US2421919A - Telephone routing system - Google Patents

Description

2 Sheets-Shae; l

R. c. AVERY Filed April '29, 1944 TELEPHONE ROUTING SYSTEM June 10, 1947.

M E M n INVENTORY ELI/ very BY Q i a g a 5 ATTORNEY June ZIOQ 1947.

R. c. AVERY TELEPHONE ROUTING SYSTEM Filed Aprilj'29, 1944 2 Sheets-Sheet 2 vary IN VEN TOR E: EEYE N v @N a Patented June 10, 1947 TELEPHONE ROUTING SYSTEM Robert Campbell Avery, Jackson Heights, N. Y., assignor to American Telephone and Telegraph Company, a corporation of New York Application April 29, 1944, Serial No. 533,314

20 Claims. 1

This invention relates generally to telephone exchange systems and, more particularly, to panel-type dial systems in which connections between subscribers lines terminating in different ofiices of the telephone exchange are established by automatic switching mechanisms over groups of interofiice trunks.

In exchange areas having a large number of local oflices, it is not alwayseconomical to provide direct groups of trunks for interconnecting any two ofiices of the exchange area, particularly where the trafiic between these offices is not heavy. In some exchange areas, therefore, it may be economical to route some or all of the traffic between certain oifices through a tandem office which may serve as a common intermediate distributing point.

It has heretofore been proposed to render more efiicient the trunking arrangement between any two oifices by reducing the number of direct trunks between those two offices and by automatically re-routing the remaining traffic between these oflices to a tandem ofilce where this trafiicmay then be routed to the desired oflice. To accomplish the re-routing of calls through a tandem oflice in systems employing central office senders which control the setting of selector switches to establish connections, each sender must receive a signal in order that it may take the necessary steps when such re-routing is required to re-route succeeding connections for that trunk group over a diiierent trunk group extending to the intermediate tandem office. Each sender must also receive another signal to reemploy the direct route between the two oflices when the direct route again becomes available.

It is an object of this invention to provide a re-route signal when all trunks of the direct trunk group'extending between two oflices are busy. It is another object of this invention to provide a re-route signal when all but a predetermined number of the trunks of such a group are busy. Any arrangement to accomplish the latter object will prevent the loss of certain calls which would otherwise occur because of the time interval after selections by the sender are started and before the selector switch controlled by Y the sender is actually positioned on the terminals of the desired trunk. A sender may be given information to route a call to a trunk of a direct trunk group extending to a desired office and may proceed to cause the selection of a trunk in such a group when there is only one such trunk idle. If the signal to the sender to reroute succeeding calls should be delayed until after the seizure of the last idle trunk such calls would not be completed if these were initiated during the interval after the sender starts its selections and before the selector switch controlled by the sender completes contact with the terminals of the last idle trunk. Hence, the selector switches operated to complete such calls to the desired direct trunk grou would be driven to the overflow position. To guard against any such contingency, it is desired to signal the sender for the re-routing of calls when all but a predetermined number of the trunks, such as one or two of the trunks, for example, are busy.

It is a further object of this invention to set the re-route signal in response to the action of the selector switches as they reach a predetermined terminal near the top of the trunk group contacted by the selector switch.

Another object of this invention is to automatically set the re-routing signal as required and to release it automatically upon the release of the previously busy trunks. This object may be accomplished by the action of a test circuit common to one or more trunk groups, the test circuit being arranged to test the trunks of each of the groups in regular order and to control the re-route signal for each such trunk group. Thus, the test circuit may operate to complete calls by means of the original route, or it may re-route these calls, all of which may be controlled by the information reaching the common test circuit. The use of a test circuit common to a plurality of trunks or trunk groups will materially reduce the amount of equipment-heretofore required for re-routing calls.

Another object of this invention is to provide a common test circuit for testing different parts of the trunk group at different times, then to record the results of such tests and thereafter to set the re-route signal in accordance with the cumulative results of the tests of all of the parts of the trunk group.

Another object of this invention is to provide arrangements for transmitting calls through different tandem centers so that calls which would be normally routed over a trunk to any one of such tandem centers may be transmitted over trunks to other of such centers whenever traffic conditions warrant such a change in routing.

The apparatus of the invention is designed to set the re-route signal or to restore the conditions to normal when calls for a particular trunk. group are in progress in the decoders without interference with such calls. The ap paratus is arranged to efiect a change in the 3 routing information after a route relay in a decoder has operated and before the decoder has completed its functions.

As is disclosed in Patent No. 1,862,549, dated June 14, 1932, to R. Raymond et al., the establishment of connections from an originating office to any other ofiice of the exchange area is made possible by district and office selectors which are set to select an idle trunk of the group of trunks extending to the office in which a desired subscribers line terminates by a sender common to all subscribers lines of the originating office. -A plurality of senders, any of which may be taken for use by a calling line, is provided-each sender having registers for registering'both the 'ofiice and numerical designations of a desired line dialed by a calling subscriber, and controlling apparatus for the setting of the district selector and an office selector to extend a connection to the office in which the desired line terminates and for controlling other apparatus in the selected office for completing the connection. A plurality of decoders is also provided common to all of the senders, an idle one of which becomes associated with a sender taken for use by a calling subscriber. The decoder is provided with office code registers to which the ofiice code is transferred from the registers of the associated-sender and is also provided with as many route relays as there are trunking points to which calls may be trunked from the originating oiiice. The combined setting of the office code registers of the decoder determines which one of these route relaysshall be operated for any particular call. The operated route relay is then instrumental in-establishing in the associated sender'a registration for directing the sender to control thesetting of the district and office selector switches or the setting of the district selector alone toselect a group of trunks extending to the ofiice in which the desired line terminates.

In accordance with the present invention each trunk group extending directly from one oflice to another office to which it is desirable to'apply alternate routing through a tandem office or other type of switching center, is provided with a cut-in relay or relays for connecting the-sleeve terminals of the trunks to a test circuit common to a number of such trunk groups. The cut-in relay or relays are operable by the common test circuit 1 which is arranged to determine the busyand idle conditions with respect to the trunks of the group. Each such trunk group is also provided with a relay for setting or restoring the re-route' signal in the various decoders. operated for setting the signal in accordance with the action of the selector switch as it passes a predetermined terminal near the top of the trunk group. It may also be operated to set the signal and it may be released to restore the signal under control of the common test circuit which, a it is connected to the sleeve leads of the various trunks, determines the busy or idle condition with respect thereto. The setting of the re-routesignal in response to the action of the selector switch as it reaches the predetermined terminal'near the top of the trunk group, assures that the re-routing of traiiic will occur only if all preceding terminals of the trunk group have been'passed and found busy. Thus, the re-routing of'trafiic will occur only when all trunks below a predetermined trunk are found busy.

The common test circuit to which the sleeve terminals of the various trunks may be connected through the contacts of the cut-in relay or relays The latter relay may be I is arranged to test a number of trunks, such as I0, at any one time. The testing apparatus is also equipped with a rotary selector and an associated relay circuit so that the test circuit may successively test other similar groups of trunks. The test circuit Will be shown to control the operation of the cut-in relay for the trunks to be tested. The test circuit will also include a group of auxiliary relays, one common to each of the trunks to be tested, and each such auxiliary relay will be operated if the trunk is found busy. The auxiliary relays are arranged in a chain to close one of a number of different circuits in accordance with the busy conditions of the various trunks. One of these circuits will be closed if all of the trunks are busy. Another circuit will be closed if all but one of the trunks are busy. Still another will be closed if all but one of the trunks are busy. Still another will be closed if all but two of the trunks are busy. And so on. One or more of the latter circuits may be connected to re-route relays for controlling the routing of trafiic when such auxiliary relays have become operated.

This invention will be better understood from the more detailed description hereinafter following when read in connection with the accompanying drawing, in which Figure 1 illustrates some of the equipment which may be employed for routing and re-routing telephone calls in accordance with the principles of the invention, Fig. 2 illustrates an arrangement which may be connected to the Fig. 1 arrangement, Fig. 2 including, among other things, a common testing circuit for testing the various trunks of one or more groups of trunks shown in Fig. 1, and Fig. 3 shows some of the apparatus that may be included in a decoder when set up to practice the invention.

Referring now to Figs. 1 and 2 of the drawing, there is shown a calling subscriber SB connected to a line finder 1F, a district selector DS, an office selector OS, a district finder DF, a sender selector SE, a sender SD and a decoder connector DC, all of which are of a well-known type. In order to simplify the drawings, these devices have been shown only schematically herein. For a more complete disclosure and description thereof, ref erence may be had to the R. Raymond et al. patent already recited above. The decoder connector DC may be connected to a number of decoders which will be referred to again in regard to Fig. 3.

In response to dialing, the office code and numerical designations of the called partys telephone station (not shown) are registered in the sender SD in a well-known manner, and following the registration of the ofiice code designation, the decoder connector DC is operated to associate one of a plurality of decoders with the sender SD. Each decoder may be provided with the usual receiving registers to which an associated sender transfers the registered office code designation, a route relay corresponding to each route over which connections may be extended from the originating ofiice and transmitting registers which are selectively set by any operated route relay. The transmitting registers are employed to transfer routing information to the registers of the associated sender which in turn control the functioning of the sender to establish the desired connection over district and office selectors in accordance with the operated route relay of the decoders.

It may be assumed that a call ha been started by the sender SD and that a selector switch SS of the office selector OS proceeds to hunt over a group of terminals such a AT which may be ten in number. The sequence switch SQ will cause the relay L of the selector switch SS to become operated at certain intervals to cause the mechanism to hunt past busy sleeve terminals such as S S5. Upon reaching a grounded sleeve terminal such, for example, as S of a busy trunk, the relay L (previously operated by the closing of the sequence switch SQ) will be held operated over a holding circuit established through its right armature and make contact and extending to ground at the sleeve terminal S5. As the relay L becomes operated and held operated, it completes a circuit through its left armature and make contact to energize the updrive magnet U'P, causin the up-drive of the selector switch SS to continue past the grounded sleeve terminal in question. When a sleeve terminal is reached which does not have ground connected thereto such as S3, for example, the relay L will release, thereby causing the up-drive mechanism of the selector switch SS to stop. When this happens the brushes of the selector will be centered upon the trunk terminals in a wellknown manner to advance the calling partys line through the the available or idle trunk.

In the present arrangement of the invention a predetermined terminal such as AA of the group of terminals AT has been set aside for operating the re-route mechanism. Terminal AA is not connected to a trunk. When the selector switch SS in moving upward in search of an ungrounded sleeve terminal reaches the terminal AA, a circuit is completed from battery through the winding of relay L, the make contact of its right armature, the sleeve contact of terminal AA, then over the upper winding of relay AR to ground. Relay AR is the re-route relay of the trunk group AT. It is a fast operating relay and its upper winding is of low resistance. The ground connected to its upper winding acts as a busy condition to the hunting selector switch SS and it holds the relay L operated so as to continue the up-drive past the terminal AA. Although the up-drive continues past terminal AA, the relay AR and relay L, which are connected in series, are both operated. The hunting selector SS will proceed to seize the next upper trunk associated with the sleeve such as S7 to complete the connection from the calling partys set SB to the called oflice, provided sleeve S 1 is ungrounded. After relay AR is operated, it is locked in its operated position over a circuit which in-- eludes battery, the lower winding of relay AR,

the make contact of armature L1 of relay AR,

the normal contacts associated with armature T1 of relay CIo and ground. Relay AR also applies ground at its armature L2 to a start lead ST which is connected to the associated common test circuit shown in Fig. 2, the purpose of which will be described later. The relay AR also transfers the code leads II, I2 and I3 of three decoders (not shown) from the normal route relays connected to conductors I4, I5 and I6 which are in turn connected to the back contacts of the upper armatures T1, T2 and T3, respectively, to the alternate route relays of the three decoders connected to conductors ii, iii and I9, respectively, the latter relays being associated with the alternate route trunk group. This will be further described with regard to Fig. 3. Thus, when relay AR is unoperated, calls will be directed to the normally associated trunk group, but when relay AR becomes operated, these calls will be transferred to the trunk group for the alternate route.

6 Terminal AA of the trunk group AT preferably should be so located that there will be one terminal above it for completing the call which may have caused the operation of the re-route relay AR. If desired two or more working terminals may be positioned above terminal AA of group AT for completing calls which may have already been routed to this trunk group but have not reached it at the time when relay AR operates. The number of the additional terminals appearing above terminal AA will depend on the probability of calls being initiated during the critical interval just before relay AR operates. If desired, two terminals such as AA may be connected in parallel for causing the operation of relay AR should relay AR not be sufficiently fast to operate as the selector switch SS passes but one such terminal. When relay AR becomes operated, it will remain operated to cause the rerouting of calls in the associated decoders but relay AR will be released by the common test circuit of Fig. 2 whenever trunks become available again for such calls, as will be explained hereafter. The common test circuit of Fig. 2 is arranged to test the several trunks of group AT at one time. The common test circuit may be arranged to function only when the relay AR is operated, or it may be arranged to operate continuously whether or not relay AR is operated, or it may make busy tests at predetermined intervals. These alternatives are controlled by keys BO, IC- and KST of Fig. 2 and will be explained hereinafter.

Assuming that the busy control key B0 of Fig. 2 is operated, tests will be made to determine the busy conditions of the trunks of group AT, but these tests will be initiated only after relay AR operates, and the tests will be stopped as soon as the relay AR releases as will now be shown. Upon the operation of relay AR and the connection of ground at its armature L2 to the start lead ST, a path will be completed to energize the windings of relays STA and C10, this path including the grounded start lead ST, the contacts of key BC, conductor 2|, the armature T2 of relay RL on its back contact, the winding of relay STA to battery and ground. In parallel with the winding of relay STA there is the circuit including the feed brush FBI of one are of a rotary selector, the main brush MB1 of the same are of the rotary selector, terminal 3 of the rotary selector upon which its main brush M281 may be assumed to be resting, conductor LDc, the winding of relay CIo, battery and ground. The ground connected to conductor 2| in the manner above described is also closed through the armature T2 of relay RL, to the upper winding of relay PT and battery. Conductor 2| is also connected through the armature L2 of relay STA and its back contact to the lower winding of relay PT and battery. Relay PT is employed in this circuit together with condenser K and resistances R and R1 in order to introduce a time interval in the operation of the connected circuit arrangement. With ground connected to both of its windings as above, the magnetic effect of the lower winding of relay PT is stronger than that of its upper winding and the relay is consequently set on its back contact.

it will not step the rotary .selector to its next contact until the stepping magnet becomes released. The energization of the windings of relays STA and CIO will cause the relay CIo to become immediately operated, but due to the slowoperate property of relay STA, this relay will remain released for a brief interval sufficient to insure that relay PT will be propertly positioned on its back contact.

The operation of relay CIo will transfer the locking path of relay AR previously completed through the normal contacts associated with armature T1 of relay CIo to lead 32 which is connected to ground through the back contact of armature L1 of relay RLA and the normal contacts associated with the armature T1 of relay RL. Thus, relay AR will be held operated as long as both relays RLA and RL remain unoperated. Relay CIo also connects the sleeves S0130 S9 of the various trunks of trunk group AT through the corresponding make contacts and armatures of relay CIo to the windings of relays P .to P9, respectively, the windings of each of the latter relays being also connected to ground through a battery. The latter circuits interconmeeting the sleeves of the trunks to the windings of relays P0 to P9 are shown only schematically by a dotted line 31. It will be understood that sleeve S9 will be connected by one make contact of relay 010 to the winding of relay P9, that sleeve S8 will likewise be connected through another make contact of relay CIo to the winding of relay Pa, and so on for all other sleeve contacts of the trunk group AT. Thus, the operation of relay CID will connect the various trunk sleeve terminals to the windings of the separate relays P0 to P9 which are part of the common test circuit for testing the busy conditions of the trunks associated with those sleeve terminals. Inasmuch as each busy trunk has ground connected to its sleeve terminal, the corresponding relays of the group P0 to P9 will become operated while relays of the group P0 to P9 connected to idle trunks will be released.

The several relays P0 to P9 have windings which are of high resistance so that the hunting selector OS may test the various sleeve terminals in the manner already described without any interference arising from the connection of the windings of these relays P0 to P9 to these sleeve terminals. Hence, the connection of the winding of any relay, such as P9, to the sleeve terminal S9 through the corresponding armature and make contact of relay CIo will not interfere with the action of selector switch SS and relay L in detecting the absence of ground on the terminal S9 when the associated trunk is idle. Each of the relays P0 to P9 has but one armature which is employed to control a corresponding relay of the group TTo to TT9. Thus, the several relays P0 to P9 control the'operation of the corresponding relays TTo to 'IT9 which are also part of the common test circuit.

It will be observed that when a sleeve contact such as S is found grounded due to the busy condition of the associated trunk, the corresponding relay P5 will become operated, and relay P5 in turn will operate relay TI'5. Consequently, the various relays of group TTo to TT9 will be operated or released depending upon whether or not the corresponding sleeve contacts of the trunk group AT are grounded or ungrounded. If there are ten trunks in the group and if all of the sleeve contacts So to S9 are grounded, for example, all of the relays P0 to P9 Will be operated as well as the cor-responding relays TTo to TT9. If all of the sleeve contacts, except sleeve S8 for example, are grounded, then relays P8 and TTB will be the only unoperated relays of the two groups of relays P0 to P9 and TTo to TT9 controlled by the sleeves So to S9. If there should be less than ten trunks in .the group, the contacts of relay CIo which are not associated with trunks are connected to ground so that the corresponding P and TT relays will be operated in the same manner as for busy trunks. In the present instance the contact of relay CIc corresponding to terminal AA of group AT will be so wired, causing relays P6 and TTe to be operated.

Although the winding of relay STA is connected in parallel with the Winding of relay CIo as already shown, relay STA is slow in operating to insure that relay PT has ample time to close its back contact as hereinbefore described. After this interval of time has elapsed, relay STA operates and disconnects ground from the lower winding of relay PT by the opening of the circuit through its armature L2 and associated back contact. This allows condenser K to become charged in a circuit extending from battery, through the lower winding of relay PT to ground. As this charge is taking place, the magnetic effect of the lower winding of relay PT is gradually reduced. When the efiect of the lower Winding is reduced to less than that of the upper winding, the relay PT operates.

Relay PT in operating causes relay RLA to operate in a circuit from the previously mentioned ground already connected to conductor 2|, through armature T2 of relay RL and its back contact, armature L1 of relay STA and its front contact, the armature and front contact of relay PT, the winding of relay RLA, battery and ground. The delay introduced in th operation of relay RLA by means of the condenser timing arrangement described above is suflicient to allow relays CIo, P0 to P9 and TTo to TT9 to be fully operated. Relay RLA in operating provides a ground at the make contact of its armature T2 over conductor 24 and through the armature T2 and back contact of relay RL, armature L1 and the corresponding front contact of relay STA and the armature and front contact of relay PT to lock the relay RLA in its operated position. This locking path to ground will also hold relays STA, PT and CIo and the stepping magnet SM operated. Furthermore, the operation of relay RLA will cause the energization of the winding of relay RL over a circuit which includes the armature T1 of relay RLA and its make contact, conductor 25, the winding of relay RL, battery and ground. Relay BL is of the slow operating type and, therefore, will not respond immediately after relay RLA becomes operated. The operation of relay RLA also opens a path at the normally made contacts associated with its armature L1 which previously extended to the winding of relay AR, over lead 32 and the armature T1 and make contact of relay C10, The opening of this path will serv to release the relay AR which controls the alternate route control circuit if the relay AR is operated, provided the results of the tests to be described indicate that the associated route has again become available. The operation of relay RLA also connects ground at the normal contacts of armature T1 of relay RL through its own armature L1 and make contact, through the lower armature of relay Q1 and its back contact, through the lower armature of relay Q2 and its back contact, and then through the lower armature and back contact of relay Q3 to the armature U2 of relay TT9. The connection of ground to the armature U2 ofv relay TT9 in response-to the operation of relay RLA will serve to hold relay AR operated, if it is already operated, or to operate it if it has become released, it the associated route is not available as indicated by the tests performed by relays P to P2 and relays TTo to TTs. This will now be explained.

The ground connected to the upper armature U2 of relay TTs will be applied through the various contacts of relays TTo to TTQ to one of the four conductors A0, B0, C0 or D0 in accordance with the number of relays of the group TTo to TTs that are unoperated at any one time. If all of the relays T'To to TTS are operated, as would be the case if all the trunks of group AT were busy, ground at the armature U2 of relay TT9 will be applied to the conductor AO over a circuit which includes the make contact of armature U2 of relay TT9, and then through the other armatures U2 of relays TTa'TTq TTo and their corresponding make contacts. If all of the relays TTu to TTQ except one, such as T'Is, are operated, then ground will be applied to the conductor BO over a circuit which includes the armature U2 of relay TT9 and its make contact, the armature U2 of relay TTs and its back contact, then through the armature U1 of relay TT and it make contact, and through the other armature U110)? relays TTc, TT TTo and their make contacts to conductor BO. If all of the relays TTo to TTQ are operated except two, such as TTq and TTs, ground at the armature U2 of relay TT9 will be applied to conductor CO over a circuit which includes the armature U2 of relay TT9 and its make contact, the armature U2 of relay TTa and its back contact, the armature U1 of relay TTq and its back contact, and then through the armature L1 of relays TTs, TT5 TTo and their make contacts to conductor CO. If three or more of the relays TTo to TT9 are unoperated, as for example, relays TTq, TTs and TTQ', ground at the armature U2 of relay TT9 will be applied to the conductor DO over a circuit which includes the armature U2 of relay TT9 and'its back contact, the armature U1 of relay TTs and its back contact, the armature L1 of relay TI'q and its back contact and then to conductor DO. Thus, ground will be applied to any one of the conductors A0, B0, CO and DO depending upon whether no one of the trunks of group ATis idle or one trunk 15101142, or that two trunks are idle, or three or more trunks are idle, respectively. The presence ofground on any one ofthese four conductors may be. employed in this invention to cause relay AR either to be operated or tobe released, as desired, to provide either the alternate routes or the normal routes, as will be further explained.

The conductor A0 is connected to the lower winding of the relay ARover a circuit which includes the feeder brush F352 of the rotary selector, the main brush MR2 of the same selector, terminal 3 of the arc of the rotary selector, conductor 36 to the lower winding of relay AR. The application of ground to the conductor AO therefore will operate relay AR if it should be released, or hold it operated if itis already operated. Thus, when the common testing circuit applies ground to the conductor A0, the relay AR will be energized so as to connect the alternate route to the system. Should it be desired to employ the alter- 10 nate route when all of the trunks of group AT but one are busy, as well as when all are busy, then the conductor CC wil1 be wired to the terminal 3 associated with brush MBs of the rotary selector. In that case relay OT will beoperated over a circuit which includes the feeder brush FB2 and the main brush M133 of the rotary selector and its terminal 3, conductor C0, the armature U2 of relay STA and its make contact, the upper winding of relay OT, battery and ground. When so wired, the relay AR will be energized upon the application of ground to the conductor BO whenever all of the trunks but one are busy. The circuit from grounded conductor B0 to the winding of relay AR will include in addition to grounded conductor BO, the upper armature of relay TB and its back contact, the upper armature of relay OT and its make contact, conductor 3|, the brushes FB2 and MB2 and their associated terminal 3 of the rotary selector, conductor 30 to the lower winding of relay AR. Thus, the relay AR will be operated under these conditions, if it is not already operated. Similarly, if it is desired to provide the alternate route when all of the trunks but two are busy, the conductor EE will be wired between terminal 3 associated with brush M133 of the rotary selector and the armature U1 of relay STA, thereby causing relay TB to become operated. The operation of relay TB will supply ground at conductor CO tothe lower Winding of relay AR over a circuit similar to that already described to operate that relay AR or hold it operated, whenever all but two of the trunks are busy. Relay TB also connects conductor B0 to the lower winding of relay AR if only one trunk should be found idle under these conditions. This arrangement may be extended to provide the alternate route path when any desired proportion of the trunks of group AT are busy.

It is apparent from the foregoing that the operation of relay RLA in response to the operation of relay AR opens the locking path for relay AR at the back contact of the armature L1 of relay RLA and at the same time closes a path at the make contact of armature L1 of relay RLA to apply ground to the armature U2 of relay TTa of the common test circuit so as to hold relay AR operated or to reoperate it if it has become released. This in turn will depend upon the number of trunks which have become idle, the idle condition being indicated by the application of ground to one of the various conductors A0, B0, C0 or DO. The operation of relay RLA will also complete a path at the make contact of its armature T1 to operate relay RL, as already pointed out, but the latter relay is of the slow-operate type in order to allow sufiicient time for the relay AR to become operated or released.

The operation of relay RL will apply ground at the make contact of its armature T1 to the lower winding of relay AR if relay AR is operated, over a path which includes conductor 32, the armature T1 of relay C10 and its make contact, and the armature L1 of relay AR and its make contact. This ground will lock relay AR in its operated condition. The operation of relay RL also opens the circuit connected to the back contact of its armature T2 and thereby releases relays CIo, STA, PT and RLA. The opening of the circuit of the stepping magnet SM at the back contact of armature L1 of relay RL will release the stepping magnet SM which then causes the main brushes MB1, M132 and MBs, which are mounted on a common shaft, to be moved to their next terminal 4. The reason for operating the rotary selector willbe explained hereinafter. The release CIo also closes the locking path for relay AR previously established, if relay AR is operated at this time, through the armature L1 of relay AR- and its make contact and the normal contacts-associated with armature T1 of relay CD). The release of relay CIo also disconnects the sleeve contacts So to S9 of the trunk group AT from the corresponding windings of relays P to P9- and the release of the latter relays will in turn release the corresponding relays TTo to TTt. The release of relay RLA will remove the supplementary ground previously connected by its armature T2 to the start lead ST through the path of armature T2 of relay RLA, conductors 24 and 21' and key BC. The release of relay RLA also releases relay RL by opening the circuit to its-Winding at the make contact of the armature T1 of relay RLA. Both relays RL and RLA are slow to release, however, so as to permit all elements of the common test circuit to be returned to normal before another test of the trunks-ofthe group AT is initiated.

The arrangement of this invention may be employed for testing a group of trunks of any size. For example, 30 trunks (only some of which are shown) may be arranged in three groups, such as AT, BT and CT, and three relays, such as C10, C11 and CI2, may be employed for separately connecting the sleeves of one of the three trunk groups AT, BT and CT respectively,to the windings of the relays P0 to P9. Only onere-route relay such as ARr'will be associatedwith the three cut-in relays Clo, CIr and C12, and the lower armature L1 of relay AR will be connected to normal contacts of the relay C in the manner already considered for a group of ten trunks. The terminals 1 2 and 3 associated with the main brush may be Wired through leads LDz, LDl and LD'u to the windings of the relays C12" C11 and Clo, respectively, as shown. Assuming also that the alternate route is to be made available when but two of the trunks or the three groups are idle, then conductors AA and DD will be positioned on terminals I, 2'and 3 associated with the main brush- -MB3 of the rotary selector. All of the main brushes M131, M132 and MBii may be considered to be positioned on terminal I at the beginning of a test of such a series of three trunk groups: It is noted, however, that the main brush M133 is of the bridging type; being adapted to close one of its terminal before it is completely disconnected from a pervious terminal. The other main brushes M131 and MBz are of the non-bridging type. 7

When the selector switch SS of the ofiice selector OS moves upward in hunting for an un- I grounded sleeve terminal, it will cause the relay AR to become operated assoon asit reaches the predetermined terminal AA, as already described. Ground at the armature L2 of relay AR'will' be applied to the start lead ST and will complete a circuit to operate relay 012, this circuit including in addition to the start lead ST. key BC, conductor 2!, the armature T2 and back contact of relay RL, the brushes and M131 of the rotary selector and the associated terminal I, lead LD2, the winding of relay C12, battery and ground. The relay C12 will now connect the sleeves S0 to S9 of the trunk group CT to the windings of the relays P0 to P9, respectively, over circuits similar to those generally designated 31 and the relays P0 to P9 will then control the corresponding relays TT'o to 'ITs as already described. Relays STA and 1'2 PT'will operate and cause the operation of relay RLA as before. Upon the operation ofreay S'I'A, relay AD will become operated over a circuit which includes battery, the winding of rela AD, conductor 36, the make contact and armature Us of relay STA, conductor FF. terminal l and brushes M33 and FBa-of the rotary selector and ground. The relay AD will be employedto control the relays V1, V2 and V3 and relays Q1, Q2 and Q: for counting the number of idle trunks in the trunk groups to be tested.

If one of the trunks of the group CT is found idle,the conductor will be grounded by the relays TTo to 'I'I9 in the manner already described, and this grounded conductorwill be connected through the contacts of relay AD to the winding of relay V1 for operating the latter relay. The operating circuit for relay'vi will include the grounded conductor BO, the upper armature and back-contact of relay TB, the upper armature and back contact of relay OT, th'e armature U2 and make contact-of relay AD, the upper armature and back contact of relay Q1, the winding of relay V1, battery and ground. With relay RL released, relay Q; will be short-circuited by the contacts of relay V1. That is, ground will be connected to the lower terminal of relay Q1 over a circuit which includes the make contact and armature of relay V1, the back contact and armature U1 of relay Q2, the back contact and upper armature of relay Q3, the make contact and armature L2 of relay AD, and ground and'ground will likewise be connected to the upper terminal of the winding-of relay Q1 over a circuit which includes the upper armature and back contact of relay Q1, the make contact and armature U2 of 'rela AD, the back contact and upper armature of relay OT, the back contact and upper armature of relay TB to the grounded conductor BO. However, when relay RL becomes operated in response to the operation of relay RLA, as already pointed out, the ground on conductor 130 will be removed from the make-before-break contact of relay RL and hence relay Q1 will be operated. The operating circuit for relay Q1 may be considered to include battery, the windings of relays V1 and Q1, the make contact and armature 'of relay V1, the back contact and armature U1 of relay Q2, the back contact and upper armature of relay Q3, the make contact and armature L2 of relay AD and ground. Thus, both of the counting relays V1 andQi will be operated in response to ground applied to the conductor BO by the testing relays TTo and TT9..

Should the testing relays TTc to TIa apply ground to the conductor CO instead of conductor BO, as would be the case, for example, if two of the trunks of the group CT are found idle, the relay V2 will then be operated over a circuit which includes battery, the winding of relay V2, the armature U2 and back contact of relay Q2, the make contact and armature U1 of relay AD, the back contact and armature L1 of relay CT, the back contact and armature L1 of relay 'I'B to the grounded conductor CO. The winding of relay Q2 will also be short-circuited by the contacts of relay V2. but this short circuit will be removed upon the operation of relay RL to cause the relay Q2 to be operated. Thus, the relays V2 and Q2 of the counting circuit will be operated in response to the application of ground to the conductor CO.

Should ground be applied to the conductor DO by the testing relays 'ITo to TI'9 by virtue of the idleness of three or more of the trunks of 1 3 rthe group CT,.relay.vgwillbecome operated and :thereby short-circuit the winding of relay Q3. rRelay .Qswwlll also become operated uponathe operation. of relay RL.

. While the main brushesMBnMBz andwMBz "be-controlled only when the rotary selector --reachesterminal"3,- as willbe explained hereinafter.

The counting -relays'V1, V2 andvgand Q1, Q2 and Q3 are employed to store the results of the -tests -mad -on the In trunks of the group CT so that these results may be added to the results -subsequentlyobtainedbytests on-the other trunk groups BT-and -AT-fordetermining whether or not the re-route relay AR is to be operated or -released after-the testing cycle has been completed.

1 Letit-be assumed that in testing the trunks of the groupCT only'one trunk is-found idle, there- 1 by causingthe operation of relays V1 and Q1.

" The operation of relay-RL will break the circuit of the stepping magnet thereby advancing the" main brushes MB1,-MB2 and-MBs of the rotary selector'to their next terminal designated a 2. -Moreover; relays RLA, STA, PT, C12 and RL will be released in themanner already described.

-As the main-brush M153 reaches its terminal 2,. it will hold relay AD operated over a circuit which includes battery, *the'winding of relay AD, the -conductor AA, terminal 2 and the brushes MBa and-FE: of the rotary selector and ground. By holding relay AD operated the-counting relays "Vrand Qr willbe held operated while the next group of trunk terminals BT are tested. The re- -lay- ;,Q1 will transfer the ground at the make- -before-break contacts of relay BL to the arma- -ture U1 of relay TTQ over a circuit-which in- -cludes-the armature L1 of' relay RLA and its -make contact; andthe lower armature and make -contactof relay Q1. This has the effect of increasing by One the number-oftrunks to be "found idlein group BT.

-With the main" brushes of the rotary selector on their terminals designated 2, the relay C11 "WlHIbECOmB operated over a circuit which includes ground at the armature L2 of'relay AR, the start lead ST, the key BC, conductor 2|, the armature T2 and back contact of relay RL, brushes FB1 and MB1 and terminal 2 of the rotary selector, conductor LD1, the Winding of relay CIn-battery and ground. Relay CIrwill connect the" sleeve terminals of the trunks of the group 'BT to the'windingsof relay P0 to P9, respectively,-which-in turn will control the'relays T'I'o to 'TT9 for testing these varioussleeve terminals in-the manner already described. Should "one of the trunks noW be found idle, after let us say only'one trunk-was found idle inthe previ-, ous trunk group CT; the ground applied to the --lower armature of relay Q1 by theoperation'of relay RLA, will be closed-through: make contacts of relay Q1 to armature U1 of relayTT9-and then -to conductorLCO. J'The application. of groundto conductor 00 willrcause relay V2 to operate over 1 a circuit similarto that previously traced. Re-

.la i1Qz willlikewise becomeoperated after relay RL releases. It seems hardly necessary to state that ifha second sleeveterminal-were alsofound grounded in the trunk group ET, the relaysva and Q3 would become operated due to ground being applied to the-conductor DO; but this'situa- 10' tion-will-be passed at this time; The operation of relay Q2 will open-the circuit tothe windings of relays V1 and Q1,releasing them, and in a similar -manner if relay Q3 should be operated instead of Qz,it would open the circuit eitherto'relays V1 and-Qr-or V2 andQareIeasing eitherpair of these --relaysthat mayhave been operated depending -upon thenumber of trunks-previously found idle.

Upon the operation of relay-'RL at the'end-of the test, the stepping magnet SM will become deenergized, thereby advancing the main brushes of the-rotary selector to their next terminaldesignated 3. The ground -fr0m lead ST closed to the feeder brush FB1 will then be applied through -lead LDo to the Winding of relay CIo, thereby operating' this relay. Accordingly, the sleeve terminals of the trunkgroup AT will now be connected to thewindings ofrelays P0 to P9 in the --manner already described for testing this group of trunks. The sequence of events occurring upon the operation of relay CIo have already been outlined and need not be repeated. If relay Q2 is operated, however, theground fromthe back contact associated with armature T1 of relay BL,

through operated contacts L1 of relay RLA, is

closed through the-lower armature and back contact of relay Q1, lower armature and make contact of relay Q2, to the armature L of relay TTs. This has the effect of increasing by'two the number of trunks to-be found idle in group AT. If relay Qsis operated," due to three or more trunks having previously been found idle, thecircuit described above isdisconnected from relay TTQ,

' so that relay AR cannot be, operated, regardless 1 of the number of trunks found idle in group AT.

Upon-reaching. terminal 3, the relay TB will become operated over a' circuit which includes battery, the winding of relay AD, the lower wind- Ling of relay TB, conductor DD, terminal 3, and the brushes MBs and F33 of the rotary selector and ground. This circuit includes the winding of relay AD and, therefore, relay AD is held 0P- erated while the-main brushes are on their terminal 3. Consequently, the previously operated counting relays will be held operated.

' With relay TB operated the re-route relay AR will be energized if it has become deenergized,

or will be held operated if it is already operated and if the alternate route is to be cut in when -,not more than two trunks of the three groups "AT,'BT and CT have been found idle. In this connection itis observed that the make contact of the armature L1 of the relay TB is wired to the feeder brush F132 and thence through lead 30 to the lower winding of relay AR. In a similar manner, relay AR will be operated'or held if only one trunk in the three groups should be found idle, by closure from conductor BO through the upper armature and make contact at relay T-B to the feeder brush FBz.

Were it desired to cut the re-route path into the system upon the location of but one idle trunk, the conductor BB would replace the conductor DD at terminal 3 associated with the main brush-M133. -With conductor BB onterminal: 3,

the relay OT will be operated instead ofwthe relay TB, the operating circuit for relay OT including battery, the winding of relay AD, the lower winding of relay OT, conductor BB, terminal 3 and the associated brushes MB: and F33 of the rotary selector and ground. Under these conditions the'grounding of conductor B0, in response to the location of a single idle trunk in any'of the various trunk groups AT, BT or CT, will be transferred through the upper armature and make contact of relay OT, over conductor 3| and through the brushes F32 and M32 to lead 30 and to the lower winding of relay AR. In either of the above cases, if all of the trunks in groups CT, BT and AT are found busy, ground will be connected to conductor AO which will cause the relay AR to be operated or held operated as hereinbefore described, without regard to relays TB or OT.

On the other hand, if the alternate route path is to be cut into the system only when all trunks are found busy, conductor GG will be positioned on the terminal 3 associated with the main brush MB3 of the rotary selector. Ground applied through brushes F133 and MB; to conductor GG will hold any of the counting relays operated, but relay AD will be released. The system is, therefore, flexible and may be made to respond to any desired or predetermined condition in the trunks of a large group.

Upon the operation of relay AR or its release at the end of the test of the three groups of trunks CT, BT and AT, the re-routing path may be cut in or omitted as determined by the testing apparatus. However, when relay RL operates at the end of the test, the stepping magnet SM will again be released to advance the main brushes M'Bi, MBz and M33 .to their next terminal designated 4. When this happens the relays OT or TB will be released if either of these relays has been operated under the separate conditions already referred to, and relay AD will also be released if it is operated. Likewise, all of the counting relays will be released upon the release of relay AD, or upon the removal of ground from conductor GG. If three or more trunks are found idle during the test of any group of trunks, such as CT, or on a cumulative basis during successive tests over all of the three groups CT, BT and AT, in consequence of which counting relays V3 and Q3 have become operated, the relay AR will become released and the regular route will therefore be employed for subsequent calls through the system. Should it be desired to use :the alternate route for these trunk groups when three or more trunks are found idle, it will be understood that additional counting relays and appropriate wiring may be added to the system for this purpose.

The other terminals 4, 5, 6, etc., of the rotary selector switch may be wired to additional cut-in relays such as C110, 0120, etc., and corresponding re-route relays such as ARIO, ARzo, etc., in accordance with the principles already outlined with respect to a group of trunks such as CT. One route relay and one cut-in relay may be associated with each trunk group. As the main brushes of the rotary switch are moved from terminal to terminal, the trunks of a trunk group such as DT associated with another of the cut-in relays, such as CIm, will be tested and the reroute path controlled by the re-route relay, such as ARio, may be added or omitted as determined by the testing apparatus. Further description along these lines would be merely repetitious and has, therefore, been omitted.

The rotary selector is arranged to pass by spare terminals which are not required for actuating alternate route control circuits. When such a terminal is reached, relays STA and PT operate as before, thereby causing relay RLA to become operated and in turn cause the relay RL to become operated. The operation of the relay RL will, of course, release the stepping magnet and advance the main bushes of the rotary selector beyond any such spare terminal.

To allow the common test circuit to make continuous tests regardless of whether or not any associated trunk group is busy, the keys BC and 10 will be left unoperated and key KST will be operated. In that case the operation of relay AR upon contact with terminal AA of Fig. 1 is not required and the wiring for such operation may be omitted. I

If it is desired to make tests at periodic intervals, the interrupter control key IC will be operated. An interrupter INT is shown connected through contacts 50 of the key 10 and through the upper winding of relay I to battery and ground. Hence relay I will be operated at the intervals determined by the interrupter INT. Upon the operation of relay I this relay will be looked over a locking circuit provided by its lower winding, contacts 5| of the key IC and the make contact and upper armature of relay I and ground. Ground will also be applied to the contacts associated with armatures L1 and T2 of relay RL over a circuit which includes the upper armature and make contact of relay I, contacts 52 of key IC, the normal contacts of key BC and conductor 2|. The conductor II will be wired to any particular terminal associated with the brush MBs of the rotary selector. With relay I operated, the tests will be made and repeated at the intervals determined by the interrupter INT for all selector terminals prior to the one to which conductor II is connected. When the terminal to which conductor II is wired is reached, the relay I will be released. This is because ground is applied to the lower winding of relay I over a circuit including the brushes FBs and MB3, conductor II, contacts 53 of key 10 and to the armature and lower winding of relay I, thereby short-circuiting the lower winding of relay I. The release of relay I will disconnect ground from the contacts associated with armatures L1 and T2 of relay RL and replace this ground with the ground connected to brush FBs. The new ground is applied over a circuit including the brushes FBa and M'Bz, conductor II, contacts 53 of key IC, lower armature and back contact of relay I, contacts 52 of key 10, the normal contacts of key BC and conductor 2| to the armatures L1 and T2 of relay RL. However, when the selector passes the terminals to which conductor II is connected, this ground is disconnected and therefore no further tests will be made until the interrupter again causes the relay I to be reoperated.

Fig. 3 illustrates a modification of certain of the apparatus of each decoder circuit of the telephone system to permit the setting and restoring of the normal and alternate routes at any time without'causing interference with calls already in progress. Here the two route relays designated RR1 and RRZ are shown which, when operated, establish the normal routes for calls. One of these relays, such as RRi, would be associated with the re-route relay AR of Fig. 1, and the other would be associated with a similar re-route relay, such as ARIO, for example. The

conductors l3 and [G of Fig. l are shown connectedto the code point CP1 and. one terminal of the windingof the relay RR1, respectively. The other code point P2 and one terminal of the winding of relay RRz are likewise connected to contacts of theirassociated re-route relay. These relays ERA and RRz are connected in parallel with respect to the winding of arelay GSi so that relay GSi will be operated if either of the relays RRi or RRz becomes operated. In other words, if the relay AR of Fig. 1, for example, is released, the relay RRi will be operated to establish normal routing for the calls received by the trunk group associated with relay AR, and the relay GS1 will also be operated. However, when the relay AR becomes operated, relays RRi and G81 willboth be released.

Fig. 3 also shows the conductor l9 associated with relay AR of Fig. 1 connected in series with the Winding of an alternate route relay RP and another relayGSz which is similar to relay GSI. Relays RP and G532 will be operated only when the corresponding re-route relay AR has been operated. It is to be noted that both relays GS1 and G82 are of the slow-release type. Consequently, upon the operation of relay AR, the relay GS1 will remain operated for a predetermined time interval so that both relays G81 and GSz will simultaneously hold their back contacts open. Hence, the connection of ground at the armature of relay GSz to the point RLX will be interrupted for the time during which it takes relay GSi to become fully released. The terminal RLX is connected in the ground supply circult which is part of the apparatus for advancing the sender (not shown). Thus the operation of the sender will be delayed and this will allow sufficient time for the route relay RRI to release, and in-turn to release any'relays which it may have operated for routing the call tothe normal trunk group. However, upon the release of the relay GSI calls'will be routed under control of the re-route relay RP.

While this invention has been'shown and described in certain particular embodiments merely for the purpose of illustration, it willbe understood that the general principles of this invention may be applied to other and widely varied organizations without departing from the spirit of the invention and the scope of the appended claims.

What is claimed is:

1. In a'telephone system, the combination of a plurality of trunk over which call may be transmitted, a plurality of testing relays each corresponding to one of said-trunks, means for simultaneously connecting said testing relays to the corresponding trunks, each testing relay being energized when its corresponding, trunk be comes busy, 2. re-route relay, and means con- 18 termined number of said testing relays are simultaneously operated.

3. In a telephone system, the combination of V a plurality of trunks each having a sleeve ternecting said re-route relay to said testing relays to operate said re-route relay when a predetermined number of said trunks become simultaneously busy.

2. In a telephone system, the combination of I minal, said trunks being divided into separate groups, a plurality of testing relays each or which correspond to one of the trunks of a group, an intermediate relay having a plurality of contacts each of which connects the sleeve terminal of one of said trunks to the winding of the corresponding testing relay, a re-route relay, means controlled by said testing relays for registering the number of trunks that are idle in all of said groups, and means controlled by said registering meansfor operating said re-route relay when the number of idle trunks is below a predetermined number.

i. In a telephone system, the combination of a plurality of trunk sleeve terminals corresponding to difierent telephone trunks, a selector adapted to hunt successively over said sleeve terminals to locate an ,ungrounded terminal, a re-route relay connected to a predetermined one of said sleeve terminals, said re-route relay being operated when. said predetermined sleeve terminal is reached by said selector, means responsive to the operation of said re-route relay to simultaneously test all of said sleeve terminals to detenmine whether said sleeve terminals are grounded or ungrounded, and'means for operating said re-route relay when a predetermined number of said sleeve terminals are grounded and for releasing said re-route relay when less than said predetermined number of said sleeve terminals are grounded.

5. In a telephone system, the combination of a plurality of trunk sleeve terminals corresponding to different trunks, said sleeve terminals being arranged in separate groups, a selector adapted to hunt successively over said sleeve terminals, a re-route relay connected to a predetermined one of said sleeve terminals and being operated when said predetermined sleeve terminal is reached by said selector, meanscontrolled by said reroute relay to separatelytest all of the sleeve terminals in the various groups in succession and to register the number of ungrounded sleeve ing circuit which may be connected to all of the trunks in any one group at one time, said testing circuit being capable of simultaneously testing all of the trunks in each group to determine the number of idle trunks in the group, means for successively connecting said testing circuit to said groups of trunks, and means for additively indicating the number of idle trunks in all of said groups of trunks.

7 Ina telephone system, the combination of a plurality oftrunks arranged in groups, testing apparatus which may be connected to all of the trunks in any one group at one time, said testing apparatus serving for simultaneously testing all of the trunks of any one group to determine the number of idle trunks in the group,

said testing apparatus including means for preventing interference with service over said trunks of the group While they are being tested, and

means for additively/ indicating the niin lgleza of idle trunks in allof said groups.

plurality of trunlisarranged in. groups, testing,v

apparatus for simultaneously testing all-of the trunks in any one-group to determinethe number of-,-idle.trunksainithe group, said testing apparatus-iincluding; means for preventing interference withiservice over. saidtrunks of thegroup while they, are being tested, means for additively indicatingthe number of idle trunks. in allof saidgroups,.. and routing means-.controlled; by the number. of idle trunks in all;ofsaidgroups 9;: In-atelephone. system, thecombination of a plurality; of trunks... which; may be transmitting telephone messages simultaneously, testing ap.- paratus for simultaneously testing all ot said trunks to determine the number of idle trunks witheutinterfering;withservice over said trunks, means for momentarily applying said testing circuitzto all of said. trunks, said testing apparatus includi-ng-,- a plurality ofv relays each having a winding. of large, impedance which is connected to. one. ofsaid trunks, and routing-means responsive to the number of idletrunks to controltherouting of calls involving; saidtrunks I V V 1.0. Ina telephone system, the combination of a. plurality oftrunks-eachhaving. tip ring and sleeve terminals, a'selector switclrfor progressivelymovingover said sleeve terminals and "stopping when it reaches an ungrounded sleeve terminal; amainrelayconnected to a predetermined sleeve terminal, said main relay beingoperated when said selector switch reaches said predetermined sleeve terminal, a testing.- circuit for simultaneously. testing.- all of thesleeve terminals to determine-the number. of ungrounded sleeve terminals, said-testing circuit includinga plural:

ityoi-relays. each of which has awinding .ofhigh.

resistance. which. is connected, to one of the sleeve terminala.v said resistance beinglof a mag.-

nitude. which. will I notsubstantially change the potential of said sleeve terminal,- and ,meanscontrolledby saidmain relay toainitiate the operationofsaid testing circuit.

1.1.. Inla telephone-system,v the combination of a pluralityoi trunks each .havingtip, ring, and sleeve terminals,- a selector switch: forprogresr sively. movingover. said sleeveterm-inalsand stop-- pingzwhen it reachesanmngrounded sleeve terminal',, a. main relay connected to.- a predetertminedflsleeve terminal, saidmainrelay; being op eratedwhen. said selector switch reaches said predetermined sleeve terminal, a testing circuit for. simultaneously testing all ofthe-sleeve terminalstosdetermine the number of ling-rounded 202 dicating. the number ofidle trunks in all of said groups of trunks,-.and meansfor routing callsreceived-over said trunks overtheir normal routes when more than a predetermined number of trunksarefound-idle and' for changing the routing of saidlcallswhen saidpredeterminednumber or less than said predetermined number of trunks are-found idle.-

13.. In a telephone system; thecom-binationof a. plurality of trunks arranged in groups; atesting circuit capable of: simultaneously testing, all of thetrunks in anyone group to determine the number of idle trunks; therein, a plurality ofj-reroute -relays one corresponding: to each group/0f trunks, means for successivelyconnecting said: testing-circuit tosaidgroups of trunks and-,for operating the corresponding re-route relays: when the number: of.- busy trunks in the respective groups. exceeds a predetermined; number;

14.- Imatelephone system; the combination .ofza, plurality; of groupsof trunks, a plurality of:'re.- route relays one'associated-with'each groupxofi trunks, a testingci-rcuit capable of; testingrall of? the trunks-,iin' any, one group-to determine: the: number, of: idle trunk-s.- therein; a rotary selector for successively connectingsaid testing, circuit;

sleeve: terminals, saiclitesting, circuit including a-.'

plurality of relays each of which has a winding of high resistance. which is connected to. one of the sleeveterminals, saidresistancebeing=- of a magnitude which. willnot. substantially change the potential of, said sleeve terminal, and means controlled byisaiditesting circuit to hold said. main relay operated when the u-ngroundedsleeve terminals are less than a predetermined numberandforreleasing said main relay, when said predeterminednumber, of fungroun-deds sleeve termimale is equaled .or. exceeded,

12., Inatelephone system, acombination of a plurality of trunks arranged ingroups, a testing circuit capable of,simultaneouslytestingall of thetrunks in any. onee group to determine the? number, of, idle trunksltherein, means for successively connectingsaid testing. ,circuitto the dif.

ferent groups oftrunks, meanssfor-additi-vely into-the different groups of trunks; means controlledby there-route relay correspondingto any trunk groupto operate the-testing.,circuit and: connect it to-the corresponding trunkgroup, and means. for controlling the operating 0f the: reroute relay; correspondingvv toa-anyutrunk groupin accordance; with the numbenoi .busy trunksregistered by the testing circuit atsaid-trunk:group.v

l5-.:.Inatelephonesystem, the combination of: a. plurality of:trunks over which calls may be transmitted; a plurality of. testing relays each corresponding, tOrOlZB ofsaid trunks; means for simultaneously connecting said :testing;v relays. to the correspondingwtrunks, a battery in. common. with all; of; said testingrelays, thetwindings of said testing relays: being of. high resistance, so

that the potential applied to the terminals of the trunks will not be substantially changed by the connection. thereto of said testing. relays, each testingrelay being energized when its correspondingfltrunk. becomes, busy,.a re-route relay, and; means foroperating said're-route relay when a predetermined number of said testingrelays are simultaneously operated:

16-. In a-- telephone system, the combination of a plurality of groups of trunks, a plurality of'reroute relays, a plurality of cut-inrel-ays; a testing circuit having" a' plurality of testingrelays equal in number to the number-of trunks in'any one group, oneof the re-routerelays-and oneofthe cut-in relays being associated-witheach group" of trunks; each-cut-in'relay interconnecting the trunks of any one groupwith-the"corresponding* testingrelays-of-the-testing circuit, and means responsiveto the simultaneous operation of. a predetermined numberof testing relays connectedto-tlie trunks of-any one group to operate the corresponding re-route relay and to release said re-route relay when the number of testing relays simultaneouslyoperated fall-s below-said predetermined number.

1 7; Inatelephone system, the combination of arpluralityotlgroups oftrunks, aplurality of reroute. relays: a: plurality of'cu-t in relays, a test ing. circuitzthaving a. plurality of testing relays equal. in'znumber to the number of trunks-in any one;.group,..one. reeroute. relay. and one-cut-i-n relay being associated with each group 'ot 4 trunks, each :re;-route.relayacontrolling the'routing of thetrunks of the associated group, a, rotary selector, and means controlled by each re-route relay for stepping said rotary selector and for connecting the trunks of each group to the relays of said testing circuit through the contacts of the corresponding cut-in relay, said re-route relay being also controlled by said testing circuit.

18. In a telephone system, the combination of a plurality of groups of trunks each having a sleeve terminal, a plurality of re-route relays one corresponding to each trunk group and being connected to a predetermined sleeve of the trunk group, a selector which may be progressively moved over the sleeve terminals of the trunks of a group, said re-route relay being operated when said selector reaches the sleeve terminal to which the re-route relay is connected, means controlled by said re-route relay for registering the number of idle trunks in the corresponding trunk group, and means for changing the route of said trunks when the registered number of idle trunks is less than a predetermined number,

19 In a telephone system, the combination of a plurality of groups of trunks each having a sleeve terminal, a plurality of re-route reiays one corresponding to each trunk group and being connected to but one of the sleeve terminals of the corresponding trunk group, a plurality of se lectors each capable of moving progressively over the sleeves of the corresponding trunk groups, each re-route relay being operated when the corresponding selector reaches the sleeve terminal to which the corresponding re-route relay is con-- nected, a testing circuit, and means controlled by the said re-route relays for connecting said testing circuit to the various trunk groups for determining the number of idle trunks in said trunk groups.

20. A claim according to claim 14 in which there is additional means for changing the routing of the trunks of any group when the idle trunks as registered by said testing circuit is less than a predetermined number.

ROBERT CAMPBELL AVERY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,030,412 tearn Feb. 11, 1936 1,674,411 Ostline June 19, 1928

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