US2332878A - Dial operated crossbar selector - Google Patents

Description

Oct. 26, 1943.

E. VROOM I DIAL OPERATED GROSSBAR SELECTOR Filed June 19, 1942 6 Sheets-Sheet 1 INVENTOR E. VROOM ATTORNEY Oct. 26, 1943. E VRQOM 2,332,878

DIAL OPERATED CROSSBAR SELECTOR Filed June 19, 1942 6 Sheets-$heet 2 wv /vmq E l ROOM 0 6, 1943- E. VROQM 2,332,878

DIAL OPERATED OROSSBAR SELECTOR Filed June 19, 1942 6 Sheets-Sheet 3 INVENTOR By EVROOM AT TORNEV Oct. 26, 1943. VROOM 2,332,878

DIAL OPERATED CROSSBAR SELECTOR Fi led June 19. 1942 6 Sheets-Sheet 4 FIG. 5

HWE/WOk E VROOM 8) 5S ATTORNEY Oct. 26, 1943. E VROOM DIAL OPERATED CROSSBAR SELECTOR 6 Sheets-Sheet 5 Filed June 19, 1942 B l-H I wvavron E. VROOM A TTORNEV Oct. 26, 1943.

E; VROOM 4 DIAL OPERATED CROSSBAR SELECTOR Filed June 19, 1942 6 Sheets-Sheet 6 INVENTOP y E. VROOM ATTORNEY Patented Oct. 26, 1943 DIAL GPERATIED CROSSBAR SELECTOR Edward Vroom,.0ssining, N.1Y., assignor. to Bell Telephone Laboratories, Incorporated, New York, N. Y, a corporation of New York Application June 19,1942, Serial No. 447,677

(o1. mo -22) g p g 6 Claims. This invention relates toswltching systems and particularly to dial controlled switches for use in communication systems.

The object of the invention is to operate a crossbar switch inthe manner of a stepeby-step switch so that'it willproperly selectan idle outgoing linefrom a group designated by a train of dial pulses. 5 This broad object has been accompllshc'd' heretofore and particularly in a manner disclosed. inthe application of S. B. Williams, Serial no. 420,445, filed November 26, 1941. The present invention is an improvement of the Williarns system. l

On an incoming call, an incoming train of pulses will set a register which designates a particular group of outgoing lines. Thereupon a select magnet having control over all-the outgoing lines of that group will be operatediand a train of circuit operations fortesting such outgoing lines will be started. There is provided a group of cold cathode tubes equalin number to the number of outgoing lines in a group; Each ofthese tubes is inserted in a circuit between the sleeve of an outgoing line and the sleeve of the link circuit to which it would be connected. If either the sleeve of the outgoing line or the sleeve: of thelink indicates a busy condition the tube will remain inert.

It only a single tube in this masscircuit connection is connected between an idle link andan idle outgoing line the tube will ignite and control the making of a permanent connection between such link and such line.- If a plurality oftubes arcincluded in possible circuits only one will igniteand the others will be inductively locked out. In general, this inductive lock out system is disclosed in the application of M. E. Mohr, Serial No. 377,998, filed February 8, 1941.

.A feature of theinventionds' the means for making a mass circuit connection of the test tubes between the outgoing lines of theselected group and the links to which they might be connected.

Accordingly, after theselect magnet of the desighated group is operated the operating circuits for all the hold magnets are closed. If any link which mi ht be used is busy, then thehold magnetwill already have been operated and no further connection willbe established. If, however, the hold magnet is cooperated at the time, then the operation thereof will cause the corresponding contact set of the crossbar switch to operate so that the mined the operated hold magnet-controlling this sleeve of the corresponding outgoing line will be point willbe maintainedand all others will be released.

Another feature of the invention is the means for simultaneously extending all idle links to corresponding outgoing lines of the selected group and then releasing all such established connections excepting the one chosen for service.

In case an all-trunks-busy condition is encountered an idle link will be temporarily employed for connecting a proper signal to the incomin line. Since the arrangement is such that there are more outgoing; lines in a group than I there are incoming lines there will always bean idle link even though an idle" outgoing line can not be found and hence this idle link isemployed for the purpose stated of sending back analltrunks-busy signal. v

Other features'will appear hereinafter.

The drawings consist of six sheets having six figures which arranged as indicated in Fig. 1 form a complete circuit diagram of a step-by-step selecting stage accommodating eighteen incoming lines and two hundred outgoingtrunks. H

The eighteen incoming lines are arranged in two groups, the first nine lines indicated in Fig. 2 and the second nine lines indicatedinFig. 3. Thus line I is the first and line 2 is the last of the first group and line 3 is the first and line 4 is the last of the second'group. All of these lines appear .in asingle crossbar switch of well-known construction and each line appears in a vertical row of sets of permanently paired contacts. and each has individual to it a hold magnet. Thus hold magnets 5, 6, l and 8 correspond to lines I, 2, 3 and 4, respectively. The contact sets 9 representing the'first and the contact set Ill representing the last are individual to the line I.

Each horizontal row of contact sets is controlled by a select magnet such as the magnet I l con trolling thecontact sets 9, l2,,l3, l4, l5 and I 6 and the magnet l1 controlling the contact sets Ill, l6, I9, 20, 2| and 22.

The contact sets 9, l2 and 13 by way of example serve to connect any of the incoming lines of the first group to a link 23 which. may be divided for appearance in the switch'of Fig. 4 into the two branches 24 and 25.

This switch is known as a twenty by ten foure wire switch since there are twenty verticals and ten horizontals and since each contact set has four pairs of permanently pairedcontacts.

The outgoing switch is a twenty by ten six-wire switch. Each contact set has six pairs of permanently paired contacts and accommodates two row. Theselect magnet 35 representing the digit 1 and the select magnet 35 representing the digit control the first and last horizontal rows respectively. The hold magnets 81, 58, 39 and.

49 control the first, second, nineteenth and twentieth verticals respectively.

All of the trunks in the first horizontal row will have the digit 1 as their first digit and all of the trunks in the last horizontal row will have the digit 0 as their first digit. The second and third digits of each trunk will be appropriately numbered to indicate its position in its group of twenty trunks. Thus trunks I5! and I52 will each appear in contact sets. 27 and 28 and-likewise trunks CH9 and 925 will each appear in contact sets 33 and 34. Thus each contact set accommodates two outgoingv trunks and each trunk appears in two contact sets. I a

The nine incoming lines of the first group have a register circuit individual thereto shown in Fig. 5 and the nine incoming lines of the second group havea similar register'circuit indi- I vidualthereto shown in Fig. 5. y

Fig. -'7 shows the common test circuit for select,- ing a combination of an idle link and the idle trunk. This circuit consists essentially of twenty cold cathode tubes, each of which is connected to a corresponding trunk of aselected group when the timecomes to make a selection as will appear hereinafter. v

In operation, when an incoming 'line, which may be a trunk from a line switch or aselecting stag e,is seized, ground will appear on the sleeve and a bridge will appearacross the tip and ring.

circuit thereof is almostimmediately opened by the operation of relay 4!. The original circuit for relay 4-3 may be traced from ground, arma ture 5 and front contact of relay 42, back contact and armature of relay 4!, back contact and armature 2 of relay 44, winding of relay 43 and winding of magnet 45 to battery.

'Also upon the operation of relay 42 a circuit is closed from the ground used for the operation of relay-42 through armature 4 and front contact of relayv 42 to the winding ofrelay 4| to battery which results in theoperation of relay 4I"and the opening of the original circuit of relay 43. However, before relay 43 can release a circuit is closed from ground through the winding of dial tone transformer 41, front contact and armature I of relay42 to the tipof the line, thence through the bridgeon-the calling line to the ring of the line, armature 2 and front contact of the normal contacts of the line relay 48, armature I and back contact of relay 49, armature 2i and front contact of slow releasing relay 43, lefthand winding of slow releasing relay 59, winding: of magnet 45 to battery. The device in the: broken line rectangle including magnets 45 and.

'45 is an electromechanical equivalent of a chain;

of counting relays and is clearly described in an. application, Serial No. 389,322, filed April 19,. 1941, by George R. Stibitz. When this device: has been set by the incoming pulses a ground on.

conductor 5| will be connected to one of ten.

conductors such as conductor 52 representing the digit 1 or conductor 53 representing the digit 0.

.At the end of the dialing period the slow releasing relay 5!) is released and this results in thewinding of relay 49 to battery tocause the energization of relay :49. It may be noted that relay 49 for the first group and relay 54 for the second group interact in such a way that when, for instance, relay 49 hasbeen operated relay 54 cannot be operated and vice versa. This is a wellknown non-interference means. Also in this connection non-interference within the group is provided. Thus while line I is calling, the remainder of the lines are temporarily. disabled by relay 4I.

Upon the operation of relay 49, the digit regis tered on the stepper is rendered effective to 0p erate the crossbar switch of Fig. 4. Thus a cir-- cuit may be traced from ground, the front contactand armature 3 of relay 49, conductor 55,.-

back contact and armature 2 .of relay 56, con-- ductor' 5i, through the circuit of the stepper to conductor 52 (assuming digit I to have been register'ed) to the select magnet 35, thus causing the operation of this select magnet. All'contact setsin the first horizontal row are thus prepared foroperation.

Ground on conductor 55 is also extended directly to the winding of relay 57 and through armature 4 and back contact of relay 58 to the winding of relay 59. Relays 57 and 59 operate, relay 51 to connect the sleeve circuits of the links, such as 23, to the test tubes and relay 59 to prepare for the operation of the select magnets iii relay 42, winding of line relay 48 to battery.

and the transfer relays of Figs. 2 and 3. Ground on conductor 55 is also extended through back contact and armature 2 of relay 55, back con-. tact and armature 3 of relay 58 to the winding of relay 60 which connects a ground to the lower winding of every other one of the hold magnets such as magnets 37 and 39. The companion relay BI under control of relay 54 at another time will similarly operate the" remainder of the hold magnets such as magnets 38 and 45.

The operation of relay 69 causes the operation of relay 62 whereby the sleeves of the various three-wire circuits controlled by the now operated hold magnets are connected to the test electrodes of the test tubes.

Let it be assumed that the hold magnet 31 was found operated in connection'with an already established connection to trunk 992. In this case contact set 31 would be found operated and a ground wouldappear on the sleeve of trunk 992 r leading through the front contact and armature 3 of relay 62 to resistance 63 andthence to a test electrode of tube 54. A ground on this test electrode will prevent the tube from firing and assaeest thus mark the :portion of the link. controlled by theupperthree contacts of zthesse't 3| as' unavailable foruse.

Now since :the portion of the link through the contacts of transfer .relay I65 is busy it is :evi-

dent 'thatthe :portio'n leading from the "contacts of relay 66 "cannot be used; Therefore, the test tube connected to this other portion of the link must be prevented from firing. 'If trunk MI is idle then 'a 'minus battery will be "connected to the-sleeve'thereof andthis .may be traced through from contact and armature -l ofrelay 6-2 to resistance 61 leading to a test 'electrode of tube 68. It will be noted that a plus battery isconneoted through resistance 69 to the other test electrode of :this tube. If this -is left unafiected the "tube may fine. However, since thelink is actually busy a ground from the sleeve thereof may be traced through conductor 10, armature l "and front contact of relay to the test 'electrodeof tube -68, thus shunting the plus battery supplied through resistance 69 and preventing the-firing oftube-68. r l p -Let it now be assumed that link 23 is idle but that trunk isbusy. Hold magnet39 in operating will cause the-operation of cont-act setg29,

it beingassumed that trunk 420 is busy through theoperation of 'contact set or in some other switch. Ground on the sleeve of trunk 120 will be extended over the front contact and armature 4 of relay 62 to resistance H leading to a, test electrode of tube '02, thus preventing this tube from firing. "l he sleeve of link '23 is nowconnectedthrough conductor -13, armature -2 and frontcontactofrelay 51 to the test electrodebf tube "14 normally supplied with arplusbattery potential through resistance 15. The other test electrode. connected through resistance '76 and thence over armature l and front contact of relay-624m the-sleeye of idle trunk H9 -willhave a minus battery: potential on it and the tube. should fire. Sincethe sleeve conductor --HJ is free of ground tube 14 -will fire- In these-tubes the test electrodes areshown withiarr-owheads and when a minus potentialappearson one'and a plus po tential on-theother the tubeatends :tobreak down. With a stream of ions passing between the :two test electrodesiane tube becomes ionized so that a current flow will be established between the main anode shown as a small oblong in the upper part of the tube and the working cathodes shown as -smallcircles neer the bottom part of the -tube. With twenty of these tubes simultaneously searching -for the combination of an idle trunk and an av-ailable idle link it. will very often happen thata pluralityof =tubes will attempt to-op erate at once. -However,:one will fire and thereafter the resistanee' drop :dueto therresistance of relay 66" will reduce the potentialon the anodes of all the tubes to the point where no other one mayoperate-even though it ihas-been sufficiently ionized by its-test electrodes. i

The inductance of the winding-of relay 56 will preventmore than onetubefrom operating. Due to the fact that when a circuit is closed through an inductance 1 the attainment of a stable value of current delayed, the natural difference in the operating time characteristics of thetubesis exaggerated because as current begains to flow through the winding of relay 56 the potential on the anodes of thetubes begins to drop and this change adverselyafiects the striking capabilities of the tubes. Hence whereitherevis-aninherent difference in the striking times of the different tubes this inherent-difference is developed by the ahead of lthere'st and of courseasr'soon as itlfires it will :lock out the :restrdue 170 the sudden lowering of the potential on the anodes below 'the point where any tube will fire. Let it now be assumed that tube lfl'whichrhas formed an idle trunk and an available idle lin'e succeeds in striking to the exclusion of all "other tubes which may have found similar conditions. A circuit will be-established from battery, front contact andrarmature 4 of relay -49, conductor 16, winding of relay-56, the'anode'of tube l4,'to the two working cathodes of this tube. The'circuit may be traced from the right-hand cathode of thistube ov'er conductor "H, the left-hand winding of relay 26, to the front'contact and armature3 of relay thence through resistance l:8,-to a "battery whichrwillaid the battery connected to the front contact-of "armature 4 of relay;49. Conductor I! also leads through the right-hand winding of select magnet H. so that this select magnet as wellas relay 25 now becomesoperated.

front contact and :armature '2 of relay 59170 the upper winding-of hold-magnet 39 tomaintainthis hold magnet operated after the 'remainderare released by the release of relay 60.

Due to the current flow through the winding of relay 5'6 this relay now "becomes :operated and the ground from conductor 55 is disconnected from .the'a'rmature 2 ofrelay 56 andfromconductor 5|. and relay t0 so that the temporarily operated sets of cont'acts in the switch of Fig. '4 are-released. J The'contact'se't 29 however is maintained operated and as will shortly appear this-is held under control of. the holding -c'onductor 78. Upon theoperation of relay5$ a circuitis closed over the front contactand armature I of relay 56 to the winding of relay 88 and thisusam'e ground is-nowe'xtend'ed by the operation of relay' liflover its front contact and armature 3, the frontcontact 'and armature 2+of relay '49 itothe windings of relay 44 and hold magnet 5 inp'arallel. Hold magnet 5 causes the operation of contactset '9 so that the incoming line 'l is now extended.

through this contact set to link "23, through the front contacts of relay "26, to 'link -25 and through the "contact "set 29 to trunk H9. The ground on the sleeve of the incoming line is connected through the 'front contact or hold magnet '9 "to the conducto "13 and the s'leeve of the outgoingtrunk N9. Ground 'on conduc'tor'l3 allows relay '25 tolock to-the sleeve of the trunk. In the samemanner that relay 4| immediately returned a ground back over'the sleeve of the incoming line I, similar means associated with the trunk "l l 9 will return agrou'nd' whichwill hold the hold na'gnet 5, and the'relay 25. The hold magnet "39. ishe1d from ground connected by the upper contact ofthe set 9 to thehold wire '18.

Due to the operation of relay .44 the circuit .of

the ,first slow relay 43. is opened which now results in the release of all "common apparatus.

Relay 43 releases and opens the circuits of the register-magnets '45 and 46 thus restoring this apparatus to normal. Relay. 419 is released and the relay 42 is released. Relay 4'2 disconnects the incoming. line lifrom the register circuit so that .After the release of the line relay. 48 releases. relay 4!! relay- 44 also releases. I

In the circuitsof Fig. 7,; ground being removed This t releasesxthe select magnet -35 y from conductor 55, relay ill andrelayl 59 release thus disconnecting the test electrodes of the tubesfrom the sleeves of the links and" the working cathodesfrom the upper windings of the holding magnets of Fig. 4. Battery being removed from conductor 16, relayfifi is released and the tube of relay 53 to battery. Thiscircuit of course is closed under ordinary conditions, but due to the slow operating characteristics of relay- 58 does not become eiiective until the operations heretoforedescribed have been completed. Under the all-trunks-busy condition however, no tube will fire andin time relay 58 will become energized and will lock directly to-the ground on conductor 55, Thereupon'the all-trunks-busy relay 85 will become energized. This relay connects .negative battery through-resistance 8! tothe resistances-82 and 83 thus placing an artificial idle indicating condition on the ten tubes indicated b tubes 64 and 12. Since relay 58 through its operation has released relay 60 this potential will be effective. Also since'there are but nine incoming lines in'a group and there are ten links someone of the tubes 64 to 12 will fire. Let us assume'that tube '52 operates.

however will-be effective to energize the lefthandvvinding of select magnet H. The circuit for the left-hand winding of 'relay 84 is closed at the same time but therelease of relay 59 prevents its operation. i i 1 -.Upon the operatiomof tube'flZ the relay 56 operates and this in turn causes the operation of relay 88 .so that now a circuit is closed from ground on conductor 55 through-armature 3 and front contact of relay 86, front contact and armature Zof relay 8%, armature 5 and front contact of relay 45, armature and back contact of -relay-85,-winding of holding magnet Bl, to loattery. At the same time holding magnet 5 is operated as hereinbefore described so that now contact sets 9 and I? are operated and the incoming line l ;is.conneoted through link 23 to the circuit 8i, Upon the establishment of this connection the release of the common apparatus will be started. Q

A connection is now established from ground, winding of transformer 88 supplying an all-, trunks-busy tone, the; tip of the line over the bridge from the tip to the ring ofthe line, winding or relay 89 to battery. All-trunks-busy tone is thus transmitted to'the subscriber on the incoming line. Relay,89 operates and extends a ground through the contacts of relay 90 to hold the hold magnet 81. The subscriber in hanging up releases relay 89 whereupon the apparatus is returned to normal.

If at the time ground is put on armature 5 of relay 49 the tenth vertical circuit 9| is being used, relay 85 will be operated and hence this ground will cause the operation of relay 9!). This will open the holding circuit of magnet 81 thus Relay 59 having been released, hold magnet 39"cannotbe operated. The otherwvorkinghathode of tube '12 encountered the all-trunks-busy condition 'an opportunity to be connected to the source of tone 88. If only one line .usesthe circuit 9| then relay Willremain operated without effect until the subscriber hangs-up-and releases relay 89. What is claimed is: i '1. A dial operated crossbar switch, comprising a coordinately arranged field of sets ofupermanentlypaired contacts operated through the temporary operation of one of a plurality of selecting magnets arranged in one coordinate direction and, during'the period of operation of said selecting magnet, the operation of one of a plurality. of holding magnets arranged in another coordinate direction, incoming lines one each multipled to all-said sets of contacts under control of a holding magnet, a group of equivalent outgoing lines appearing in the sets-of contactsunder control of each said selecting magnet, means for selectively operating said selecting magnets,-' means for selecting an idle one of said outgoing lines of the group under control of said selectively operated selecting magnet, comprising a plurality of discharge tubes, means to temporarily connect said tubes" each in a circuit to an outgoing line and a corresponding incoming line, means responsive to the busy condition of either said outgoing or incoming line to prevent the operation of the corresponding tube, lockout means for preventing more than one tube connected to a pair of idle lines from operating,

and means responsive to the operation of one such tube for establishing a connection'between the corresponding incoming and outgoing lines and dismissing said test circuit connection,

2; A dial operated crossbar switch, comprising a coordinately arranged field of sets of permanently paired contacts operated through the terriporary operation of one of a plurality of selecting magnets arranged in one coordinate direction and, during the period of operation of said selectingmagnet, the operation of one of a plurality of holding magnets arranged in another coordinate direction, incoming lines one each multipled to all said sets of contacts under control of a holding magnet, a group of equivalent outgoing lines appearing in the sets of contacts under control of each said selecting magnet, means for selectively operating said *selecting magnets,

means forselecting an idle one of said outgoing lines of the group under control of said selectively operated selecting magnet, comprising means to temporarily operate all said holding magnets to establish a testing circuit, a testing circuit, and means responsive thereto for releasingall of said holding magnets except the one corresponding to the selected'outgoing'line.

-3. A dial operated crossbar switch, comprising acoordinately arranged-field of sets of permanently paired contacts operated through the temporary operation of one of a'plurality of selecting magnets arranged in one coordinate direction and, during the'period'of operation of said selecting magnet, the operation of one of a plurality of holding magnets arranged in another coordinate direction, incoming lines one each multipled to'all said sets of contacts under control of a holding magnet, a group of equivalent outgoing lines appearing in the sets'of contactsunder control of each said selecting magnet, means for selectively operating said selecting magnets, means for selecting an idle one of said outgoing lines of the group under'control of said i selectively operated selecting magnet, comprising means to temporarily operate all remaining unoperated holding magnets to connect said outgoing lines to corresponding idle incoming lines, a test circuit, means to connect said test circuit to said incoming lines, means responsive to the busy condition of either an incoming line or a corresponding outgoing line to hold the corresponding portion of said test circuit unresponsive to said connection, a lockout means for holding all remaining portions of said test circuit except one unresponsive to said connection, means controlled by the response of said excepted one portion of said test circuit for holding the corresponding holding magnet energized and for releasing all other holding magnets, not otherwise operated.

4. A dial operated crossbar switch, comprising a coordinately arranged field of sets of permanently paired contacts operated through th temporary operation of one of a plurality of selecting magnets arranged in one coordinate direction and, during the period of operation of said selecting magnet, the operation of one of a plurality of holding magnets arranged in another coordinate direction, incoming lines one each multipled to all said sets of contacts under contral of a holding magnet, a group of equivalent outgoing lines appearing in the sets of contacts under control of each said selecting magnet, means for selectively operating said selecting magnets, means for selecting an idle one of said outgoing lines of the group under control of said selectively operated selecting magnet, comprising means to temporarily operate all remaining unoperated holding magnets to connect said outgoing lines to corresponding idle incoming lines, a test circuit comprising a cold cathode tube for each incoming line, means to connect said tubes to corresponding incoming lines to test for the busy or idle condition of either said incoming lines to said temporarily connected outgoing lines, a lookout circuit for allowing the operation of only one tube of the remainder connected to idle paired incoming and outgoing lines, and means responsive to the operation of one such tub-e for dismissing said test circuit and releasing all holding magnets corresponding to idle paired lines except the said holding magnet corresponding to the selected one of said idle paired lines.

5. A dial operated crossbar switch, comprising a coordinately arranged field of sets of permanently paired contacts operated through the temporary operation of one of a plurality of selecting magnets arranged in one coordinate direction and, during the period of operation of said selecting magnet, the operation of one of a plurality of holding magnets arranged in another coordinate direction, incoming lines one each multipled to all said sets of contacts under control of a holding magnet, a group of equivalent outgoing lines appearing in the sets of contacts under control of each said selecting magnet, means for selectively operating said selecting magnets, means for selecting an idle one of said outgoing lines of the group under control of said selectively operated selecting magnet, comprising a plurality of discharge tubes equal in number to the number of outgoing lines appearing in, the sets of contacts under control of each said selecting magnet, means comprising a circuit established by the holding magnet for each outgoing line for simultaneously starting all of said tubes associated with an idle outgoing line and an idle incoming line, and lockout means for preventing the firing of more than one of said tubes forselecting an idle outgoing line and for connecting such selected idle outgoing line to an idle incoming line.

6. A dial operated crossbar switch, comprising a coordinately arranged field of sets of permanently paired contacts operated through the temporary operation of one of a plurality of selecting magnets arranged in one coordinate direction and, during the period of operation of said selecting magnet, the operation of one of a plurality of holding magnets arranged in another coordinate direction, incoming lines one each multipled to all said sets of contacts under control of a holding magnet, a group of equivalent outgoing lines appearing in the sets of contacts under control of each said selecting magnet, means for selectively operating said selecting magnets, means for selecting an idle one of said outgoing lines of the group under control of said selectively operated selecting magnet, comprising a plurality of discharge tubes equal in number to the number of outgoing lines appearing in the sets of contacts under control of each said selecting magnet,

means comprising a circuit established by theholding magnet for each outgoing line for simultaneously starting all of said tubes associated with an idle outgoing line and idle incoming line, a source of tone, and means responsive to the prior operation of all said holding magnets for connecting said source of tone for indicating an all-lines-busy condition to one of said incoming lines.

EDWARDVROOM.

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