US2291040A

US2291040A - Switching system

Info

Publication number: US2291040A
Application number: US377990A
Authority: US
Inventors: William H T Holden
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1941-02-08
Filing date: 1941-02-08
Publication date: 1942-07-28
Anticipated expiration: 1959-07-28
Also published as: FR938845A; CH252653A; GB550210A

Description

July 28, 19 w. H. T. HOLDEN SWITCHING SYSTEM Filed Feb. 8, 1941 8 Sheets-Sheet l at GE m 59m 99m MGR m 6R R0 ma 3U wwfi MH W W m 3 v9 3 H c: I- 1 E 5 Ililllll? M QQ I: L \Q 09 H r -6 B Q W ll? L, T3 N 5 o\\ \O\ 1-- L w\\ lllll E 2 1 g m\ m9 K Q My A 3 6Q A T TORNEY y 1942- w. H. T. HOLDEN 2,291,040

SWITCHING' SYSTEM Filed Feb. 8, 1941 v 8 Sheets-Sheet 2 SECONDARY SWITCH 2 PRIMARY SWITCH 204 /NVENTO/? W H. T. HOLDEN ATTORNEY y 1942- w. H. 'r. HOLDEN SWITCHING SYSTEM Filed Feb. 8, 1941 8 Sheets-Sheet 3 o 0 o c o a O o W Q Q N uvvavron W H. I HOLDEN WW ATTORNEY y 28, 1942- 1 w. H. 'r. HOLDEN 2,291,040

SWITCHING SYSTEM Filed Feb. 8, 1941 8 Sheets-Sheet 4 MA RA E R5 INVENTOR m H r HOLDN WWWW? ATTORNEY y 1942- w. H. T. HOLDEN 2,291,040

SWITCHING SYSTEM Filed Feb. 8, 1941 8 Sheets-Sheet 5 VENTOR M. H. 7.' HOLDEN A T TORNE Y W. H. T. HOLDEN SWITCHING SYSTEM Filed Feb. 8, 1941 July 28, 1942. 2,291,040

8 Sheets-Sheet 6 INVENTOR M. H 2' HOLDEN WWW ATTORNEY FIG. 7

July 28, 1942.

w. H. T. HOLDEN 2,291,040

SWITCHING SYSTEM Filed Feb. 8, 1941 8 Sheets-Sheet '7 III INVENTOR W H. 7. HOLDEN ATTORNEY y 23, 2- w. H. T. HOLDEN 2,291,040

SWITCHING' SYSTEM WWTWW ATTORNEY Patented July 28, 1942 SWITCHING ,SYSTEM William H. T. Holden, Woodside, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application February 8, 1941, Serial No. 377,990

8 Claims. (Cl. 179- 18) This invention relates to switching systems and particularly to those used in the establishment of telephone and other communication connections.

The objects of the present invention are to simplify these switching systems, to increase their speed of operation, to reduce the cost, and to obtain other improvements therein.

In automatic telephone systems where large groups of lines have access through switches, such as line-finders or line switches, to trunk groups it has been proposed heretofore to arrange these switches in primary and secondary formation with linksfor extending the lines to the trunks and also to arrange the switches and lines in coordinate rows or groups, such as horizontal and vertical groups, for the purpose of obtaining an orderly inter-connection of the lines with the trunks. In such a switching system it has usually been necessary to provide relay chains and rather involved control circuits for serving the lines having calls thereon, for preventing interference between simultaneous calls in the different groups, for insuring selection in preferred horizontal and vertical groups, for selecting idle groups of trunks, and for selecting idle links having access to these trunk groups.

Accordingly, a feature of the invention is a system in which the relay selecting and testing mechanisms are replaced by electronic discharge devices which perform the selecting and testing functions with greater speed and with less complication in the control circuits. More specifically a plurality of discharge tubes are provided for the horizontal groups of lines and switches, and a second plurality of tubes are provided for the vertical groups. The presence of calls in these groups causes impulses to be applied to the corresponding tubes. These impulses have different phases characterizing the different horizontal groups, so that only one horizontal-group tube can discharge at a time, the discharged tube preventing other tubes from discharging by the simple expedient of altering the potential on their electrodes. Impulses of different phases are also applied to the vertical-group tubes, and the first tube to discharge in the series prevents any other tube from operating.

Another feature of the invention is a system in which the several groups of trunks to which a calling line, chosen to be served by the horizontal and vertical group selections above mentioned, has access are tested simultaneously with the links extending respectively between the chosen line and said trunk groups, said test being are applied to the corresponding test tube.

tubes. If the trunk group and the link having access to such group are both available, impulses The impulses applied to each tube difier in phase from those applied to other tubes, so that only one tube can discharge in the event there are two or more trunk groups available at the time the test is made.

A further feature is a testing system in which the lines of a selected vertical group are tested by a series of discharge tubes to determine which line shall be served provided two or more are calling at the time. The selection is determined by applying impulses of different phases to the tubes, the tube that is first to discharge serving to render the others inoperative.

These and other features of the invention will be described more fully in the following specification.

In the drawings accompanying the specification:.

Fig. 1 is a diagrammatic illustration of a frame of primary and secondary line switches for extending calling lines to idle trunks;

Figs. 2 to 8, inclusive, when arranged as illustrated in Fig. 9, show the detail circuits for controlling the operation of the primary and secondary line switches;

Fig. 2 shows a subscriber's line appearing in a primary switch and also shows a secondary switch for extending the lines to idletrunks;

Fig. 3 shows the horizontal-group tubes and relays for selecting a horizontal group" to be served;

Fig. 4 shows the vertical-group tubes and-relays for selecting a vertical group to be served. This figure also shows the tubes for testing the line links between the primary and'secondary line switches;

'Figs. 5,6- and 7 disclose the testing tubes for selecting a group of trunks to serve the calling line and for selecting an idle trunk therein. Figs. 5 and 6 also show relays for closing'the operating circuits of the switch magnets; and

Fig. 8 shows the testing tubes for choosing a particular line to be served when a number of lines are calling simultaneously.

Referring to Fig. 1, the frame of line switches illustrated therein comprises ten horizontal groups or levels of primary line switches of which three are shown, the No. 0 level I00, the No. 5 level llll, and the No.9 level I 02. Each of these primary line switches serves ten'subscribers lines, the lines appearing in the vertical rows of conperformed by means of a plurality of discharge tacts ofthe switches. The line links appear in the horizontal rows of contacts, which are multiplied from switch to switch in the corresponding horizontal group. The line switches are also arranged in vertical groups, and there may be as many as ten of these vertical groups. In the present disclosure, however, it will be assumed that there are seven vertical groups of primary switches, including the No. vertical group I03, the No. 1 vertical group I04 and -the No. 6 vertical group I05. Thus the primary switches I01, I 00, I09 constitute the horizontal group I00; primary switches IIO, III, I I2 constitute the horizontal group IOI; primary switches H3, Ill, III constitute the horizontal group I02; and other primary switches not shown constitute the remaining horizontal groups of switches. The lines appearing in each primary switch may be regarded as a subgroup of the corresponding horizontal group.

Ten secondary switches H6, H1, Ill are also provided on the frame, and the interconnecting line links serve to extend calls from each horizontal group of primary switches to each of the secondary switches. For example, the ten links I I! extend respectively from the horizontal group I00 to each of the ten secondary switches I I6, I I1,

III. Similarly, the ten links from each of the other nine horizontal groups extend respectively to the ten secondary switches. Each of the secondary switches has a group of ten outgoing trunks extending to succeeding selector switches and a group of ten incoming trunks for inward calls. For example, the outgoing trunk groups I20, I2I', I22 extend to district selectors and serve calls originating on the subscribers lines appearing in the primary line switches. Incoming calls to these subscribers lines are completed over the incoming trunks I23, I24, I25.

The primary and secondary line switches disclosed herein may bed the well-known crossbar type. The structure of these switches is disclosed in the patent to Reynolds, No. 2,021,329, of November 19,1935. Inthe present disclosure the invention is illustrated in the circuits and devices for controlling the operation of these primary and secondary line switches to extend calling lines to idle trunks leading to succeeding switches. Although common register senders are used for receiving the designations from the calling lines and for controlling the selective operation of the selector switches, these senders have not been included in the present disclosure. They could, of course, be readily provided, and an understanding of the manner in which the senders are associated with the selected idle trunks may be had from the patent to Busch et al., No. 2,224,251, of December 10, 1940.

A general description will now be given of tlie circuits and equipment shown in detail in Figs. 2 to 8. The subscriber's line 200 and nine. other lines not shown appear in the vertical rows of contacts in the primary switch I The line link 202 extending from the primary switch 20I appears in a horizontal row of contacts in the secondary switch 203. A second line link 204 extends from the primary switch 20I to another secondary switch notshown, Similarly the line link 20! extends from a primary switch, not shown,

and appears in the horizontal row of contacts in the secondary switch 203. Two trunks are shown appearing in the vertical rows of contacts of the secondary switch 203. The trunk 20 serves a subscriberfs line when calling and extends it, therefore, to a succeeding district selector switch. The trunk 201 serves to extend incoming calls to the subscribers lines and it leads, therefore, from an incoming selector switch. Trunk 200 is one of a group of ten similar trunks appearing in the secondary switch 203, and these ten trunks all extend to the same frame of district selectors.

The subscriber's line 200 is provided with an individual line tube 200 of the space discharge type in lieu of the usual line relay, and the same is true of all other. lines in the system. The starting electrode 200 of the tube 200 is connected through a resistance 2" and contacts of the hold magnet 2 to the conductors of the subscriber's line. The cathode H2 is connected over a group start conductor 2Il and through a resistance 300 and a contact of relay 400 to the negative pole of battery "I. When the subscriber makes a call, current flows from battery 22] over the loop of line 200 in a circuit including resistor 220. The drop across resistor 22! raises the electrode 200 to a positive potential, whereupon the control gap 200-2I2 ionizes. Conductor 2I0 is common to the tubes of the ten lines served by the primary switch "I; hence this conductor is multipliedto all ten of the cathode elements. In a similar manner the ten line tubes associated with each of the remaining six primary line switches in the horizontal row are connected to a common start conductor corresponding to the conductor 2I3. There are, therefore, seven of these start conductors for each of the ten horizontal rows or a total of seventy in all, and these start conductors all derive their current supply from battery I.

The purpose of these start conductors is to ionize the start tubes of the group 305, of which there are ten, one for each horizontal group of line switches. Two ar shown, tube 3" serving the No. 0 horizontal group and tube 30'! serving the No. 9 horizontal group. The tube 200 is provided with seven start electrodes which serve the seven start circuits (including circuit 2|!) individual to the seven line switches of the No. 0 horizontal group. For instance, the circuit 2I3 extends through a condenser "I and resistance 102 to one of these start electrodes, and in like manner the other six start circuits extend through corresponding condensers and resistors to the respective start electrodes of tube 3". And the same is true of the start circuits for the remaining nine horizontal groups. The remaining three start electrodes of each of tubes 30! are connected together and serve as a collector as will be explained-more fully hereinafter.

The anodes of the line tubes are connected to an impulse generator 221 designed to produce a series of impulses often-different phases. The impulse lead 2 for a particular phase (phase No. 0) is connected in multiple to the anodes of the tubes of all lines occupying the No. 0 position in all of the primary line switches.- It may be assumed that line200 occupies the No. 0 vertical row of contacts in the switch "I; therefore, the impulse lead 2 is connected to the anode III of line tube 200. Theimpulse lead 2Ii for phase No. 1 is similarly connected to the anodes of the tubes of all lines occupying the No. 1 vertical row of contacts in all of the vertical line switches. And the same is true of the impulse leads for the remaining eight phases. When, therefore. a line tube has its control gap ionized and the generator 221 delivers a positive impulse to the anode, such as anode 2i! of tube 200, the tube discharges its maingap, and an impulse of positive polarity appears on the start conductor 2I3.

The impulse generator 221 which produces the phase impulses for operating the line tubes, and the impulse generators to be described herein after in connection with other parts of the control circuits disclosed herein, may be of any suitable type, such as those shown in the application The No. horizontal-group tube 3 has a start-,

ing electrode 3|2 which is connected over conductor 308 to the collector electrodes of the tube 306. The other start electrode 313 is connected to the impulse lead 3 which delivers negative impulses of phase No. 0 from the impulse generator 349. The cathode 3l5 is connected to the negative pole of battery, and the screen electrode 3 I 6 is connected through a common resistance 3 I I to the battery supply lead 3l8. Finally the anode of the tube 3 is connected through the winding of the associated horizontal-group relay 320 to the battery supply conductor 3l3. The electrodes of the remaining horizontal-group tubes are connected in a similar manner, the starting electrodes corresponding to electrodes 3I3-35l extending respectively to the impulse conductors for the successive phases. The screen electrodes 3I632l of the ten tubes are connected in multiple and through the common resistance 3|! for the purpose of looking out all tubes as soon as one of them has operated. This lock-out function will be explained more fully hereinafter.

The ten horizontal-group relays 320-32l serve to extend the seventy start conductors through to the vertical group tubes 405. There are seven of these vertical-group tubes 406-401, one for each of the seven vertical groups of primary line switches. The start conductor 2l3 pertaining to the line'switch 20I in the No. 0 vertical group and in the No. 0 horizontal group is extended by way of condenser 322 and conductor 323 through the contacts of horizontal-group relay 320 over conductor 324, condenser 406, resistor 409 to the starting electrode 4| 0 of the verticalgroup tube 406 pertaining to the No. 0 vertical group. Likewise the other six start conductors 325 pertaining to the other six primary switches in the horizontal group No. 0 and in the respective vertical groups are extended through contacts of relay 320 to the start electrodes of the other six tubes of the group 405. One of these, start conductor 326, pertaining to the No. 6 vertical group, is shown leading through the contacts of relay 320 thence over conductor 321, condenser l, resistor 4l2'to the start electrode 4 I3 of the No. 6 vertical-group tube 401. The other start electrodes 4l4--4l5 are connected in multiple and through a common resistor 416 to the negative pole of battery. Theanodes 4ll-4l8 are connected respectively to the windings of associated vertical-group relays 9-420 to the battery supply conductors 42l and M8. The screen control electrodes 422-423 are connected in multiple through a common resistor 424 to the battery supply conductor 42l. The purpose of the group of tubes 405 is to select one vertical group of lines for service to the exclusion of other vertical groups having simultaneous calls;

and the purpose of the associated relays 4 I 9420 is to prepare operating circuits for the ten select magnets of the primary switch in the chosen horizontal and vertical groups.

Having chosen to serve the lines in a primary line switch of a particular horizontal group and of a particular vertical group by operating, the corresponding one of the start tubes 305 and one of the horizontal-group tubes 3l0 and the associated relay and by operating the-corresponding one of the vertical-group tubes 405 and the associated vertical-group relay, it is now necessary to make a joint test of each line link serving the chosen primary line switch and the group of trunks reached by the line link. This may be seen more clearly byyreferring to Fig. 1. If, for example, the line switch 3 is chosen to serve the group of ten lines '-"appearing therein, then a test is made of the ten line links I26 and the ten groups of trunks reached respectively by these line links. A line link is not available for extending a call if it is busy or if all of the trunks to which it has access are busy. However, all links that are idle and have access to trunk groups containing idle trunks are available. and a choice is made of some one of these available paths in a manner to be described hereinafter.

The line links are tested by a group. of ten tubes 430, each of the tubes 43l--432 serving ten links. The ten links served by the tube 431 are those which have access through a particular secondary switch to the associated group of trunks. Therefore, the start electrodes of this tube represent respectively a link in each of the ten horizontal groups. Likewise the ten start electrodes of tube 432 represent respectively a link in each of the ten horizontal groups. And the same is true of the other eight tubes in the group 430. Corresponding electrodes in each of the ten tubes are connected in multiple relation. When the horizontal group is chosen, such as the horizontal group represented by the tube 3 and relay 320, a circuit is closed for applying potential to the start electrodes, one. in each of the ten tubes 430, corresponding to the ten line links serving the chosen horizontal group. This circuit may be traced from the positive pole of battery 328 through the contacts of group relay 320, conductor 329 thence in parallel through

resistors

425, 426 and 421, 426 and through the other sets of resistors (not shown) to the starting electrodes of the ten tubes of the group 430. Thus a test circuit is prepared for the ten line links that serve the chosen group of lines. The positive battery potential on the start electrodes of these tubes indicates that the associated links are idle. However, any links that are busy at the time the test is made have ground potential on their test or sleeve conductors, and this ground potential is impressed upon the start electrodes of the tubes to identify them as busy links. For example, if the link 202 is busy, ground potential on the sleeve conductor 2" is extended by way of conductor 2! and resistor 426 to the associated start electrode of tube 43I. Similarly, if the link 204, is busy, ground potential is extended over conductor 2l9 and through resistor 426 to the start electrode representing this link in the tube 432. The anodes of the link test tubes 430 are connected respectively to the ten phase impulse conductors over which positive impulses are supplied from the generator 433. The cathodes of these tubes are connected branches which includeresistors lll, Ill, Ill,

etc. When all trunks are idle, all of these branches are open, and each trunk taken in use causes the closure of the associated resistance branch. The several resistance branches are connected over a common conductor IlI through a common resistor lll to the negative pole of battery Ill, and the conductorlll connected to the other terminals of the branch resistors, extends over conductor III through the contacts of relay lll tothe positive pole of battery lll. The conductor 'IlI is also connected tothe midpoint of the secondary winding of the transformer H2 and thence in simplex over conductors III to the midpoint of thewinding of transformer Ill over conductor'lli to the start electrode of tube Ill, which is one of a group of ten tubes 120 individual respectively to the ten groups of trunks. The primary winding of transformer H2 is connected in multiple through the back contacts of relays individual to the ten trunks of the group, thence over conductors I23 to the ten phase impulse leads of generator I24. The ten trunk relays I25, I26, 121, etc., are normally deenergized when their respective trunks are idle and are in an operated condition when the trunks are busy. Each idle trunk, therefore, in the group of ten is indicated by the presence of impulses of the corresponding phase in the primary winding of the transformer H2. If all of the trunks are busy theresistance branches Ill, Ill, Ili are all open under the control of operated trunk relays, such as the relays I25, I2l, 121, etc., and no circuit exists for the flow of current over conductors Ill and II I. Therefore, the full negative potential of battery Ill is impressed on the start electrode I2l of the tube Ill which represents the trunk group. Under this condition the tube Ill does not ionize since,

aswill be explained hereinafter, the cathode I2! is also at negative battery potential. If one trunk in the group is idle and the rest are busy, a single one of the branches is closed, and current flows from battery lll over conductors Ill and Ill and through the closed branch to battery Ill. The flow of current in the resistor lll makes the electrode 1" more positive but not enough so to cause the ionizationof the tube Ill. If,-however, two or more trunks are idle in the group, the amount of current flowing produces a drop in resistor Ill suificient to cause the ionization of the tube Ill.

In like manner the remaining trunk groups, each of which is represented by a test network lll, "2, are connected to the phase impulse leads of generator I2l and are tested for the presence of two or more idle trunks by the respective test networks and the individual tubes of the group I2l.

The matching testfor determining an idle line link having access to a group of trunks containing an idle trunk is performed by the tubes ll2 and lll under the simultaneous control of the tubes lll and I2l, and the result of this test is ten tubes lll. Each of the tubes of the group lll represents a line link and the group of ten trunks to which the link has access. For example, the No. tube lllrepresents the No. 0

5 line link of any horizontal group and the group of ten trunks accessible to the No. 0 link, and the No. 9 tube I02 represents the No. 9 link and the associated group of trunks. To explain the matching test of links and trunk groups assume 10 that each of the trunk groups represented by the test nets Ill and Ill has two or more idle trunks therein and that the trunk group represented by the test net "2 has only one idle trunk. On this assumption tube lll has its control gap ionized by means of a circuit traceable from conductor 101, which is now positive, through the windings of transformers H2 and Ill, resistor Ill, control gap I2l--I2l, conductor Ill, conductor I32. winding of transformer lll, conductor go lll, normal contact of relay ill to the negative pole of battery ill. Also the tube I2I ionires in a similar circuit. Tube Ill, however, does not ionize since the current flow through a single resistance branch of thenetwork "2 does not make the conductor Ill sufllciently positive. The

anodes of the tubes I2l are connected respectively to impulse leads of generator 125 which delivers positive impulses of the ten phases No. 0 to No. 9. When, therefore, the impulse generator Ill proso duces the next impulse of phase No. 0 a positive potential is applied to the anode Ill of tube Ill, and current flows from the anode to the cathode I2l over conductor Ill to the negative pole of battery lll as previously traced. The current impulse of No. 0 phase flowing through the pri-,

mary windlng of transformer lll induces an impulse of like phase but of negative polarity in the secondary winding, and this negative impulse is applied to the start electrodes lll and I of the tubes "2 and ill. If at this instant the line link,such as the link 202, having access to the trunk group represented by the test net Ill is idle, the start anode ll! of tube lll is at positive potential, and ionizing current is flowing from the electrode lll to the cathode lll and thence through inductor 431 to the negative pole of battery-lll. At this same instant, namely, when the transformer lll is delivering a negative impulse to the electrodes lll and ill, a positive impulse 01110. 0 phase is applied over impulse lead lllto the anode lll. Since the tube lll is ionized, current ilows from the anode lll to the cathode lll thence through inductor lll to the negative pole of battery lll. This current flow produces a positive, impulse on conductor lll which is applied to the start electrodes lll and lll of the tubes 2 and lll. Therefore, these tubes ionize their control gaps, and a circuit is now closed from impulse conductor lll of phase 50 No. 0 of generator lll, anode lll, cathode lll,

resistance lll, to the negative pole of battery ll2. This flow of current produces an impulse of positive potential which is transmitted through condenser lll over conductor lll, normal contacts of relay lll and conductor lll, resistor lll to the start electrodes lll, 501, etc., of the ten tubes lll. Since the impulse generator lll is delivering animpulse of No. 0 phase at this particular instant, this No. 0 phase impulse appears on impulse lead ill, and the other start electrode Ill 01' the tube lll is raised to positive potential. The start gap lll-509 of the tube lll now ionizes. The positive potential on conductors lll and 42! is applied by way of conregistered by the operation of one of the group of ductor Ill and common resistor Hi to the control grids 5I2, 5| 3, etc., of the tubes 500. Therefore, transfer takes place in the tube 50!, and current flows from the positive pole battery over supply conductor 5l0, winding of the district group relay 5, anode 5l5, cathode M6 to the negative pole of battery. Thus the single tube 50l of the group 500 is selected and operated to identify an idle line link having access to a group of trunks containing at least two idle trunks.

.Current also flows through the common resistor 5H and the control grid 5l2 to the cathode 5l6. This flow of current in the common resistor produces a drop of voltage which lowers the positive potential of the control grids of the remaining tubes 500 to prevent another one from discharging even though other links and associated trunk groups are available for use.

When a moment later the impulse generator 135 reaches phase No. 1, an impulse is delivered over lead 131 to the anode of the ionized tube 12!, and a current impulse flows through transformer 604. The transformer 604 delivers a negative impulse of phase No. l to the

tubes

602 and 603, and one of these tubes may ionize its. control gap provided at the same instant an impulse of phase No. l is flowing through one of the tubes of the group 430 indicating the idle condition of the line link having access to the trunk group represented by the network 10L 'The ionized tube 602 delivers an impulse to the starting electrodes of the tubes 500, but the corresponding tube in the group 500 cannot ionize at this time since it is locked out by the potential drop in the resistor 5| l as a result of the operated condition of the tube 50L Two

tubes

602 and 603 are used to secure a sufiicient deionization interval between successive impulses, and for this reason oddnumbered impulse phases are connected to anodes of tube 803 and even-numbered to 602. For longer intervals between pulses one 10 anode tube could be used, or for shorter intervals 3 or more might be needed.

The trunk group relays 5H, 511, etc., serve to complete operating circuits for the switch ma nets and also to extend the test circuits for the trunk groups through to the trunk test tubes 6 I 5. The operation of one of these relays, therefore, such as the relay 5, prepares the testing circuits for the group of trunks represented by the network 100. If the trunk represented by relay 125 is one of the two trunks of the group assumed to be idle, this relay is in a deenergized condition, and an impulse is delivered over the following circuit as soon as the impulse generator 124 reaches phase No. impulse lead 138, conductor I39, resistor 140, normal contacts of relay 125 through the primary winding of transformer H2 to ground. This impulse is induced in the circuit H3 and is in turn induced in the secondary winding of the transformer H4, causing a current impulse to flow over a conductor Ml through the contacts of relay 5, conductor 5l8, normal contacts of relay 400, conductor 2, resistor BIG and thence in multiple to the start electrodes 6, BIB, etc., of the ten tubes 0|5, each of which represents one of the trunks in the group of ten. These start electrodes 6", H8, etc., are raised to positive potential in the phase No. 0. At this same instant an impulse of negative polarity of phase No. 0 is applied from generator 632 over the impulse lead 6|9, resistor 820 to the other start electrode 62| of the tube 622. Tube 622 alone ionizes its control gap, and, since the control grid 635 is supplied with positive potential by way of conductors M0 and 823, transfer takes place to the main anode and current flows from the positive pole of battery over conductor 823 through the winding of relay 624, anode 025, cathode 626 to the negative pole of battery. Current also flows through the common resistor 621 in circuit with the control grid 035, and the voltage drop across the resistance lowers the potential of the control grids of the remaining tubes to prevent a second one from discharging. The flow of current in the anode circuit operates relay 624, which identifies the selected idle trunk in the group and prepares an operating circuit for the corresponding hold magnet of the secondar switch 203.

Having selected the horizontal and vertical groups to be served by operating the corresponding relays, such as relays 320 and M9, and having chosen an idle line link and an idle trunk in the group served by the line link, it now becomes necessary to choose the particular one of the ten subscribers lines in the primary line-switch that is to be served first, a number of which may be calling simultaneously. This line selection is made by the group of ten test tubes 800, each tube representing one of the ten lines in the chosen primary line switch. The start electrodes 801, 802, etc., of these ten tubes are connected individually to the phase impulse leads of generator 803. The

other start electrodes

804, 805, etc., are connected in multiple through the common resistor 806 and condenser 801 to the impulse lead 808 which will be described presently. The cathodes 809, 8|0, etc., are connected in multiple through the common resistor 8 to the negative pole of battery M2. The anodes M3, 8, etc., are connected in multiple to the common conductor 8l5. The collector electrodes BIG, 8, etc., are connected individually to the electrodes of a corresponding group of tubes 820,

there being one of these tubes for each of the tubes in the group 800.

The impulse conductor 808, which is connected to the starting electrodes of the tubes 800, is an extension of the starting conductor 2l3 individual to the group of ten subscribers lines in the chosen primary line switch. This extended start circuit may be traced from conductor 2I3 over condenser 322,'conductor 323, contacts of horizontal-group relay 320, conductor 324, contacts of vertical-group relay M9 and thence to conductor 808. If a plurality of subscribers lines are calling simultaneously in the group of ten lines, impulses of the corresponding phases are transmitted from the impulse conductors 2H,.

2 l6, etc., through the individual line tubes to the start conductor 2l3 and thence to conductor 808 and to the starting electrodes of the tubes 800.

'If, following the completion of this extended start circuit, the first impulse to be transmitted is the impulse of No. 0 phase, the'tube 8l8 alone ionizes its control gap since impulses are applied to its control electrodes simultaneously. The No.

0 phase impulse transmitted over the start conductor 008 to the control electrode 804 of tube 8l8 and to the corresponding control electrodes of the remaining ninetubes isof positive polarity; whereas the impulses delivered by the impulse generator 803 are of negative polarity. Therefore, a positive impulse is applied to electrode 804 and simultaneously a negative impulse to electrode The ionization of the control gap of tube all! transfers to the main anode as soon as the anode circuit'is closed following the measurement of a time interval. This interval, which will be described presently, is introduced to give the select magnets of the primary line switch time in which to operate.

At the time relays I20 and 4|! operate a circuit is closed from the negative pole of battery through the winding of connecting relay 220, conductor 323, contacts of relay 220. conductor 324, conta cts'of relay 9, conductor 808, winding of relay 8 l9 to the positive pole of battery. Relay ll! operates and closes the circuit from positive pole of battery thence over contacts of relay 332, which is being held operated as will be explained more fully hereinafter, contacts of release relay 333, conductors lit, 42l and Bill, contactsof operated relay conductor H9, contacts of relay H9, resistor 82L condenser I22 to ground. The condenser 822 charges and measures an interval of time, at the end of which suflicient potential is applied across the electrodes I23 and 2240f tube 825 to ionize the same. Thereupona circuit is closed from positive battery potential on conductor 5| 9 through the winding of relay I26 through the anode and cathode gap of the tube 825'to the negative pole of battery. Relay .2 operates and extends positive battery supply conductor 5|! to conductor "5 and thence to the anodes of the tubeslllfl. As soon, therefore, as the control gap 80l8lil of the tube 8 ionizes as above described, ionization transfers to the main anode 8H. and current flows in a circuit traceable from the positive pole of battery thence over supply conductor BIS, contactsof relay 828,

conductor 8| 5, anode H2, cathode 8" through the common resistor OH to the negative pole of battery 8| 2. The voltage drop across the resistor 8H lowers the potential of the cathodes of the remaining tubes of the group ill! to prevent another one from discharging subsequently if the corresponding lines are in a calling condition.

After the tube 8i 8 discharges, current flows from the anode M3 to the collector electrode BIG thence through resistor 82! and resistor ill to the negative pole of battery 2. The flow of current in resistor 821 increases the positive potential of the electrode of tube 829. At this time the other control electrode 830 is connected by way of conductor 83L contacts of relay 220, winding of the primary hold magnet 2 to the negative pole of battery. It will be recalled that relay 220 is included in the circuit with relay BI! and is operated at this time. Tube 82! ionizes, and a circuit is now closed from the positive pole of battery 832, secondary winding of transformer 832. winding of relay 834, conductor "5, anode 838 of the ionized tube 829, cathode l3. and thence as traced through the winding of the hold magnet 2 to the negative pole of battery. The current flowing in this circuit, which is the sum of the current supplied by the direct current batteries and the alternating current generator 831, causes the operation of the primary'switch hold magnet 2 H to complete the connection from the calling line through to the selected idle trunk. Upon the extension of the connection to the selected trunk, ground potential is applied to the sleeve conductor of the trunk in any well-known manner to maintain the hold magnet 2| l energized. When the alternating current source t3! enters the next opposing alternation the current flowing in the circuit of tube 829 is insumcient to maintain the main discharge from anode to cathode. The tube quenches and does not again discharge since the cathode "I! is now maintained at ground potential by way of the test conductor of the selected trunk. The relays I and "I measure an interval of time and then bring about the release otthe controlling circuits.

If at the time the testis made of the trunk groups no group'has two or more idle trunks therein a further test is made to determine whether there is any group having a single idle trunk. This reserve test is made under the control of the tube l2. and the associated relay 3. In order that a more complete explanation maybe given of the operation of the system, the assumption will now be made that the subscriber's line 2ll-initiates a call and is connected through the primary and secondary line switches 2M and 2.2 to an idle trunk. It will also be assumed that line 2 is the No. 0 line in the primary switch at the intersection of the No. 0 horizontal and verticalgroups on the line switch frame. When the subscriber of line 200 closes-his line loop, the individual line tube 208 ionizes in the manner hereinbefore described, and an impulse oi positive'polarity of No. 0 phase superimposed on direct current of negative polarity is applied to the start conductor 2". The impulse circuit may be. traced from the negative pole ofbattery 3, normal contacts of relay 4", conductor 4, impulse generator 221, thence over the No. 0 phase impulse lead 2, anode 2|, cathode M2 to the start lead 2". The impulse produced by the generator 221, which is of positive polarity, appears on the start lead 2|! and is applied through condenser ill and resistor "2 to the start electrode I of the start tube 3". Since the cathode I" is at negative potential, ionization occurs, and a discharge circuit is closedfrom the pomtive pole of battery I", interrupter 331, when closed, left winding of relay 332, anode III, cathode I" to the negative pole of battery 3.

The interrupter ,1" is designed to close the discharge circuit above traced at successive intervals and to maintain it closed for a period suflicient to permit the operation of the control circuits and the completion of the connection from the subscriber's line to the trunk. During this closed period of the interrupter, relay 332 is operated to close certain control leads, and the relay is made somewhat slow to release to provide a sumcient interval following the opening of the interrupter. If the interrupter 331 is open at the time the subscriber initiates the call, the phase impulses continue to occur in the start conductor 2i! until the interrupter 331 closes, at which time the main discharge circuit of the tube ll becomes eflective as above described. The relay I32 operates in response to the flow of current in the discharge circuit of tube 306 and connects the positive pole of battery I through the normal contactsot release relay 233 to the battery supply conductors m, 2|, Ill and 23.

The potential set up on the collector electrodes 3 is applied over conductor ill to the start electrode N2 of tube ii I; and, when generator 3 applies the next impulse of No. 0 phase to lead 3, the tube 3 ionizes its control gap and the discharge transfers to the main anode, resulting in the operation of the associated horizontal-group relay 320. The operating circuit for relay 320 may be traced from conductor ill, which has positive potential thereon, winding of said relay, anode Iii, cathode ii! to the negative pole of battery 2. As hereinbefore explained, the discharge of tube 3 causes current to flow in the circuit of the control grid ill to disable the other tubes of the group IIO.

When the horizontal-group relay 320 operates, the start lead 2!! is extended over condenser 322 and conductor 323 to conductor 324, which further extends through the condenser 406 through resistor 449 to ground and through resistor 409 to the start electrode 410 of the vertical-group tube 406. The tube 406 lonizes its control gap, and immediately current flows from the positive pole of battery over conductor 421 through the winding of the vertical-group relay 419, anode 411, cathode 445 to the negative pole of battery 446. Relay 419 operates to identify the vertical group containing the calling primary line switch 201. Current also flows between the gride 422 and the cathode 445 in a circuit including the common resistor 424, and the voltage drop across this resistor makes the grids of the other tubes less positive, thereby preventing a second one from transferring its ionization to the main anode.

If two vertical groups are calling at the same time and the calling lines therein occupy the same position in the line switches, impulses of the same phase are delivered over the start wires to the electrodes 410-413 of the corresponding tubes 406401. Due to the varying characteristics of the tubes, one of them will increase its ionizing current to the transfer value in advance of the others. This flow of transfer current, although small in value, sets up a voltage across the common resistor 416 which lowers the potential on the start electrodes of the other tubes, thus impeding the ionization of a second tube until the first one has transferred its ionization to the main anode.

The horizontal-group relay 320 in operating closes a circuit from ground over conductor 343, contacts of relay 320, conductor 344 through the winding of the select magnet 221 of the secondary switch 203. The select magnet 221, which is individual to the horizontal row of contacts in which appears the single line link 202 extending from the'calling primary switch 201, prepares the contacts in this horizontal row for connecting the link to any one of the group of ten outgoing trunks, such as the outgoing trunk 206. Relay 320 also closes an operating circuit for relay 330 which may be traced from battery, resistor 345fwinding of relay 330, contacts of relay 320, conductor 346 to ground at the normal contacts of relay 400. Relay 330 applies battery potential over the leads 628 to the trunk group testing circuits shown in Fig. '1 preparatoiyto the performance of the group test to determine which groups have idle trunks therein. Moreover, the horizontal-group relay 320 applies positive battery potential over conductor 329 to one of the start electrodes in each of the ten line.

link test tubes 430-.

Assume that the group ing the trunk 206 to which access has two or more idle trunks therein and that this group of ten trunks is represented by the testing network 100. Further, it may be assumed that trunk 206 is one of the idle trunks and i re resented by tirat relay 126 represents another of the idle trunks in the group.

Relays

125 and 126 are, therefore, in a released condition, and the circuit branches 104 and 105 are closed. As herembefore explained, the current flowing from battery 331 over conductor 111 through branches 104 and 105 in parallel and resistance 106 to the negative pole of battery 109 produces a potential drop across resistor 108 which is applied to the electrode 128 of the cause the ionization of said tube. As soon thereof ten trunks includthe line link 202 has the individual relay 125 and tube 116 and is suflicient to rent flows in the main discharge gap of the tube 116 producing an impulse in the transformer 604 which causes the application of a negative potential to the

electrodes

606 and 601 of the

matching test tubes

602 and 603. If at this same instant the line link 202 is idle, no ground potential is applied over conductor 216 to the starting electrode 435 of the tube 431, and the starting gap of this tube is in an ionized condition when the positive impulse of No. 0 phase is applied over the impulse lead 439 to the anode 440 of the tube. Hence, discharge current flows in the main gap of tube 431, and an impulse is applied over conductor 434 to the

electrodes

600 and 601 of the

tubes

602 and 603. These tubes ionize their starting gaps; and, since at this same instant a positive impulse of No. 0 phase appears on generator lead 608, current flows over anode 606, cathode 610, resistor 6H and the negative pole of battery 612. This flow of current impresses an impulse on conductor 614 which is applied over the circuit previously traced to the

start electrodes

506, 501, etc., of the ten trunk group tubes 500. At the same instant that this impulse appears on the

electrodes

506 and 501, an impulse of No. 0 phase is applied over generator lead 508 to the electrode 509 of the tube 501. This tube discharges to the exclusion of the remaining tubes and identifies the selection of the trunk group containing the idle trunk 206. The operation of the tube 501 results in the flow of. current from conductor 510 through the winding of relay 514, anode 515, cathode 516 to the negative pole of battery 521. Current also flows between control grid 512 and cathode 516 in a circuit including common resistor 51 1, and the drop across this resistor lowers the potentials on the grids of the remaining tubes to prevent a second one from operating subsequently. Relay 514 in operating closes a circuit from ground over conductor 522,

contacts of said relay, conductor 523, resistance 345 to battery. This circuit shunts the relay 330, and the latter releases to remove battery potential from the conductors 628 so that the trunk groups may be available for test at other line switch frames. Before relay 330 releases, however, to remove battery potential from conductor 110, the relay 514 closes a substitute circuit from the positive pole of battery 524, conductor 525, contacts of relay 514, conductor 526 to conductor 110. Thus battery potential is maintained on the test circuits of the selected trunk group until the connection to an idle trunk therein is completed. The operated trimk group relay 514 also completes a circuit for energizing the primary switch select magnet 222. This circuit may be traced from battery through the winding of said magnet, conductor 223, contacts of the vertical-group relay 419, conductor 441, contacts of relay 514 to ground. The magnet 222 operates and prepares the contacts in the horizontal row in which the selected line link 202 appears.

Next, an idle one of the trunks in the chosen group isv selected for use. If all trunks in the group except those represented by relays and 126 are busy, one of these two trunks will be selected depending upon the phase of the impulse generator 124 at the time relay 514 is operated. If, following the operation of relay 514 an impulse appears on the No. 0 phase lead 136 prior to the appearance of an inpulse on the No. 1 phase lead 143, the trunk represented by relay 125, namely, trunk 233, is selected. The

positiveimpulse of phase No. on lead 133 is applied over conductor 133, resistor 133, normal contacts of relay 123 to the primary winding ing electrode 32| of the tube 322. This tube,

ionizes, and current flows from the positive pole of battery over conductor 323, winding of relay 523, anode 325, cathode 323 to the negative pole of battery 323. Current flowing through the common resistor 321 in the circuit of the control grid 635 alters the potential oirthe control grids of all other tubes to prevent a second one from operating. Relay 323 in operating closes an op-- erating circuit for the secondary switch hold magnet 223, which is individual to the selected idle trunk 233. This circuit may be traced from battery through the normal contacts of relay 333, conductor 333, operating winding of select magnet 233, conductor 223, contacts of relay 323, conductor 333, contacts of relay |3 to ground. I

The lines in the primary line switch 23| are now tested to determine which ones are calling and to select one of these calling lines for connection to the selected trunk 233. Since line 233 is in a calling condition, the start conductor 2|3 is extended through condenser 322, conductor 323, contacts of relay 323, conductor 323, contacts of relay 3| 3, conductor 333 through condenser 331 through resistor 333 to ground and through resistor 333 to the

start electrodes

333, 333, ,etc., of the ten line test tubes 333. Since the multicontact relay 223 is connected in multiple to the conductor 323, it operates in series with relay 3| 3 which is connected to the conductor 333. The relay 223 is individual to the primary line switch 23| and serves to extend the operating circuits of the hold magnets through to the cathodes of the ten corresponding discharge tubes of the group 323. Relay 3|3 in operating extends the conductor 5| 3 through resistor 32| to the condenser 322. Condenser 322 charges at a rate depending upon the resistance 32| and measures an interval sumcient to permit the operation of the primary switch select magnets. At the end of this interval the tube 323 ionizes and discharge currentflows to operate the relay 323. Relay 326 extends the positive pole of battery over conductor 3|3 through the contacts of relay 323 and conductor 3|5 to the anodes of the tubes 333.

Upon the next appearance of a positive impulse of phase No. 0 over the start conductor 2|3 and thence over the path previously traced on the start electrode 333 a negative impulse of the same phase is applied over impulse lead 333 to the start electrode "I, and the tube 3|3 ionizes. Current now flows in the anode-cathode circuit, the remaining tubes are disabled by the voltage drop in the common resistor 3| I, and the voltage drop across the resistor 321, as a result of current flow in the circuit of the collector electrode 3|3, applies a positive potential to the electrode 323 of the tube 323. Tube 323, therefore, ionizes its control gap since negative potential is applied over conductor 33! to the electrode 333. Current now flows in the circuit traceable from the positive pole of battery 332, secondary winding of transformer 333, relay 333, conductor 333, anode 333, cathode 333, conductor 3", contact 01' relay 223, winding oi the primary switch hold magnet 2 to the negative pole of battery. The current flowing in this circuit is increased and diminished by the-value of the alternating current component produced by the generator 331. This current is suilicient to operate the hold magnet 2, which completes the establishment of the connection from the calling line 233 over the link 232 through the secondary switch 233 to the selected idle trunk 233. In the well-known manner the trunk 233 returns ground potential to hold the magnets 223 and 2| andthis ground potential is further extended over the conductor 33| to the cathode 333 of the tube 323. When the component of the generator 331 next opposes the battery 332, the voltage drop between the anode and cathode of the tube 323 is insuiilcient to maintain ionization since ground potential has now been applied to the cathode. Therefore, the tube 323 quenches and does not reoperate as long as ground potential is maintained on the cathode thereof.

At the time relay 333 operates it closes an obvious circuit for the operation of slow-release relay 333. When relay 333 releases at the time the tube 323 quenches, a circuit is closed from ground over the back contact of relay 333, front contact of relay 333, conductor 333, conductor 3, winding of release relay 333 to battery. Relay 333 operates and opens the supply circuit from battery 333. Thus all of the operated relays and tubes in the line switch control circuit are restored to their normal condition. After an interval relay 333 releases and opens the circuit of release relay 333.

At the time relay 323 operates a circuit is closed from the positive pole of battery over conductor 3| 3, contacts of relay 323, resistor 332 to the condenser 333. The condenser 333 is charged in series with the resistor 332 and measures an interval of time suilicient to permit the controlling circuits to complete their functions. If release of the controlling circuits does not proceed in the regular manner, the condenser 333 reaches its charged condition, and the tube 333 ionizes. Current now flows from the positive pole of battery over conductor 3|3, relay 335 through the tube 333 to the negative pole of battery 333. Relay 333, which is slow to release, closes a circuit from ground through the contacts of operated relay 333, conductor 331, operated contacts of relay 323, operated contacts of relay 333 to conductor 3". This operatesthe release relay 333 and brings about the release of the controlling equipment as above described.

The timing

condensers

322 and 333 are discharged by circuits closed through the back contacts of relays 3", 323, 333 and 333.

' If the initial test of the trunk groups indicates that no group has two or more idle trunks therein, a reserve test is made to determine whether any group has a single idle trunk. The reserve test is made at the end of a measured interval which is long enough to enable the test and selection of any group that is found having two or more trunks. This interval is measured by a circuit traceable from battery supply conductor 3",

conductors

332 and 333, resistor 333, contacts of relay 333, conductor 331, resistor 321 to condenser 323. At the end of the interval the condenser reaches its charged condition, and full positive potential is applied through resistor 529 to the electrode 530 of tube 520. Tube 520 ionizes since the other control electrode 53l is connected through the winding of relay 503 to the negative pole of battery 504. Current now flows in a circuit from the positive pole of battery over conductor 623, anode of the tube 520, cathode 53!, winding of relay 503 to the negative pole of battery 504. Relay 503 operates in this circuit and increases the negative potential applied over conductor 605 to the electrodes of the tubes of the group 120'. This increased potential is applied from the negative pole of battery 532 in series with battery 504 through the contact of relay 503, resistor 533, contact of relay 503 to conductor 805. Hence the voltage drop across the electrodes of any one of the tubes 120 is sufficient to ionize the tube if one of these electrodes is connected to a testing network 100 having onlyone of its branches closed, indicating that only one of the trunks in the group is available.

The line switch frames disclosed herein also serve to complete incoming calls to the called subscribers lines. For a general understanding of the operation of primary and secondary crossbar switches for the completion of both outgoing and incoming calls, reference may be had to the patent to W. W. Carpenter, No. 2,089,921, granted August 10, 1937.

One of the terminating switch controlling markers 450 is illustrated diagrammatically in the drawings. Whenever the marker 450 seizes the line switch frame for the purpose of completing a terminating call to one of the subscribers lines, relay 400 is operated over a circu t extending from the marker over conductor 45! through the normal contacts of relay 332 50 volts positive, and 50 volts negative are suitable peak impulse amplitudes.

What is claimed is:

1. In combination, a plurality of switches arranged in groups, groups of lines appearing respectively in said groups of switches, a plurality of space-discharge devices, one for each of said groups of lines, a source for producing potentials oi diiiferent phases, said phases characterizing respectively the different groups of lines, means including said source responsive to the lines oi! each group for applying to the corresponding discharge device impulses of the phase characterizing said group to cause the discharge of said device, means efiective when any one of said devices discharges to prevent the discharge of any one of the other discharge devices, and means responsive to the discharged device for controlling the operation of the switches of the associated group.

2. In combination, a plurality of switches arranged in groups, groups of lines appearing respectively in said groups of switches, a plurality of discharge tubes, one for each group of lines, means responsive to the lines of each group for applying to the corresponding tube impulses of potential to cause the discharge thereof, the impulses applied to the different tubes being of different phases to cause a single one of said tubes to discharge at a time, means effective when any one of said tubes discharges to prevent the discharge of any other tube in response to the potential impulses applied thereto, and means responsive to the discharged tube for controlling (no outgoing call being extended at this time) conductor 358 to the winding of relay 400 to battery. Relay 400 operates and removes battery potential from conductor 2I3 and similar conductors to prevent the operation of any one of the line tubes in case one of the subscribers initiates a call. Relay 400 also opens other control leads that are used when outgoing calls are being extended. In a similar manner the extension of an outgoing call prevents the terminating marker 450 from seizing the line switch frame.

This is accomplished by opening the energizing circuit oi relay 400 at the contacts of relay 332.

The phase impulse generator leads disclosed in various parts of the drawings may be connected to separate generators, all of which are in synchronism, or to the same impulse generator. These phase impulses are produced during each cycle of an alternating current source of any suitable frequency. As has been explained, some of these impulses are of positive polarity and others are of negative polarity. Generally speaking the discharge tubes disclosed in the drawings are designed to ionize their control gaps when a voltage is applied thereto which is equal to the sum of the positive and negative potential impulses, whereas a single impulse is insuiiicient. Large amplitude pulses are required in the anode circuits, as in tube 208, in which case the direct current component of anode voltage is insufiicient to maintain a. discharge, while the sum of the direct voltage and the peak impulse voltage in the anode, less thedrop in the tube while conducting will result in an impulse of normal amplitude across an output impedance. I have found that suitable values are 75 volts and the operation of the switches of the associated group.

3. In combination, a plurality of switches arranged in groups, groups of lines appearing respectively in said groups of switches, a plurality of space-discharge tubes, one for each group of lines, means for applying a control potential to all of said tubes, means responsive to any line in each of said groups for applying to the corresponding tube operating potential impulses to cause the discharge of said tube, the impulses applied to the different tubes being of different phases to insure that only one tube discharges at any time, means effective when any one of the tubes discharges for altering the control potential on the remainder of said tubes to render them unresponsive to said operating potential impulses, and means controlled by the discharged tube for causing the operation of the associated switches.

4. In combination, a plurality of lines arranged electrically in coordinate horizontal and vertical groups, switches for serving said lines, a series of discharge tubes individual respectively to the horizontal groups, a second series of discharge tubes individual respectively to the vertical groups, a source of potential of different phases, each phase characterizing a difierent one of said horizontal groups of lines, means responsive to a calling line in any horizontal group for causing the application to the corresponding tube of potential impulses from said source of the phase characterizing said horizontal group to effect the discharge of said tube, means efiective when any one of said horizontal-group tubes discharges for rendering the remaining horizontal-group tubes unresponsive to potential impulses, means responsive to the discharge of a tube individual to a horizontal group of lines for applying potential impulses to the tubes individual to said vertical groups, means responsive to the discharge of one of lines appearing in said switches, a plurality of I groups of trunks, a plurality of link circuits accessible to said switches for extendingsaid lines to said trunk groups, said link circuits having access respectively to said trunk groups, means for performing a matching test between each link circuit and the trunk group to which it has access, said means comprising a plurality of spacedischarge tubes, one for each link and its associated trunk group, means for producing potential impulses of diflerent phases characterizing the idle links, means for producing potential impulses of diflerent phases characterizing the idle trunk groups, and means for applying to said tubes potential impulses of the phases representing idle links'that match with idle trunk groups to cause the discharge of one of said tubes, means responsive to the discharge of one vof said tubes for disabling the remaining tubes, and means controlled by the discharged tube for causing the operation of said switches to extend a connection over the chosen idle link and trunk group.

6. In combination, a plurality of switches, a plurality of lines appearing in said switches, a number of trunk groups, a number of links accessible to said switches for extending said lines to said trunk groups, said links having access individually to the respective trunk ing mechanism comprising a series of discharge tubes for making a matching test of the links and the associated trunk groups, said tubes serving individually the respective 'links and their associated trunk groups, means for producing potential impulses of different phases characterizing the respective links. means'tor transmitting to said testing mechanism impulses of the phases representing idle links, means for producing potential impulses of difierent phases characterizing the respective trunk groups, means for transmitting to said testing mechanism potential impulses representing trunk "groups having an idle trunk therein, means resposive to the potential impulses of like phase transmitted roups, a test- 85 charged tube to said testing mechanism for causing the application of potentials of like phase to said discharge tubes to cause the discharge oi one of said tubes representing an idle link having access to an idle trunk, and means responsive to the dis- !or causing the operation 01' said switches. 7. In combination, a plurality of switches, a plurality 01' groups of lines appearing respectively in said groups. of switches, each group oi'lines divided into subgroups which appear respectively in the individual switches of the corresponding group, means for selecting a group of lines to be served to the exclusion oi. other line groups, means for selecting a subgroup of lines in the chosen group to be served to the exclusion of the other subgroups in said chosen group, means for producing potential impulses of diflerent phases representing the different lines of the chosen subgroup, a series of dischargetubes each representing one oithe lines of said subgroup, means for applying said potential impulses to said tubes to cause the discharge 01' one of said tubes, and means controlled by the discharged tube for operating the associated switch to serve a particular. line represented by said tube.

8. In combination, a plurality of switches, a plurality of groups of lines appearing respectively in saidgroups of switches, each group of lines divided into subgroups which appear respectively in the individual switches of the corresponding group, means for selecting a group 01" lines to be served to the exclusion of other line: groups, means for selecting a subgroup of lines in the chosen group to be served to'the exclusion of the other subgroups in said chosen group, means for producing potential impulses oi diflerent phases representing the diil'erent lines of the chosen subthe phases operation of ro p,

a series 01' discharge tubes each representing one of the lines of said subgroup, means for applying to said tubes potential impulses of characterizing calling lines in the chosen subgroup. for causing the discharge or one 0! said tubes to the exclusion oi the other tubes, means controlled by the discharged tube for disabling the remaining tubes, and means controlled by the discharged tube for causing the the associated switch to extend the chosen line. WILLIAM H. 'I'. HOLDEN.