US2310452A - Switching system - Google Patents

Description

Feb. 9, 1943. A. MEACHAM ETAL 2,310,452

SWITCHING SYSTEM 7 Filed June 5, 1941 6 Sheets-Sheet l F IG 5 FIG. 7

FIG 4 FIG 5 FIG 2 FIG. .9

l I g /NVENTOR$: MI, 5V

A TTORNEY b- 1943- A. MEACHAM ETAL 310.452

SWITCHING SYSTEM Filed June 5. 1941 6 Sheets-Sheet 2 LAMEACHAM m/vE/vmRs. C M PARKER By WW K A T TOR/V5 V Feb. 9, 1943. L. A. MEACHAM ETAL SWITCHING SYSTEM Filed June 5, 1941 6 Sheets-Sheet 3 /Nl/ENTORS;

By I T: 1

ATTORNEY NE a Ref. 9, 1943. A. MEACHAM EI'AL SWITCHING SYSTEM Filed June 5, 1941 v6 Sheets-Sheet 4 Q MIVENTORS.

C l/ PARKER A T TOP/V5 V Feb; 9, 1943. 1.. A. MEACHAM El'AL 2,310,452

SWITCHING SYSTEM Filed June 5, 1941 6 Sheets-Sheet 5 L. A. MEACHAM WVENTORS. Gym/"(ER WWW A 7' TORNE Y Feb. 9, 1943. A. MEACHAM ETAL 2,310,452

SWITCHING SYSTEM Filed June 5, 1941 6 Sheets-Sheet 6 IMPUL .S'E GENERA TOR LAMEA M mn/E/vroRs. CVPARKER WWWKW A T TORNEV ?tente Feb 1943 SWITCHING 'SYSTEM Larned A. Meacliam, Summit, N. 3., and Carlyle V. Parker, Washington, D. 0., assignors to Bell Telephone Laboratories, Incorporated, New

York, N. Y., a corporation of New York Application June 5, 1941, Serial No. 396,687

7 Claims.

This invention relates to switching systems and particularly to such systems when used for establishing telephone or other communication connections.

The objects of the invention are to simplify these switching systems, to minimize the use of common controlling equipment, to increase the speed of operation, 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 links for 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 interconnection of the lines with the trunks. In such a switching system, and particularly when crossbar switches are used, 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 insurin selection in preferred horizontal and vertical groups, for selecting idle groups of trunks, and for selecting idle links having access to these trunk groups. Also in these prior systems it has been necessary to provide each group of primary and secondary switches, such as those occupying a, frame, with a considerable amount of equipment that is used in common by all switches on the frame.

Accordingly a feature of the present invention is a system in which the relay selecting and testing mechanisms are replaced by electronic discharge devices which perform the testing, selecting, and other controlling functions with greater speed, and in which much of the common controlling circuits and equipment have been eliminated. More specifically a calling line in one of the primary switches is extended over a link to a secondary switch and thence to an idle trunk in the group accessible to such secondary switch by selectively operating with impulses of difi'erent phases one of a series of discharge tubes to select the horizontal group of primary switches containing the calling line; by utilizing this tube to apply potential to the switch magnet operating circuits of all links extending from the chosen horizontal group to the respective secondary switches, each operating circuit including the operating magnets associated with a given link on all primary switches in the group and the magnet of the secondary switch to which the corresponding link extends; by operating by means of phase impulses one of a series of tubes representing a trunk group having idle trunks therein and an idle link extending from the calling horizontal group of primary witches to the secondary switch having access to such trunk group; and by utilizing the operated tube to apply potential at the chosen secondary switch to the magnet operating circuits of all links extending to said secondary switch from each of the several primary switch groups. Thus at the primary switches the magnet operating circuits are prepared for all links extending from the chosen horizontal group to the several secondary switches, and at the chosen secondary switch the magnet operating circuits are prepared for all links extending thereto from the several primary horizontal groups; and the result is that one and only one of these circuits is completed to effect the desired connection, the choice of the idle link and trunk group from other links and groups, which may be busy or idle at the time, being made by applying phase impulses to the selecting tubes.

Another feature of the invention is a system of this kind in which the extension of a calling line to an idle trunk results in the discharge of a tube common to the trunk group to cause the release of the operated tubes and circuits associated with the frame, whereby the switches and links of the frame become available to other calling lines.

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

In the drawings accompanying the specification:

Fig, 1 is a diagram of a frame of primary and secondary switches, illustrating in particular the energizing circuits of the switch operating magnets;

Figs. 2 to 6 when arranged as indicated in Fig. 7 disclose the detailed circuits and equipment for a portion of one of the line switch frames illustrated in Fig. 1;

Fig. 2 shows a horizontal row or group of primary line switches;

Fig. 3 illustrates another horizontal row of primary switches and also shows the horizontal group selecting equipment for choosing the horizontal group to be served;

Figs. 4 and 5 show two of the secondary line switches to which the links incoming from the several horizontal primary groups have access and which have access to respective outgoing groups of trunks; and- Fig. 6 shows the common controlling mechanism for performing the link and trunk testing operations and for releasing the circuits after use.

Referring first to Fig. 1, the frame of crossbar line switches illustrated therein comprises ten horizontal groups or rows of primary line switches, of which two are shown, the No. group I00 and the No. 9 group IOI. Each horizontal group may have any desired number of primary switches therein, it being assumed in the present case that seven primary switches are provided in each of the ten horizontal groups. Two of the seven primary switches I02 and I03 are shown in the horizontal group I00, and two primary switches I04 and I05 are shown in the group IOI. Each of these primary line switches serves ten subscribers lines, the lines appearing in the vertical rows of contacts of the switches. For example, lines I06 and I0I appear in the primary line switch I02, and lines I08 and I09 appear in the primary line switch I03.

The line links which interconnect the primary line switches and'the secondary line switches appear in the horizontal rows of contacts in the switches of the difierent primary horizontal groups. There are ten of these links for each horizontal group, and all ten links are multipled to appear in the horizontal rows of contacts or all of the seven primary switches.

Ten secondary switches are also provided on the line switch frame, there being one of these secondary switches for each horizontal group oi primary switches. switches are shown, the No. 0 switch H0 and the No. 9 switch III. The primary and secondary switches are interconnected by the line links in such a manner that all ten primary groups have access to all ten secondary switches.- This is accomplished by extending the ten lin links from each horizontal group respectively to the ten secondary switches. Stated in another way, each secondary switch receives a single link from each of the ten primary groups.

The ten secondary switches have access respectively to ten outgoing groups of trunks, to which the calling lines are connected through the joint operation of the primary and secondary switches. For example, the trunk group I I2 appears in and is accessible to the secondary line switch I I0, and the trunk group I I3 appears in the secondary line switch III. Eight other trunk groups appear respectively in the remaining eight secondary switches not shown in the drawings.

Each of the primary and secondary switches on the frame is provided with ten select magnets, one for each horizontal row of contacts, for preparing the switch for operation and with ten hold magnets individual respectively to the vertical rows of contacts for holding the contacts in their operated condition after the select magnets have been released. For example, the select magnets I I4 and I I5 are individual respectively to the No. 0 and No. 9 horizontal rows in the switch I02. Similarly the select magnets H6 and II! serve the No. 0 and No. 9 horizontal rows of contacts in the switch I03. The two line link H8 and H9 of the horizontal group I20 appear respectively in the No. 0 and No. 9 horizontal rows of switches I02, I03, etc., of the horizontal group I00. Similarly the select magnets I2I, I22 of switch I04, and I23, I24 of switch I05 control the No. 0 and No. 9 horizontal rows of contacts in these switches, in which the line links I25 and I26 of the group I2! appear. The links of the Two of these secondary group I20 appear respectively in the No. 0 horizontal rows of contacts in the ten secondary switches. the link H8 in the No. 0 row of switch H0 and the link H9 in the No. 0 row of switch III. These horizontal 'rows in switches H0 and III are controlled by the selectmagnets I28 and I29. The links of the group I2I appear in the No. 9 horizontal rows of contacts in the respective secondary switches and are controlled by the select magnets thereof, such as the magnets I30 and I3I of switches H0 and III, respectively.

As will be described more fully hereinafter, the select magnets of the primary and secondary switches are operated in series circuits. For example, the select magnets H4, H6, etc., of all primary switches and the select magnet I28 of the secondary switch pertaining to the N o. 0 line link II8 are included in a series circuit. One end of this circuit is closed through contacts I32 to the positive pole of battery, and the other end of this circuit is closed through contacts I33 to the negative pole of battery. Likewise the select magnets of the primary switches andsecondary switches pertaining to each of the remaining links are included in series circuits. All of the select magnet operating circuits for a horizontal group of links, such as the group I20, are connected in series parallel relation through the contact I32 to the positive pole of battery. Similarly all oi the select magnet operating circuits incoming to any particular secondary switch, such as the switch IIO, are connected in series parallel relation through the contact I33 to the negative pole of battery. By closing the contact I32 at the primary switches and the contact I33 at the secondary switches a single circuit is completed through the windings of magnets I I4, II6, etc., of the primary switches and the magnet I28 of the secondary switch IIO. In this manner the select magnets of the primary switches and the secondary switch having access to the calling line and to an idle trunk respectively are energized simultaneously to prepare these switches for operation.

The hold magnets I34, I35, I36, I31, etc., of the primary switches serve to hold these switches in their operated condition after the select magnets have been released. Similarly the hold magnets I38, I39, etc., of the secondary switch I I0 (and the same is true of the other secondary switches) serve to hold the operated contacts in their closed condition after the release of the select magnets.

The primary and secondary line switches disclosed herein may be of any well-known type of crossbar construction, one suitable type being disclosed in the patent to Reynolds, No. 2,021,329 of November 19, 1935.

A general description will now be given of the circuits and equipment shown in detail in Figs. 2 to 6. The No. 0 horizontal group of primary switches on the frame includes the No. 0 switch 200 and the No. 6 switch 20I, there being. as above explained, five similar intermediate primary switches. Ten subscribers lines, including line 202 and 203, appear in the vertical rows of contacts of switch 200 and are provided with individual line discharge tubes, including the No. 0 and No. 9 tubes 204 and 205. Similarly the No. 60 and No. 69 lines 206 and 20! appear in the last switch 20I in the group and are provided with corresponding line tubes 208 and 206. There are, therefore, seventy subscribers lines appearing in the horizontal group and seventy individual line tubes comprising the group 2I0.

In like manner each 01' the remaining nine,

The ten horizontal groups of primary line switches are provided with a group of ten space discharge tubes 305. one for each of the horizontal groups. These tubes serve to select one of the horizontal groups for service to the exclusion of other groups having calling lines therein. The tube 306, for example, is individual to the No. horizontal group shown in Fig. 2 and when operated renders this group effective and excludes the remainin groups. The other tube shown, tube 301, is individual to the last' or No. 9 horizontal group shown in Fig. 3.

The tubes of each of the line groups 2| 0, 3|0, etc., are operated selectively by means of multiplex signal impulses. These impulses are produced by the impulse generator 308 which has seventy output circuits 309. The generator 308 is energized by a source of alternating current 3 of any suitable frequency and serves to produce momentary impulses of seventy different phases during each successive cycle of the source 3| l. The impulses of the different phases, which are referred to herein as phase impulses, appear successively in the impulse circuits of the group 309 and are applied to the control electrodes of the respective tubes of the group 2E0 and similarly to the electrodes of the respective tubes in each of the other groups. For example, impulses of the No. 0 phase appear in the conductor 3l-2, which is connected to the starting anode 2!! of the No. 0 tube 204 and in multiple to the starting anodes of the corresponding tubes in the other nine groups. Impulses of phase No. 9 appear in impulse conductor 3|3, which is connected to the starting anode of theNo. 9 tube 205 in group 2l0 and to the corresponding tube in the other groups. Impulses of phases No. 60 and No. 69 appear respectively in impulse conductors 3l4 and 3|5, these conductors being connected to the starting anodes of tubes 208 and 209 and tubes 303 and 304 and to corresponding tubes in other groups. When two or more lines in any oneof the horizontal groups initiate calls the line tube corresponding to one of these lines operates depending upon the phase of the impulses at the time the calls are originated and excludes or looks out all of the other tubes in the group. Thus only one line tube in any horizontal group is permitted to operate at a time. This will be explained more fully hereinafter.

The horizontal group tubes 305 are also operated selectively by phase impulse delivered by the generator 308. Since there are only ten of these horizontal group tubes only ten of the group of impulse generator leads 309 are required. Any ten of the seventy leads may be used depending upon the phases desired. As shown in the drawing, however, the first ten impulse conductors are connected to the starting electrodes of the ten tubes of the group 305. For example, the No. 0 impulse conductor 3l2 is connected to the starting anode 3|6 and the No. 0 horizontal group tube 306, the No. 9 impulse conductor M3 is connected to the starting anode 3|! of the No. 9

horizontal group tube 307, and the eight intermediate phase impulse conductors are connected to the corresponding intermediate tubes. of the group 305. When two or more. horizontal groups are calling at the same time, one of the tubes 305 operates to prefer its horizontal group and to exclude the others.

The currents of the different phases may be obtained from the source 3 in any suitable manner, and the momentary impulses delivered to the circuits 309 for operating the tubes may also be produced by suitable impulse generating means. For example, impulse generators designed to produce momentary impulses of either positive or negative polarity are disclosed in the, application of W. H. T. Holden, Serial No. 361,536, filed October 17, 1940; and W. H. T. Holden Patents 2,288,815, of June 9, 1942; and 2,252,766, of August 19, 1941.

The ten links outgoing from the horizontal group shown in Fig. 2 extend respectively to the ten secondary line switches. For example, the link 212 appears in the secondary line switch 400, and link 2l3 appears in the secondary line switch 500. The other eight links not shown in this group appear in the corresponding secondary line switches. In like manner the remaining groups of link circuits are distributed amongthe secondary line switches 400, 500, etc., the links 3| 8 and 3| 9 shown in the last horizontal group appearing respectively in the secondary switches 400 and 500.

The No. 0 secondary line switch 400 has access to a group of ten trunks, including trunks 4M and 402, outgoing to some selective switching stage beyond. The No. 9 secondary switch 500 has access to a similar group of trunks which includes the trunks 50! and 502, and the same is true of the eight intermediate secondary switches. Thus the subscribers lines appearing in the ten different horizontal groups of primary switches have access through these switches and intermediate links and the secondary switches to any one of ten groups of outgoing trunk circuits.

The secondary switches of the frame are provided with a common group of testing tubes 600, there being one of these tubes for each of the outgoing trunk groups. The first of these tubes, tube 60 l, is individual to the group of trunks outgoing from the switch 400 and serves simultaneously to test these trunks for an idle one and to test the link incoming to the switch 400 from the calling group of primary switches. The last tube of the group 600, tube 602, is individual to the group of trunks outgoing from the last secondary switch 500 and serves simultaneously to test these trunks and to test the link incoming to the switch 500 from the calling group of primary switches. The matching test made by the tubes of the group 600 is registered on the tubes of group 605. There is one of these tubes in the group 605 for each of the ten groups of outgoing trunks, and the tube that is finally operated as a result of the matching test by the group 600 indicates that there is an idle trunk in the associated group and that there is an idle link having access to this trunk from the primary switches in which the calling line appears.

The test of the trunk group is made by a circuit extending through the contacts of the sleeve relays individual to the trunks of the group. Considering the trunk group appearing in the switch 400, the test circuit for this group extends from the battery lead 403 through the normal contacts in parallel of the sleeve relays 404, 405,.

of all idle trunks, thence through parallel resistances 406, 401, conductor 408, resistor 803 to battery and ground. If all trunks are busy, this circuit is open, and no current flows through the resistor 503. Therefore, no potential is applied through resistors 506 and 601 to the starting anode 600 of the corresponding tube 609. However, if two or more of the trunks are idle, the amount of current flowing through resistor 603 results in theapplication of a potential of a predetermined value to the anode of the tube 608. This potential alone, however, is not sufilcient to ionize the control gap of the tube. Simultaneously with the foregoing test of the trunk group a test is also being made of the particular one of the ten links M2, M8, etc., which extends to switch 400 from the calling group of primary switches. If the group of Fi 2 is calling, for example, the link 2 I2 is tested by means or a circuit extending over the conductor 2l4 through resistor 409, conductor 4 I to one of the multiple starting anodes oi the tube 60L If the link 2l2 is idle. positive potential appears on this anode, and the gap including the anode and the cathode H0 is ionized. If, however, the link U2 is busy, the usual ground potential -on the sleeve con ductor H5 is applied over conductor 0 to the starting anodeof the tube 6M, and the control gap is not ionized. The test of the other nine groups of trunks and the simultaneous test of the respective links having access to these trunks are made in a similar manner by means of the corresponding tubes of the groups 600 and 605. For example, the last group of trunks, including trunks 5M and 502, are tested by means of a circuit extending from the battery supply conductor 403 through the back contacts in parallel of the sleeve relays 503, 504, of all idle trunks,

discharges in a circuit traceable from the battery supply conductor 403. resistors 822 and 823, anode 824, cathode 8l0, resistor 828 to the negative pole of battery 628. Assume also that a moment later an impulse of the corresponding phase is applied by the generator 615 to the impulse conductor 821. This impulse passes through the discharged tube GUI, and current flows through the resistor 025. This added flow of current .as a result or the impulse causes the application 01 an increased positive potential through condenser 620, resistor 801 to the starting anode 000 of the tube 009. This increased potential applied to the anode 008 is sufiicient to ionize the control gap formed by the anode 608 and the starting cathode 029. Thereupon the tube 009 discharges and signifies the selection of the associated trunk group and its matching idle link. Thev operated tube 009, as will be explained more fully hereinafter, also looks out the remaining tubes of the group 605 to prevent the selection of another group of tnmks for serving this particular call. As the generator 6|! proceeds to transmit impulses of succeeding phases to the remaining conductors of the group BIB, corresponding impulses are applied to the cathodes of all tubes of the group 600 that are in a discharged condition because they represent qualified trunk groups that are reached by idle links from the calling primary group. However, these impulses are ineffective to cause the operation of a second one oi! the tubes 60!, in case the associated trunk groups have idle trunks therein, for the reason stated, namely that the operation of the first tube 609 to select the associated group and link automatically renders the remaining tubes of the group 605 ineffective.

Since the positive potential applied to anode 600 must be enough to operate the tube it two trunks are idle and must not be enough to operate the tube falsely if several trunks are idle,

the application of a potential of a predetermined value to the anode GIZ of the tube 6l3. At the same time the link having access to this trunk group, such as the link 213, is tested by means of a circuit extending over conductor 2 i 6, through resistor 508, conductor 509 to one of the multiple starting anodes of tube 602. If the link2i3 is idle, positive potential appears on this anode and the gap including the anode and the cathode H4 is ionized.

As a result of these tests, therefore, each tube of the group 600 corresponding to a trunk group which is reached by an idle link has its control gap ionized; and each tube of the group 605 corresponding to a trunk group having two or more idle trunks therein has a positive potential applied to its starting anode. Since one trunk group and its matching idle link should be selected to the exclusion of other groups, the operation of tubes 605 is controlled by phase impulses delivered from the impulse generator Eli. The ten impulse conductors SIG to which the generator SIB delivers impulses of ten different phases, are connected through condensers 0H, 8 to the main anodes of the ten tubes of the group 600. The first one of the tubes 600 to receive an impulse from the generator M5 after the foregoing test has been made causes the op eration of the corresponding tube of the group 605. Assume for instance that the control gap oitube 60! is ionized because the associated trung group contains two or more idle trunks. {is soon as the control gap ionizes, the main gap the potential on conductor 408 is held below a predetermined value by means of the battery 88!, which .has its positive pole connected through the varistor 062 to conductor 400. Thus, no matter how many trunks are idle, the potential of anode 600 is held to such a value that the phase impulse delivered to the tube 60! is needed to operate tube 609, having in mind the positive potential applied to cathode 620 from battery 0" through resistor 664.

When all trunks are busy, battery ii! is disconnected from conductor 408. To avoid the possibility, under this condition, of tube 608 ionizing on the combined voltage of batteries 663 and 005 a varistor 606 is connected between conductor 400 and ground.

It was mentioned above that the trunk groups are first tested to determine whether they have two or more idle trunks. In case no trunk group is found having as many as two idle trunks a reserve test is then'made to determine whether any group has a single idle trunk. This test is made by the tube 0 and the associated circuits. At the time the matching test is started battery potential is applied to conductor I20, and current fiows through the resistor "I to charge the condenser 032. After an interval or time has expired the condenser "2 reaches a predetermined charge, and the control gap 0! the tube 030 formed by the electrodes 0" and 034 ionizesa Current now flows in the main discharge circuit of the tube traceable from battery supply conductor 2|. conductor III. winding of relay 030,

anode 631, cathode 634 to battery and ground.

Relay 636 operates and applies direct ground potential to the starting cathodes 629, 638, etc., of the tubes of the group 605. With direct ground potential on the starting cathodes of these tubes the control gap of any tube will ionlze if a single trunk in the group is idle. In other words, a single idle trunk results in current flow over conductor 403 through the closed contact of a single one of the sleeve relays, such as relay 404, resistor 406, conductor 408, resistor 603 to battery. The intensity of this current flow through resistor 603 causes the application of a potential to anode 608 which is somewhat less than the potential applied thereto when two trunks are idle. However, the voltage produced across the gap 608-629, when the generator 6! delivers its phase impulse to the tube 60 I, and with electrode 629 at ground potential, is suflicient to ionize the gap. Thus the reserve test results in the selection of any group having a single idle trunk matching with an idle link.

When one of the tubes 605 operates to indicate the selection of the trunk group, an asso ciated relay 639, 640, operates to prepare for the selection of an idle one of the trunks in the of the common equipment as will be described hereinafter. The condenser 4|3 discharges through the relatively low resistance 65! and cannot assume a sumcient charge to cause the group. Relay 630, for example, closes a circuit from negative pole of battery through its contacts, conductor 64!, resistor 435 to the starting cathodes of the tubes of group 4I2, there being one of these tubes for'each of the ten trunks 40!, 402. The startinganodes of the tubes 4! 2 are connected through the normal contacts of the sleeve relays 404, 405 to the ten phase impulse leads 642 outgoing from the impulse generator 6l5. Therefore, as soon after one of the relays 639, 640 operates as the generator 6l5 delivers an impulse of the phase corresponding to the first idle trunk the corresponding tube 4!2 ionizes and prepares for the operation of the associated hold magnet of the switch 400. The selection of an idle trunk in any one of the other groups, such as, the group outgoing from the secondary switch 500, proceeds in the same manner under the control of the associated relay, such as the relay 640 individual to the tube 6! 3.

The impulse generator 6!5 may be similar to the generator 308; or, if desirable, the impulse leads 6!6 and 642 may be connected directly to the output leads of generator 308 for obtaining pulses of the required phases for application to tubes 600, 605 and M2.

The common matching and testing mechanism shown in Fig. 6 is released, as soon as it has performed its functions, underthe control of a group of release tubes 645. There are ten of these tubes 646, 641, etc., one for each of the ten trunk groups. The starting anodes of these tubes are connected through resistors and the front contacts of the sleeve relays'of the trunks to condensers. For example, the anodes of tube 646 are connected through resistors 648, 649 through the contacts of relays 404 and 405 to condensers M3 and M4. When the testing operation commences, battery potential is applied to conductor 403. and condensers M3 and M4 assume a charged condition. Later when the trunk is selected and the sleeve relay operates, the potential of the associated condenser is applied to the starting anode of the tube 646, and this tube ionizes its control gap. The main discharge gap of the tube 646 is then ionized, and current flows through the release relay 650. Relay 650 brings about the release reionization of the tube 646 for the reason that the resistor 4! 5, through which the condenser charges, is considerably higher .in value than the resistor 6I5, through which it is constantly discharging while the trunk 40! is in use. Therefore, other trunks in the same group may be selected subsequently, and the tube 646 operates as soon as each trunk is selected in turn to cause the release of the common mechanism.

A description will now be given of the detailed operation of the system, assuming for this purpose that the subscribers of lines 202 and 30! initiate calls at about the same time. When the subscriber of line 202 removes his receiver from the switchhook, thus connecting a low resistance across his line, an obvious circuit is closed through the back contacts of the hold magnet 2!! including the resistor 2!8. The current flowing through the resistor 2! 8 places a negative potential on the starting cathode 2!!! of the associated tube 204, and this tube ionizes as soon as a positive impulse is delivered by the generator 308 over conductor 3!2, resistor 220 to the starting anode 2| As soon as the tube ionizes, current flows from the positive pole of battery 6l9, conductor 62!, resistors 22! and 222, anode 223, cathode 224, winding of the hold magnet 2 I! to the negative pole of battery. The current flowing in this circuit is not enough to cause the operation of the hold magnet 2 1. The voltage drop, however, produced by the current flowing in resistors 22! and 222 lowers the potential of the anodes of the remaining tubes in the group 2!0 to a point where no other one of these tubes can operate. Prior to the operation of the tube 204 the condenser 32! associated with the horizontal group tube 306 is in a charged condition. The charging circuit for this condenser may be traced from the positive pole of battery over conductor 62!, resistor 22!, condenser 32!, resistor 322 to ground. At the instant the tube 204 discharges sudden current flow through the resistor 62! lowers the positive potential applied to the condenser 32!, resulting in the application of a negative potential through the resistor 323 to the starting cathode 324 of the tube 306. This negative potential is applied to the cathode 324 until the condenser 32! has time to adjust its charge in the circuit including the resistors 322 and 22!. This interval is sufliciently long to permit the impulse generator 308 to apply a positive impulse over conductor 3! 2 and resistor 325 to the starting anode 3!6 of the tube 306.

In a similar manner the initiation of a call on line 30! causes the operation of the associated line tube 303. Tube 303 causes the application of a negative impulse to the starting cathode 33! of the horizontal group tube 301, and this potential remains on the electrode 33! sufiiciently long to permit the generator 308 to deliver a positive impulse of the corresponding phase to the starting anode 3!!.

Depending upon the phase of the generator 308 at the time the cathodes 324 and 33! are rendered negative in potential, an impulse of positive potential will be applied first to the anode 3!6 of the .tube 306 or to the anode 3!! of the tube 301. Assuming that the phase is such that the next impulse is delivered over the conductor 3!2 to the anode 3!6, thetube 306 ionizes its control gap 3I6-324, and current now flows from the positive pole of battery 610, conductor 62l,conductor 326, winding of the common relay 321, anode 328, cathode 323, group relay 330 to the negative pole of battery. Relay 330 operates to indicate the selection of the No. group of primary switches 200, 20l for service; and the flow of current through the common impedance 321 lowers the potential on the anodes of the remaining tubes of the group 305 to. a value such that no one of these tubes can operate. Therefore, the tube 301 representing calling line 30| fails to operate when the generator 308 delivers the corresponding impulse to the anode 3H, and the subscriber's line 30l must wait until the other line has been served.

When the horizontal group relay 330 operates, circuits are prepared for the select magnets associated with the ten link 2|2, 2l3 outgoing from the primary group serving the calling line 202 to each of the ten secondary switches 400, 500 on the frame. The preparation or these select magnet circuits consists in connecting the positive pole of battery to them. The circuit for the select magnets associated with the first link 2|2 may be traced from the positive pole of battery 6l9, conductor 62l, conductor 326, contacts of relay 330, conductor 332, through the winding of select magnet 225, thence over conductor 2| 4 through the windings of the corresponding select magnets, including magnet 226, of the other six primary switches, thence through the select magnet 6 associated with the link 2l2 in the secondary switch 400 to the common conductor M1. The circuit for the select magnets of the link 2l3 may be traced as before over the common conductors 62l, 326, contacts of relay 330, common conductor 332 thence through the windings of select magnets 221, 228 and over conductor 2l6 through the select magnet 5l0 associated with the link 2|3 in the secondary switch 800 to the common conductor 5| l. Likewise parallel circuits may be traced from the positive pole of battery 6! 9 through the select magnets 01 all of the remaining eight links outgoing from the primary group to the remaining eight secondary switches not shown. These circuits, however, are not yet effective to cause the operation of any of these select magnets since the common conductors 411, 5| l, etc., at the secondary switches are open and will remain open until it is determined which trunk group is to serve the calling line 202.

When the common relay 321 operates, it also extends the positive pole of battery 6l9 by way of conductor 62! through the contacts of relay 321, conductor 333, conductor 403, thence through the back contacts of the sleeve relays of all idle trunks in all of the ten trunk groups outgoing from the secondary switches 400, 500. As hereinbefore explained, "these parallel circuits through the back contacts of the sleeve relays extend through the resistors 606, 601 and 652, 653, etc., to the starting anodes of the test tubes 605. Battery potential on the conductor 333 is also extended over conductor 4! to the main anode 624, 654, etc., of the ten tubes 600. Each one of the tubes 600 representing a trunk group that is reached by an idle link extending from the calling group is ionized as above ex-' plained; and, when the generator 6l5 applies a positive impulse through the discharged tube 600 to the anode of the associated tube 605, the latter tube operates to indicate the selection of a trunk group having an idle trunk therein and accessible to an idle link from the calling primary group. Assume that the tube 600 is the one operated, indicating that the trunk group including trunks 40l and 442 has been selected, the operation of tube 600 results in the closure oi! a circuit from the positive pole 0! battery 6". conductor 62L conductor 636, impedance element 656, anode 651, cathode 656, winding of relay 639 to the negative pole of battery.

When relay 630 operates it connects the negative pole of battery to the select magnet operating .circuits of all ten links incoming to the switch 400 .irom the ten different primary groups of switches. It will be noted that only one oi these select magnet operating circuits has been prepared by the connection of positive battery thereto, namely, the select magnet operating circuit for the link 2l2 coming from the calling primary group containing the subscribers line 202. Hence this one select magnet circuit is closed while the remaining circuits remain inefiective. This circuit may be traced from the negative pole of battery through the middle contacts of relay 633 to the conductor 4" thence as previously traced through magnet 4", conductor 2l4, magnets 226, 226, conductor 332 to the positive pole of battery BIS. Similarly relay 639 partially completes parallel circuits for the remaining nine links incoming to the secondary switch 400. One of these may be traced over the common conductor 1 through the winding of select magnet 4l9, conductor 420 through the select magnet 334 and other select magnets 01 the primary switches to the common conductor 335. These circuits, however, are open at the primary switches, and none of them is fully closed. Primary select magnets 205, 226, etc., and secondary select magnet 4", however, are energized in the circuit above traced to prepare for the establishment of the connection over the selected link 2l2 to one 01 the ten trunks in the group outgoing from the switch 400.

The operation of relay 630 also prepares for the test of the ten trunks in the selected group to choose an idle one thereof for use. Relay 639 at its inner contacts closes a circuit over conductor 64! through the resistor 435 to the starting cathodes of the tubes 412. Assume that the first trunk 40l in the group is idle and that the phase of the impulse generator H5 is such that the next impulse is delivered to the impulseconductor 659. This impulse is applied through the resistor 42I through the back contacts of the sleeve relay 404 01' the idle trunk 40I, conductor 422 through resistor 423 to ground. This impulse causes the application oi positive potential to the starting anode 424 01 the tube 426, which is individual to the trunk 40L Since the starting cathode 426 is at negative potential, the gap 424-426 ionizes when the impulse is delivered, and the main discharge gap ionizes resulting in the flow of current in the following circuit: positive pole of battery 6l9, conductor 62!, resistor 421, anode 428, cathode 420, wind ing of the hold magnet 430 to the negative pole of battery. Current flowing in this circuit alone is not sufllcient to energize the magnet 430. However, as soon as the select magnet 4| 6 has been operated as above described, a parallel circult is traceable from conductor 62| through the closed contacts 'of magnet 6, common impedance 43l thence through the tube 425 and the magnet 430 to battery. The magnet 430 operates in this circuit and closes the contacts of the switch to complete the connection from the incoming link 2l5 to the selected outgoing trunk 40L The flow of current through the common impedance element 43l lowers the potential of the anodes of the remaining tubes of the group 2 to prevent another one from operating in response to the succeeding phase impulses delivered thereto by the impulse generator 6l5.

At the time the primary select magnet 225 operates as above described a parallel circuit is closed from the battery supply conductor 62| through the closed contacts of magnet 225, common impedance element 229, anode 223, cathode 224 of the tube 204 and winding of the hold magnet 2|! to battery. The added current flowing through the parallel circuit just traced causes the energization of the hold magnet 221. Magnet 221 closes the contacts of the switch 200 to complete the connection from the calling line 202 to the selected link 2l2'.

Thus the calling subscriber's line 202 is extended through the switch 200 over the link 2|5 through the switch 400 to the idle trunk 40L The relay 432 operates in an obvious circuit in series with the loop of the subscriber's line 202 and closes an operating circuit for the sleeve relay 404. Relay 404 operates and applies ground potential to the sleeve conductor 433. The hold magnet 2|! of the primary switch is now maintained energized in a circuit traceable from battery through the winding of the magnet contacts of the switch 200, sleeve conductor 2l5, contacts of switch 400 to the grounded conductor 433. The hold magnet 430 of the secondary switch 400 ismaintained energized in a circuit from battery through the windings of sa d magnet through the grounded conductor 433. The grounded conductor 433 is extended through contacts of the switch 400 thence over conductor to the start anode ofthe test tube 60l, thus identifying the link 215 as busy when the next test is made.

The operation of the sleeve relay'404, indicating that the connection has been completed to the trunk '40l, causes the connection of the charged condenser M3 to the start ng anode of the release tube 646. The tube 646 discharges and release relay 650 operates in the main discharge circuit thereof. Relay 650 operates the relay 620, which in turn disconnects the ma n battery supply from the conductor 62!. The operated tubes and relays of the common control mechanism are now released to restore this mechanism to the common use of the switch frame. The release of'tube 306 and relay 330 reconditions the tubes 305 for subsequent operation and causes the release of the operaed select magnets 225, 226 and M6. The release of tube 609 and relay 639 causes the restoration of the tubes 605.

As soon as the horizontal group tubes 305 are released and the common mechanism is restored to the common use of the frame, the calling subscribers line 30! becomes effective in competition with other lines that may be calling in the meantime to seize a horizontal group of primary switches and to initiate the testing operations above described for extending a call to an idle trunk in one of the groups.

It will be obvious that numerous modifications may be made in the details of the system herein disclosed. It may also be noted that the discharge tubes illustrated may be of any suitable types such as those commonly known as gas filled cold cathode tubes.

What is claimed is:

1. In combination, primary switches, secondary switches, lines appearing in said primary switches, trunk groups appearing in said secondary switches, groups of links interconnecting the primary and secondary switches, operating magnets for said switches, a magnet operating circuit for each link including in series a primary switch magnet and a secondary switch magnet,-electronic devices for said primary switches, means for operating one of said devices to select a group of links for serving one of said lines, electronic devices for said trunk groups, means for operating one of said trunk group devices to select a group of trunks to serve said line, means controlled by the operated electronic devices for applying potential to the magnet circuits of the links of the selected group outgoing from the primaryswitches and to the magnet circuits of the links incoming to the selected trunk group to efiect the energization of the magnets of a single link interconnecting a primary switch in which said line appears and a secondary switch in which the selected trunk group appears, and means controlled by the energization of said magnets for establishing a connection from said line over the selected link to an idle trunk in said group.

2. In combination, primary switches, secondary switches, lines appearing in said primary switches, trunk groups appearing in said secondary switches, groups of links interconnecting the primary and secondary switches, operating magnets for said switches, a magnet operating circuit for each link including in series a primary switch magnet and a secondary switch magnet,- a plurality of discharge tubes individual respectively to said link groups, means for applying impulses'of difierent characters to said tubes for selectively operating one of them to determine the link group to serve a calling one of said lines, a plurality of discharge tubes individual respectively to said trunk groups, means for applying mpulses of different characters to said latter tubes for selectively operating one which represents a, group having an idle trunk and accessible to an idle link in the selected group, means controlled by the operated tubes for applying potential to the magnet circuits of all links of the selected group outgoing from the primary switches and to the magnet circuits of all links incoming from the several primary groups to the second ary switch having access to the selected idle group of trunks to cause the energization of the magnets of a single link interconnecting aprimary switch in which said calling line appears and a secondary switch in which the selected trunk group appears, and means controlled by the energization of said magnets for establishing a connection from said line over the selected link to an idle trunk in said group.

3. In combination, primary switches, secondary switches, lines appearing in said primary switches, trunk groups appearing in said secondary switches. groups of links interconnecting the primary and secondary switches, operating magnets for said switches, a magnet operating circuit foreach link including in series a primary switch magnet and a secondary switch magnet. a plurality of discharge tubes individual respectively to said link groups. means for applying impulses of difierent phases to said tubes for selectively operating one of them to determ ne the link group to serve a calling one of said lines, a plurality of discharge tubes individual respectively to said trunk groups, means for applying impulses 01 different phases to said trunk group tubes for selectively operating one 01' said tubes representing a group having an idle tmnkand accessible to an idle link in the selected group or links, means controlled by the. operated tubes for applying potential to the magnet circuits of the links of the selected group outgoing from the primary switches and to the magnet circuits of the links incoming from the several primary groups to the secondary switch having access to the selected idle group of trunks to cause the energization of the primary and secondary switch magnets of, a single link in the selected link group, and means controlled by the energized magnets for establishing a connection from the calling line to an idle trunk in the selected group.

4. In combination, primary switches, secondary switches, lines appearing in said primary switches, trunk groups appearing in said secondary switches, groups of links interconnecting the primary and secondary switches, operating magnets for said switches, a magnet operating conductor for each link extending from a primary switch to a secondary switch, said conductor including in series a primary switch magnet and a secondary switch magnet, a plurality of electronic tubes serving respectively said groups of links, means for operating one of said tubes selectively to choose the corresponding link group to serve one of said lines, a plurality of electronic tubes serving respectively said groups of trunks, means for operating selectively one oi? said trunk group tubes representing a group having an idle trunk and accessible to an idle link in the selected link group, means controlled by the operated link group tube for connecting at the primary switches battery potential to the magnet operating conductors 01' all links in the selected link group, means controlled by the operated trunk group tube for connecting at the secondary switches battery potential to the magnet operating conductors of all links incoming from the several link groups to the secondary switch having access to the selected idle trunk group, thereby causing the energization oi the primary and secondary switch magnets for a single one or the links in the selected group, and means controlled by the operation of said magnets for extending a connection to an idle one of th trunks in the selected group. a

5. In combination, primary switches arranged in groups, secondary switches, each group of primary switches having a plurality of links outgoing therefrom, the links of each primary group appearing in all primary switches of the group and appearing respectively in said secondary switches, each primary and secondary switch having an operating magnet for each link -appearing therein, a magnet operating circuit for eachlink including in series the operating magnet of each primary switch and the operating magnet of the secondary switch, trunk groups accessible respectively to said secondary switches, electronic devices serving respectively said primary switch groups, means for selectively operating one of said primary group devices to select the associated primary switch group for extending a connection, electronic devices serving respectively said trunk groups, means for selectively operating one of said trunk group devices to select a trunk group which has an idle trunk and which is reached through the secondary switch by an idle link in the selected primary group, means responsive to the operated primary group electronic device for applying a potential to the magnet circuits of the links of the selected primary switch group, means responsive to the operated trunk group electronic device for applying potential to the magnet circuits of the links incoming to the corresponding secondary switch from the primary groups for eflecting the operation of the primary and secondary switch magnets for the single link extending from the selected primary group to the selected trunk group, and means controlled by the operated magnets for completing a connection to an idle trunk in the selected group.

6. In combination, primary switches arranged in groups, lines appearing in said primary switches, secondary switches, each group of primary switches having a group of links outgoing to all of said secondary switches, trunk groups accessible respectively to said secondary switches, a plurality of line discharge tubes, means for operating said tubes selectively by phase impulses to determine the particular calling line in a group or primary switches to be served, link group tubes individual respectively to said groups of links, means for operating one of said link group tubes selectively to determine the group oi primary switches to serve one of its calling lines to the exclusion of other primary switch groups,

trunk group tubes individual respectively to said groups of trunks, means for operating one of said trunk group tubes selectively to determine the trunk group to serve the calling line, operating magnets for said primary and secondary switches, a magnet operating circuit for each link including in series the operating magnet of each primary switch in which said link appears and the operating magnet of said secondary switch in which said link appears, means controlled by the operated link group tube for applying potential to the magnet operating circuits of all links of the group selected to serve a calling line, means controlled by the operated trunk group tube for applying potential to the magnet operating circuits of all links incoming to the secondary switch having access to the selected trunk group, thereby energizing an operating magnet for a' single link in each oi. the primary switches in which said link appears and the operating magnet of the secondary switch in which said link appears, and means controlled by one of the operated primary switch magnets and by the operated secondary switch magnet for establishing a connection from the calling line chosen for service over the selected link to an idle trunk in the selected trunk group.

7. In combination, primary switches, secondary switches, lines appearing in said primary switches, trunk groups appearing in said sec ondary switches, groups or links interconnecting the primary and secondary switches, operating magnets for said switches, a magnet operating circuit for each link including in series a primary switch magnet and a secondary switch magnet, a plurality of discharge tubes individual respectively to said link groups, means for applying impulses of different characters to said tubes for selectively operating one of them to determine the link group to serve a calling one of said lines, a plurality oi! discharge tubes individual respectively to said trunk groups, means for applying impulses of difierent characters to said latter tubes for selectively operating one which represents a group having an idle trunk and accessible to an idle link in the selected group means controlled by the operated tubes for anplying potential to the magnet circuits of all links of the selected group outgoing from the primary switches and to the magnet circuits of all links incoming from the several primary groups to the secondary'switch having access to the se ected idle group of trunks to cause the energization of the magnets of a single link interconnecting a primary switch in which said line appears and a secondary switch in which the selected trunk group appears, means controlled by the energization of said magnets for establishing a connection from said line over the condition.

LARNED A. MEACHAM. CARLYLE V. PARKER.

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