USRE22413E - Rcijxz - Google Patents

Description

Dec. 21, 1943. vo Re. 22,413

TELEPHONE SYSTEM Original Filed Feb. 7. 1938 14-Sheets-She et 2 FIG. 3-

@SUBSCRIBEg LINE CIRCUIT f l SUBSCRIBERS [NE CIR UIT 2 2 AUTOMATIC EXCHANGE TRUNK I INE cmc urr I I RA392 INVENTOR. JOHN H. VOSS "M, M m

ATTORNEY .5,

Dec. 21, 1943.

Original Filed Feb. '7. 1938 J. H. VOSS TELEPHONE SYSTEM 14 Sheets-Sheet 3 JOHN H. VOSS BY m ATTORNEY5.

Deg. 21, 1943. .J.H. voss TELEPHONE SYSTEM Original Filed Feb. 7, 1938 14 Sheets-Sheet 7 mmhu Dec. 21, 1943. J. H. voss Re. 22,413

TELEPHONE SYSTEM Original Filed Feb. '7, 1938 14 Sheets-Sheet 8 00 mm m P u u u hm U INVENTOR. 3; gammy; 3 JOHN H.VOSS J BY l I\ m o co co co o no A w My aim, Jg w uu uu uuuuuuu ATTORNEYS Original Filed Feb. 7, 1938 14 Sheets-Sheet 9 INVENTOR. JOHN H. V055 BY M W, 9m

ATTORNEYS.

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INVEN IOR. Jol -1N H. VOSS WW, M M

ATTORNEYS.

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Dec. 21, 1943. J. H. voss TELEIHONE SYSTEM Original Filed Feb. 7, 1938 14 Sheets-Sheet 14 mmiu 330 mmEu mwio

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mwiu 7 Q i E H i i m a i i jooim INVENTOR. JOHN H. V055 BY wm m M ATTORNEYS Reismed Dec. 21, 1943 can:

ramrnonn srs'rniu John H. Von, Downers Grove, Ill, assignor to Automatic Electric Laboratories, Inc a corporation of Delaware Original No. 2,252,809, dated August 12, 1941, Se-

rlal No. 189,121, February 7, 1988. Application for reissue January 25, 1943, Serial No. 478,549

110 Claims.

The present invention relates to telephone systems and has for one of its objects the provision of improvements in small automatic systems of the type adapted for installation in small towns or villages.

An automatic telephone system of the character noted is generally known as a community automatic exchange or C. A. X and usually has a line capacity of one hundred lines or less. Certain of the subscribers lines may be oi the metallic, two-conductor variety while others of the subscribers lines may be of the well-known ground return type. The metallic lines are usually of the multi-party type and bridged or divided code ringing is utilized for selectively signaling the substations thereon. In order to provide toll service to the subscribers, provisions are also made for trunking calls to and irom one or more distant exchanges, One improved allrelay exchange of this type is disclosed and claimed in the copending application, Serial No. 158,443, Frank Kessler, filed Feb. '3, 1938. and the present invention is directed more particularly to further improvements in the circuits illustrated and described in this copending application. In brief, in the system as disclosed in the Kessler application, the subscribers lines and trunk lines of the system extend to the finder and connector switches of each of a plurality of all-relay finder-connector links, common equipment including a relay type linkdistributor or allotter being provided for assigning idle ones of the links to successive calling ones of the lines in a definite order. The finder switches of the various links are fully directive in character; that is, all lines hunting operations are eliminated through anarrangement whereby only the group and unit relays corresponding to a particular calling line are caused to operate when a link is taken for use. For the purpose of decreasing the cost of the links, the number of unit relays provided in the finder and connector portions of each link are reduced by half through the provisions of a subgroup relay. By providing such subgroup relays, each unit relay of each line selecting finder or connector switch is rendered operative to select either of two lines depending upon the operated or non-operated condition of the associated subgroup relay. In the particular disclosed arrangement of the finder switches, however, there is a possibility of a busy line being momentarily connected through to the link circuit of a seized link. to the annoyance of the conversing parties, due to the fact that no apparatus is provided for insuring the operation of the subgroup relays before the operation of selected ones of the unit relays. In the improved trunk selecting facilities of the system disclosed in the above-referred-to copencling application, no provisions are made for fully utilizing the unit relays of the various connector switches to accommodate the selection of trunks arranged in groups of more than five lines, these relays only being individually operative to select for use a single corresponding trunk when the corresponding trunk is idle. Also, no distinction is made between the trunk lines and the subscribers lines as regards the method of arranging the line circuits individually associated therewith. Provisions are made for the selection of coded ringing current for signaling desired substations on the multi-party lines, the code selection being entirely performed in any operating link through the response of the link circuit apparatus to a code selecting digit which follows the line selecting digits required for the selection of the desired called line.

It is a further object of the present invention to provide for use in a system of the character noted an automatic line selecting switch of the ail-relay type which is fully directive in character, which requires a minimum number of relays both in the switch itself and in the associated common equipment, and which is operative to preclude the connection of more than one line to its associated link circuit during a line selecting operation.

It is another object of the invention to provide an improved arrangement of the allotter or distributor equipment associated with the all-relay finder-connector links, whereby a new link is substantially instantaneously assigned to the use of a calling line in the event a first assigned link fails to find a calling line, and which functions in a manner such that there is no possibility of more than one link seizing the same calling line at the same time or more than one calling line being seized by one link.

It is a further object of the invention to provide an improved line terminating circuit which is so arranged that the line associated therewith is immediately marked as busy in the connector switches to which it extends when a calling condition is created on the line and before the finder switch of an assigned link functions to seize the line.

It is an additional object of the invention to provide an improved finder-selector link circuit arrangement whereby either of two diiferent line circuit arrangements may be employed in terminating difierent ones of the lines extending to the link.

It is a further object of the invention to provide in a link 01 the character described an improved arrangement for determining the type 01 ringing to be utilized in ringing over a selected called line and for determining the code 01 the ringing current to be projected over the selected called line.

It is a further object of the invention to provide, in a link circuit arranged to provide divided ringing on multi-party metallic lines, an arrangement for preventing the ringing current source from being short-circuited when ringing over a selected line or the ground return type.

It is another object of the invention to provide improved apparatus for registering the operation of a link each time the link is involved in aconnection.

It is a still further object of the invention to provide an improved arrangement for selecting idle ones of a group of trunk lines accessible to a link, which arrangement is such that the number of selecting relays utilized in each of the several links is reduced to a minimum.

The features of the present invention are illustrated as being incorporated in an exchange having an ultimate capacity or one hundred lines, ten of which are two-way trunk lines extending to a distant exchange and the remaining ninety of which are subscribers lines. The various lines individually terminate in line circuits which are differently connected and arranged depending upon whether the terminated line is a trunk line or a subscriber's line. In accordance with one feature of the invention, the control apparatus of any assigned finder-connector link is so arranged that the line circuit associated with any calling one of the lines is caused to operate when the assigned link finds the calling line irrespective of which type of line is responsible for the seizure of the link. More specifically, each line circuit which terminates a subscriber's line is of the usual type wherein busying potential is only applied to the private or control conductor of the line following the operation of an assigned link to seize the associated line when a calling condition is created thereon. In each of the line circuits terminating a trunk line, an improved arrangement is utilized which insures that the terminated line will be marked as busy immediately a calling condition is created thereon. To this end, each of the last-mentioned line circnits has embodied therein a line relay which operates to apply a predetermined busying potential to the private or control conductor of the terminated line when a call is initiated on the line. Also, each 01 the last-mentioned line circuits is provided with a cut-oil relay which is operative to disconnect the associated line relay from across the associated line only in response to the application of a, predetermined potential to one of the talking conductors of the associated line. In order to make each link circuit universally efl'ective to control either form line circuit, there is provided therein control apparatus for applying busying ground potential to the control conductor 01' a seized calling lin and also to the line condoctor of the link line by way of which the cutoff relays of the trunk terminating line circuits are energized.

For the purpose of selecting idle ones of the links for the use of calling ones of the lines and for the Iurther purpose of controlling the operation of the finder switch of any assigned link to seize the calling line to which it is assigned, there is provided in association with the several links common equipment including a group of allotter relays individually corresponding to the links and a group of auxiliary relays individually corresponding to the unit relays oi. the respective finder switches. The allotter relays are connected and arranged sequentially to operate to select idle ones 0! the links and are individually provided with looking or holding circuits which are so arranged that th locking circuit for each relay is opened in response to the operation of the next succeeding relay. The allotter relays are arranged to control start circuits individually extending to the various links. These start circuits are successively prepared during the sequential operation 01' the allotter relays and an improved arrangement is provided for maintaining the start circuits open independently of the allotter relays during the sequential operation of these relays. In order to prevent more than one of the allotter relays from inadvertently being locked operated in a link assigning position at the same time, there is provided an additional relay which is operative to prevent two 01' the locking circuits from being held completed at the same time for a prolonged time interval. Provisions are also made in the allotter for assigning a new link to the use of a calling line in the event a first assigned link fails to find the line. To this end and in accordance with a further feature of the invention, there is provided a relay which is conditioned to operate from one position to a second position in response to the assignment of one of the links to the use oi a particular calling 'line. and the allotter relays are arranged to respond to the operationsoi this relay to its second position to assign a second link to the use of the calling line. This relay is constructed to have an operating characteristic such that the time required for it to operate from its first to-its second position, following the conditioning there- 01 for such operation, exceeds the time normally required for the finder switch of one o! the links to seize a calling line. The circuit arrangement is such that ii the first assigned link is successful in finding the calling line the relay is prevented from operating to its second position.

The arrang ment oi the unit relays provided in each 01' the finder and connector switches is similar to that employed in the system of the above-referred-to copending application in that the lines of each line group are divided into subgroups and each unit relay is operative to select either of two different lines individually included in diiterent subgroups depending upon the operated or non-operated condition of an associated subgroup relay. In accordance with a further feature oi! the present invention, however, the operation 01' any unit relay provided in any one 01' the finder switches and corresponding to a line to be selected is made dependent upon the prior operation of the associated subgroup relay in any case where the selection of the line requires that both the subgroup relay and th unit relay, corresponding to the line, be operated. More particularly, each finder switch is arranged so that the subgroup relay provided therein operates when a. calling line is included in one subgroup of lines and, upon operating. completes the operating circuit for the one unit relay which is operative to select the calling line, whereas this one unit relay is caused to operate independently of the subgroup relay when it is included in the other subgroup of lines. By this arrangement there is eliminated any possibility of a busy line momentarily being connected through to the link circuit of an assigned link.

In accordance with a still further feature of the invention the connector switch of each link is provided with a subgroup relay which is connected and arranged to cooperate with the unit relays of the switch in selecting idle ones of the group of ten trunks terminating at the switch. More particularly, two sets of relays are provided in association with the trunk lines, which relays are so arranged that an idle trunk line is selected immediately a previously selected trunk line is made busy. The function of any one 01 the connector switches on a trunk call is, therefore, to pick out a trunk which has previously been selected by the two sets of relays noted. To this end. the control apparatus provided in each link is arranged to cause the operation of the one of the unit relays which is operative to select a preselected idie trunk, without causing the operation of the subgroup relay, so long as certain of the trunk lines are idle, and is arranged to cause the operation of both the subgroup relay and one of the unit relays when more than a predetermined number of the trunk lines are busy.

Each link is provided with a group of impulse counting relays which function to control the connector group and unit relays during the operation Of the connector portion of the link to select a desired called line. Two successive operations are required on the part of the impulse counting relays to cause the selection of a desired called subscriber's line, the first line selecting operation of the counting relays being eflectlve to cause the selection and operation of the group relay in the connector switch which corresponds to the particular group of ten lines including the called line, and the second operation being utilized to cause the selection and operation oi the particular unit relay corresponding to the called line, or the operation of this relay and its associated subgroup relay. For the purpose of preventing the ringing current source from being short-circuited in ringing over a subscriber's line of the ground return type, improved apparatus is provided for preventing the ground conductor of a line of this type from being connected to the ungrounded output terminal of the source during the ringing operation. At least a, portion of the subscribers lines are of the multi-party type and certain thereof are arranged for divided code ringing. Others of the multi-party lines are arranged for bridged code ringing. This means that each link must be equipped to provide code ringing of either type and that suitable provisions be made for selecting the type of ringing as well as the particular code of the ringing current to be projected over the called line. In accordance with another feature of the invention, apparatus is provided in each link which responds to the operation of the associated impulse counting relays during the dialing of a digit preceding the line selecting digits to determine whether bridged or divided ringing will be utilized in ringing over a selected one Of the subscribers lines. This apparatus also functions at least partially to select the code of the ringing current to be projected over the selected line. More specifically. the operation of this apparatus is dependent upon the number of impulses comprising the first digit dialed into the counting relays from a calling substation and, if the digit comprises ten impulses. it does not operate. In the case of a ten-impulse first digit, certain 01 the control apparatus embodied in the link circuit functions to cause a preselected idle one of the trunk lines terminating at the connector portion of the link to be selected for the use of the calling subscriber by the connector portion of the link.

7 For the purpose of registering the operation of the links, there are individually provided therein registering or recording devices, each of which is arranged to operate only in response to the operation of the associated counting relays when a first-received series of impulses transmitted thereto comprises more than one impulse. By this arrangement the operation of a link is only recorded when it is taken for use by a calling line and a series of impulses is actually transmitted thereto from a calling substation associated with the calling line.

Fm'ther features of the invention pertain to the particular arrangement of the circuit elements whereby the above-outlined and additional operating features are attained.

The novel features believed to be characteristic of the invention are set forth 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 Fig. 1 illustrates the general arrangement of a system to which the present invention pertains, Fig. 2 illustrates the mode of combining the succeeding figures 1' the drawings to form a unified system, and Figs. 3 to 14. inclusive, illustrate the details of a system having incorporated therein the features of the invention as briefly outlined above.

Referring now more particularly to Fig. 1 of the drawings, the system there shown comprises a small community exchange Ill and a second automatic exchange 30 interconnected by a group of two-way trunk lines, one of which is indicated in the drawings at It" as terminating at a. repeater circuit 20 forming a part of the apparatus located at the exchange l0. Also extending to the exchange III are a plurality of subscribers lines, three of which are indicated at H, i2 and it. Each of the enumerated subscribers lines is illustrated as being of the multi-party type having two or more telephone substations associated therewith. Thus, two telephone substations A and F are diagrammatically shown as being connected to the line H, two additional substations B and C are shown in association with the line II and two substations D and E are illustrated as being connected to the line it. Each of the substations connected to the various subscribers lines may comprise the usual hand set telephone having embodied therein a dial controlled impulsing device of conventional arrangement. Each of the lines extending to the exchange Ill terminates in a line circuit through which it is rendered accessible to the finder and connector switches included in the exchange. Thus, the line H terminates at a line circuit 2i through which it is connected to the contact springs of a finder switch II and a connector switch 26. which two switches are connected together to form a finder-connector link. Similarly, the line l6 terminates at a line circuit 22 and the illustrated two-way trunk It interconnecting the exchanges it and 30 terminates at a line circuit 23.

The exchange II is designed to serve a relatively small number of subscribers lines, specifically 90, and the trunk lines of a ten-line trunk group and, accordingly, all of the subscribers lines and trunk lines are arranged to be served by any one of the plurality of finder-connector links included therein. Obviously, the number of links required for satisfactory service will depend upon the volume of telephone traffic but, taking into account the character of the exchange, ten links are considered adequate for reasonably good service. In order to assign the links to successive calling lines in a definite order so that a link is always available for the use of a calling line, excepting. of course, those instances when all of the links are simultaneously busy, there is provided certain common equipment 25 which includes a link distributor or allotter. The several links also have commonly associated therewith ringing, tone and timing apparatus which is diagrammatically shown at 21;

Neglecting for the present the details of the circuit arrangement, and considering briefly the operation of the system, when a subscriber, such, for example, as the subscriber at the telephone A, initiates a call over the line I l to a second subscribers telephone, such, for example, as the telephone C, by lifting his receiver from its hook, the line circuit 2| operates to cause the finder-connector link previously assigned by the distributor to the calling line, to be started in operation to seize the calling line. Assuming that the link 24, 2B is delegated by the distributor to set up the connection, the finder portion 2! of the link operates to select the calling line and to connect it through to the conductors of the link line incoming to the connector portion 26 of the link. When the calling line H is seized and extended by way of the finder switch to the associated connector switch, it is marked as busy in all of the other connector switches and the control equipment of the seized link is automatically conditioned to respond to the impulses of the first series of impulses transmitted from the impulsing device at the calling substation A. Since the subscribers lines are of the multi-party type, provision must necessarily be made for selectively signaling the desired called substation on any selected line. With the arrangement to be described more in detail hereinafter, coded ringing is utilized to accomplish this end. It is further contemplated that certain of the lines may be arranged for bridged ringing and others of the lines may be arranged for divided ringing and, in accordance with one feature of the present invention, the link circuit apparatus of any seized link is arranged to respond to the impulses of the first digit dialed at the calling substation to select the type of ringing to be used and further, if bridged ringing is selected, partially to select the code of the ringing current subsequently to be projected over the called line. Further in accordance with this feature of the invention, the operation of the link circuit apparatus during the first digit determines whether the selected line will be a called subscribers line or a trunk line extending to the distant exchange. Also, if the number of impulses making up this first digit is more than a predetermined number, a registering device provided in the link is caused to operate to register the operation of the link.

In the case under consideration wherein the call is to be extended to the subscriber's line It, the connector switch 26 will respond to the impulses of the first digit to select the type of ringing to be utilized and to cause the operation of the link to be registered or recorded. If the called line It happens to be one of the lines arranged for bridged ringing, the first digit also results in the partial selection of the ringing code to be used in ringing over the called line.

The connector switch 26 responds to the impulses of the second digit dialed at the calling substation by selecting the group of ten lines which includes the called line It. Thereafter, and in response to the dialing of the third digit, the called line I6 is selected from the selected group of lines. When the called line is selected. the control apparatus included in the link circuit functions to test this line for the purpose oi determining the busy or idle condition thereof.

Busy tone current, as generated by a portion of the common apparatus 21, is not returned over the p extending to the calling subscriber's substation until the fourth or code selecting digit is dialed at this substation. When this digit is dialed, the link circuit apparatus functions to select the particular code of the ringing current to be projected over the called line for signaling the substation desired by the calling party. At the conclusion of this digit and if the called line is busy, busy tone current is returned over the loop to the calling subscriber's substation to indicate to the calling party that the desired connection can not immediately be completed. Ii, on the other hand, the called line is idle at the time it is selected, ringing current of the selected code corresponding to the called substation C is projected over the called line for the purpose of energizing the signaling devices individually provided in association with at least a portion of the substations on the called line. In this connection it will be understood that, if divided ringing is utilized, the ringing current is projected over the side of the line to which the ringing device at the desired substation is connected. On the other hand, if bridged ringing is selected as the type of ringing to be used, ringing current is conducted over both sides of the called line to energize all of the ringing devices individually provided at the substations connected to the line. During the ringing operation, ring-back tone current is applied to the loop extending to the calling substation to indicate to the calling party that the desired substation is being signaled. Finally, a connection between the calling and called substations is completed when the call is answered at the substation C. As pointed out with particularity hereinafter, the arrangement of the link circuit apparatus is such that a link utilized in setting up a desired connection is released only when the connection is cleared out at the calling substation.

The switching apparatus located in the exchange Ill may also be utilized in setting up a connection between two substations connected to the same line, the sole function of the apparatus in this regard being that of selecting and applying ringing current to the calling line of the proper code to signal a party at the called substation. The discharge of this function necessitates the seizure of the calling line by an idle link, all in the manner outlined above. Thereafter, and following the dialing of the four digits of the called substation directory number at the calling substation, the calling party restores his receiver to its hook. When the calling party hangs up, the finder portion of the operated link is released, and ringing current of the selected code identifying the called substation is projected over the calling line. If the calling line is one of the lines arranged for divided ringing, the link circuit apparatus functions to project a reverting call signal ringing current over the line conductor which is not used for conducting ringing current to the signal device at the called substation. This reverting call signal ringing current energizes the ringing device at the calling substation to indicate to the calling party that the called substation is being rung. When the call is answered, the operated link is released and the line circuit 01' the line in service operates to busy the line, thereby to prevent its seizure by the connector portion of another link on a second call to one or the substations on the line. Finally, the line circuit 01' the line in use is restored to normal when the connection is cleared out at the calling and called substations.

In the operation of the link to select one of the group 01 trunk lines interconnecting the exchange it and the exchange 38, only one digit comprising a predetermined number of impulses is dialed at the calling substation. The connector portion or the seized link responds to the impulses of this one digit to select a preselected idle one the group of trunk lines. More specifically, these lines have associated therewith trunk preselecting apparatus which functions to select an idle line for use immediately a previously selected line is taken for use. Hence, at the conclusion of a trunk selecting digit dialed'at a calling substation, the connector portion or the seized link immediately operates to seize the preselected idle trunk. When the trunk line is seized, the calling line is switched through by way of the selected trunk line and its associated repeater circuit to the terminating apparatus located in the distant exchange. In this connection, it is noted that two sets of link line conductors are provided in the link circuit or each link, one or which is devoid of all voice frequency coupling plements, such, for example, as condensers or repeating coils, and is utilized in extending a desired connection by way or one of the trunk lines. Following the extension of the connection to the trunk line terminating apparatus located in the distant exchange 30, the connection may further be extended to the desired called line through the dialing of the regular directory number corresponding to the desired line at the calling substation. The manner in which the connection is released following the establishment of a connection to the desired line depends upon the nature of the repeater circuit associated with the trunk line which is used in setting up the connection.

Since the trunk lines interconnecting the exchanges HI and 30 are of the two-way type. it will be understood that any one of these lines and the switching apparatus included in the exchange Ill may be utilized in the completion of calls originating or extended by way of the exchange 30 and which are to be extended to one of the subscriber's lines terminating at the exchange Ill. The mode of operation of the equipment in the exchange HI following the extension of a call thereto by way of one of the trunk lines noted is substantially similar to that set forth above with reference to the connection established between the substations A and C, it being observed that each of these trunk lines terminates in a line circuit connected and arranged in a novel manner which is diflerent from the arrangement or the line circuits individually terminating the connection is entirely under the control of the calling party whose line terminates at the distant automatic exchange.

Considering now in greater detail the apparatus embodied in the exchange In and involved in the above-described sequence of operations, and referring to Figs. 3 to'l4, inclusive, there are illustrated the elements of the line circuits respectively terminating the lines II, It and 50, of the finder- connector link 24, 26, and of the common equipment 25. There are also diagrammatically illustrated those portions of the apparatus 21 which are essential to an understanding of the operation of the system. The line circuits individually terminating the subscribers lines are of more or less conventional arrangement. More particularly, the line circuit 2| comprises a line relay R300, a cut-ofi relay R3"), and a line lockout relay R320 connected and arranged to operate in the usual manner. The corresponding relays for the line circuit 22 are indicated at R300, RSHI' and R320. This last-mentioned circuit is indicated as having associated therewith a pay station tone source 368 which is connected through a condenser "I to impress a pay station tone current on the control or private conductor C306 01 the line It for the purpose of indicating to a supervising operator in a distant exchange as, for example, the exchange 30, that the substation B is of the pay station type. This pay station tone circuit is shown in association with the line It for illustrative purposes only, it being pointed out that a circuit of this form is provided only in conjunction with those of the lines terminating at the exchange III which are of the single party pay station type. Since the apparatus for applying pay station tone current to the control conductor C306 may be entirely conventional in character, it has been only diagrammatically illustrated.

The line circuits individually terminating the respective trunk lines are identical in character and are diil'erently connected and arranged from the line circuits terminating the subscribers lines. In brief, the line circuit 23 comprises a line relay R350, a cut-off relay R390, a lookout relay R340 and a switching relay R330. The primary diiierence between this line current and the subscribers line circuit 2|, for example, pertains to the method of busying the associated lines to calls incoming thereto. With this illustrated circuit arrangement of the line circuit 23, the line relay R350 is operative to impress a line busyi g potential on the private or control conductor C359 immediately a calling condition is created on the associated line, and the cut-oil relay R390 is operative over a circuit including the positive talking conductor C351 0! the line. With the subscribers line circuit 2|, on the other hand, a line busying potential is only applied to the control conductor C306 following the operation of one of the finder switches to seize the line, and the cutoffrelay R3 I ll is only operative when the busying potential is applied to the control conductor C306.

Each of the finder switches is of the fully directive all-relay type, group and unit relays being provided for performing the line selecting functions. The lines terminating thereat are arranged in groups oi ten lines and the lines of each group terminate at the contact springs of one group relay. More particularly, the finder switch 24 comprises ten group relays corresponding to and including the two relays R318 and R380 individually having contact springs for terminating the conductors of ten lines. While the group relays may be or any desired commercial type having the required number 01' contact springs, preierably these relays are of the improved i'orm disclosed and claimed in the copending application Serial No. 109,813, Herbert F. Obergi'ell, flied November 9, 1936. The lines of each group are divided into two equal subgroups and through the provision of a subgroup relay in each finder switch a group or live unit relays is suilicient to permit the selection of any desired one of a selected group of ten lines. Thus, the tinder switch M comprises a subgroup relay R410 and live unit relays corresponding to and including the two relays R400 and Bill, each of which unit relays is operative to select either or two lines in a selected group of ten lines depending upon the operated or non-operated condition of the subgroup relay Rllll. The finder switch 24 also includes a pair 01 switching relays R440 and R! which are arranged to be controlled by way or the common equipment illustrated in Figs. 5 and 6 and are operative to initiate the operation of the finder switch to seize a calling line.

The common equipment shown in Fig. 5 includes a plurality of auxiliary relays which are arranged to be energized over start circuits controlled by the line circuits associated with the respective lines terminating at the exchange l0. These relays are selectively operative in response to the selective completion oi the start circuits and individually iunction to condition for operation a particular group relay and a particular unit relay embodied in the finder portion of an assigned link. More particularly, ten auxiliary group relays including the two relays R5"! and R5" are provided, which individually correspond to the finder group relays oi each link. A group of five auxiliary unit relays, including the two relays R530 and R5, are similarly provided which individually correspond to the unit relays provided in the respective finder switches and are operative to condition the corresponding linder switch unit relays for operation. The energization oi the subgroup relay in the finder portion of any assigned link is dependent upon the operated or non-operated condition oi a subgroup relay R5 provided in the common equipment, this latter relay also being arranged to be energized over those of the start circuits which are adapted to be completed in the line circuits terminating the lines included in predetermined ones of the line subgroups. Specifically, the arrangement oi the apparatus is such that when a calling condition is created on one oi the lines terminating at the exchange, start circuits are completed by the line relay associated with the calling line for causing the operation of the one of the auxiliary group relays R5", R520, etc., which corresponds to the group 01 lines including the calling line and for energizing the one of the auxiliary unit relays R530, R540, etc., which is operative to select the calling line. It the calling line is in one subgroup of its particular group the auxiliary subgroup relay R500 is not energized over the completed start circuits, but it the calling line is in the other subgroup, R500 is energined and operates to condition the subgroup relay embodied in the finder switch 01' the selected link for operation. In order to give the finder switch otanyassignedllnkampletimetoseizethecalling line to which it is assigned and to connect the line through to the connector portion of the link before the auxiliary group and unit relays are reconditioned to respond to another call, the slowto-release relay RIM is provided. which relay is operative immediately a link starts to seize a calling line and functions to open the operating circuits for all of the auxiliary grou and unit relays.

The link alloter or distributor is illustrated in Fig. 6, as comprising a group of ten allotter relays R810, R610, R880, R890, etc., which individually correspond to the ten links corresponding to and including that illustrated, an all-links busy relay RIOI, a link start relay R604 and six additional relays Rilll, R620, R830, R640, R650, and R680. Of these six additional relays, the slow-to-release relay Bill and the relay R680 are provided for the purpose of opening the locking circuits ior all of the allotter relays when two or more 0! the allotter relays are inadvertently locked operated in the link assigning position at the same time, and for automatically causing a single allotter relay corresponding to an idle link to be operated to its link assigning position following interruption of the allotter relay locking circuits. The relay R820, the slow-to-release relay R630, and the slow-to-operate relay R840 cooperate to cause the allotter relays to assign a second idle link to the use of a calling line in the event a first assigned link is found defective and fails to operate properly in response to the operation of the start relay R604. The relay R628 also functions to control the all-links busy relay R"! in a manner such that a multiple holding circuit is provided for maintaining RG00 operated during the operation of an assigned link to seize the calling line, which holding circuit is opened to permit R600 to restore following the complete operation of the assigned link to seize the calling line and in the event the assigned link is the last available idle link. In addition, the relay R62! functions to hold open the link start circuits individually extending to the various links during the operation of the allotter relays to select an idle link. In order to insure the continued operation of R500 for a short interval after the allotter relays have operated to select an idle link, the slow-to-release relay R6" is provided.

The link circuit proper is illustrated in Figs. 7 to 10a of the drawings. The link line "I incoming to the link circuit terminates in a repeater I and the talking conductors thereof are arranged to be included in the operating circuits for two line relays R'Hil and Rlli. The relay RHll is the usual impulse responsive relay and in combination with the two additional relays R1" and R forms an impulse correcting network for repeating impulses transmitted thereto from a calling substation over an impulsing circuit to the slow-to-release control relay R860 and the impulse counting relays illustrated in Figs. 10 and 10;. The link hold relay is indicated at R180. this relay being arranged to be held energized by a slow-to-release slave relay R150. The relay Rlllli is the usual switching through relay which is energized only when the link is utilized in extending a connection by way of one of the trunk lines to the distant exchange 30. The battery reversing relay is indicated at R115, this relay being arranged to operate in the usual manner when a call is answered at a called substation, and functions to control the operation 01 a slave relay R which actually performs the battery reversing operation. Any selected called line is electrically balanced through the provision of an impedance element ill having an impedance substantially equal to that of the battery reversing relay R115. For the purpose of preventing the link from being permanently tied up through seizure by a defective line or a line having a call-

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