US2443945A - Revertive ringing automatic telephone system - Google Patents

Description

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REVERTIVE RINGING AUTOMATIC TELEPHONE SYSTEM original Filed Nov. 2o, 1944A 1o sheets-smi 1o upa 7a4 700 aol 70o .900 C 709 CTB gram rate classes of apparatus by cables carrying the utilized in establishing the def Patented June A22, 1948 ItEvaItTIvI; RINGING AUTOMATIC -TELEPIIQNE sYsTEM- v John I. Bellamy, Brookfield, Ill., assigner to logg Switchboard and Supply Company,

Kel.- Chicago, Ill., a corporation of Illinois Original application November 20, 1944, Serial No. 564,293; vDivided and this application ctober 26, 1945, Serial'No. 624,784

I4 claims-L.l (ci. 179-11) verting call is handled by transmitting ringing current back to the calling line, from the connector in use, rather than by making a connection forward to the calling line as a called line, which arrangement is of particular utility in a trunking system of the disclosed type employing a plurality of switching devices in tandem to extend connection from a connector to a called line.

Other objects and features will appear hereinafter.

This application is a divisionof my application for Automatic telephone systems, Serial No. 564,293,'illcd November` 20, 1944.

GENERAL DESCRIPTION It has been chosen to illustrate the invention as applied to a. unit serving yone-hundred subscriber lines (including individual lines and tenparty lines) and employing switching equipment and control and trunkingr arrangements generally as disclosed in my prior Patent 2,354,660, issued August 1, 1944. The switches employed in the line unit disclosed in this application are assumed however to be of the improved construction disclosed in my (so-pending application for Automatic telephone switches, Serial No. 524,816, med March 1, 1944. l

The drawings Ofthc accompanying drawings,

Figs. 1 to 5 are circuit diagrams showing suf iicient of jthe circuit arrangements of the lmproved U-line unit to enable them to be understood;

Fig. 6 is a combined trunking and cabling diashowing the interconnection of the sepatrunks and links .aired connections to and from the lines served by the unit;

. Fig. 7 is a fron view of the improved 10D-line unit, showing the relative locations of the various lll . the finder trunk and the ondary 2 pieces of .apparatus mounted on the front face thereof Fig. 8 is a rear view of the unit showing the relative locations of the several pieces of apparatus mounted on the rear face thereof; and

Fig. 9 is a layout showing the way in which the sheets on which Figs: 1 to 5 are drawn should be assembled for a ready understanding thereof.

Figure 1 Fig. l shows one of the one hundred lines (#I I)` served by the unit, together with the individual line circuit of such line and switching apparatus which may be employed as line-finding apparatus to extend such line, when calling, over a certain one of several paths to a inder trunk FTI. Such nder trunk extends directly to a connector trunk CTI when the unit disclosed serves as the complete exchange. When such unit serves as :one unit of a larger exchange employing selectors, the direct connection between connector trunk is removed, and the finder trunk is extended by way of ldotted conductors |08` to a selector, while the connector trunk is then reached by way of dotted conductors |09 incoming from the selectors.

The individual line circuit of the illustrated line #Il includes line relay IOI, lockout relay 02, and cutoff relay ID3.

Primary switch IA is one of fifty line primary switches IA to 0E shown in trunking diagram in Fig. 6, and shown locationally in Fig. 7. It is one of ilve switches, IA to IE, serving the first 10- line subgroup, lines II to I0; Primary switch IA includes a hold magnet |04 and ten selective stackups Iof contacts, of which stackup I (through which the associated line #II reached) is one, the nine remaining selective stackups of the switch being omitted plicity.

Primary switch IA is accessible to line-secswitch A2, and others in the same secondary subgroup, by Way of the two-way line link LIA. Such link is reached through the illustrated stackup I of secondary A2, which has nine other selective stackups, each of which gives access to aline link extending to the corresponding one of the primary subgroups 2 to 0 of Fig. 6.

Fig. 1 shows also the first and last of ten subgroups of common equipment TGI and TGO. Subgroup TGI serves the rst ten-line subgroup (including the illustrated line,#II), While sub-'- group TGO serves the tenth subgroup of lines, including line #00, Fig. 3. The equipment in subgroup TGI includes subgroup lockout relay II I,

for sim,- i

' 3 which is operable as hereinafter explained to lock the associated ten 'lines temporarily out of finder service under certain conditions; connecting relay I2| operable only during line-finder action to -connect the ten local units leads I to of the first ten-line subgroup to the common units leads UI to U0; connecting relay |3I, which is operable both during line-finder action and during connector action to associate the five primary switches serving the associated ten-line subgroup with the line controller of Fig. 5, which controls the operation :of the'concerned switches during line-finder action and during connector action; and a meter M I which is eiecti've to record traffic data specific to the concerned ten-line subgroup.

At this pointv it may be noted that, for the convenience of manufacture and installation, as well as for the sake of compactness, the equipment illustrated in Fig. 1 as comprising tens subgroup TGI is mounted on a plate shown in Fig. 7 arranged to be mounted opposite the five primary switches IA to IE which serve the associated tenline subgroup, and on which plate are also mounted the three relays |0| to |03 comprising the line circuit :of the illustrated line #I I, as well as the nine remaining three-relay groups comprising the line circuits of the nine other lines of the associated ten-line subgroup. The arrangement of each of the remaining tens subg-roups TG2 to TGO is similar. y

Fig. 1 also shows the units-select magnets, being the select magnets which control the ve primary select shafts 109 (Fig. 7) serving the line primary group of switches. Only the rst two and last two (UMI and UM2, UMS and UMD) of the ten unitsselect magnets are illustrated in Fig. l. These ten magnets are controlled over ten units leads UI to U0, by ten units-preference relays UPI to UPU, respectively.

Fig. 1 also shows the first two and the last two 1 of the tens-select magnets TMI to TMO, which control the'iive select shafts H0 (Fig. 7) of the secondary group of switches. They are controlled respectively by tens-preference relays |3I to |40.

Figure 2 Fig. 2, parts 1 and 2, shows the connector CI, to which the connector trunk C'I'I extends, and from which the distributor trunk DTI extends to the distributor DRI of Fig. 3. Among other things, this connector includes equipment for supplying transmitter current to -the calling line, equipment for recording the tens, units, and party digits in the called number, "and equipment for supplying Aringing and transmitter current to the called line. The portion of connector CI shown in part 1 of Fig. 2 includes control relays 20| to 2|I and the so-called talking condensers 2|2 and 2 I3, while the portion of the connector CI shown in part 2 of Fig. 2 includes the tens, units, and party registers TR, UR, and PR.

. Of thev control relays oi' connector CI, relay is termed a timer relay, in that it cooperates with the common timerv 200 to release the connector CI at any time when such connector has been held for a predetermined interval (15 seconds, for example) during which no effective use is made thereof, such as dialing or conversation;

Relay 202 is a supervisory relay whose principal function is to reverse the direction of current ow over the incoming talking conductors when the call is answered;

Relay 203 is the line relay, being directly con- 4 trolled over the calling line in the manner common to connectors;

Relay 204` is the slow-restoring release relay, controlled by line relay 203 to prepare the connector for operation and to maintain the connection;

Relay 205 is the so-called back-bridge relay, being controlled directly over the called line when the call is answered;

Relay 20B is the so-called ringing relay, which is operated under the control of ringing interrupter 24| (Fig. 2, part 2) at the beginning ,of a ringing cycle to place the connector in ringing condition, and which is restored during the socalled silent interval to permit back-bridge relay 205 to respond when the call is answered;

Relay 201 is the so-called switching relay, being controlled from the line controller of Fig. 5 to effectuate the completion of the connection and to prepare for the ringing of the called line, in the event that the called line has been found to be idle and an idle connection path thereto exists;

Relay 200 is the so-called busy relay, being controlled from the line controller of Fig. 5 to place the connector in busy condition and return a. busy-tone signal to the calling line if the called line is busy or if for any reason a connection thereto must be denied;

Relay 209 is the so-called tens-transfer relay, being arranged to operate following the transmission and termination of the impulses constituting the tens digit to prepare for the reception of the units digit;

Relay 2|0 is the so-called units-transfer relay, in that it is arranged to operate upon the termination of the units digit to transfer the circuits into condition for the reception of the party digit;

Relay 2|| is the so-called chain relay, being included in the preference chain passing through the contacts of similar relays of the other connectors. It is arranged to operate at the termination of the party digit to temporarily indiidualize the connector CI with the line controller of Fig. 5.

Digit registers TR, UR, and PR (part 2 of Fig. 2), are controlled respectively over impulse conductors 223, 221|, and 225 to record the tens, units, and party digits in the called number. Each of these registers is preferably an electromagnetic counting device of the type disclosed in my copending application for Electromagnetic counting devices, Serial No. 493,312, led July 2, 1943, being illustrated as of the type shown in Figs. 21 to 25 thereof, wherein a. single control magnet is provided with a hold winding H, and with an impulse winding I effective to cause the counting contacts to close successively in a wave-like operation responsive to the successive impulses of a series.

Certain features residing in the connector CI of Fig. 2 not brought out hereinbefore include the following:

(1) Release relay 204. which must be slow-acting to enable it to remain operated during successive restorations of line relay 203 pursuant to impulse transmission from the calling line, is rendered capable of remaining operated with a much heavier spring load than it could otherwise carry by virtue of a self-locking circuit therefor through resistor 2|1, havinga resistance sufficiently low to carry substantial current during the time line relay 203 is restored, but insuilicient to maintain the relay operated after its momentary slow holding action (residing principally in the diagrammatically illustrated conducting sleeve underlying its windings) has subsided. Additionally, in

e order to secure a, powerful initial energization to give a fast operation, a second winding is provided on relay 204, which is disconnected by contacts 6 2|0 is arranged to be restored by back-bridge relay 205 to act as a ring-cutoff relay, to prevent further operation of ringing relay 206i`n the event that the called subscriber replaces his receiver ahead of the calling subscriber;

(4) Further relay economy is introduced by arranging that switching and busy relays 201 and 208, which are normally operated in the alterna tive, are both operated to place the connector in reverting-call condition, lwhen one subscriber on a party line calls another subscriber on the sam line;

(5) Tens-transfer relay 209 has a third use. After operating, rst as a tens-transfer relay, and second as a party-transfer relay, it is employed as a hang-up (or ring-start) relay during reverting calls, restoring uponthe replacement of the receiver on the concerned line to initiate the ringing operation;

(6) A still further feature is that, when a reverting call is made, no forward connection is established from the connector to the called line, ringing current being transmitted back to the calling line (which is then also the called line) over the connection established initially therefrom to the connector. This is accomplished by the cooperation of busy relay 200 and the tripleduty tens-transfer relay 209,. as will be explained' subsequently. This' arrangement makes unnecessary the withdrawing of a further secondary switch such as BI (Fig. 3) and a further primary switch such as 0B from service during the concerned interval, as well as insuring that a reverting-call connection will not fail because of a temporary all-busy condition of the terminating trunks or line links between the connector and the 'called line.

The equipment shown in the lower part of Fig. 2,' part 2, includes the common ringing interrupter 24|, uwhich controls the associated common leads L and PU and controls the common one-ring and two-ring relays 242 and 243. The particular system of party-line ringing illustrated in this application includes ve common ringing leads FI to F5 eachof which is supplied with a' detail hereinafter.

ringing relay 200, as will be described more' in This common arrangement avoids the necessity'of provisions individual respectively to the several connectors, often used to accomplish the same result.

Figure 3 Fig. 3 shows switching apparatus which may be employed to extend a connection, by connector action, from connector CI of Fig. 2 over a certain one of several paths to one of the one hundred lines (#00), as a called line.

The distributor DRI of Fig.' 3 is one of the thirteen distributors DRI to DRI'3 of Figs. 6 and 7. Each of these distributors is a three-wire, twenty-point switch of the type disclosed in my previously noted application,4 Serial No. 524,816, having a circuit arrangement as shown in Fig. 17 of such application. Each such switch has eleven selectively operable stackups of contacts, of which stackups 2 and are shown in Fig. 3. For se lecting any one of the first ten outlets of the distributor DRI, the corresponding one of its stackups I to I0 is operated without operation of stackup I I. When any one of the eleventh to twentieth outlets ofthe distributor DRI is to be selected, one or another of the stackups I to I0 is actuated, along with stackup I I. For selection of the stackup, or stackups, to be operated in one of the switches of the distributor group, the six selecting shafts 1|| of Fig. "I are provided. They vcorrespond respectively to the shaftsshown in Fig. 1 of the above-noted application 524,816) at S|`-2, S3-4, S56, S1-8, SSM-10, and S11.

The distributor DRI has access to secondary switch BI by wayof terminating trunk TT2; through its illustrated stackup 0, secondary switch BI has access to primary switch OB by way of the associated line-link LOB; and through its illustrated stackup Il, primary switch OB has access to line #00.

Line, lockout, and cutoff relays 30|, 302, and

303 of the line circuit associated with line #00 correspond respectively to relays |0| to |03 (Fig. 1).

Figure 4 Fig. 4 shows the twelve distributor select magnets DMI to DM2 which control the select shafts 1| (Fig. 7). Magnets DMI'to DM|0 correspond resepectively to the'stackups to |0 of any of the thirteen distributors DRI to DRI'S (Figs. 3, 6, and

, 7); magnet DMII corresponds to the eleventh separate frequency of ringing'current, as by separate generator such as 246.

On a ten-party line, the rst I iveparties are signalled by a single, or one-ring, application of 4 the respective frequencies Fi to F5, such application being made by the common one-ring relay 242 to the first ve generator leads G| to G5. Signalling of the last five parties on a IO-party line is accomplished by a two-ring application, by relay 243, of the respective frequencies FI to F5 to the last five generator leads G6 to G0. A feature of the ringing-interrupter arrangement is that resistors 244 and 245,' to which the ringing leads GI to G0 are connected except during actual ap-v plication of ringing current, serve to drain theI condensers on the called line to prevent a disagreeable discharge of such condensers into the calling line upon the subsequent restoration-of or switching stackup of any one of the distributors; and magnet DM|2 is an additional magnet employed to rotate the sixth selecting shaft of the distributor group in the opposite direction to that in which it is rotated by magnet DMI Magnet DM|2 is a magnet not disclosed in my prior application Serial No. 524,816. It may be located immediately to the right of magnet MII in Fig. 1 of such application, to cause the sixth selecting shaft to be rotated in the opposite direction at any time when selection of the eleventh stackup is not to occur. The utility of this arrangement is to secureinvariably the closure of the distributor off-normal contacts, DON I2, associated with the sixth shaft of the mechanism. Off-normal contacts DON I, 2; DON

' DON to cause such conductors' to be joined only (Serial No. V

7 when one or another of the rst iive shafts has been moved ofi-normal, along with the sixth shaft of the distributor group. This provision is utilized in the control of the circuit operations in a. manner to be pointed out subsequently.

Fig. 4 also shows the circuit arrangement of the units-sleeve connector USL` and the tens-sleeve connector TSC. These two sleeve connectors cooperate during the setting up of a connection from any connector to the called line to connect incoming conductors CLS in cable 552 to the sleeve conductor of the called line, such for example as conductor SI I, the sleeve conductor of line #I'I (Fig. 1) and conductor S00, the sleeve conductor of line (Fig. 3). The hold magnet of USC is shown at 40|, while the hold magnet of TSC is shown at 402. These magnets are operated over conductor S0 in cable 552 only when the connection being handled by the line controller of Fig. is one involving connector action, las distinguished fromone involving line-finder action.

The selective action of sleeve connectors USC and TSC is automatically controlled in accordance with the designation of the called iine, for sleeve connector USC is an additional switch in the line-primaryfgroup as shown in Fig. 7, and sleeve connector TSC is an additional switch in the line-secondary group. USC is controlled by select shafts '|09 of the primary group along with primary switches IA to 0E, while TSC is controlled by shafts 1|0 of the secondary group along with secondary switches AI to E5.

By providing a, direct and immediate test path from the line controller of Fig. 5 to the called line, the sleeve connectors make possible a more orderly and eflicient handling of connector calls, rendering it unnecessary to extend a connection through a distributor, and matched secondary and primary switches, to a called busy line; they enable reverting calls to be handled even though all forward paths be busy between the connector and the called line; and they enable the secondary and distributor switches to be reduced from fourwire switches to three-wire switches.

Fig. 4 also shows trailic meters 4| I to M9, which are controlled from the line controller of Fig. 5 to record the disposition of each call received by the line controller, according to whether the line controller is called in for i'lnder action or is called in for connector action, and according to whether the received call is disposed of normally or is lost because there is no available path for its completion. Of these meters, 4|| and M2 record the total calls; 4|3 and 4M record the finder calls; and 4|5 and 4|6 record the connector calls. The rst meter in each pair records the calls received in the concerned category, while the second meter of the pair records the number of such received calls as are lost.

Meters 4|1 to 4|9 are assigned to recording of lost calls, whether finder calls or connector calls, according to the existing traino condition resulting in the loss, as will be later described.

Figure 5 Fig. 5, consisting of parts l, 2, and 3, shows the line controller previously referredto. This line controller is temporarily called in for finder action by any ten-line subgroup containing a calling line. It makes the necessary tests, and determines which switches are to be operated to extend the calling line to an idle ilnder trunk. The line controller of Fig. 5 is also temporarily called in for connector action by any connector at which the called number has been completely dialed. It

then makes the necessary tests, and directs the extension of a connection to the called line.

The line controller is connected by conductors in cable 55| with the equipment in Fig. 1 and with the distributor-select magnets of Fig. 4; it is connected through the conductors in cable 552 with the sleeve connectors USC and TSC of Fig. 4; it is connected through conductors in cable 553 with the traic meters 4| I tc M9 of Fig. 4; it is connected through conductors in cable 554 with each connector, such as CI of Fig. 2; and it is connected, through conductors shown at the top of part 3 of Fig. 5, to each of the twenty-eight line secondary switches shown in Figs. 6 and "I, including secondary A2 of Fig. 1 and secondary BI of Fig. 3.

The line controller of Fig. 5 consists principally of forty-eight relays, 50| to 548. Of these relays, 50| and 502 are employed only for trafc-meter control, relay 50| operating on each call when there is an idle line link in the concerned ten-line subgroup, and relay 502 operating each time there is an idle trunk of the concerned category (finder trunk or terminating trunk).

Relays 503 to501 comprise the connector group, being rendered operative during each connector call, but remaining unaffected during finder calls. Of these relays, relay 503 tests the busy or idle condition of the called line, over the sleeve conductor thereof to which relay 503 is connected by sleeve connectorsUSC and TSC (Fig. 4) relay 504 is the so-called idle relay, operated by relay 503 when the called line is found idle; relay 505 is the so-called busy relay, being operated through back contacts of idle relay 504 each time the called line is found busy; relay 50B is the test-cutoff rel.,lay, operated each time relay 505. operates; and

relay 507 is the transfer relay. which operates in the connector chain to transfer the circuit arrangements of the line controller from finderaction conditon to connector-action condition.

Relay 508 is the start relay. It operates each time the line controller is called in, whether for finder action or for connector action.

Relays 509 to 5| I comprise a cycle timer, which is operated eachtime start relay 508 operates. It passes through a timed cycle of operations, which serves to clear out the line controller upon 'its completion, provided the line controller has not previously cleared out in normal manner.

Relays 5|2 and 5|3 are the clearout and operate relays. Relay 5|3 normally operates to swi-tc the call through. Relay 5|2 normally operates under the control of operate relay 5|3 to clear out the line controller upon the restoration of the latter relay, but operates alternatively from the associated cycle ltimer or from the connector group of relays under certain conditions.

Relays 5I4 to 5I9 comprise the choice allotter, which responds to each operation of start relay 508 to shift the choice to the next succeeding one of the secondary subgroups A to E. Relay 5|4 is a so-called driver relay, while relays 5|5 to 5|9 are arranged to operate successively in a counting operation as will be subsequently described.

Relays 520 to 525 comprise the matching group. Of these relays, 520 to 524 control the matching in secondary subgroups A to E respectively, subject in each case to there being an' idle secondary switch in such subgroup of the concerned category (finder action or connector action), and subject further to there being an idle primary switch in the concerned primary subgroup accessible from such secondary subgroup. Relay 525 is a so-called reserve-control 9 relay, which operates in a manner to be subsequently explained to permit the use ot the last idle secondaryswitch in a subgroup. of the category with which the current call is concerned.

Relays 525 to 530 are primary test relays, being controlled over leads PA to PE in cable 55|, which leads are connected by apparatus as disclosed in Fig. 1 to the respective primary switches A to E in the primary subgroup concerned in any call, nder action or connector action.

Relays 53| to 533 aretest-control relays. Relay 53| is operated on each finder call to connect the control leadsof the even-numbered secondary switches (assigned to ilnder action) in each secondary subgroup to respective ones of the secondary test. relays; relay 532 operates on each connector call to connect the control leads from the odd-numbered secondary switches (assigned to connector action) respectively to the secondary test relays; and relay 533 operates on each cali to disconnect test ground from the secondary test relays when matching (hereinafter described) has been effected.

Relays 534 to 548 are the secondary test relays, connections to which are controlled by relays 53| to 533. Three secondary test relays sufllce for each secondary subgroup, inasmuch as only three switches of a secondary subgroup are involved in any one call, nder action or connector action as the case may be. This arrangement efiects a considerable saving in secondary test relays, as will be appreciated.

Figure 6 In the combined trunking and cabling diagram shown in Fig- 6, it will be observed that the fifty line-primary switches are divided into ten subgroups of ilve switches each, subgroups 1 to 0 respectively. Each subgroup of primary switches serves a separate ten-line subgroup of the`100- line group served by the disclosed improved line unit. For example, lines AII to I areserved by primary subgroup I; lines 2| to 20 are served by primary subgroup 2^, and so fourth. The ten vertical lines shown extending across the l ive switches A to E of any primary subgroup represent rthe ten subscriber lines in the corresponding tenline subgroup.

As shown in Fig. 1, and in Fig. 3, the conductor pair representing any line may be attached to the appropriate-terminal contacts of the rst primary are reached from the switches comprising secondary subgroupE.

As noted, each of the primary switches, together with its associated line link, is employed both for finder action and for connector action. On the other hand, ea-ch of `the secondary switches is definitely assigned to either finder action or connectoraction. The assignment arrangement selected is such that the odd-numbered secondary switches in any subgroup are employed for connector action, while the even-numbered secondary switches are employed for finder action. The first five finder trunks FTI to FTS extend from secondary switch 2 in the respective primary subgroup of switches, one such line link for each switch. These line links are designated LIA to LIE for primary subgroup I; L2A to L2E for primary subgroup 2; and lso forth, to LOA to LIiE for primary subgroup 0. The usual primarysecondary spread is employed-between the primary and secondary switches. Accordingly, the ten links LIA to LOA, extending to the .A primary switches in the ten primary subgroups, are reached through secondary subgroup A; the ten line links LIB to LOB, extending to the B switches in the ten primary subgroups, are reached from secondary subgroup B; and so forth, to the ten line links LIE LOE, extending to the E switches in the ten primary subgroups, which subgroups A to E; the second five iinder trunks 'FP6 to FT|0 extend respectively from -secondary switch l in the respective secondary subgroups A to E; while the final three finder trunks FTI I to FTI3' extend` respectively from secondary switch 6 in subgroups A to C.

When the unit disclosed is the complete exchange, selector cables 606 and 601 are omitted,

and local cable 602 is employed to extend finder trunks FTI to FTI3 respectively to connector trunks CTI to CTI3 by way of terminals of connector trunk block CTB. But, when the unit disclosed is merely one of a number of similar units of the exchange, one or more stages of selectors are employed through which originated calls are routed to one or another of the line units according to the destination of the call. ln this event, local cable 602 is omitted, and selector cables 606 and 601 are installed. When used, cable 606 carries the thirteen finder trunks FTI to FTI3 to a corresponding number of selectors, while cable 601, when used, carries thirteen connector trunks, incoming from the selectors to the connectors by way of connector trunk block CTB.

Instead of a selector switching stage as disclosed in my previously noted Patent 2,354,660, the unit herein disclosed is particularly adapted to cooperate with an installation employing one or more selector switching stages of the improved type disclosed in my co-pending application for Selector switching systems, Serial No. 531,949, filed April 20, 1944, now Patent No. 2,400,530, issued May 21, 1946.

From the connector trunk block CTB, thirteen connector trunks CTI to CT|3 extend by way of cable 603 to connectors CI Vto' CI3, respectively, of which connector CI is shown in detail in Fig. 2. From the connectors CI to CI3, thirteen distributor trunks DTI to DTI3 extend, 4by way of cable 604, to distributors DRI to DRI3, respectively. As previously noted, each o the distributors is a three-wire twenty-point switch, having access to twenty three-wire outlets. Of the twenty outlets of the distributor group, numbers 1 to 15 are employed, numbers 16 to 20 being unused. n

The distributors have access in common to fifteen terminating trunks TTI to TTIE, extending by way of cable 605 to the respective odd- `switches 5 in subgroups A to E.

It is to be noted that fifteen terminating trunks TTI to TTIS are employed to handle the trailic passing through the thirteen connectorsCI to 11 CI3, and the thirteen distributors DRI to DRIB. Consequently, except when one or more distributor trunks may be temporarily withdrawn from service, as when the secondary switch to which a terminating trunk extends is being repaired, at least two terminating trunks remain idle at a time when-the maximum number (13) of connections are established through the connectors CI to CI3. By this arrangement, the likelihood of being able to lfind an idle path to the called line through one or another of the secondary subgroups `and the corresponding primary switch of the called subgroup is greatly increased with a minimum increase in equipment. The only increase in cost represented by an increase in terminating trunks is the cost ofthe added connector-action ysecondary switches, switches 5 in subgroups A and E in the illustrated arrangement. It is also clear that the ve unused outlets (16 to 20) of the distributor group can be extended to ve additional connector-action secondary switches if desired (one in each secondary subgroup) at a very small cost in equipment, if experience indicates that this is desirable. In this event, the five added connector-action secondary switches would be reached from the distributor group by way of terminating trunks TTIB to TT20 (not shown).

Calls extended to the terminating trunks are completed, through the operation of the secondary switches to which such terminating trunks extend, and thence to the respective called lines, by way of line links in cable 001 and primary switches in the concerned subgroups.

Figures 7 and 8 As previously noted, Fig. 7 shows a front view, and Fig. 8 shows a rear view, of an upright frame 100 on which the equipment and wiring comprising the disclosed 1D0-line unit are supported. Frame 100 is not shown in complete detail as its specific construction forms no part of this invention. It may be noted that in one physical embodiment of the unit, the frame 100 is of rectangular plan view having overall dimensions: 33 inches in width, 20 inches in depth, and 6 feet, 8V.; inches in height. Such a frame is conveniently constructed of angle iron sections welded together, with upright members as shown in Fig. '7A

providing three columnar spaces for the mounting of face equipment thereon.

The face equipment mounted in the left-hand column of Fig. '7 includes mounting plate 10|, on which the ten units-preference relays UPI to UPO (Fig. 1) are mounted, and ten similar tensgroup mounting plates TGI to TGO, each of which,

mounts the line-circuit relays and common equipment specific to a subgroup of ten lines.

The layout of the equipment on the first mounting plate TGI is shown in detail in Fig. '7. Six line-circuit relays are mounted yin each of the ve rows provided in plate TGI. The first three spaces in the uppermost roware occupied by linecircuit relays IOI to |03 of Fig. 1, associated with line #l of the first ten-line subgroup. The three line-circuit relays for the second line of the subgroup may be mounted in the three remaining spaces in the first row of plate TGI. The second, third, fourth, and fifth horizontal rows on plate TGI may car'ry the line-circuit relays for the remaining eight lines of the ten-line subgroup. The lowermost row of equipment on plate TGI comprises meter MI and relays |21, ISI, and

||I of Fig. 1.

The equipment mounted in the second columnar space in Fig. 'l comprises the primary group of switches, controlled by select shafts 108.

These shafts are. supported between upper andlower brackets 102 and 103, which carry the select magnets and off-normal contacts as disclosed in the previously noted application, Serial No. 524,816. The units sleeve connector USC of Fig.

-4 ls mounted as the flrst switch in the group immediately below upper shaft bracket 102, and is followed by the fifty line-primary switches IA to 0E.

It will be noted that the ten plates TGI to TGO are each mounted substantially opposite the five -primary switches which serve the associated tenline subgroup, permitting short direct connections from the bank multiplies to the respective line circuits, through uniform pre-formed cables.

The equipmentmounted in the third column in Fig. 7 includes the secondary group of switches, controlled by the five select shafts 1 I0, which extend between upper and lower shaft brackets 10d and 105. Tens sleeve connector TSC of Fig. 4 is mounted as the first switch in the secondary group, immediately below upper shaft bracket 106, and is followed by the twenty-eight secondary switches AI to E5.

Immediately below the group of secondary switches, lies the distributor group, controlled by the sixselecting shafts 1II, extending between upper and lower shaft brackets 106 and 101. The thirteen distributor switches DRI to DRI3 are mounted between these shaft brackets.

The remaining portion of the space in the third face-equipment column of Fig. 7 is occupied by fuse panel 108, which carries the required fuses, such as IIO (Fig. l), through which current is supplied from the ungrounded pole of the exchange battery to the various consuming circuits of the unit.

Connector-trunk block CTB of Fig. 1 is mounted behind the fuse panel 108, as is shown in dotted outline in Fig. rI. This block serves as a fixed terminal point for such conductors as extend from outside the unit to equipment mounted on gate 800.

The equipment mounted at the rear of the frame 100 is shown in Fig. 8. In order to give ready access to the wiring lying between the equipment mounted on the front of the frame (Fig. 7) and the equipment mounted on the rear (Fig. 8) the equipment at the rear of the frame is mounted on the above-noted gate 800, hinged at I and 802 to enable it to be swung out of the way when desired. Connections to the equipment mounted on the swinging gate 800 include the trunk conductors in cables 603 and 604 of Fig. 6, together with miscellaneous connections such as battery, ground, generator leads, and the like. All such connections are by way of exible cable conductors.

The uppermost space on gate 800 is occupied by the illustrated mountingplate LC extending entirely across the open space provided by the gate, and carrying principally relays 50I to 548 of the line controller shown in Fig. 5. Of these relays, 50l to 52| are mounted in a row along the top I' of plate LC; relays 522 to 542 are mounted in a row along the middle of plate LC; and relays 503i` to 548 are mounted in the rst six spaces along the bottom row on the plate. Immediately following relay 548, ringing interrupter relays 242 and '243 of Fig. 2 are mounted, and these relays are followed in turn by the nine traiiic meters MI to M9 of Fig. 4. l

The remaining equipment carried on gate 800 comprises the thirteeny connectors CI to CI3, of

-The frame '|00 as illustrated in Figs. 7. and 8 extends sufficiently to the front and to the rear to include within its confines all equipment mounted thereon. Such a. frame may be fully enclosed by top and sidecovers, and by front and rear doors, attached directly thereto.

It will be observed that the plates on which the equipment comprising connectors CI to C|3 are mounted fall short of taking up the entire width of gate 800. Where connectors are employed re-A quiring additional equipment, such as a. greater number of control relays, the length of the connector mounting plates may be increased as de sired, up to the full width of the gate.

It will be noted that the unit as disclosed in Figs. 7 and 8 makes no provision for the mounting of common'equipment such as timers |42 and 200, ringing interrupter 24|, generators such as 246, the current-supply battery, and its charging equipment. It will be understood, of course, that such equipment is ordinarily .provided at a separate location, as on a so-called power board.'

Where the exchange installation employs a number of units such as the one disclosed, and one or more groups of selectors, the power-board equipment is more or less common to all of the units and to the selectors, for which reason it would be an unnecessary duplication to provide space on a line unit for such equipment.

DETAILED DESCRIPTION A. Line #11 calls line #00 Operations involved in a call from line #11 as shown in Fig. 1 to line #00 as shown in Fig. 3 will now be described. Each of the illustrated lines #l1 and #00 is shown as a party line by the conventionally indicated common connections extending from the conductors thereof. One of a l possible ten substations on line #1l is shown at II5, and one substation on line is shown at Mil. To make a call, a subscriber at a substation such as I I on line #11 removes his receiver; waits for dial tone; and then dials the desired threedigit number, such as number 001.l l

A1. Marking the calling line f When the receiver (not shown) is removed at calling substation II5, the usual direct-current bridge is closed across the conductors of the calling line, operating line relay IOI through contacts of cutoff relay |03; Line relay |0I grounds the associated start conductor ST, common to the ten lines of the first subgroup, and it also grounds 14 the associated units conductor I individual to the calling line.

A2. Tens selection Assuming that the line controller of Fig. 5 is currently idle, the grounding of start conductor ST associated with tens subgroup TGI of Fig. 1, closes a circuit for tens subgroup relays .I2I and ISI and start relay 508 in series, as follows: from ground on start conductor ST in tens subgroup TGI, through contacts of relay III, the winding of relay |2I, lower winding of relay Ill, normally closed contacts of relay ISI, the chained innerlower contacts of the ten relays |40 to I3I. nderchain-in conductor F-CH--IN in cable 55|, chain contacts 4 of transfer relay 501, contacts 2 of clearout relay 5|2, andthence to battery through thewinding of start relay B08. Relays I 2|, III, and 508 operate in series overthe above-traced circuit. Start relay 508 starts the line controller into operation with results to be hereinafter described.

At its middle lower armature, relay I3I locks its lower winding directly to conductor F-CH--IN to maintain the above-traced circuit intact after the control chain is broken at its inner lower contacts. The chain-end conductor F-CH-END is now isolated to temporarily preclude operation of any/ further tens subgroup relays, or of relay 501'.

Relay |3I connects common control conductors PA to PE in cable 55| to control conductors A to E in tens subgroup TGI, being the respective control conductors of the primary switches IA to IE in the rst primary subgroup. This permits test relays 52E to 530 in the line controller to operate in accordance with the busy or idle condition of the respective concerned primary switches, as will be described hereinafter. Additionally, relay I3I connects the common meter conductor M to the meter MI pursuant to recordation of the disposition of the initiated nder call.

Finally, at its lower contacts, relay I3I closes a circuit for the associated tens magnet TMI, to effect tens selection in the secondary group of switches,y wherein the rst primary subgroupis thereby selected.

A3. Units selection When relay I2I operates over the above-traced circuit, it connects the ten units conductors I to IJ, associated respectively with the ten lines of the rst subgroup, to the lcommon units conductors UI to U0,. which extend respectively to unitspreference relays UPI to UPU. With units conductor I in tens subgroup TGI grounded,units preference relay UPI is operated over conductor UI through the illustrated preference chain circuit. Upon operating, relay UPI closes a locking tcircuit for itself independent of the remaining relays in the chain, and opens the preference chain to preclude operation of anyfurtherrelays thereof for the time being. -This arrangement prevents two or more units selections from being made in case two or more lines in the same tens subgroup are calling at the same time. At its upper contacts, relay UPI closes a circuit for units-select magnet UMI, causing units selection to be accomplished in the primary subgroup of switches, wherein line I of the calling subgroup is thereby selected.

A4. Tens and units selections locked 'I'he eiected tens and units selections are locked in, independent of the continued energiza- 15 tion of line relay |I, partly as an aid to the iurther operations, and partly to prevent false or partially completed nder actions from resulting from momentary bridging or grounding of al A5. Finder switch operation As soon as the described tens and'units selections have been accomplished, ground is extended to off-normal conductor PS-ON in cable 55| (through off-normal contacts SON I, 2 and PON I, 2) to inform the line controller of Fig. 5 tha-t tens and units selections have been made.

Assuming that primary switch IA and secondary switch A2 shown in Fig. 1 are both idle, these two switches may now be operated by the line controller of Fig. (in a manner to be hereinafter explained)l to extend calling line #II to ',flnder trunk FTI. The operating circuit for primary switch IA is from ground applied to conductor PA in cable 55| by the line controller, and thence through contacts of relay I3I, primary conductor A of subgroup TGI, make-busy contacts |05, and thence to battery through the winding of hold magnet |04 of primary switch IA. Units-select magnet UMI having been operated, the operation of hold magnet |04 results in the closure of the illustrated stackup of switch IA to extend the tip, ring, and sleeve conductors of the calling line to thecorresponding conductors of line link LIA. The initial operating ground potential for hold magnet |04 is n-ow applied, through the sleeve contacts of stackup I of primary switch IA, to sleeve conductor S of the multiple associated with the f calling line, thereby closing an operating circuit for cutoff relay |03 in series with lockout relay |02. Relays |03 and |02 accordingly operate, whereupon line relay IOI restores because it is disconnected by cutoff relay |03.

The associated conductors ST and I are thereby ungrounded, but this causes no immediate re-y sponse, for ground is maintained on each of these conductors (by relays |2| and UPI) by the abovedescribed selection-locking connections.

The ocrating circuit for hold magnet |06 of secondary switch A2 is as follows: from ground on the associated control conductor CA2 (grounded by the line controller as will be subsequently explained), through make-busy contacts |01, left-hand winding of hold magnet |05, sleeve conductor S of nder trunk FTI and connector trunk CTI, back contact 4 of release relay 204, make-busy contacts 2I4, and thence to battery through the associated resistor. Tens-select magnetY TMI having been operated, the operation of hold magnet |06 results in the closure of the illustrated stackup I of switch A2, to extend the conductors of line link LIA respectively to the tip, ring, and sleeve conductors of nder trunk FTI and connector trunk CTI. f

A moment later, holding ground is placed on the sleeve conductor of the established connection, by the connector (CI) providing a holding circuit for relays |02 and |03, and for magnets |04 and |06. Such holding circuit maintains the connection after the line controller clears out y current.

I2I, III, and UPI, and magnets TMI and UMI to restore. The holding circuit for secondary switch A2 is from ground on the sleeve conductor of the established connection, through the locking contacts of hold magnet |06 (the so-called offnormal-stackup of the switch), and the right` hand winding of magnet |06, to battery on lead B of the finder trunk FTI, suppliedy through fuse IIO.

A6. Preparing connector C1 for operation The connection has been extended as described, through primary switch IA and secondary switch A2, -to connector CI (Fig. 2). In the connector CI, line relay 203 (connected to the incoming tip and ring conductors, through normally closed contacts oi busy relay 208 and of supervisory relay 202) operates over the calling line and closes a circuit for both windings of release relay 204 in parallel. Current flow through these two windings provides a powerful initial energizatlon of the release relay, resulting in fast operation thereof.

Upon operating, relay 204 disconnects its upper winding at contact 6 to reduce the holding At its contacts 1, relay 204 applies ground potential to local conductor 2I6, thereby 'preparing locking circuits for relays 201 to 2I0,

closing a holding -circuit for registers TR and UR, and establishing an auxiliary circuit for its own lower winding, by way of resistor 2I1. At armain the manner to be hereinafter described. 'I'he ture 4, relay 204 disconnects the associated idleindicating battery-supply resistor and applies the above-noted holding ground potentialy to the incoming sleeve conductor S in trunk CTI, such potential being obtained through normally closed contacts of chain relay 2II and of timer relay At its contacts 8, release relay 204 prepares the impulse circuit for the digit registers, while at its contacts 3, it applies dial tone to the tip talking conductor from the common dial-tone lead DT, by way of contacts of relays 2I0, 209, 201, and 206, and condenser 2|5. The dial-tone signal, thus applied to the calling line, informs 4the calling subscriber that he may now dial the digits of the desired number.

A7. Setting tens register TR When the calling subscriber operates `the dial of his calling device (not shown) in accordance with the first digit of 4the desired number, line relay 203 is restored momentarily a number of times corresponding to the value of the digit. Release relay 204, being slow-restoring, remains operated during dialing. The previously noted auxiliary holding circuit, through resistor 2|1, is effective to assist the normal holding provisions of relay 204 in maintaining operated the comparatively heavy contact load.

Each time it :restores incidental to the dialing of a digit, line relay 203 applies ground potential, through contacts 8 of release relay 204, to impulse conductor 2|1. During the dialing of the rst digit, conductor 2|1 extends through contacts 9 of relay 201, contacts 8 and 1 of relays 2I0 and 209, to branch conductor 223, extending to .the impulse winding I of magnet 25| of the tens register TR. Tens register TR accordingly receives from one to ten impulses, depending upon the value of the first digit.

In the assumed example, the rst digit is 0, wherefore it is represented by ten impulses. Re-

' cording contacts I to 0 of register TR close succontact pair opening on the receipt of the next succeeding impulse. Accordingly. contacts of register TR are closed at the end of the nrst digit 0, contacts I to 9 having reopened successively.

Off-normal contacts ON of register TR close upon the delivery of the first impulse and remain closed until the register is cleared out. The clo'- sure of contacts ON of register 'I'Roccurs with line relay 203 in -restored condition during the delivery of the first impulse of the series. Each time line relay 203 reoperates thereafter during the dialing of the rst digit, a circuit is momentarily closed as follows for tens-transfer relay 209: from ground through the left-hand front contact of line relay 203, conductor 22|, off-normal contacts ON of register TR, conductor 221, normally closed contacts controlled by armature 6 of relay 209, contacts 6 of relay 2|0, and thence to battery through the winding of relay 209. Relay 209 is preferably slow-operating by virtue of the comparatively heavy spring load carried thereby, and by virtue of a comparatively inductive winding. As a result, relay 209 cannot operate during the delivery of series of impulses, for line relay 203 remains operated only momentarily between impulses.

When line relay 203 comes to rest in an operated condition at the conclusion of the dialing of the initial digit (0, in the assumed example), tens-transfer relay 209 operates over the abovetraced circuit. Upon so doing, it closes a local locking circuit for itself at its contacts 6, at the same time opening its initial circuit to free conductor 22| for further similar control. At its contacts I, relay 209 disconnects dial-tone lead DT so as to terminate the application of the dialtone signal at the termination of the dialing ofthe rst digit, in accordance with usual practice. At its armature 1, relay '209 disconnects impulse conductor 2|1 from branch conductor 223 of register TR and transfers it to the Asimilar conductor 224 extending to the register UR.

A8. Setting units register UR When the units digit is dialed, the impulses delivered to impulse conductor 2 |1 pass over branch conductor 224 (through armature 1 of relay 209 and its front contact), wherefore units register UR responds as noted in connection with tens register TR to record the units digiti).

When line relay 203 comes to rest in an operated condition at the termination of the dialing of the units digit, units-transfer relay 2|0 (similar to '203) operates over the following circuit: from ground on conductor 22|, off-normal contacts ON of units register UR, con-ductor 228, normally closed contacts controlled by armature 1 of relay 2|0, and thence to battery through the winding of relay 2|0. Upon operating, relay 2 l0, at its armature 1 and associated contacts, locks itself to local conductor 2||i independent of conductor '228; at armature 8, it disconnects impulse conductor 2|1 from units branch '224 and transfers it to party branch 225, extending to party register PR. At its contacts 9, relay, v2| 0 applies ground potential to holding conductor 22B local .to party register PR, register TR and UR being held over branch 222 of grounded conductor 2|6.

At its contacts 6, units-transfer relay 2|0 opencircuits and restores the locked tens-transfer relay '209, so as to enable the latter relay to be reoperated as a party-transfer relay.

A9. Setting the party register PR tor 225 to the party register PR whereat contacts from ground on conductor 22|, olf-normal contacts ON of party register PR, conductor 229, and thence to battery through the winding of relay 209. Upon reoperatingrelay 209 does not lock operated, as its locking circuit through its armature 6 and associated contacts has been interrupted at contacts 0 of relay 2 l0.

A10. Calling in the line controller The line controller of Fig. 5 is now called in responsive to the closing of a circuit for chain relay 2| of the connector through contacts 5 of transfer relays '209 and 2|0, subject to the line controller being currently in idle conditon. This cir- `cult is as follows: from ground, through contacts 5 and 8 of relays 201 and 203, contacts 5 of p relays 209 and 2|0, winding of chain relay 2|'|,

normally closed contacts controlled by armature 5 thereof, chain-end conductor 2|8 (common to all connectors Cl to Cl3), chain contacts on the chain relays of all connectors, including contacts 6 of relay y2||, chain-in conductor CH-IN of cable 554, winding of transfer relay 501 of the line controller, normally closed contacts controlled by armature 5 lof relay 501, conductor F-CH-END of cable 55|, chain contacts of relays such as |40 and |3|, conductor F-CH-IN in cable 55|, chain contacts 4 of transfer relay 501, contacts 2 of clearout relay 5|2, and thence to battery through the winding of start relay 508.'

Relays 2| I, 501, and 508 operate in series over the above circuit.

At its armature 5 and associated contacts, relay 501 locks itself to the origin point of the chain (through contacts 2 of relay 5|'2 and the winding of relay 508) independent of conductors F-CH END and F-CH--IN in cable 55|, while at its contacts 4, it disconnects conductor F-CH--IN to preclude the operation of any of the iin-der control relays such as |3| and |2| (Fig. 1) for the time being.

Relay 501 adapts the line controller for connector action as will be subsequently described, while start relay 508 initiates the required cycle of operations.

In the connector Ci chain relay 2| i, on operating, locks itself to the associated chain-in conductor at armature 5 and its front contact, at the same time isolating its winding and chain-in conductor CH-IN from the local chain-end cond uctor 2|0 to preclude the operation for the time being of the chain relay in any other connector.V At its contacts 2, chain relay 2|| locks ground on its upper winding terminal independent of contacts of relays 201 to 2 l0 to insure that relay 2I| remains operated until restored by the line controller; at its armature and associated contacts, it disconnects the normally applied source of holding ground potential from the incoming sleeve conductor and substitutes ground potential over sleeve lead SL in cable 554, having to do with the reverting-call busy test; at its contacts 3 and 4, it connects switching and busy re- `lays `201 and 208 respectively to the switching and busy leads SW and BU in cable 554; and at contacts 1 and 8, itgrounds conductors 23| and 232 of the tens and units register TR and UR, and thereby grounds the selected one of the tens ,conductors in cable 241 and the selected one of the units conductors in cable 248.

fact is determined by the A11. Tens and units selection conductor M to the tens group meter M; it connects common primary control conductors PA to PE to control conductors A to E of the tenth primary subgroup TGO; and at its lower contacts, it closes an operating circuit for the tenth tensselect magnet TMO.

A12. The called une is busy If line #00 of Fig. 3 is busy when called, such line controller of Fig. 5,whichthereupon grounds conductor BU in cable 554, closing a circuit through contacts 4 of relay 2|I for busy relay 208. Thereupon, relay 208, at its armature 9, removes ground from holding conductor 222 to permit registers TR and UR to clear out, and closes a self-locking circuit to conductor 2|6, and at its contacts 0, it opens the initial energizing ground connection to chain relay 2II, leaving such relay locked through its contacts 2.

A moment later, when the line controller opens conductor CH-IN in cable 554 incidental to the clearing out, chain relay 2|I is open-circuited and restored. With contacts 8 of busy relay 208 now open, relay 2II cannot reoperate when conductor CH-IN is reclosed at the line controller. Since switching relay 201 has not operated, no operating ground potential has been extended to the sleeve conductor S of the distributor trunk DTI, wherefore there has been no forward extension of the connection from the connector Cl toward the called line.

At its contacts 2, busy relay 208 connects the common busy-'tone lead BT through back con-A tacts I of relay 206, condenser 2I5 and contacts 3 of relay 204 to the tip talking conductor, thereby transmitting a busy signal back to the calling line. The calling subscriber is now expected to replace his receiver to permit the established connection to clear out. Such clearing out is accomplished in a manner similar to that to be hereinafter described.

A13. The called line is idle If the line #00 of Fig. 3 is idle when called,

- the line controller of Fig. proceeds to match a y A14. Connector switch. operation When the necessary selections have been effected by the line controller, control conductors CBI and B of the switches BI and 0B (Fig. 3) are grounded along with switching conductor SW in cable 554.

'The grounding of conductorn associated wan primary switch 0B closes a circuit, through make- 4busycontacts 308, for holding magnet 301, whereupon the illustrated stackup 0 of such switch is closed, having been selected by magnet UMO (Fig. 1).

Responsive to the placing of ground potential on control conductor CBI, a circuit is closed, through contacts 306, for the hold magnet 305, whereby the illustrated stackup 0 of switch BI is closed, having been selected by magnet UMD (Fig. 1)

Responsive to the grounding of conductor SW in cable 554, switching relay 201 operates, through contacts 3 of relay 2|I. It closes a self-locking circuit to conductor 2I6 at its contacts 1; it ungrounds conductor 222 at its contacts 8 to permit registers TR and yUR to clear out; it prepares a circuit for ringing relay 206 at its contacts 6; it opens a point in the initial circuit of chain relay 2 I'| at its back contact 5; and at its contacts 4, it

extends the grounded incoming sleeve conductor, through contacts 5 of busy re1ay`208, to sleeve conductor S in distributor trunk DTI, thereby closing a circuit for hold magnet 304, which includes conductor B (supplied with battery through fuse IIO) Hold magnet 304 now closes the selected stackup 2 of the distributor DRI, thereby extending conductors T, R, and S of trunk DTI, through the back contacts of stackup |'I and through the three upper pairs of stackup 2 of the distributor, to the selected terminat-y ing trunk TF2, whence the connection extends as illustrated to the called line. The ground potential on sleeve conductor S of distributor trunk DTI is now effective to hold magnets 304, 305, and 301 operated as well as to hold operated cutoi and lockout relays 303 and 302 of the called line circuit, such relays having previously been operated over the associated test conductor S00 by the line controller.

A moment later, chain relay 2II restores responsive to the temporary removal of energizing potential from conductor CH-IN in cable 554, incidental to the clearing out of the line controller. With back contact 5 of relay 201 maintained open, relay 2II cannot reoperate when conductor CH-IN is subsequently reclosed.

00 Upon operating,

A15. Ringing the called substation Following the described operation of switching relay 201, as soon as the common pick-up lead PU is next Vgrounded momentarily, by ringing interrupter 24| at the beginning of the'next ringing cycle, a circuit is closed kfor ringing relay 206 from lead PU, through contacts 3 of units-transfer relay 2I0, contacts 6 of busy relay 208, normally closedy contacts controlled by armature 4 of relay 206, and contacts 6 ofswitching relay 201. n ringing relay 206 locks itself to locking conductor L in substitution for pick-up llead PU, remaining locked over conductor L throughout the remaining portion of the ringing cycle, at the end of which conductor L is temporarily ungrounded to permit ringing relay 206 to restore temporarily before the next ringing cycle starts. If no response is obtained before the next succeeding ringing cycle begins, ringing relay 206 is operated again when pick-up lead PU next becomes grounded.

As a further result of its operation, ringing relay 206 disconnects conductors T and R in distributor trunk DTI from talking condensers 2I2 and 2|3, and from the windings of back-bridge relay 205, and connects themy to the ringing cir-,

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