US1809086A - Telephone system - Google Patents

Description

.1. H. Voss E1' AL 1,809,086

TELEPHQNE SYSTEM Original Filed June 29. 1928 2 Sheets-Sheet haw Y H H w .l TM. R. I I j J I I. SSS@ Q .WS

June 9, 1931.

I Jahn H. Unss BarnaPdDLzZ'lls June 9, 1931. J. H. Voss ET AL. 1,809,086

TELEPHONE SYSTEM original Filed June 29, 192s zsneets-sneet 2 -Iniranur's- Jahn H. Ua ss n Ber'nar'dIlI/:Iills MM Err-y.-

Patented June 9, 1931 UNITED STATES PATENT YOFFICE JOHN H. VOSS, OF BERIVYN, .AND :BERNARDI` D. WILLIS, OF OAK PARK, ILLINOIS, AS- SIGNORS TO RESERVE HOLDING COMPANY, OF KANSAS CITY, MISSOURI, A .CORPORA- TION OF DELAWARE TELEPHONE SYSTELI Application le .Tune 29, 1928, Serial No. 289,137. Renewed September 6, 1930.

The present invention relates to telephone systems in general, but is concerned more particularly with telephone systems employing non-numerical switching apparatus of the preselecting plunger type, such apparatus being commonly known as plunger line switches and being ordinarily employed in automatic telephone systems between subscribers lines and first selectors.

The broad general object of the present invention is the production of new and im proved circuit arrangements whereby the operation of the line switches is improved; the likelihood of two line switches taking the same trunk is practically eliminated and the circuits themselves are considerably simplified.

Among the new features disclosed herein, the following may be pointed out:

(a) The contacts of the line relays of the line switches are arranged in a chain so that when two of them operate simultaneously only one can extend an operating ground to the associated pull down winding, with the result that only one line relay can close an effective circuit at a time. This avoids the soscalled double-plunging7 evil.

The line relays of the line switches have been provided with contacts for placing a guarding potential on the associated test conductor leading to the connector bank multiple and commonly called the private normal conductor immediately upon the operation of the line relay. This serves to make the line busy immediately and to eX- tend ground forward over the so-called release'trunk conductor of any trunk taken for use, thereby simplifying the master Yswitch operation as will hereinafter appear. Itis to f be noped that a special circuit arrangement has been devised to pre-vent a buzzing interaction between the line relay and the bridgecut-off relay of the line switch in case the line switch is prevented from operating by a lino switch in a receding portion of the o 3- P e i erating chain circuit.

(c) Another feat-ure lines in the pro-vision of a delay relay connected in the source of the operating chain and arranged to delay the operation of any line switch for a small fraction of a second following the operation of the line relay sonas to avoid trouble due to a line switch attempting to reoperate before its plunger is completely restored, this arrangement being needed only when the socalled self-aligning plungers are used. y

(d) Another feature of the arrangement is that, since the line relay places a guarding potential on the release trunk conductor of any trunk seized by a line switch, the master switch may make its regular busy test over the release trunk conductor leaving the fourth conductor free to be used as a trunk conduct-or over which a metering operation or any other desired operation may be carried out.

(e) Another feature of the arrangement is that the alarm circuit has been'revised so that in case the chain circuit goes open an alarm is brought in responsive to the falling back of a chain-test relay, while the alarm circuit', Vas a whole, has been revised in the master switch so that the so-called supervised-ground relay, which has at times been the source of considerable annoyance, is done away with. v

(f) Another' feature lies in the provision of an arrangement in the primary master switch of a 'meter for recording the number of times that the master switch tests all trunks and finds them busy, a delay device being provided for preventing the operation of the meter at other times.

(g) Another feature relates to the provision of a secondary line switch provided with a line relay operable over a calling subscribers line in the same way as the line relay of a primary line switch, secondary line switches having been operated heretofore over the fourth trunk conductor by ground potential extended to such conductor upon the operation of thel primary line switch. By this arrangement, the secondary line switch may be provided with the same chain and holding arrangement as the primary line switch and the same simplified master switch may be used. v

(h) Another feature is the provision ofa separate trunk conductor, a kick-off conductor which incidentally does not pass lUO through the line switch bank but goes directly to the bank of the master switch, for each trunk line from a primary line switch to a secondary line switch, which conductor is grounded at the secondary group responsive to an all-trunks-busy condition so as to cause all trunks incoming to a sub group to test busy to the primary master switches when there are no further idle trunks' outgoing from thesecondary sub groups.

(i) Another feature lies in the arrangement provided for rendering busy normally the trunks common to all line switches served by two master switches and for rendering the common group idle to the two master switches separately, only as the trunks individual to theline switches served by such master switches become all busy. A particular feature of this arrangement is that the busying means is applied to the kickoff conductor above mentioned.

Another feature relates to the provision of a relay included in the operating chain out of one master switch to the line switches controlled thereby and having contacts included in the operating chain of the master switch controlling an associated group of line switches, when the two line switches have access to trunksin common so as to prevent simultaneous operation of line switches in the two groups.

(7c) Another feature relates to the provision of circuit arrangements such that the holding ground potential is taken away from the line relay contacts of a busy secondary sub group so that, in case another line switch is taken for use after all trunks have become busy and before the primary master switches have responded to the condition, the primary line switch seizing the last trunk is not provided with ya holding circuit, so that it releases and reoperates taking a trunk to an idle sub-group.

Other objects and features incidental to the carrying out the foregoing ones will become apparent upon a further perusal of the specificatlon in connection with the accompanymg drawings.

` General description Referring now to the drawings comprising Figs. 1 and 2, they show by means of the usual circuit diagrams a sufficient amount of equipment in an automatic telephone system Vembodying the features of the invention to enable the invention to be understood.

line switch SLS and others of the same sub group.

Fig. 2 is a combination circuit drawing and trunking layout which indicates the interconnection of two master switches when graded trunking is employed between two primary groups by means of tie so-called individual-trunk and common-trunk method.

In reference to Fig. l, it may be pointed out that the primary line switch PLS, in common with the other line switches presided over by the master switch shown in the drawings has access to ten trunks, one of them being the trunk comprising conductors Set-8 and extending to the secondary line Vswitch SLS. It is common practice to distribute the trunks from a primary group so that each trunk extends to a different secondary sub group, this distribution being effected at the intermediate distributing frame I. D. F. A cross-connect block C. C. B. is provided adjacent to the line switches and the master switch, at which point the trunk terminals coming from the line switch banks may be jumpered to the terminals of the cable leading to the I. D. F. It is at the cross connect block C. C. B. that the proper connections are made so that any trunk may be individual to one line switch group or common to two line switch groups as trafic conditions may warrant.

The circuits of the primary master switch have been shown in full, but the circuits of the secondary master switch have not, as the secondary master switch may be identical with the primary master switch. The eXternal connections to the two master switches, however, differ somewhat, as indicated. The conductors leading out from the primary master switch lead to terminals numbered 51-57. The secondary master switch is shown with the same terminals numbered 51-57. The primary master switch is shown in the position in which it comes to rest on trunk 10 just at the end of an operation of a solenoid 7l.

Deia/Zed description The invention having been described generally a detailed description of the operation of the apparatus shown will now be given. For this purpose, it will be assumed that the subscriber at substation A initiates a call.

Outgoing @all Then the usual receiver is removed at substation A, a bridge is placed across the line conductors 2l `and 22, operating line relay 24 of the primary line switch PLS,

ground potential being connected to conducswitches in the chain as well as terminal 53 of the primary master switch, contacts ot' the open-ground relay ot the inastcr switch, lower winding of the delay relay, contacts of the start relay, and terminal 5.7 of ,the master switch.

The resistance ot the lower winding ot the delay relay is suiiciently high. to prevent the operation of the line switch until the delay relay operates and places the upper winding in multiple with its lower winding. The operation of the delay relay is retarded by the upper winding thereof which is normally short circuited by the contacts ot the relay, permitting current to circulate through the winding with the same effect as is produced by the usual copper collar.

Then the delay relay operates, which'it does after a sui'iicient time interval has elapsed to permit any partially aligned plunger that might be just restoring and aligning to come to rest, the magnet of the line switch operates the plunger arm 27 and the l. C. O. arm 26, and over the above traced circuit. The plunger arm 2T forces the springs 28-31 into engagement with their` respective contacts, while at the contact springs shown to the lett of magnet 25 and operated by arm 27 the holding winding of magnet 25 is connected to the grounde conductor 23. rllhe operation ot the bridge-cutoff arin 26 results in ground being disconnected `from conductor 2l and inthe line relay 24 being disconnected from conduct lr Line relay 24, being slow cting, remains operated for an interval.

Then connection is made withV a trunk line upon the operation of the line switch, theA ground potential extended from the grounded J- 1 conductor 23 through springs 29 to trie rex/L. i lili; le l.' Twan o C lease t r il conductor esuls in closing a circuit. through the associated contact in the upper master switch bank and through test mmf t1 'l' "ittleL t uipcr i or ne upper wincing o io star relay. The start relay, at its inner armature, removes ground from the chain circuit and places ground on conductor 74. As soon as ground is thus removed Irom Athe chain circuit, no Jfurther line switch can start to opeate, even though it may occupy a position in the chain circuit nearer the source than the line switch PL The opening ot the chain ot course opens the circuit or" the lower winding of magnet 25, leaving the magnet operated through its upper winding 'from conductor 23 and through contacts now closed by arm 27.

rllhe delay relay now falls back in preparation for the next call.

As a result of placing the ground potential on conductor 74, the locking relay is operated to unlock the masterfswitch segment so as to allow it to advance. rhe locking relay, at its outer contacts, operates the open-groun l relay. This relay is arranged to be quick to operate, but, upon operating, it closes its upper winding at its upper contact so as to render it slow to release.' t its lower contacts the onen ground rela opens a further a l e y L lay falls back, opening the circuit of the locking relay at its inner armature, at the same time preparing to reground the open ground conductor through the delay relay. The locking rela-y immediately falls back and relocks the master switch at the associated locking segment, at the same time opening the circuit ot the open-ground relay. The openground relay falls back after a slight interval and replaces the ground potential, applied through the dela-y relay, to the openground conductor. The slow action of the open-ground relay is necessary to allow the line relay of the line switch to fall back so as to prevent a reoperation ot the delay relay.

its a result of the seizure of the trunk line. comprising conductors Sel-37 and eX- tending irom the banks of the primary line switch PLS to the secondary line switch SLS through the cross-connect block and intermediate distributing tra-me, a ground potential is applied as above mentioned from the grounded conductor 23 to the conductor 35,'closing a. circuit through the holding winding of magnet 42 of -the secondary line switch SLS. The holding winding is so designed that the current flow there- -hrough is not suiicient to operate the plunger arm 43. rllhe calling line is extended over conductors 34 and 37 to the secondary line switch, with the result that line relay 4l operates thereover and at its upper arniature applies a holding ground potential to conductor troni the common conductor 76. This holding potential is applied be- 'ore relay 24 of lLS falls hack.

At its lower armature, relay 41 closes a circuit from the open-ground conductor through the contacts of the preceding line switches in series for the pull down winding ot magnet 42, operating the arm 43 to press the springs 44-47 into engagement with their respective associated contacts.

rlhe arm 43 operates the contact springs to the left thereof to disconnect ground and the winding of relay 4l from the talking conduotors. Relay 41 falls bac-k after a slight interval.

Responsive to the above operation of the secondary line switch in seizing the preselected tirst selector, the said first selector is prepared for operation in the usual manner, placing ground on its release-trunk conductor to hold the two line switches operated.

Responsive to the placing of ground potential upon the release trunk conductor of the selector when the line switch SLS operates, the secondary master switch is operated in the manner described above in connection with the primary master switch to preselect another idle trunk, ground potential being removed in the same manner from the open-ground conductor during the preselecting operation.

By way oi explanation, it may be pointed out that the subscriber sub-station A, after obtaining connection with the first selector, dials the digits in the desired number in the usual manner, whereupon the connection is extended through the firs selector and other automatic switching apparatus to the called line.

It will be noted that there is a meter Ml associated with the line switch of calling line and that there is an electro-polarized relay associated with the trunk line between the primary line switch and the secondary line switch. Current starts to flow through the two windings of this relay as soon as the trunk line is seized, but the current flow through each winding is of a relatively small value so that current flow through either winding alone is not sucient to operate the relay. Furthermore, when the trunk line is first seized the current flow through the upper winding is in such a direction that the effect thereof, neutralizes the effect of the current flow through the lower winding so that the relay is not operated. The resistance 33 is provided in shunt of the upper windings so as to permit talking current to by-pass the relay in case a free connection is established and so as to permit the switchoperating impulses to by-pass the relay.

. Vhen the called subscriber responds, the current flow in the talking connection extending back to the calling line is reversed, whereupon the electro-polarized relay 32 responds and closes a circuit at its lower armature for the meter Ml, operating the meter to register the completed call. At its upper armature, relay 32 short circuits its upper winding and the resistance 33 so as to afford a better talking circuit and so as to prevent the relay 32 from deactuating when the current flow is again reversed back to normal when the called subscriber replaces his receiver at the end of the conversation. The current flow through the lower winding of the relay is suiicient to maintain the relay operated after it has once pulled up under the combined inuence of both windings.

When the receiver is replaced at substation A after the conversation has been completed, the ground potential is removed from the release-trunk conductor of the established connection in the usual manner, whereupon the current ceases to flow through the magnets 25 and 42 of the two line switches shown, permitting the line switches to restore and their plungers to re-align. The meter Ml resto-res when the line switch PLS releases after the electro-polarized relay 32 falls back responsive to the removal of ground potential rom release-trunk conductor.

Incoming @all 0n a call incoming to the line of substation A, ground potential is applied to privatenormal conductor 23 by the seizing connector, energizing magnet 25 through its upper winding. The bridge-cut-otf armature 26 is operated to clear the called line of its linerelay bridge, but the plunger arm 27 is not operated due to the relatively weak pull exerted by the magnet 25 when energized through the upper winding.

AZZ Secondary trunks busy Assuming now that a call comes in and operates a secondary line switch, the line switch SLS for example, and that the trunk seized thereby is the last idle trunk, when relay 64 operates responsive to the seizure of the trunk it removes the ground potential at the last point 'from the circuit of the common alltrunks-busy relay 63, whereupon relay 63 falls back and at its upper armature it operates the kick off relay G5 which places a ground potential on test conductor .38. Relay is provided with a separate contact for each secondary line switch controlled by the same secondary master switch, so that each trunk from incoming to the secondary sub group has its kick off7 conductor grounded so that any master switch standing on a trunk incoming to the group is caused to move on and o-ther master switches are prevented from stopping on any trunk incoming to the group regardless of whether such trunks are actually in use or not.

Relay 65 also removes the ground potential from the common conductor 76 so that, in case another trunk incoming to the secondary group were seized before the kick off operation became effective, the holding circuit ordinarily provided by the line relay thereof to the release trunk conductor is opened, causing the primary line switch to release. It will be understood that the time required for relays 64 (for example), 63, and 65 to operate after the last trunk is seized is long enough to allow the usual holding circuit to be established in the selector. Therefore only unsuccessful connections are released, When the primary line switch releases under this condition, the primary master switch has already moved to a new trunk and the self aligning plunger of primary line switch realigns itself onto the new trunk, and the line switch reoperates, taking a trunk to another secondary sub group. The result is that the call is not lost but is rerouted in this inanner. n

At its inner armature, relay 63 closes a circuit for the meter h/I4, operating the meter to record the all-trunks-busy condition. As the further result of the release of relay G3, it removes ground potential from terminal 57 of the secondary master switch, thereby preventing unnecessary operation of the master switch on account of the fact that the start relay, even though operated at this time no matter where the master switch is standing, is unable to close a circuit for the locking and trip relays, as may be seen from reference to the primary master switch.k

lWhen the all-trunks-busy condition is terminated upon a trunk line outgoing from the secondary group becoming idle, a relay such as relay 64e falls back, reoperating the alltrunks-busy relay 63, whereupon the meter M4 is restored; ground potential is reappled to terminal 57', and the kick off relay G5 is released.

Master-switch operation Referring now to the primary master switch, it will be noted that the master switch is provided with two fingers, 1 and 10. IV hen the master switch has trave-led counter clockwise under spring control until its wipers s and on the test terminals of trunk 1, finger 1 of the master switch closes the associated contacts. preparing a circuit for the the trip relay. By this time, finger 10 of course has moved away from the locking spring of the trip relay so that the locking spring is resting with its catch upen the extended spring of the trip relay, preparing to locking the spring relay mechanically. lVhen trunk 1 is seized under this condition. the start relay closes a circuit for the locking relay and a branch circuit is closed at. this time for the trip relay through the springs held in engagement by finger 1 of the master switch, whereupon the trip relay operates and locks itself mechanically so as to remain operated'until the master switch has been brought over to trunk ten.

At its inner contact, the .trip relay closes a circuit for the solenoid 71, and at its outer contact it closes a circuit for holding the lock relay operated. Responsive to the energiza.- tion of the solenoid 71, the master switch is restored to the position shown in the drawing whereupon linger 10 unlocks the 'trip relay which has been latched in an operated position, whereupon the trip relay is restored,

7 opening the circuit of the lock relay and the circuit of the solenoid. The lock relay catches on the associated segment holding the master switch in this position unless the trunk tests busy.

Metering and supervision It will be noted that the vibrating-contact relay 58, which is of the type shown in Fig. 1 of Patent 1,641,256 granted to Erickson, September 6, 1927, operates in multiple with the lock relay each time the lock relay operates, but due to the vibratory action of the springs of relay 58 the meter M2 is not operated unless the operation of the master switch is prolonged, as for example when thereareA ne trunks remaining idle. The meter M2 records the number of times that all trunks are tested and are found busy.

It will be noted that the alarm conductor 75 is grounded by the chain-test relay, by the trip relay, by the locking relay, and by the start relay, with the result that conductor 75 is grounded at one or more point-s eyery time the master switch operates. Ordinarily, the ground potential does not remain on conductor 75 long enough for the slow alarm, common to several master switches and connected to terminal 51 through the individual lamp L1, to operate and sum*- mon an attendant. The attendant however is summoned when the master switch continues to operate for some time. It will be understood of course that the slow alarm requires a longer time to operate than is required for the operation of meter M2.

It will be understood also that the chaintest relay fails to reoperate following the operation of aline switch in case the chain is not reclosed. In this case, of course, ground potential remains on conductor 75 and the alarm is caused to operate.

The meter M3 is a so-called peg-countmeter and is connected through terminal 56 to contacts of the delay relay so that it operates on every call when the delay relay pulls up to permit the operation of the primary line switch.

In case it is desired to meter onlythe number of times when connections are attemptedY during an all-trunks-busy condition, the right-hand terminal of relay 58 is connected to ground instead of to battery, and the left hand terminal of the relay is connected to terminal instead of to terminal 54, as indicated by the dotted connection, in which case relay 58 operates each time a call is attempted when the open ground conductor is switched over to relay 58 by the operation of the open ground relay. In case this condition exists only momentarily, as during the regular operation of the master switch, the meter is not operated, but is operated in case a call comes in when no further trunks are available. y

Referring now to the secondarymast'er IOS switch, it will be noted that the slow alarm is not connected directly to terminal 51', as in the case of the primary master switch, but the vibrating-contact type relay 61, the same type as relay 58, is connected to terminal 51 and permits the slow operating relay 62 to operate only after the vibrating contact comes to rest after relay 61 operates.

In case an abnormal condition arises within the secondary master switch so that. the conductor corresponding to the conductor 75 of the primary master switch and leading out to terminal 51, the terminal 51 of the secondary master switch remains grounded, indicating trouble within the master switch. Relay 62 operates as above pointed out and closes a circuit for the kick-off? relay 65 so as to prevent further calls from coming into the group while the secondary master switch is not functioning properly. The circuit for the slow alarm thro-ugh the individual lamp L2 is closed at the upper contacts of relay 62.

It will be noted that the peg-count meter M5 is connected to terminal 56 of the seconda-ry master switch, while the terminals 54 and 55 are left dead. It will be noticed further that the conductors associated with the lower bank of the secondary master switch are not used, as it is not necessary to provide kick-ott test conductors for the secondary master switch.

Referring now to the trunk line extending from the bank of the primary line PLS to the secondary line switch SLS, it may be pointed out that it is sometimes desirable to operate the meter individual to the calling line a variable number of times, depending upon the destination of the connection, in which case the contact of the lower armature of the electro-polarized relay 32 is disconnected from ground and is connected to conductor 36, in which case this conductor is carried along separately through the switches to such meter-control apparatus as may be provided.

In case it is desired, the electro-polarized relay may be omitted entirely, and the conductor 36 may be connected to the contact of spring 30 directly as indicated by the lower dotted jumper.

The vprovision of the spare conductor 36 through the connection is made possible because the primary master switch is able to test over the release-trunk conductor 35 as this conductor is grounded immediately upon seizure by the line relay 24 of the primary line switch PLS.

Referring now to Fig. 2 it will be pointed out how the so-called individual and common trunking arrangement is applied to the trunk lines extending from the banks of two groups of primary line switches. Two primary master switches are shown, and two of the trunk lines associated with each master switch are shown. Trunk lines 2 and 3 are shown as`- sociated with the upper master switch, while trunk lines 4 and 5 are shown associated with the lower master switch. Each trunk line has its conductors brought out to the cross-connecting and block C. C. B., whereat jumpers are run to the cables leading to the I. D. F. Trunk 2 is individual to the irst line-switch group, and trunk 4 is individual to the second group, while trunks 3 and 5 are connected together through jumpers on the cross-connect block and lead through the cable set 7 and a jumper on the I. D. F. to the same secondary line switch. It will be noted that the cable sets 6 and 8 are cross-connected on the I. D. F. to first selectors directly instead of being cross-connected to secondary line switches.

Each individual trunk is provided with a relay such as relay 14 through the contacts of which a ground potential is supplied to rclay 15, which is common to all the relays such as 14 of the same group. Relay 15 maintains the local branch of the kick-off conductor of the common trunk disconnected from the main conductor and maintains the local branch grounded, preventing the master switch from stopping on the common trunk.

Vhen all the relays such as 14 are operated, relay 15 falls back and removes the ground potential from the local branch of the kick-o conductor of the common trunk and reconnects the local branch to the main conductor. The common trunk may now be used in the usual manner.

The common trunk, of course, is still busy to the other master switch and remains so until such time as all trunks individual to the other master switch become busy, at which time relay 17 is caused to 'fall back owing to the operation of all relays such as 16, giving the lower master switch access to the common trunk. Only one common trunk is shown, but the relay 15, it will be understood, is provided with suficient contacts to handle several common trunks.

Since both master switches may stand on the same trunk at the same time it is desirable to provide an arrangement for preventing a line switch of one group from seizing a trunk at the saine time that the trunk is being seized by a line switch of the other group. This is done by providing the series relay 13, in series with the open-ground conductor of the upper primary master switch with contacts in series with the open-ground conf ductor of the lower master switch. Relay 13 operates when a line switch associated with the upper master switch is being operated and prevents a simultaneous operation of a line switch associated with the lower .im

switches are standing on the same trunk or not, but it was not thought to be suiiiciently objectionable to take the trouble of providing arrangements for rendering the series relay operative only when needed.

It will be noted of course that the trunk lines 9 and lO extending from the right hand side of I. D, F. to first selectors are not provided with kick-ott conductors, the corresponding terminals on the left hand side of the l. D. F. being lett dead.

Although each jumper is indicated by a single line, it will be understood that each jumper contains as many conductors as there are contacts embraced by the brackets between which the jumper symbol extends.

What is claimed is:

l. In a telephone system, a plurality ot non-numerical switches each having a line relay, a circuit for each switch closed by the associated line relay, and series contacts on all said line relays arranged in a chain so that when a pluralityT ot line relays are operated only one can close the circuit ot the associated no-n-numerical switch, a signal, and means for operating said signal responsive to an open condition in the contact chain.

2. In a telephone system wherein the subscribers7 lines are arranged to be extended by line switches presided over by a co nmon master switch and wherein the line switch operating contacts of the line relays ot the line switches are series connected in an operating chain, a signal, and means responsive to an abnormal condition within the master switch or to an open condition of the contact chain outside ot the master switch for operating said signal.

' 3. In a telephone system wherein a line switch individual to a line is provided with a. line relay operable over the associated line when a. call is originated thereon and is provided with a cut-od relay operable over a j test conductor when the line is tested on an incoming call and found to be idle, and contacts on the line relay effective when the line relay is operated to disconnect the said test conductor from said cut-oill relay and for connecting it to a source of guarding potential.

t. In a telephone system employing a plurality otl line switches and provided with a master switch common to said line switches to preselect an idle one of a plurality or" trunk lines, an o eratiug chain circuit for vidual to each line switch, an operating chain circuit for any line switch including contacts ot its individual line relay and contacts of the line relays ot all preceding line switches, and contacts in the master switch for opening the chain circuit to all line switches whenever no idle trunk is preselected, a signal, and means tor operating said signal responsive to said chain circuit being opened either at the master switch or at any line switch tor longer than a predetermined interval. y

6. ln a telephone system wherein line switches of the plunger type are presided over by a mast r switch and have access to a plurality ot 4our-conductor trunk lines, contacts in the bank oi' each line switch arranged to extend a connection to each conductor ot a trunk line, trunk testing means included in the master switch arranged to test said trunk line through the medium of the said releasetrunk conductors thereof independent of the said fourth conductors.

7. ln a telephone system wherein nonnumerical switches are presided over by a master switch and wherein an operating cir cuit tor the non-numerical switches is provided with operating current through contacts associated with the master switch, means eiilective while the master switch is operating to preselect an idle trunlf tor altering the current-supply circuit so that its resistance is too great .tor any non-numerical switch to be operated thereover, and means responsive to an attempt to operate a non-numerical switch over said circuit at such time for registering the attempt.

8. In a telephone system wherein nonnumerical switches are presided over by a master switch and wherein an operating circuit tor the non-numerical switches is provided with operating current through contacts associated with the master switch, means effective while the master switch is operating to preselect an idle trunk tor altering the current supply circuit so that its resistance i-s too great for any non-numerical switch to be operated thereover, means responsive to an attempt to operate a non-numerical switch over said circuit at such time tor registering the attempt, and a timing device arranged to prevent immediate registration ot an attempt to operate a non-numerical switch while the master switch is operating until sufficient time has been allowed. to enable a trunk to be preselected it one is idle.

9. ln a telephone system wherein calling subscribers lines are extended to switching` apparatus through the medium of primary and secondary plunger-type line switches, the secondary line switches being accessible to primary line switches, and the switching apparatus being accessible to the secondary line switches, a separate primary line switch for each calling line, a line relay individual to each primary switch operable over a calling line to operate the line switch indivi-dual thereto to seize a secondary switch, and means including a line relay individual to the seized secondary line switch and operable over the calling line for operating the secondary line switch to seize a trunk line eX- tending to the switching apparatus. V

10. In a telephone system, a plurality of non-numerical switches presided over by a common master switch and having access to a plurality of trunk lines, means including automatic switching apparatus for seizing any one of said non-numerical switches, means responsive to such seizure for operating the seized switch to seize an idle trunkline, and means including` a chain circuit extending through contacts of the non-numerical switches in series for preventing simultaneous seizure of the same trunk line by two non-numerical switches.

ll. In a telephone system, two groups of non-numerical switches, an operating cir cuit for any switch in either group including` chain contacts in any preceding switches of the same group arranged to prevent simultaneous operation of two switches of the saine group, and a relay connected in the operating circuit of the line switches of one group and arranged to open the operating circuit of all the line switches in the other group when a line switch in the first group is operated so as to prevent simultaneous opera tion of line switches in the two groups.

12. In a telephone system wherein nonnumerical switching apparatus is accessible to automatic switching apparatus and has access to trunk lines, each of said non-numerical switches being so arranged that when it is seized it closes a holding circuit for the seizing apparatus momentarily and prepares a holding circuit for itself and the Vsaid apparatus over a trunk line when it seizes the same, means eective when there are no trunks idle to the non-numerical switches for rendering the. remaining idle non-numervical switches busy to the said automatic switching apparatus and for opening the temporary holdin@- circuit at any non-nib merical switch at which it happens to be closed.

13. In an automatic telephone system wherein primary and secondary line switches are operated in series relation by calling lines to extend such lines to the numerical switching' devices, the secondary line switches being arranged in groups, means within each group for rendering the remaining idle line switches of the group busy to t-he primary line switches when the last numerical device accessible to such group becomes busy, and means eifective in case a primary line switch seizes an idle secondary line switch of a group after the last numerical device becomes busy and before the said busying means becomes effective for releasing the primary and secondary line switches and for reoperating the primary line switch to seize a secondary line switch in a diiferent group.

14:. In a telephone system wherein a line switch individual to a line is provided with a line relay operated over the associated line when a call is originated thereon and is provided with a cut-off relay operable over aV test conductor when the line is tested on an incoming call and found to be idle, contacts on the line relay eective when the line relay is operated to disconnect the said conductor from the cut-off relay, and contacts in the line switch effective when the line switch operates to reconnect the test conductor to the cut-off relay.

15. In a telephone system wherein a line switch individual to a line is provided with a line relay operable over the associated line when a call is originated thereon and is provided with' a cut-ofil relay operable over a test conductor when the line is tested on an incoming call and found to be idle, contacts on the line relay effective when the line relay is operated to disconnect the said test conductor from the said cut-off relay and to connect it to a source of guarding potential, and contacts effective when the line switch operates for reconnecting the test conductor to the cut-o relay.

16. In combination,two groups of plunger line switches controlled by two master switches, respectively, means for operating any one of the line switches in either group, means controlled by either master switch for preventing operation of any line switch in the same group, and means for preventing simultaneous operation of any two of said line switches.

17. In combination, a group of line switches arranged to be controlled by a master switch, a second group of line switches arranged to be controlled by a second master switch, means for operating any line switch in either group including circuit arrangements controlled by the associated master switch, and means for preventing a line switch in the second group from operating while a line switch in the first group is being operated.

18. In combination, two groups of plunger line switches having access to trunk lines and controlled by two master switches, respectively, trunk lines individual to one group of primary line switches and extending to numerical switches, trunk lines individual to the other group of primary line switches and extending to numerical switches, and a group of trunk lines common to both groups of primary switches and extending to secondary non-numerical switches having access to numerical switches.

19. In a trunlring system, two groups of primary trunlring switches, secondary nonnumerical trunking switches, numerical switches, trunk lines individuall to one group of primary switches and extending to numerical switches, trunk lines individual to the other group of switches and extending to numerical switches, a group of trunk lines common to both groups of primary switches and extending to secondary non-numerical switches, and trunk lines leading from said secondary switches tonumerical switches.

20. In an overflow trunking system, two groups of plunger line switches, each group being controlled by a separate master switch, two groups of trunk lines individual to the two groups of line switches, respectively, overflow trunk lines common to both groups of line switches, cross-connecting blocks ar-1 ranged adjacent the groups of line switches to enable the ratio of individual trunks to overflow trunks to be changed readily, a group of relays connected to the individual trunks outgoing from the lirst group of line switches between the line switches and the cross connecting blocks, a similar group of relays similarly associated with the trunk lines individual to the second group of line switches, and means controlled by said relays for causing the associated master switches to permit a call to be extended through an overflow trunk line by a line switch only when all-of the trunk lines individual to the group containing such line switch are busy.

2l. ln combination, a plurality of groups of plunger line switches, trunk lines accessible to said groups of line switches, some of said trunk lines being individual to one group while others are common to two groups of line switches, and facilities for running jumper connections to alter the ratio of individual and common trunks accessible to a group of line switches, said facilities including cross-connecting jumper blocks located adjacent the groups of line switches.

22. In a trunking system for use in an automatic telephone system, selector switches, secondary line switches, an intermediate distributingframe, trunk lines vextending from terminals at the intermediate distribut-ing frame to said selectors, trunk lines extending from terminals at the intermediate distributing frame to said secondary line switches. groups of primary line switches, cross-connecting blocks associated with said groups, trunk lines outgoing from said line switches to said cross-connecting blocks, and conductors connecting other terminals of said cross-connecting blocks with other terminals on the intermediate distributing frame, facilities for ruiming jumpers at the cross-connecting blocks whereby trunk lines may be extended individually from a. line switch group to the intermediate distributing frame and whereby trunk lines from two line switch groups may be extended over common conductors to said intermediate distributing frame, and facilities for running jumpers on said intermediate distributing frame whereby trunk lines individual to a group of'line switches may be extendedto selector switches, `while trunk linesk common to two groups of line switches i common trunk line non-selectable by one of said master switches, while rendering it seisc-table by theother master switch.

2i, In atrunking system, two groups of plunger line switches, two controlling master switches associated with said groups, respectively,v two groups of trunk lines individual to said two groups, respectively, an additional trunk line common to both groups, means effective to render said com'- mon trunk line non-selectable to both master switches, and means responsive to all trunks individual to one master switch and the associatedline switches becoming busy for rendering the common trunk line selectable by such master switch, while maintaining it non-selectable tothe other master switch.

25. ln a trunking system, a group of plunger line switches having a master switch associated therewith, trunk lines individual to said group of line switches, another' trunk line common to said group of line switches and to other switches, means for rendering said trunk line normally non-selectable to said master switch, and means responsive to all of said individual trunk lines becoming busy for rendering said trunk line selectable to said master switch.

26. In a telephone trunking system, a group of line switches controlled by a master switch, a group of trunk lines `each oomprising a plurality of conductors and acceslll) lll() sible to said line switches certain of the conline switches to which said trunk lines extend by. way of jumpers on said intermediate distributing frame, additional trunk lines accessible to said groups of secondary line switches, each of the primary-to-secondiso ary trunk lines having an additional conductor therein extending back from the secondary group to the bank of a, master switch andl excluding the banks of the associated primary line switches, and means eiective when all trunk lines accessible to a secondary group of line switches are busy for placing a guarding potential on the said additional conductors of the trunk lines incoming thereto so as to make such trunk lines busy to the primary master switch.

9.8. In a trunking system, a plurality of groups oi' primary plunger line switches, a master switch associated with each group of primary line switches, a plurality of groups of secondary line switches, a group of trunk lines accessible to each secondary line switch, each or" said groups of trunk lines being smaller in number than the group of line switches to which it is accessible, trunk lines extending from said groups of primary line switches to said groups of secondary line switches, mea-ns including an intermediate distributing frame located between the primary group and the groups of secondary line switches for enabling each trunk line outgoing from a given primary group to be extended to a line switch in a different secondary group, means for operating said primary line switches under the control of the associated master switches to take said secondary line switches for use through the medium of said primary trunk lines and for operating said secondary line switches to take for use the trunk lines accessible thereto, each of said primary trunk lines comprising a plurality of conductors extending to a secondary line switch -from the banks oi the primary line switch and comprising an additional conductor extending from the bank of the master switch to the secondary group and excluding the banks of the primary line switches, and mea-ns effective when all of the trunk lines accessible to a group et secondary line switches become busy by placing a guarding potential on the said additional conductors of all trunk lines incoming to such secondary group.

29. In a telephone system, a group of secondary line switches controlled by a master switch, a. trunk line incoming to e-ach of said secondary line switches conprising talking conductors and a release trunk conductor, means including automatic switching apparatus for making connections to said secondary line switches by means of said talking conductors and said release trunk conductors, there being an operating magnet for each of said secondary line switches, an operating conductor extending from the secondary master switch through chain contacts on the line relays of all the secondary line switches, circuit connections for operating the line relay of the secondary line switch over the incoming line conductors when the secondary line Leonesa' switch is taken for use, and means controlled by the line relay for placing a guarding and holding potential on the release trunk conductor of the incoming trunk line and for closing an operating circuit for the operating magnet of the secondary line switch over said operating conductor and for opening the connection between the operating conductor and the succeeding secondary line switches of the group.

30. In a secondary plunger-type line switch for use in a trunking system in connection with a master switch common to said secondary line switch and to a plurality of others, a plunger-0perating magnet, line conductors incoming to said line switch, a line relay connected in circuit with the incoming line conductors, contacts controlled by said line relay when it is operated over the incoming line conductors to close a circuit ior said plunger-operating magnet, an armature on said magnetv for operating the plunger of the line switch, and contacts mechanically operated by the plunger armature for disconnecting said line relay Yfrom in circuit with the incoming line conductors.

31. In a trunking system, a group of plunger line switches controlled by a master switch, a separate line incoming to each of said line switches, each line switch having a plunger controlled 'by said master switch and having a magnet provided with an armature to operate the associated plunger, a line relay for each line switch connected to a talliing conductor of the associated incon'iing line and arranged to be'operated thercover to close an operating circuit for the associated plunger-operating magnet, and contacts operated by the plunger-operating armature of the magnet to disconnect the line relay from the incoming line conductor.

In witness whereof, I hereunto subscribe my name this 27th day of June, A. D. 1928. JOHN H. VOSS. In witness whereof, I hereunto subscribe my name this 27th day of June, A. D. 1928.

BERNARD D. VILLIS.

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