a further point in the above-traced operating circuits for the release magnets 415 and 346. At RA245, the relay R240 interrupts the operating circuit for the hold relay R300. At its armatures RA24I to RA244, inclusive, the relay R240 connects the talking conductors C264 and C266 of the trunk I to the conductors of the calling line, thereby to establish a full metallic loop circuit between the calling substation and the repeater associated with the selected trunk line. In response to the extension of the calling loop circuit to the repeater associated with the selected trunk line, the line and hold relays embodied in this repeater operate in the usual manner to connect multiple ground potential to the private conductor of the trunk and thus ground the private wiper 483. As a result, a multiple holding circuit is completed for the relay R240, this holding circuit extending by way of the grounded wiper 403, the normal post springs 418, C211, RA248, RA246 and the two windings of R240 in series to battery. In addition, multiple ground is impressed upon the control conductor Cl 08 in response to operation of the line and hold relays embodied in the repeater associated with the selected trunk line. This multiple ground path extends from the grounded wiper 483 by way of the normal post springs 418, C211, RA248, C218 and RA421 to the control conductor Ci00. Thus,

' has been released.

The relay R240 in operating, opens, at its armatures RA24I to RA244, inclusive, the loop circuit as initially extended through the windings of the line relays R260 and R320, causing these two relays to restore. The relays R320 and R300 in releasing open the holding circuit for the relay R3l0. The relay R300 in restoring, also disconnects multiple ground from the control conductor CNS and. interrupts the operating circuit for the relay R220. Thus, the relays R220, R250, R300, R3l0 and R320 are caused to restore in response to the switching-through operation of the relay R240. Following the release of these relays no further operation of the apparatus embodied in the connector switch 25 occurs until the repeater associated with the selected trunk line has been released.

It will be understood from the foregoing explanation that if the first trunk of the selected group is idle, the busy testrelay R420, the rotary magnet 348 and the relay R360 are not further energized after the wipers 480 are rotated to engage the set of bank contacts terminating this trunk. Accordingly, no trunk hunting operation occurs. Shortly following the selection of the first trunk the relays R360 and R350 sequentially restore, the latter functioning to open the path short-circuiting the upper winding of the switching-through relay R240. The relay R240 now completes its operation to perform the functions described above.

After the loop circuit has been extended to the repeater associated with the selected trunk line, the calling subscriber at the substation A may proceed to dial the additional digits of the directory number designating the desired substation, assuming that this repeater is associated with a trunk extending to a distant exchange of the automatic type. The manner in which the connection is further extended to the desired line and the talking circuit is ultimately established is entirely conventional and well understood in the art. When the connection is cleared out at the calling substation A, the control apparatus of the repeater associated with the selected trunk operates to disconnect the private conductor of this trunk from ground. As a result, ground potential is removed from the control conductor Cl08, causing the deenergization and release of the relays R2l0, R240 and R310. The relays R2l0 and R240 in releasing, complete the previously traced operating circuit for the release magnet 346, whereby the wipers 480 of the Strowg'er switching mechanism are restored to normal in the manner previously explained. When these wipers are restored to normal, the. normal post springs 262, 321 and 418 are respectively disengaged. The removal of ground potential from the control conductor C|00 also causes the release of the finder switch 20 and the line circuit I3 in the usual manner.

Restricted service As pointed out previously, provisions are made in the system for providing restricted service, which excludes free central office trunk calls, to certain subscribers of the system who pay a low flat fee for service and for providing unrestricted central oflice trunk service to other subscribers who pay a higher flat fee for the service rendered. In order to prevent the restricted service subscribers from gaining access to the trunks of certain specified groups, such, for example, as the trunks extending to the central automatic exchange, the connector switch 25 is provided with normal post springs 325 and 326 which are arranged to be operated to the off-normal position to open the above-traced automatic cut-in circuit when the wipers 480 are positioned opposite a level of contacts terminating trunks of a restricted group.

Assuming that a subscriber at the restricted substation B, after initiating a call and being assigned the link comprising the finder switch 20 and the connector switch 25, dials the digit designating a group of restricted trunks, the wipers 480 of the connector switch are elevated to a position opposite this group of trunks in the exact manner previously explained. When the wipers 480 are moved to this position the normal post springs 325 are disengaged and the springs 326 are moved into engagement. In addition, the normal post springs 262 are engaged to prepare the operating circuit for the relay R240, the normal post springs 321 are engaged to complete the previously traced circuit for energizing the lower winding of the relay R330 in series with the lower winding of the relay R440, and the normal post springs 418 are engaged to prepare the previously traced control circuits. In the case under consideration it is assumed that the extra control contact of the contact set terminating the line I! in the finder switch 20 is not wired. Accordingly, no holding circuit is completed for the relay R330 when this relay operates. With the normal post springs 325 disengaged, the previously traced automatic cut-in circuit for energizing the rotary magnet 348 in parallel with the two relays R350 and R360 is' interrupted, and with the springs 326 in engagement a circuit is prepared for energizing the lower winding of the busy test relay R420 in series with the parallel connected windings of the relay R350 and the rotary magnet 348. Shortly following the end of the digit, the relay R340 restores, to complete, at RA344, the previously traced locking circuit for the relay R440. At RA343 and RA344, the relay R340 opens the circuit for energizing the lower winding of the relay R330, causing the latter relay to restore. At RA345 and RA332, the relays R340 and R330 complete the above-mentioned circuit for energizing the busy test relay R420. This circuit extends from ground by way of RA30I, C316, RA451, C391, RA345, C396, the lower winding of R420, C395, RA3|4 and the normal post springs 325 to RA332 where it divides, one branch extending through the winding of the magnet 348 t battery and the other branch extending by way of RA36I and the winding of R350 to battery. Due to the relatively high resistance of the lower winding of the busy test relay R420, the

' rotary magnet 348 and the relay R350 are insufliciently energized to operate when this circuit is completed. The busy test relay R420, however, operates to complete, at RA421, the previously traced locking circuit for itself. At RA422 and RA023, the relay R020 opens the pulsing circuits to the rotary magnet 348 and the operating magnet 414 of the minor switch 410. At RA424 and RA426,the relay R420 opens two points in the previously traced operating circuit for the line switching relay R550. At RA428, the relay R420 prepares the operating circuit for the relay R200. At RAdZi, the relay R420 completes the previously traced path for connecting the busy tone current source to the talking conductors of the established connection extending back to the calling substation B, whereby the usual busy tone signal is reproduced by the receiver provided at this substation to inform the calling subscriber that the connection cannot be obtained. The manner in which the connection is released when the call is abandoned at the calling substation will be apparent in view of the preceding explanation.

In the event the above-described call intended for a trunk of a restricted group originates at a substation arranged for unrestricted service, the relay R320 is caused to restore during the dialing of the trunk selecting digit at the calling substation, whereby the relay R3l0 is released to provide a path in shunt with the normal post springs 325. As a consequence, the automatic cut-in of the wipers 38!) followed by the trunk hunting operation previously described proceeds in the exact manner explained above. Thus, the substation A, which is arranged for unrestricted service, is provided with a calling device comprising an extra pair of shunt springs 28 which are arranged to connect the positive side of the line 50 to ground when the dial of the calling device is moved-off-normal. With this arrangement if a call intended for extension over a restricted trunk originates at the substation A and is routed by way of the link comprising the finder switch 20 and the connector switch 25, the calling subscriber upon dialing the digit designating the desired group of trunks completes a path for short-circuiting the winding of the relay R320 immediately the dialing operation is started. This path may be traced as extending from ground by way of the shunt springs 20, the positive conductor (H02 of the line i0, the positive line Wiper of the finder switch 20, CI06, RA202, RA244, RA252, C261, the winding of R320, the off-normal springs 325, RAM! and the secondary winding of the transformer 338 to ground. When thus short-circuited, the relay R320 restores, to open, at RA32I, the prepared holding circuit for the relay R3l0. When thereafter the wipers 480 of the connector switch are elevated one step in the vertical direction, the off-normal springs 306 are disengaged to interrupt the operating circuit for the relay R310. The relay R3l0 now restores, to open, at RA3I4, a-point in the above-traced circuit for energizing the lower winding of the busy test relay R420 in series with the multiple connected windings of the rotary magnet 348 and the relay R350, thereby to prevent the busy test relay R420 from operating when the relay R340 releases shortly following the end of the digit. At RA3I3, the relay R3l0 completes an obvious path in shunt with the normal post springs 325. With this branch path completed the automatic cut-incircuit for energizing the relay R360, the relay R350 and the rotary magnet 348 in multiple is completed shortly following the end of the first digit and in response to the release of the relay R340. This circuit now extends from ground at RA30I by way of C316, RA451, C391, RA345, C396, RA441 and 0398 to RA3|3 where it divides, one branch extending through the winding of the magnet 348 to battery, the second branch extending by way of RA36I and the winding of R350 to battery and the third branch extending 'by way of RA312, C38l, RA423, RA422, C318 and the winding of R360 to battery. These three circuit controlling devices operate to perform the functions previously described, among which is the operation of the wipers 480 into engagement with the first contact set of'the selected level through operation of the rotary magnet 348. From this point on the manner in which the automatic trunk hunting proceeds, the connection is ultimately established and the operated switch train is released are exactly the same as described previously.

From the above explanation it will be apparent that the relay R3l0 is caused to restore in response to the dialing of a first digit at any calling substation connected with the connector switch 25 and arranged for unrestricted central office trunk service, regardless of the character finder-connector link illustrated and the other similar links. It may also be desirable to restrict certain of the local lines terminating at these links or certain of the substations on such local links from all trunk calls. For example,

it may be desirable to restrict the substation B and other substations associated with the line ll so that only local call connections can be completed therefrom without the assistance of an operator. As explained previously, the extra control contact terminating the conductor CI09 of the line H is, for this purpose, connected to the negative terminal of the exchange battery through a resistor 29. With the line circuitl i arranged in this manner, if a one-digit trunk call is initiated on the line H and extended to the connector switch 25 through the finder switch 20 and the single digit designating the desired group of trunks is dialed atthe calling substation, the wipers 480 of the Strowger switching mechanism are. elevated to a position opposite,

the level of bank contacts terminating the desired group of trunks in the manner previously described. When the wipers 480 are moved to this position, the normal post springs 232, 321 and 410 are respectively operated into engagement. At the springs 321, the previously traced circuit is completed for energizing the lower winding of the relay R330 in series with the lower winding of the relay R443. The relay R330 in operating, completes a holding circuit for itself, this circuit extending from ground through the upper winding of R330 andby way of RA33I, CI01, the extra control wiper of the finder switch 20, Cl09 and the resistor 29 to battery. When this circuit is completed the relay R330 is held operated until the finder switch is released. At RA332, the relay R330 in operating, opens the previously traced automatic cut-in circuitfor energizing the rotary magnet 348 in parallel with the two relays R350 and R360 and prepares an alternative circuit for energizing the lower winding of the busy test relay R420 in series with the parallel connected windings of the magnet 348 and the relay R350. This latter c rcuit is completed shortly following the end of the digit when the relay R340 restores and extends from ground by way of RAI, C316, RA451, C391, RA345, C396, the lower winding of R420, C395 and RA3|4 to RA332 where it divides, one branch extending through the winding of the magnet 348 to battery and the other branch extending by way of RA30I and the winding of R350 to battery. As indicated above, the magnet 348 and the relay R350 do not operate when energized in series with the lower winding of the busy test relay R420. The busy test relay, however, operates to perform the functions previously described, among which is the transmission of the usual busy tone signal to the calling substation. The manner in which the two operated

switches

20 and 25 are released when the call is abandoned at the calling substation will be clearly apparent from the preceding explanation, it being pointed out that the relay R330 is deenergized and restores during the release of the finder switch 20 when the extra control conductor CI09 is disconnected from the conductor CI01 to interrupt the holding circuit for this relay.

If the above-described trunk call initiated on the subscribers line H is initiated at a substation, such, for example as the substation C arranged for unrestricted trunk call service, the line relay R320 is short-circuited during the dialing of the trunk selecting digit through operation of the extra shunt springs 3| to ground the positive line conductor Cl04 of the line H. The relay R320 in releasing, opens the holding circuit for the relay R3l0 so that when the wipers 480 ar elevated one step in the vertical direction to cause disengagement of the ofi-normal springs 306 the relay R3l0 is deenergized and restores. This relay in releasing, opens, at RA3 I 4, the above-traced circuit for energizing the lower winding of the busy test relay R420 in series with the parallel connected windings of the rotary magnet 348 and the relay R350, and completes, at RA3I3, the path in shunt with the break contacts controlled by the armature RA332 of the relay R330. With this path completed, operation of the relay R330 is without effect to open the automatic cut-in circuit and thus prevent the connector switch 25 from seizing an idle one of the trunk lines of the selected group in the manner previously explained.

Release on pcrmanents As indicated previously, provisions are made in the connector switch 25 for forcibly releasing this switch and the preceding finder switch 20 in the event it is seized over a line having a permanent calling condition thereon or over a line having a call initiated thereon and the calling subscriber fails to complete dialing; fails to receive an answer from the called substation and to abandon the call; fails to release the connection after the called subscriber has disconnected; fails to abandon the connection after receiving a busy tone signal; and in the case of reverting calls, fails to receive an answer within a specified period or to release the connection by establishing an answering loop circuit over the line involved. These provisions include the usual time cut-off and time pulse generating apparatus, which is common to the several links, and a link release relay R230. More specifically, the time cut-off and time pulse generating apparatus is arranged to be started in operation in response to the application of ground potential to the timer start conductor C201. This apparatus functions periodically to impress ground pulses upon the time pulse lead C251 and normally maintains ground potential upon the time cut-off lead C235. A predetermined time interval after each ground pulse is applied to the time pulse lead C251, the apparatus functions in .a well-known manner to disconnect ground from the time cut-off lead C235 for a predetermined period which is in excess of that required for any operated connector switch fully to release.

From the foregoing explanation it will be recalled that when the finder switch 20 and the connector switch 25 are seized over a calling line, the relay R2 l 0 operates to prepare the operating circuit for the link release relay R230 andv the relay R220 operates to impress ground potential upon the timer start lead C201, thereby to initiate operation of the time cut-01f and time pulse generating equipment in the event this apparatus is not already operating. If the desired connection is fully established between the calling substation and the called substation before a ground pulse is impressed upon the time pulse lead C251, the back bridge relay R250 is operated, to open, at RA255 and RA256, the operating and holding cir cuits for the link release relay R230. Accordingly, the link release relay is rendered ineffective forcibly to release the connection. Moreover, if the connector switch 25 is seized over a calling line, such, for example, as the line I0, which is arranged for unrestricted service, the link release relay is rendered ineffective forcibly to release the operated switch train. In this case the time cut-01f and time pulse generating apparatus, upon operating to impress ground potential upon the time pulse lead C251, completes an obvious circuit for energizing the upper winding of the link release relay R230. This relay in operating, completes a path for short-circuiting its lower winding, which path extends by way of the grounded extra control conductor C l I I, the extra control wiper of the operated finder switch 20, the conductor CI01, RA33I, C214, RA234, RA256 and the lower winding of R230 back to ground. At RA23I and RA232, the relay R230 provides a substitute holding ground on the conductors C21l and C108, which substitute ground is derived from the grounded time cut-oft lead C235. When, however, the ground pulse impressed on the time pulse lead C251 is terminated, the link