US2201986A

US2201986A - Telephone system

Info

Publication number: US2201986A
Application number: US214351A
Authority: US
Inventors: Raymond E Collis
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1938-06-17
Filing date: 1938-06-17
Publication date: 1940-05-28
Anticipated expiration: 1957-05-28

Description

May Z8, 1940- R. E. coLLls l TELEPHONE SYSTEM 4 Sheets-Sheet 1 R. E. COLLIS TELEPHONE SYSTEM May 28, 1940.

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ATTORNEY May 28, 1940@ R. E. coLLxs 2,201,986

TELEPHONE SYSTEM Filed Jne 1'7, 1938 4 SheetlS-Sheet 3 //V RECOLL/S R. E. COLLIS TELEPHONE SYSTEM May 28, 1940.

Filed June 17, 1938 4 Sheets-Sheet 4 j A TTOR/VEV Patented May 28, 1940 UNITED STATES PATENT OFFICE TELEPHONE SYSTEM npplicanon June 17, 193s, serial No. 214,351

8 Claims.

rIhis invention relates to telephone systems and particularly to those employing automatic switching equipment for establishing connections.

The objects .are to increase the accessibility of trunk groups; to obtain a more uniform distribution of the load over the trunks of a group; and in other respects to enable a more efficient use of the trunk groups and of other parts of the system. 10 It has been proposed in the past, where large trunk groups are involved, to subdivide the trunks into smaller groups and to test these subgroups successively until an idle trunk is found. If, however, the total group contains a large number of trunks, the `time required to test all of the several subgroups may be considerable, and consequently other calls for the same group may be required to wait correspondingly.

According to a feature of the present invention it is possible to make eicient use of a large trunk group by dividing it into a plurality of first-choice subgroups and a single second-choice subgroup and providing means whereby each call for the group causes a preferred one of the first-choice subgroups to be tested for an idle trunk, followed by a test of the second-choice subgroups in case the first-choice subgroup is busy. Following each call for one of the trunks in a group, the

preference is shifted from one first-choice subgroup to the next, so that each new call causes a different one of the rst-choice subgroups to be tested, followed always by a test of the single second-choice subgroup in case no idle trunk is found in the preferred subgroup. This arrangement makes it possible to utilize the trunk groups and the controlling equipment to best advantage. By repeatedly shifting the preferred first-choice subgroup, each new call coming in has a favorable chance of finding an idle trunk in the preferred subgroup, and, if all trunks of the preferred subgroup are busy, the probability is even greater than an idle trunk, if there is one anywhere in the entire group, will then be found in the second-choice subgroup. By omitting to test successive subgroups after having tested the preferred subgroup, the holding time of the equipment is reduced considerably, and the waiting time of other calls is not increased unduly.

Another feature of the invention is a system in which a plurality of switch-controlling mechanisms, such as markers, have common access to the trunk groups, in which each of the markers is arranged to test first the preferred subgroup and then the second-choice subgroup on each particular call, and in which each marker autotion:

matically shifts the preference so that when it receives the next call for the same or another trunk group, it will test the succeeding rstchoice subgroup and then the common secondchoice subgroup. 5

The foregoing and other features of the invention will be described more fully in the following detailed specification.

In the drawings accompanying this specifica- Figs. 1 to 4, when arranged in the order illustrated in Fig. 5, disclose a portion of a central omce telephone system.

Fig. 1 illustrates diagrammatically the subscribers line and automatic switches for extend- 15 ing the line to outgoing trunk circuits. This figure also illustrates a central office sender and certain connecting mechanism.

Figs. 2, 3 and 4 disclose a part of a switchf.

controlling marker. 20

Fig. 2 illustrates diagrammatically the trunk testing mechanism of the marker and showsin detail a stepping or allotting circuit for shifting the preference from one subgroup of trunks to another; 25

Fig. 3 shows a grouping circuit, controlled by the allotting circuit of Fig. 2, for shifting certain control conductors in accordance with the allotment of the trunk subgroups; and

Fig. 4 shows certain control relays and circuits 30 cooperating with the circuits of Fig. 3.

In the system illustrated herein the invention is applied to one of the switch-controlling markers which are used particularly for controlling switches of the cross-bar type. The sub- 35` scribers lines, such as line wil, in a system of this kind appear in frames of cross-bar line switches. One of these frames llll is illustrated', comprising a number of primary cross-bar switches 102 and a number of secondary cross-bar 40 switches H13. The line switches have access by way of district trunk circuits H14 and H15 to frames of district selector switches. One of these frames H16 is illustrated, comprising primary cross-bar switches I and secondary cross-bar 45 switches 108. The district frames in turn have access to oce selector frames, such as frame |89. The ofce frame lil@ comprises a plurality of primary cross-bar switches H0 and secondary cross-bar switches Ill, H2, H3, H4. 50

The outgoing trunk groups over which calling lines are extended, either to terminating equipment in the same exchange or to distant exchanges, appear in the terminals of the secondary oiiice selector switches. These trunk groups are 55 divided into a number of subgroups, and the switch-controlling markers, one of which is illustrated in Figs. 2, 3 and 4, are provided with testing mechanism for making a test of the trunks of the subgroups to determine an idle one.

The calling subscribers lines also have access by way of the district selector trunks itil, M5 to frames of sender selector switches. One of these frames hlt, comprising primary cross-bar switches H5 and secondary cross-bar switches IIE, is illustrated. The sender selector switches serve to extend the calling lilies automatically to idle senders, such as the sender il?. The senders in turn have access to the markers by way of marker connector mechanisms H3.

Since the marker serves to test the trunks of the desired outgoing group to locate an idle one and also causes the selective operation of the district and oiice selector switches, it is necessary to provide connecting means for associating the marker with the outgoing trunk groups and with the district and ofce selector frames. This is effected by connecting devices, such as the frame connector H9 shown partially in detail in Fig. 1.

l/Vhen a calling subscriber removes iis receiver to make a call, his line it is automatically extended by the line switches it! and i533 to an idle district trunk it terminating in primary district switch lill and also appearing in the sender selector fraL e it. The initiation of the call also causes the operation of the sender selector switches to extend the calling line through to an idle register sender il?, The subscriber dials the wanted number, which is recorded on the. sender l l. Thereupon the sender causes the operation of a marker connector H8 to connect the sender to an idle one of the switch-controlling markers. The recorded information in the sender is transferred to the registers of the marker, and the marker makes us-e of this information to determine which one of the trunk groups is to be used in extending the subscribers call.

Assume that the desired trunk group outgoing from the oice switches subdivided into four subgroups. Assume also that the irst subgroup |20 is represented by the trunks iEl and 22; that the second subgroup l23 is represented 'o trunks i255 and it; that the third subgroup E2G is represented by trunks 22? and E22; and that the fourth subgroup 129 is represented by trunks |30 and ESE. These subgroups may comprise any convenient number of trunks, and they may be distributed in any suitable manner over the secondary switches of the office frames. As soon as the marker determines the desired trunk group, it causes the operation of the frame connector H9 to connect the marker testing mechanism 2.4i! to the test conductors of the particular one of the subgroups that happens to be preferred at the time. After an idle trunk has been found the marker will proceed to test the link circuits of the district and ofce frames Hit and 38 to extend the district trunk circuit Mld through to the chosen idle outgoing trunk. The marker is then released, and, if the sender has not already been released, it will restore to normal as soon as it has completed its further functions in connection with the extension of the call.

Inasmuch as the invention relates particularly to the marker, 'much of the entire system has been omitted, and only those parte of the marker itself are shown which are necessary to an understanding of' the invention. In the following descrptions reference will be made to numerous operations which are not fully disclosed in all ol their details. Wherever such references occur, it will be understood that the circuits and equipment so referred to are already known, and in particular reference is made hereby to the following for a detailed description of all operations and equipment referred to herein but not specilically disclosed: Carpenter Patent 2,093,117, September 14, 1937; and Carpenter application Serial No. 214,356, iiled June 17, 193

The cross-bar switches employed in this system may be of any suitable ty 4e, s' ch as the one dis-- closed in the patent to Reynolds, No. 2,021,329 of November 19, 1935.

The marker is provided with a relay P9 that operates and releases each time the marker is taken in use to serve a call. Also the marker is equipped with four pairs of control relays 213 and 2M, and 2%, 2E? and 2li), 255i and 220, the function of which is to connect ground consecutively, after each call to each of the conductors 2M, 263i, 292, 263, 2l 255, 2%, 28?, 223, 2&9, 2i@ and 2li in the order named, thereby causing the consecutive operatic respectively, of the grouping relays S, 35i, 352, 334, 385, 336, iii'l, 398, 359, 3m and 3H. One of the control relays, 2i@ or 2id, responds to each operation each release of relay 239, and these relays in turn control the functioning of the other control relays, as will be hereinafter described. The grouping relays are connected as shown to provide for three rst-choice subgroups of trunks. This is obtained by connecting in multiple the armatures of relays 3M, 305 and 359, relays 36E, SM, 361 and 3H), and relays 3751395, 35S and 3H. Connections for the three subgroups are brought out over conductors 3 l 3l 3, Elfi and l 5; 315, 351,358 and SIS; and 324, 325, 32S and 32?. Two of the conductors of each subgroup extend as shown to the frame connector H9, and the other two conductors to the testing mechanism 2d@ of the marker.

Conductors

321, 222, 323 are energized through contacts of the ground supply relay i512 and route relay tti. These ccnductors are lnultipled, as indicated, to the make contacts of the grouping relays 39@ to fill, inelusive, and the contacts of route relay lll?! are multipled, as indicated, to cor esponding contacts of' other route relays, not shown, which are associated with relay 492.

With this arrangement, when relay 358 operates in response to the engagement of the marker for a call, conductors 320, $52!, 322, 323 are connected respectively to conductors 3l2, SH3, 3M', 395, to establish the test for the first subgroup of trunks 525i. In like manner, when relays Bill and 392 operate. the test connections for the second and third subgroups respectively are established. When relays 383, 335 and 399 operate subsecuently, a test connection for the first subgroup is again made. Similarly, relays 39?, BH1 and 353, 3l! operate te again. make connection to the second and third subgroups. With this arrangement the control conductors for the same subgroup of trunks are connected at the operation of every fourth relay. With total of twelve grouping relays, three subgroups of trunks may be connected to the grouping circuit as shown. It is to be understood that the grouping relays may be connected also to provide for grouping a different number of subgroups, as for example, four, six or twelve subgroups. For four subgroups, the make contacts of every third relay would be multipled; for six subgroups, the contacts of every second relay; for twelve subgroups none of the make conand 402.

tacts would be multipled, each set of contacts controlling a different subgroup of trunks.

The trunk testing circuit Efiii is arranged to test a maximum of forty trunks, twenty trunks appearing on one oflice frame of a pair and twenty trunks on the other frame. Assuming that the grouping relays of Fig. 3 are multipled to provide for twelve first-choice subgroups, the maximum number of trunks is 480. With a maximum of orty trunks in an associated second choice subgroup the maximum total number of trunks for a group of trunks extending to a called destination is 520.

Relay 40| is the route relay corresponding to the first-choice subgroups of trunks which extend to the called oice. 1t operates after the oiice code is dialed and connects the four

control conductors

320, 32l, 322 and 323 tothe ground supply relay 462. One conductor is used to operate the proper pair of frame connectors, such as connector HS, to effect the connection of the marker to the pair of office frames on which is located the subgroup of trunks extending to the called oiice. Two other conductors are used to mark the beginning and the end of the trunk subgroup in relation to its connection with the trunk testing circuit 240. A fourth conductor is used for the operation of the trunk level relays associated with the frame connectors il@ of the pair of office frames upon which is located the subgroup of trunks extending to the called ofi'lce. These relays connect the marker test leads |32 to the test conductors associated with certain horizontal levels of the oflice switches. On these levels are located the subgroups of trunks leading to the called destination. To recapitulate, one conductor determines the frame location of the trunk subgroup, a second and third conductor the limits of the trunk subgroup, and a fourth conductor the horizontal levels to which the subgroup is connected, thereby completely determining the trunk subgroup.l

When all the trunks of a first-choice subgroup of trunks are found busy, trunk busy relay {lili} is operated, causing the operation of relays 4u? Relay 402 opens the circuit of the

control conductors

32D, 32l, 322 and 323 and effects the operation of the second-choice route relay 404. This relay connects conductors 498, 409, Ml) and 4l l, to determine the second-choice subgroup in the same manner as the iirst-choice subgroups were determined. In this case the control conductors are connected' directly to the ofce frame and trunk testing circuit without going through the grouping circuit of Fig. 3. If all trunks of the second-choice subgroup are busy then relay 406 is operated, disconnecting control conductors 403, Mill, lill and 4M and effecting the operation of a route relay li'f, which may be associated with an alternate route group, or with an overiiow group of trunks.

The operation of the system will not be described in detail. It should be stated here that any one of the relays 2l3, 2|4, 2id, 2&5, 2li', 2MB, 2|9 and 220 may, in the course of the regular functioning of the marker, be left operated after a call; for this description, however, it is assumed that all relays at first are normal. When relay 23B operates due to the markers engagement for a call, relay 2I3 operates in a circuit traced from battery through resistance Ml, winding and normally closed contacts of relay 2l3, and lower contacts of relay 23D to ground. Relay Zls locks to ground 2l2, on its make contacts. Relay 230 also, `with its upper contacts, connects ground to conductor 20B, through middle break contacts of relay 214, break contacts of relays 2 l5, 220 and 2I8. Relay 300 operates in this circuit and connects the group of conductors 329, including

conductors

32B, 32E, 322 and 323, to the group 3%, comprising conductors M2, M3, 314 and 3&5, respectively. Since the group 33t relates to the first subgroup of trunks, the selection of this group of conductors determines that the first subgroup of trunks l2ll are preferred for this call.

Route relay 4U! also operates, selectively, at this time in response to the oice code dialed at the calling station and connects conductors 325i through upper contacts of relay 46E to sources of batteryand ground as indicated. The multiple straps shown connected to contacts of relay 4M, indicate that other route relays that may be used for other trunk destinations are also connected to the upper break contacts of relay mi. The operation of route relay lill completes circuits to establish the testing connection for the first-choice subgroup of trunks i2@ that extends to the called office. One of these circuits extends from battery over conductor iii-i, contacts of relay 482 and 4M, conductor 32d, contact of relay Stil, conductor SI2, multi-contact relay its of frame connector H9 to ground and also through the corresponding relay of the mate frame connector. Another circuit leads from ground over conductor 4H, contacts of relays 4t2 and Mii, conductor 323, Contact of relay SSE?, conductor 3I5, contact of operated relay i3d, relay l35 to battery. A multiple of this circuit extends to the mate frame connector. The remaining two conductors 32i and 322 of group 329 are extended through relay Se@ and over conductors 3&3 and 384 to the testing mechanism 24H) to determine which of the testing relays thereof are needed to test the subgroup of trunks |20. With relays 34 and l35 0i connector H9 operated, the test leads i371 associated with subgroup Iii) are extended over conductors |32 to the marker testing mechanism 248. And the same is true of the other half of subgroup l that appears on the mate frame. If an idle trunk is found in subgroup im, the marker ccnnects itself tol district frame ill and operates the district and office switches lill, i538, lill, and lll to extend calling line lil@ to the selected idle trunk. The marker is then released as well as all other common equipment.

With the marker released, relays 49E, 350, and 230 are deenergized. vRelay 2i3, however, remains operated and relay 2 ld operates. The operating circuit for relay 224 may be traced from battery through resistance 222, winding of relay 2l4 and operated make contacts of relay 243 to ground. Before relay 23u released, ground on conductor 229, through lower outermost break contacts of relay 2M, kept this relay shunts-d. When relay 2|4 operates it closes a circuit for operating relay 2|5, which circuit may be traced from ground on the inner make contacts of relay 2M, through upper inner break contacts of relay 229, normally closed contacts and winding of relay 2l5, and resistance 223 to battery. Relay 2l5V locks to ground on the upper, outermost break contacts of relay 22E?. Relay 2lb does not operate now due to the shunt connection through its own break contacts. At the end of the iirst call, with relay 23E released, relay 35i! is released, and relays 2l3, 2M and 2l5 are operated.

- Assume now that the marker is again engaged on a second call. When relay 230 operates, it

connects ground with its contacts to conductor 229, thence through lower outermost make contacts of relay 2id. to the left terminal of resistance 22|, thereby shunting relay 2|3 which releases. Relay 2|li remains operated, however, in a circuit completed through the normally closed contacts of relay 2|3 to conductor 229. By reason of the continuity contacts of relay 2|3, the transfer from ground 2l2 to conductor 229 is made Without opening the circuit of relay 2|4. A circuit is now closed for grounding conductor 25| and operating relay 35|. This circuit is traced from ground through the upper make cont cts of relay 230, lower middle make contacts of relay 2i lowermost break contacts of relay 226, lower break contacts of relay EIB, lower break contacts of relay 225i to conductor 20|. Relay now operates, and, assuming that relay liti has functioned as before described, conductors 329 are now connected to the respective conductors 2K5, 2|?, 2|8 and 2|9 of group 33|, which prepare for testing the second first-choice subgroup of trunks |23. In this case a circuit is extended from battery on conductor Mii, over conductor 320, contacts of relay 30|, conductor Sie, thence to relays, such as relay |34 on the pair of oice frame connectors individual to the frames in which the subgroup |23 appears. As illustrated, this subgroup appears in frame |99 and its mate frame (not shown); hence relay |31? and the corresponding relay of the mate connector are operated. The circuit from ground over conductor lll?, however, now leads through contacts of relay 30|, conductor 3|9, thence through contacts of relay i3d (Fig. l), winding or' relay E36 to battery. With relays |36 and it` operated, test leads |38 associated with subgroup |23 are extended to the marker testing relays 22u. In due time the marker is released.

When this occurs, relay 236 releases for the second time, and relays 38| and 2|4 release. Relay if; in releasing removes the shunting ground for relay 2|@ which now operates and operates relay 2li in a circuit from battery through resistance 225, winding and normally closed contacts of relay 2 l'l, and upper inner make contacts of relay M to ground. Awaiting the operation of relay 23B for a third call, relays 2|3, 2|4, 2|8, 2|3, and 220, are normal, and relays 2|5, 2HE and 2H are operated.

When the marker is next seized and relay 23|) operates for the third time, relay 2| 3 operates as before described. A circuit is now closed for grounding conductor 222, which circuit may be traced from ground on the upper contacts of relay 23S, middle break contacts of relay 2|4, make contacts of relay 2 l 6, uppermost break contacts of relay 2i?, to conductor 202. Relay 302 operates and connects conductors 329 to conductors 3251, 325, 32% and 327 of group 332, thereby establishing a testing connection for the third first-choice subgroup of trunks, relay iH again operating as before described. In this instance circuits over conductors 324 and 32'! cause the operation of relays |34 and |39, respectively, to

associate the marker testing mechanism with subgroup |25.

When the marker is again released and relay 23S releases for the third time, relay 322 releases, relay 2i@ operates and closes a circuit for shunting relay 2i5. This circuit is traced from ground on the innermost contacts of relay 2M, upper inner break contacts of relay 220, make contacts of relay 2|@ to the lower winding terminal of relay 255. Awaiting the fourth operation of relay 239, relays 2|3, 2li, 2|6 and 2|? are operated, and relays 2|5, 2|8, 2 I9 and 22|] are normal. When the marker is next seized and relay 230 operates for the fourth time, relay 2|3 is shunted as before, and a circuit is closed for operating relay 3&3, which may be traced from the upper contacts of relay 232, through middle make contacts of relay Elfi, lower outer make contacts of relay 2.!6, and upper normally closed contacts 0f relay 2m, over conductor 223 to the Winding of relay 303. Relay 363 in operating, by reason of the multiple connection of its armatures with those of relay 3D3, establishes again a testing connection for the first subgroup of trunks |20. When relay 239 releases for the fourth time, relay 38S releases, relay 2M releases as before, relay 216 releases due to the removal of the holding ground on the inner contacts of relay 2| 4, and relay 2|8 operates due to the removal of the .shunting ground connected through the inner contacts of relay 2 iii and the lower middle break contacts of relay 2li?. Relay 2 i8 in operating closes a circuit for operating relay 2|llv which is traced from ground on the lower, innermost contacts of relay Zia, through normally closed contacts and winding of relay Zig, and resistance 227 to battery. Awaiting the fifth operation of relay 233, relays 213, 2M, 215, Zl and 220 are normal and relays 2|?, 2|8 and 2li) operated.

When relay 236 operates for the fth time, relay 2 3 operates as before and at the same time a circuit is closed for operating relay 304. This circuit is traced from ground on the contacts of relay 238, through middle break contacts of relay 2M, break contacts of relays 2|6 and 223, lower outermost make contacts of relay 28, over conductor 2M and through the winding of relay Slll to battery. Relay Bfi in operating, establishes the testing connection for the second subgroup of trunks |23, in the same manner as did relay 36|, due to the multiple connection of the armatures of the two relays. When relay 23 releases for the fifth time relay 2M. operates as before and closes a circuit for operating relay 255, which may be traced from ground on the innermost contacts of relay 2id through upper inner break contacts of relay 22, normally closed contacts and winding of relay 2|5, and resistance 223 to battery. Awaiting now the sixth operation of relay 230, relays 2|3, 2513, 2|E, 2|T, 2|8 and 2|9 are operated and relays 2HE and 226 are normal. When relay 23B operates for the sixth time, re lay 2|3 releases as before described, and a circuit is closed for operating relay 335, which circuit is traced from ground on the contacts of relay 230, through middle make contacts of relay 2M, lowerrnost break contacts of relay Zl, lower make and break contacts of relays 2|8 and 22D, respectively, over conductor 225 and through winding of relay to battery. Relay 305 operates and connects the third subgroup of trunks |26, as did its multipled relay 3D2. When relay 232 releases for the sixth time, relay 305 releases. Relay 2|4 releases as before described and causes the operation of relay ZES by removing the ground connected through the upper break contacts of relay 22D, and the break contacts of relay 2|5. Relay 2|B in operating causes the release of relay 2|?. The shunting circuit effecting this release may be traced from ground on the inner make contacts of relay 2| 6, through make contacts of relay ZES to the winding of relay 2H. Awaiting now the Seventh operation of relay 230, relays 2|5, 2| 6, 2| 8 and 2I9 are operated and relays 2I3, 2I4, 2II and 220 are normal. v

When relay 230 operates for the seventh time, it causes the operation of relay 2I3 as before described and closes a circuit for operating relay 308, which circuit may be traced from ground on the contacts of relay 230, through middle break contacts of relay 2M, make contacts of relay 2| 6, uppermost make contacts of relay 2l8, over conductor 226 and through the winding of relay 306 to battery. Relay 366 in operating connects conductors 330 associated with the first subgroup of trunks to conductors 329. When relay 23D releases for the seventh time, relay 306 releases, and relay 2M, as before described, operates. The operation of relay 2M connects a shunting ground to the winding of relay 2i 5, through the circuit before traced after the third release of relay 230. Relay 2I5 releases, and now relays 2I3, 2M, 2I6, 2I8 and 2|!! are operated and relays 2I5, 2H and 22D are released, awaiting the eighth operation of relay 230.

Relay 230 operating for the eighth time operates relay 301 in a circuit traced from ground on the contacts of relay 23E), middle make contacts of relay 2M, make contacts of relay 2I6, upper inner make contacts of relay 2I8, over conductor 207 and through the winding of relay 30'! to battery. Relay 301 functions to connect the associated conductors of the second subgroup of trunks. Relay 230 in operating also effects the release of relay 2I3 as before described. When relay 23E] releases for the eighth time, then relay ZILI releases as before and releases relay 2 lli. Relay 2 I6 in releasing releases relay 2 I8 by opening its holding circuit which extends from ground on the inner contacts of relay 2I6, through normally closed contacts of relay 2H, winding of relay ZIB and resistance 226 to battery. Relay 2 I8 in releasing removes a shunting ground from the winding of relay 22B, which operates. This ground extends from the lower, innermost contacts of relay ZIB through break contacts of relay 228 and resistance 223 to battery. Await ing the ninth operation of relay 230, only relays 2id and 22E) are now operated.

*When relay 232 operates for the ninth time, a circuit is closed for operating relay 308, which circuit may be traced from ground on the contacts of relay 230, through middle break contacts of relay 2M, break contacts of relay 216, lowermost make contacts of relay 220, break contacts of relay 2I8, over conductor 208 and through winding of relay 323 to battery. Relay 3&8 in operating functions the same as did relays 3s2 and 3135, connecting the control conductors of the third subgroup to Fig. 4. Relay 225i! in operating operates relay 2I3 as before, and in releasing for the ninth time releases relay 32B and effects the operation of relay 2id. Relay 2M in operating operates relay 2V! in a a circuit from ground on the inner make contacts of relay 2M, through upper inner make contacts of relay 229, normally closed contacts and winding of relay '2H and resistance 225 to battery. Awaiting the tenth operation of re lay 230 relays 253, 2M, 2|?, 2I9 and are operated and relays 2I5, 2l@ and 2l8 are released.

When relay 23@ now operates for the tenth time, a circuit is closed for operating relay 3M, which is traced from ground on the contacts of relay 23il,.through middle make contacts of re- `lay 2M, lowermost break contacts of relay 2I6,

lower break contacts of relay 2I8, lower make contacts of relay 220, over conductor 209 and through winding cf relay 3639 to battery. Relay 3DS in operating effects the necessary connections for the first subgroup of trunks. Relay 230 in operating causes relay 2I3 to release as before described, and when relay 23D releases for the tenth time relay 30B releases and, as hereinbefore described relay 2M again releases. The release of relay 2M causes relay 2I8 to operate, due to the removal of the shunting ground from its winding. Relay 2 I8 in operating causes relay 2l@ to release due to a shunting ground from the lower inner make contacts of relay 2 I8, through make contacts of relay 22!! to the winding of relay 2id. Awaiting now the eleventh operation of relay 2552, relays 2 I3, 2 I4, 2 I5, 2 I6 and 2m are normal, and relays 2H, 218 and 220 are operated.

When relay 23d operates for the eleventh time, ground on the make contacts of relay 230 is connected through middle break contacts o'f relay 2M, break contacts of relay 2I5, lowermost rmake contacts of relay 22d, lower make contacts of` relay 2m, over conductor 290 and through the winding of relay 350 to battery. Relay 3 I!! in operating connects the control conductors of the second subgroup of trunks to Fig. 4. Relay 236 in operating also operates relay 2I3 as before. When relay 232 releases for the eleventh time, relay 3l!! releases, and relayv 2M operates as before. Relay 2li! in operating shunts the winding of relay 2H, causing this relay to release. Awaiting now the twelfth operation of relay 23B, relays 2I3, 2I4, 2I8 and 22@ are operated and relays 2I5, ZIE, 2I'I and 2l9 are normal.

Relay 230 in operating for the twelfth time closes a circuit for operating relay 3H which may be traced from ground on the contacts of relay 23B, through middle make contacts of re lay 2M, lowermost break contacts of relay 2I6, lower make contacts of relays 2I8 and 220, over conductor 2H and through the winding of relay 3II to battery. Relay 3l! in operating connects the control conductors, associated with the third subgroup of trunks, to Fig. Il. Relay 230 in operating also shunts down relay 2I3. When relay 239 releases after its twelfth operation, relay 3H releases and also relay 2M. Relay 2l!! in releasing, releases relay 2I8 and relay 2I8 releases relay 22). All relays 2l3 to 22u inclusive are now normal, having completed one cycle of operation. When relay 23|! againA operates, another cycle, similar to that hereinbefore described begms.

With the arrangement just described it is seen how the various first-choice subgroups of trunks of any group are connected in rotation on successive calls. If all the calls in an oflice were handled by one marker and the relays 300 to 3II were used only for one group of trunks, then there would always be a consecutive use of the subgroups. Relays 300 to 3! l, however, may be provided with additional sets of four contacts, not shown, to handle subgroups of other trunk groups.

Assume now, using the same marker, that relay Sill? operates to connect subgroup No. 1 of a trunk group extending to an office A, and then on the next call relay 3M operates and connects subgroup No. 2 of another trunk group extending to another office B, using, as aforesaid, an additional set of four contacts. Under this condition on the next call relay 392 will operate and now if the call is to be directed to the o'ice A associated subgroup No. 3 will be connected, the connection of subgroup No. 2 being skipped for this cycle of operation. Subgroup No. 2 for oce A might also be skipped at this time, when another marker is connected to handle a call and its corresponding control circuit of Fig. 2 should be set by chance, to operate its relay 30 l, or some succeeding relay of the series. However, in the consecutive use of the various trunk subgroups, over a considerable period of time the number of times each subgroup is used will be equal approximately to the average use of all the subgroups.

The arrangement before mentioned, whereby, if the trunks of a first-choice subgroup are all found busy the marker immediately engages a single second-choice group of trunks extending to the called oflice, will now be described in detail.

Assuming now that the test of the trunks of a first-choice subgroup, which was made following the operation of the rst-choice route relay 0l indicates that all trunks of this subgroup were busy, then relay G is operated from ground on the contacts of relay 403. This latter relay operated when the marker found all trunks of the first-choice subgroup busy. The operation of relay 400 effects the release, through a circuit not shown, of the district link connection established for the aforesaid first-choice subgroup of trunks. Relay 400 also closes a circuit for operating relay 401, which circuit may be traced from ground on the contacts of relay 400, through lower inner contacts of first-choice route relay 40, lower ;break contacts of relay 402 and winding of relay 401 to battery. Relay 407 operates and locks through its lower contacts to ground on the contacts of relay 400. Relay 40'! operates relay 402, which locks to ground M3. The lower make contacts of relay llll'l are arranged to close before its upper contacts close, to insure that the operating circuit of relay 40T is not opened by relay 402 before the locking circuit of relay 0l is closed. The operation of relay 402 disconnects the four control conductors 329 which were used in establishing the testing arrangement for the rstchoice subgroup of trunks. Opening these conductors releases the connection to the pair of office switch frames and releases the group start and group end relays of the trunk testing circuit, not shown, which is thus restored to normal. At this time relay 403 releases which in turn releases relay 400, thereby releasing relay 401; this closes a circuit for operating the second-choice route relay 401i. This circuit may be traced from battery through the winding of relay 04, lower outer contacts of relays lill! and 402 to ground through the break contacts of relay Llill.

Relay 1305iextends conductors 400, 009, H0 and 4l I, which relate to the second-choice subgroup of trunks extending to the called oiiice, these conductors being extended independently of the grouping relays of Fig. 3. These conductors now engage the same pair of once frames as before,

' `or another pair of frames, upon which the trunks of the second-choice subgroup of trunks may be located. Also, the proper trunk level relays are operated and the limits of the trunk group are indicated by operating the associated group start and group end relays. One of the circuits thus closed leads from battery over conductor llll, contact of relays 000 and (5525, conductor 008, thence to the winding of frame connector relay ll, and to a similar relay on the mate connector. It should be noted that conductor 408 would lead to a different pair of frame connectors if the second-choice subgroup l29 were located on other frames. Another of the circuits closed by route relay 401i leads from ground on conductor M9, contacts of relays 406 and 404, conductor dll, contact of relay i3d, winding of relay llll to battery. Relays l34 and lill extend the test conductors |42 of the second-choice subgroup l2?) to the testing mechanism 240. If this secondchoice subgroup should also be found bus, then relays 403 and 400 operate again and effect the operation of relay 505 through the lower inner make contacts of relay 404 and the lower break contacts of relay @06. Relay 405 operates relay 406, thereby opening the circuit establishcd for

conductors

400, 409, H0 and llll, and locks to ground on the contacts of relay 000. Relay 60S locks to ground 4M. When relay 400 finally releases, relay 405 releases and then a circuit is closed for relay M2, which circuit may be traced from battery through the winding of relay M2, lower outer make contacts of relays 40d and 405 to ground on the break contacts of relay 405. Relay lll is a route relay, with contacts not shown, which is used to establish connection to an alternate route of trunks or to an overflow route, if the alternate route is not provided. The additional relays necessary for the alternate and overfiow groups of trunks are not shown. When these groups are subdivided the four control conductors extending from the contacts of each corresponding route relay are carried through additional contacts, not shown, of the grouping relays of Fig. 3 so as to equalize the use of these subgroups in the same manner as the use of the rst-choice subgroups was equalized.

What is claimed is:

l. In combination, trunk groups, each of which is divided into a plurality of rst-choice subgroups and one second-choice subgroup, a marker, means effective when a call is made lor one of said groups for causing the marker to test rst the trunks of a preferred first-choice subgroup and then the second-choice subgroup to find an idle one for use, and means in the marker for changing the preference from one rst-choice subgroup to another each time the marker is used.

2. In combination, trunk groups, each of which is divided into a plurality of first-choice subgroups and one second-choice subgroup, a marker, means effective when a call is made for one of said groups for causing the marker to test first the trunks of a preferred first-choice subgroup and then the second-choice subgroup to find an idle one for use, and a sequence circuit in the marker operative each time the marker is taken in use for advancing the preference from one rst-choice subgroup to the next.

3. In combination, trunk groups, each of which is divided into a plurality of first-choice subgroups and one second-choice subgroup, a plurality of markers, any one of which may serve to extend calls to any of said groups, means effective when a marker is seized for a call in one of Said groups for causing the marker to test a particular one of the first-choice subgroups in preference to the others, and means in each marker operative each time the marker is used to shift the preference from one to an- P other of the first-choice subgroups in each group.

In combination, trunk groups, each of which is divided into a plurality of first-choice subgroups and one second-choice subgroup, means effective when a call is made for one of said groups for testing the trunks of a particular one of the first-choice subgroups in preference to other irst-choice subgroups, means effective to shift successively the preference from one to another of the first-choice subgroups of a group, and means operative if all the trunks of a preferred rst-choice subgroup are found busy for causing a test to be made of the second-choice subgroup.

5. In combination, a group of trunks divided into a plurality of first-choice subgroups and a second-choice subgroup, a controlling device, means for seizing the controlling device for making calls in the trunks of said group, means responsive to a call for said group of trunks for connecting the controlling device to one of said first-choice subgroups in preference to others for making a test of the trunks in said preferred subgroup, means for causing the controlling device to release from the rst-choice subgroup if all trunks are busy and for connecting it to said second-choice subgroup to test the trunks therein, and means operative each time the controlling device is taken in use for shifting the preference from one to another of said first-choice subgroups.

6. In combination, trunk groups, each of which is divided into a plurality of first-choice subgroups and one second-choice subgroup, the number of rst-choice subgroups differing for different groups, means effective when a call is made for any one of said groups for testing a preferred first-choice subgroup, means operative eac'h time a call is made for any group for shifting the preference from one first-choice subgroup to the next, and means eiective if all trunks of the preferred first-choice subgroup are busy for causing a test to be made of the corresponding second-choice subgroup.

'7. The combination in a telephone system of automatic switches, trunk groups appearing in said switches, each group being divided into a plurality of first-choice subgroups and a secondchoice subgroup, a marker for controlling the selective operation of said switches to extend calls over said trunks, register means in said marker for receiving and registering the designation of a desired one of said trunk groups, means effective when the designation of one of said groups is registered in the marker for causing the marker to test first the trunks of a preferred first-choice subgroup and then the trunks of the second-choice subgroup to find an idle one for use, and means in the marker for changing the preference from one rst-choice subgroup to another each time the marker is used.

8. The combination in a telephone system of automatic switches, trunk groups appearing in the terminals of said switches, each group being divided into a series of first-choice subgroups and a single second-choice subgroup, a marker common to said switches for controlling the selective operation thereof to extend calls over said trunks, means for transmitting and registering in said marker the designation of a desired group of said trunks, means effective when the designation of one of said groups is registered in said marker for causing the marker to test the trunks of a preferred first-choice subgroup and to select RAYMOND E. COLLIS.