US2091127A - Telephone system - Google Patents

Description

1937. R; TAYLOR El AL 2,091,127

TELEPHONE SYS'IFEM Filed Nov. 22, 1934 9 Sheets-Sheet 1 INVENTORS REGINALD, TAYLOR GEORGE THOMAS BAKER ATTY.

Aug. 24, R T YL R r AL A 2,091,127

TELEPHONE SYSTEM Filed NQV. 22, 1934 9 Sheets-Sheet 3 I out INVENTORS REGINALD TAYLOR GEORGE THOMAS BAKER ATTY.

Aug. 24, 1937.

R. TAYLOR El AL 2,091,127

TELEPHONE SYSTEM Filed Nov. 22, ,1954

9 Sheets-Sheet 4 REGINALD TAYLOR GEORGE THOMAS BAKER ATTY.

R. TAYLOR El AL TELEPHONE SYSTEM Filed Nov. 22, 1934 9 Sheets-Sheet 5 INVENTORS REGNALD TAYLOR GEORGE'THOMAS BAKER Aug. 24,- 1937.

R. TAYLOR ET AL TELEPHONE SYS TEM Filed Nov. 22, 1934 9 Sheets-Sheet 6 REGINALD TAYLOR GEORGE THOMAS BAKE ATTY.

Au 24, 1937. R T R A 2,091,127

TELEPHONE SYSTEM Filed NOV. 22, 1934 9 Sheets-Sheet 7 INVENTORS REGINALD TAYLOR GEORGE THOMAS BAKER ATTY.

Aug.24, 1937. R. TAYLOR ETAL 2,091,127

TELEPHONE SYSTEM Filed NOV. 22, 1934 9 Sheets-Sheet 8 TNVENTORS REGNALD'UWLOR GEORGE'THOMAS BAKER ATTK Aug. 24, 1937. R. TAYLOR ET AL 2,091,127

TELEPHONE SYSTEM Filed Nov. 22, 1934 9 Sheets-Sheet 9 SED I ob! H 28 H INVENTORS o5 REGINALD TAYLOR GEORGE THOMAS BAKER ATTY.

Patented Aug. 24, 1937 UNITED STATES PATENT OFFICE TELEPHONE SYSTEM Application November 22, 1934, Serial No. 754,250 In Great Britain December 6, 1933 8 Claims.

The present invention relates to telephone systems and has for its main object the provision of improved automatic switching apparatus for use in such systems for increasing the rapidity and reliability with which connections may be set up automatically. Another object of the invention is to provide improved methods of testing for the various conditions which arise during the operation of the automatic equipment.

According to one feature of the invention, a potential operated device such as a thermionic Vacuum tube or a grid controlled gas discharge tube is employed for indicating when a particular condition exists in all of a group of pieces of apparatus. Thus it may be used for operating a relay to perform a switching operation when all of a group of pieces of switching equipment become busy. This feature of the invention is of general application wherever it is desired to respond to a change in potential on a lead without connecting potential to the lead from the responding device.

Other features of the invention, however, are concerned more particularly with telephone systerns employing finder switches, and from this point of view the invention may be considered as an improvement in the system described in U. S. Patent No. 1,914,540. In this prior arrangement the primary finder switches were arranged so that those first taken into use are directly connected to first group selector switches, while those last taken into use were arranged to be connected to group selector switches in a common group over secondary finder switches. The

primary finder switches were of the two-directional type provided with vertical and rotary magnets for effecting the setting of the switches and with a release magnet for allowing the switch to restore to normal.

One of the features of the present invention relates to improved circuit arrangements, whereby the primary finder switches may be of the two-directional type in which no release magnet is provided, the restoration of the switch to nor- 45 mal being effected by the intermittent operation of one of the magnets employed for setting the switch so that the switch wipers in the complete setting and restoration operation perform a rec tangular movement.

50 Another feature of the invention relates to arrangements whereby the time required in setting up the connection may be decreased by causing the various switches to move at an increased speed. The increased speed of stepping is achieved 55 by employing self-interrupted stepping circuits to the magnets of the various switches, together with other circuit modifications which are necessary in order that the switches shall still operate accurately at the higher speed.

The invention will be better'understood from 5 the following description of one method of carrying it into effect, reference being had to the accompanying drawings comprising Figs. 1 to 10; Figs. 1 to 9 should be arranged as shown in Fig.

10 to form a complete circuit. 10

Fig. 1 shows a subscribers line circuit and also the start relays SA, SB, SC, SD and SE which are common to a group of 200 lines; Figures 2 and 3 show a primary finder circuit which is the same for both regular and overflow primary l5 finders. Figure 5 shows the group busy detecting circuit for the regular and overflow groups of finders comprising respectively the thermionic valves VAA and VAB. The start circuits to the primary control sets PCI, PCZ and PC3 are also shown. Figures 6, 7 and 8 show primary control set PCI, which is associated with regular and overflow finders over the distributor switches RPD and OPD respectively; Figure 9 shows one of the secondary start groups SS! which con- 25 trols two secondary control groups such as SCI each having access to a group of secondary finders over a secondary distributor such as SED. Figure 4 shows one of the secondary finders SFS which has access over the connections on the 30 distributing frame IDF to finders in the overflow primary group.

Referring now to the assembled circuit, it should be explained that the necessary changes of connection according to the conditions under n5 which the finders are to be used are made at the intermediate distributing frame IDF shown in Fig. 4, and in this instance the broken line connections shown are suitable for regular finders which are directly connectedv to first group selectors, while the connections shown in broken lines intercepted by dots apply to overflow finders, in which case connection with a first group selector is made by way of a secondary finder. The same convention is employed in connection with leads 23 and 28 in the upper right-hand corner of Fig. 5. For the purpose of the first part of this description it will be assumed that the finder shown is one of a regular group, and accordingly the broken line connections only will apply.

Referring now to Fig. 1 of the assembled circuit, when the calling subscriber removes his receiver to originate a call, a circuit is completed by way of his instrument circuit over conductors l and II to operate the line relay L over both its windings in series in circuit with the resting armatures R2 and k3. Relay L at armature Z1 marks the rotary bank of all finders with battery extended through the winding of relay K shunted by a comparatively low resistance l2, and at armature Z2 extends battery by way of a resistance such as YA (provided one per calling level) to mark in the vertical bank multiple the level in which the calling line is situated. It will be seen from the drawings that the vertical bank VB Fig. 2 extends to five relays, viz: SA, SB, SC, SD and SE which are provided with two windings each, one per level, and these relays are arranged to be operated from the marking battery extended by armature 12 to complete a start circuit for one or other of the primary control sets such as PCI (shown in Figs. 6, '7 and 8) having access by way of a distributor switch or switches to all finders in the group.

As fully described in the prior patent referred to, these relays are provided for the purpose of starting up the primary control sets PCI, PC2 and PC3, either separately or together, depending upon the calling conditions at any particular instance. Arrangements are also shown for altering the connections between the armatures of these relays according as twoor three primary control sets are provided. In the present case three primary control sets are assumed to be fitted and accordingly connection is made between the terminal points U26, U28 and U30, U32 as shown. If only two primary control sets are provided the terminal U32 will obviously be spare and connection is then made between terminal points U24, U26 and U28, U30.

- For the purposes of this description it will be assumed that relay SA is the one which is energized in parallel with the marking battery to the vertical bank VB and at armature sal located in the bottom left-hand corner of Fig. 5 a circuit is completed for the start relay ST of primary control set PCI which may be traced from earth at armature sal, armature 0 23, conductor I3, armature rs2, left-hand winding of relay ST to battery by way of resistance YC. Relay ST in operating at armatures st5, silt and st! prepares a circuit for a delayed alarm changeover device comprising the relays TA, TB and TC which will be described later, and at armature stl (Fig. 7) completes the following circuit for energizing relay LK providing that the regular primary distributor switch RPD has already preselected an idle regular finder: earth at operated armature stl, comparatively low resistance l4, windings of relay LK in series, armatures 15b5, (Z03, tc2, T85, 0913, wiper RI and bank contact of switch RPD, conductor l5, off-normal springs NI of the preselected finder, armature hal jumper connection at distributing frame IDF, release trunk conductor P of the associated selector to idle marking battery therein. It should be mentioned that the preselecting circuit of the regular primary distributor switch RPD functions in the following manner. If the finder engaged by the distributor is busy, wiper RI will encounter earth connected over operated springs NI via the low resistance right-hand winding of relay HB and relay G thereupon operates over the following circuit: earth on wiper R5, armatures ofr3, r35, tc2, 1103, 15b5, windings of relay LK in series, resistance l4, armatures Zlcl and '012, interrupter springs rpml and 010ml, conductor I1, winding of relay G, conductor I 6, armature 0124 to battery by way of resistance YA. Owing to its high resistance, relay G alone is operated in this circuit and at armature g! energizes the driving magnet RPM of the regular primary distributor, in a circuit including normally operated armature 1722 (Fig. 5). The magnet in energizing prepares for the advancement of the wipers Rl to R8 into engagement with the next set of bank contacts and at the conclusion of its stroke operates the associated interrupter springs rpm], thereupon opening the circuit to relay G which releases. In this manner, interaction takes place between relay G and the magnet RPM so that the wipers will be automatically rotated until the first idle finder is encountered.

In order to obviate the possibility of incorrect operation owing to a disconnection in the bank engaged by wiper RI it is arranged that when a call is initiated a test is made over this wiper for idle marking battery and if no such battery is encountered, relay LK will fail to operate and the following self-interrupted driving circuit becomes effective tomagnet RPM to advance the wipers on to the next set of contacts: earth at armature stl (Fig. '7) armatures Zkl and M2, interrupter springs rpml and opml, conductor ll, armatures stZ, 0fr2 and rfz2, winding of magnet RPM to battery. Automatic rotation therefore takes place and when idle battery is found, relay LK operates over the circuit previously described and opens the circuit of magnet RPM at the resting contacts of armature Zlcl, while at the make contacts of this armature a circuit is com pleted to the upper holding winding of relay LK which is sufficiently low in resistance to mark the selected finder as busy in the bank of the distributor switch. The operation of armature llcl also removes a shunt from relay VR which extends from its right-hand terminal over armature d111, resistance l4, armatures Zkl and 1112, interrupter springs rpm! and opml, conductor ll, armatures stZ, ofrZ, )1'22, ofc2 and $153 to its left-hand terminal. Relay VR thereupon operates in series with the lower winding of relay LK shunted by resistance M to battery in series with the driving magnet RPM. Due to the reversal of current flow in its lower winding the two windings of relay LK are now in opposition and the lower winding partly neutralizes the flux produced by the upper winding, although the resultant flux under the conditions prescribed is quite sun'lcient to hold the relay operated.

If, however, two distributor switches engage the same finder simultaneously, both relays LK in the respective control sets will operate although one may operate somewhat before the other due to slight diiferences which must always be present even between such similar relays. Consequently, with two distributors testing in on to the same finder the current in the holding winding of each LK relay will be approximately half the normal holding current so that the opposing lower winding will now almost completely neutralize the effect of the upper winding and the relays will start to release. Again due to inherent differences in the relays one will start to release first, and immediately it opens its make contacts it will restore almost full holding current to the other relay which then alone remains operated. In order that the relays VR shall not operate during the testing period described, they are preferably made slow to operate by providing them with highly inductive windings which in addition are normally shunted by a resistance, and by suitable choice of values a safe period for testing can be ensured.

When relay VR operates, it looks over its armature ml to battery in series with the driving magnet RPM, at armature 0T4 operates relay SF (Fig. 8) in series with one winding of the line relay of the associated group selector (not shown) at armature 2115 connects earth to wiper R6 of the distributor to operate relay HA in the finder, at armature UTE (Fig. 7) connects relay LK in circuit with the vertical bank VB of the finder, at armature or? (Fig. 6) connects earth to a common conductor l8 to ensure that the common relay OFZ (Fig. 5) shall not operate at this time. Relay SF in operating prepares the circuit to the cut-off relay CO (Fig. 8) which is normally short-circuited by Way of the armatures pbl and pct of the wiper testing relays PA and PB. The operation of relay HA in the finder at armatures hal disconnects idle marking battery from bank R! to busy the finder and as a result relay LK in the control set releases and at the resting contacts of its armature Zkl completes a circuit for operating relay DV over its left-hand winding. Relay DV in operating at armature do! completes a testing circuit for relay LK, this time in circuit with the right-hand winding of relay DV and at armature c1112 extends earth from armatures st! and lkl to wiper R3 to complete a self-interrupted driving circuit to, the vertical magnet VM of the finder. Intermittent energization of the magnet VM which is brought about by the selfinterrupting contacts 'vml, results in the shaft and wipers of the finder being raised to the marked level; when this is reached the vertical wiper VW picks up idle marking battery which is extended over wiper and bank R2 of the distributor switch to operate relay LK over both its windings in series. Relay LK thereupon at armature lkl opens the driving circuit to the vertical magnet and completes a holding circuit over its upper winding which is sufficiently low in resistance to guard the selected level. Simultaneously with this operation relay LK completes a circuit to its lower opposition windin in series with the right-hand winding of relay DV and the dual testing operations already described are again performed in connection with the vertical bank. It will be noticed that the right-hand winding of relief relay DV is shunted by a resistance. the value of this resistance is suitably chosen in relation to the winding resistance so that the combined resistance presented to relay LK is the same as that presented when relay VR was connected in circuit.

Assuming that no other finder is testing on the same level at this instance, relay LK will hold in spite of the opposition winding and at armature Z702 (Fig. 6) complete a circuit for operating relay RS (Fig. 8) over its lower winding, armature dvd and conductor 24, in series with the right-hand winding of the start relay ST (Fig. 6) to battery by way of the driving magnet RPM. Owing to the high resistance of this circuit the driving magnet RPM does not function. Relay RS in operating looks over its upper winding at armature T32 transfers the initial energizing circuit of relay ST to the control of armature 1702, at

armature 1193 (Fig. 8) maintains relay HA in the '1 at armature hb3 disconnects battery from the holding circuit of relay DV in thecontrol set which thereupon releases and completes the following self-interrupted driving circuit to the rotary magnet RM; earth at armature 1505 (Fig. 8) armatures m6, pal, pbl, conductor l9, armatures d116, ofcd, wiper R8 and bank contact, conductor 28, interrupter springs rmi, winding of rotary magnet RM to battery. Intermittent energization of the rotary magnet RM brought about due to the interrupter contacts rm! results in the wipers of the finder being rotated in search of the calling line which may be in either the Pl or P2 bank, depending upon whether the subscriber is located in the odd or even hundred line group.

If the subscriber is in the odd group, the test Wiper P! (Fig. 2) will encounter the marking battery when the calling line is reached and this is extended by way of the wiper and bank R3 of the distributor (Fig. '7) to operate the test relay PA in the control set.

If, however, the subscriber is located in the even group test wiper P2 will pick up the marking battery which is extended by way of wiper R5 to operate the other test relay ,PB (Fig. 8). The operation of either testing relay is arranged to open the operating circuit to the rotary magnet and thereby hold the wipers in engagement with the calling line.

If it is assumed that relay PA is the one which is operated, then armature pail (Fig. 8) opens the driving circuit to the rotary magnet and completes a holding circuit for relay PA over its lower winding in series with the right-hand winding of the start relay ST to battery by way of the driving magnet RPM, and armature paE connects earth to wiper R6 of the distributor to hold the switching relay I-IA in the finder. The operation of armature pal also removes a short-circuit from the cut-off relay CO (Fig. 8) which is of comparatively high resistance and thereupon immediately operates in series with the rotary magnet RM of the finder. Relay CO at armature col completes a locking circuit for itself by way of the comparatively high resistance YD, at armature 003 maintains relay SF on its upper winding, at armatures 004 and 005 shortcircuits the upper windings of relays PB and PA respectively, and at armatures cot and col shortcircuits both windings of the start relay ST which releases. Relay ST thereupon releases relays LK, VR and relay LK releases relay RS; upon the release of the latter relay earth is disconnected from one terminal and connected to the other terminal of the lower holding windings of relays PA and PB at armature rst. Relay PA which was holding to battery from the driving magnet RPM thereupon releases and the magnet becomes fully operated to kick the wipers of the distributor off the now busy finder when. the operating circuit is opened. Upon the release of relay PA a short-circuit is again connected across relay 00 so that it too releases and in turn releases relay SF; under this condition the resistance YD limits the current in the circuit of the rotary magnet in the finder to prevent its further operation. All relays in the control set are now restored to normal and upon the opening of the circuit of the driving magnet RPM at armature cat the magnet de-energizes and the wipers of the distributor switch are automatically advanced on to the next set of contacts.

Relay K in the calling subscribers line cir-- cuit operates in series with the test relay in the control set and when the control set releases it locks in series with the switching relay in the finder over a circuit which may be traced as follows: battery via the winding of relay K, operated armature kl, release trunk conductor P, bank and wiper Pl of the finder, armatures M3 and 72.123, springs NRE, conductor 2!, upper lowresistance winding of relay HA, conductor P to earth connected up at the controlling switch in the train. Relay K also at armatures 702 and k3 clears the calling circuit of its battery and earth connections, whereupon relay L releases after its slow period and opens the marking circuit and the start circuit to the primary con- 5 trol sets. It will be noticed that upon the release of armature ll, relay K is again connected to the private nermal conductor extending to the final selector bank multiple, but owing to the connection of comparatively low resistance earth through the upper winding of relay HA the circuit is effectively guarded against selection.

The calling circuit is now extended by way of the wipers negative 5, positive I, Pi and M! of the primary finder over resting armatures of relay HB and operated armatures of relay HA over the dotted connections at the distributing frame IDF to the associated first selector accessible over the bracketed conductors (Fig. 4). It may be however that both test wipers Pi and P2 will encounter marking battery from different calling lines at the same instant, and in this case relay PA in operating will take precedence over relay PB since the locking circuit of the latter includes a resting contact of relay PA. Operation of the calling subscribers meter MTR in Fig. 1 is effected when the calling subscriber replies by the application of potential to the metering conductor M by the controlling switch in the train.

Upon the release of the connection when the 40 calling subscriber replaces his receiver, holding earth is removed from the release trunk conductor P by the controlling switch in the train, whereupon the relevant switching relay in the finder and the K relay in the line circuit re- 45 lease. The release of the switching relay in the finder completes a self-interrupted driving circuit to the rotary magnet RM from earth connected to conductor 22 by way of common equipment arranged to give an alarm in case of fail- 50 ure to release and the wipers are thereupon automatically rotated to the 12th position. In this position the normal rotary springs NRI restore and the supporting means for the shaft and Wipers is mechanically tripped so that they fall 55 by gravity assisted by a spring and then rotate beneath the bank under the: control of this spring to the full normal position. The construction of a two-directional switch arranged to restore to normal in the manner just described is given in 60 British Patent No. 391,123.

In the fully normal position the ofi-normal springs N! and N2 again open, springs NI disconnect low resistance earth by way of the righthand winding of relay HB from the bank R! of 65 the distributor switch and replacing it by battery from the first group selector to mark the finder as idle. Springs N2 open the self-interrupted driving circuit to the rotary magnet and the magnet ceases to function.

It will be appreciated from the circuit arrangement that the operation of the rotary magnet persists during the period that the wipers are being restored vertically and moved to their normal position below the bank, and it is therefore 75 conveniently arranged that the rotary pawl is out of engagement with the ratchet during this period. If the switch fails to release within a reasonable period, the continuous operation of the rotary magnet will result in the common release signal equipment functioning after a suitable period to give an alarm of the faulty condition.

If the calling subscriber should abandon the call during the finding operation, the start relay ST will release either due to the release of the level start relays SA to SE if armature rsZ is not operated, or due to the release of relay LK if armature 1'82 is operated. It will be recollected that relay BS is operated when the calling level is found so that if the calling subscriber abandons the call before the calling level is found relay ST will release due to the release of the level start relays while if the calling subscriber abandons the call after the calling level is found relay ST will release due to the release of relay LK. Relay LK in these circumstances is holding over its upper winding to marking battery connected over the vertical bank so that when the marking battery is removed due to the release of relay L then relays LK and ST will release. The release of relay ST causes the control set and the finder to restore to normal and it will thus be seen that whenever the calling subscriber hangs up his receiver the release of the finder immediately takes place.

Consideration will now be given tothe case when all regular finders of the group in question are in use so that further calls will require to be extended over and overflow finder having access to a selector in the common group by way of a secondary finder such as SFS (Fig. 4) Owing to the fact that all the regular finders are in use negative marking battery will disappear from the common conductor 23 (Fig. 5) extending to the grid of the valve VAA whereupon the grid of this valve then becomes positive with respect to the filament and as the result of a. substantial increase in the plate circuit current produced by this condition relay RFB connected in the plate circuit then operates. The device VAA is preferably a vacuum tube having three or more electrodes, but use may alternatively be made of a grid controlled gas discharge tube or the like. It will be understood that the metal rectifier MRA provided in each finder is connected up in such sense as to prevent the earth subsequently connected tothe bank RI of the distributor from shunting the idle marking battery of other finders connected to the same common conductor 23. Relay RFB in operating short-circuits relay RFZ which it will be remembered is normally operated in series with the filament of the valve VAA so that this relay releases. Relay RFZ at armature rfzl completes the filament circuit to the other valve VAB by way of the various resistances shown, at armature rfzZ removes the short-circuit from the upper winding of relay OFC (Fig. 6) of primary control set PCI, at armatures H23 and 1724 removes a similar short-circuit from relays CFO of primary control sets P02 and PCS shown diagrammatically as dotted rectangles, at armature rfeE prepares a locking circuit for relay CFO of primary control set PCI, at armatures 1726 and Tfel prepares similar locking circuits for relays OFC in primary control sets P02 and P03 and at armature rfzB (Fig. 9) connects up earth to operate relay OS. One relay, such as OS is provided to every three secondary start groups such as SSI (Fig. 9) and controls in all 6 armatures, two of which are associated with each secondary start group that is, one to each of the two control circuits associated with each secondary start group. One of the armatures only is shown, namely 08! (Fig. 9) which on operation disconnects earth from the bank contacts of the overflow primary distributor OPD and replaces it with idle marking battery by way of the resistance YA connected in parallel with the upper winding of the start relay DS of secondary start group SSl. The other armature of relay OS associated with the secondary start group SSi is located in the other secondary control circuit.

In case regular and overflowfinders were connected to the same distributor switch, earth connected over resting contacts of relay OS would serve to mark the overflow as busy while regulars were still available.

Returning again to relay OFC in the primary control set PCi, it should be explained that although the short-circuit is removed from its upper winding this relay does not operate until the last regular finder has been fully set and the control relays are restored to normal. When this is done the driving magnet RPM of the regular primary distributor RPD is energized in the kickon circuit to advance the wipers on to the next set of bank contacts, but as all regular finders are now in use testing wiper RI will encounter earth in all positions of the bank and the preselecting control relay G thereupon immediately operates. Relay G at armature g! extends earth to the driving magnet RPM but as the armature rfzi. has opened meanwhile, the circuit is completed via the upper winding of relay O-FC which thereupon operates and is of sufficiently high resistance to prevent the operation of the magnet. Relay OFC thereupon locks on its lower winding by way of armature ofci and in a parallel circuit operates the relief relay OFR.

It will be seen from the drawings that the operation of relays CFO and OFR at armatures ofcZ, ofc3, o-fc i, 0 05, ofcS, 0 1'2, o-fr3, 0 74, 0W5 and 0 1'6 transfers the control leads extending from primary control set PM to the driving magnet and wipers of the overflow primary distributor OPD.. Similar operations take place in primary control sets P02 and PC3 so that their associated overflow primary distributors are con.- nected in circuit and all subsequent calls in the group are thereupon completed over overflow finders.

For the purpose of the description, it will now be assumed that a further call is handled by primary control set PCI and that the finder shown in Figs. 2 and 3 is one of the overflow group. It will also be understood that the connections shown in broken lines intercepted by dots will now apply. Relay ST is operated from the start circuit in the manner already described, and as the overflow primary distributor OPD is already standing in engagement with a free overflow finder, wiper 0! will pick up idle marking battery by way of the connections at the distributing frame to the resistance YA connected in parallel with start relay D8 of the secondary start group SS! (Fig. 9). Relay LK in the primary control set PC! thereupon immediately operates and locks, relay DS also operates in this circuit. After a short period relay VR operates to prepare for the vertical operation of the finder in the manner previously described. Under the present circumstances, however, and owing to the different connections at the distributing frame, relay VR also at armature m3 extendsdirect earth over the Wiper 01 of the distributor tomaintain the start circuit to relay DS in the secondary start group and also to busy this point of access to other hunting distributor switches. Earth at armature 0T3 also provides a locking circuit over the metal rectifier MRB for relays OFC and. CPR which is effective in case a regular finder becomes free meanwhile and brings about the re-operation of relay RFZ. The metal rectifier MRB is included in this circuit so as to prevent earth from armature 7725 starting up the secondary control group prematurely.

It will be understood that the secondary start groups SS! are accessible in common from the overflow primary distributor switches serving different groups of lines and are provided in numbers sufficient to cater for the maximum trafiic on these groups of lines. It is also arranged that adjacent contacts in the banks of the distributor switches extend to different secondary start groups in order to increase the possible availability, although it is conceivable that the same secondary start group may make more than one appearance in the banks of a distributor switch but only on contacts which are widely spaced apart.

Returning again to the primary control set PC! it will be seen that relay VR at armature vr i (Fig. 8) again connects up relay SF, but under these circumstances relay SF is not immediately operated and the resistance YA connected in parallel serves to connect up a marking battery to mark the overflow finder which has been taken into use in the bank multiple of the secondary finder switches SFS. From this point the operations of the overflow finder proceed as described for the regular finder and when the calling line is located one or other of the testing relays PA or PB in the primary control set operates toterminate the hunting operation. If the overflow finder taken into use is not yet found by one of the secondary finders SFS there will be no circuit for relay CO owing to the non-operation of relay SF and the control set therefore remains held in this condition.

It will be understood that simultaneously with the, hunting operation of the overflow finder to find the callin'glline, two secondary finders such as SFS (Fig. 4) are set in operation to find the overflow finder which has been taken into use. These secondary finders have no normal resting position for their wipers and it will be appreciated therefore, that the arrangement of setting two in operation simultaneously considerably reduces the time taken to find the overflow finder.

Considering the detailed circuit operations, when the start relay DS is operated from the primary control set it locks over its armature dsl to the access point S2 and at armatures ds3 and (Z35 completes a circuit to the control relays of the secondary distributor switch SED to cause this switch to select the first available secondary finder. A similar circuit is completed by way of armatures ds4 and 1186 to another secondary distributor (not shown) which also selects a further secondary finder. In order to still further reduce the finding time it is arranged that the secondary distributors perform a preselecting function which ensures that their wipers. are automatically positioned into engagement with a free secondary finder if there is one available.

. It will be seen from reference to the circuit that under normal conditions .if the secondary. finder engaged by the'distrib-utor .is busy, the

test wiper SEi will encounter earth from armature ski. which is operated at this time, and this is extended through both windings of relay DK in series, comparatively low resistance 25 shunted by the high resistance relay DR and another resistance 26, interrupter springs semi, armatures (Z83, sdi and ,fTB, winding of extremely high resistance relay iG to battery. Owing to its high resistance, relay iG is only operated in this circuit and at armature igi energizes the driving magnet SEM of the distributor. The magnet SEM in energizing prepares for the advancement of the wipers SE! to SE5 into engagement with the next set of bank contacts and at the conclusion of its stroke operates the associated interrupter springs semi, thereupon opening the circuit to relay iG which releases. In this manner interaction takes place between relay iG and the magnet SEM so that the wipers are automatically stepped into engagement with a free sec ondary finder. It therefore follows that as soon as relay DS operates, both secondary distributors SED should switch through on to idle secondary finders but in order to obviate the possibility of incorrect operation owing to a disconnection in the bank engaged by wiper SEI it is arranged that when a call is initiated a test is made over this wiper for idle marking battery connected to the P wire by the associated group selector (not shown). If no such battery is encountered relay DK will fail to operate and the following self-interrupted driving circuit becomes effective to the magnet SEM to advance the wipers on to the next set of bank contacts; earth at armature c185, armatures dki and dr3, interrupter springs semi, armature ds3, winding of the magnet SEM to battery. Automatic rotation therefore takes place and when an idle marking battery is found relay DK then operates over both its windings in series and opens the circuit of the magnet at the resting contacts of its armature dlci, while at the make contacts of this armature a circuit is completed to the holding winding of relay DK which is sufficiently low in resistance to mark the selected secondary finder as busy in the bank multiple of the distributor switches SED. The operation of armature dlci also disconnects the shunting resistance 25 from relay DR which thereupon proceeds to operate in series with the left-hand winding of relay DK which is shunted by the resistance 25, to battery in series with the driving magnet SEM. It will be recognized that relays DK and DR present a similar combination to relays LK and VR 5 in the primary control set and they are intended to perform an exactly similar function, namely the prevention of dual switching.

If it is assumed that no other secondary allotter SED is testing the same contact, relay DK will hold in spite of the opposition of its left-hand winding and after a short interval relay DR also operates. Relay DR locks by way of its armature dri, at armature drZ disconnects the shunting resistance 26 at armature dr3 opens 5 a further point in the circuit to the magnet SEM, at armature drd connects full earth to wiper SEi at the same time short-circuiting relay DK which releases, at armature dr5 extends earth over wiper SE5 to the negative conductor to prepare the associated group selector for operation, at

armature drfi prepares the testing circuit to relay FK and at armature dri extends earth over wiper SE3 to energize the driving magnet SEM. The driving magnet of the secondary finder thereupon energizes in a self-interrupted circuit controlled by the contacts sfmi to rotate the wipers SF i to SF4 in search of the overflow finder which has been taken into use. When this is found the test wiper SFi will pick up marking battery connected in parallel with relay SF in the primary control set which is extended over the bank and wiper SE4 of the secondary distributor switch to energize relay FK over both its windings in series. Relay FK thereupon opens the circuit of the secondary finder magnet at the resting contacts of its armature flci, while at the make contacts of this armature a circuit is completed to the upper holding winding of relay FK which is sufficiently low in resistance to mark the overflow finder selected as busy in the bank multiple of the secondary finder switches SP8. The operation of armature fki also removes a shunt from relay FR which thereupon proceeds to operate in series with the lower winding of relay FK shunted by a resistance 21 to battery in series with the driving magnet SFM. Again the relays PK and FR are similar in function to relays DK and DR in that they guard against the dual switching of secondary finders on to the same overfiow finder.

If it is assumed that no other secondary finder is testing on the same contacts at this instant, relay FK will hold in spite of the opposition of its lower winding and relay FR thereupon operates after a slight interval, thereby at armature fri disconnecting the opposition winding to relay FK, at armature fr2 supplements the earth to wiper SE5, at armature ,fr3 opens the shunt about its own winding, and at armature ,fr5 provides an alternative holding circuit to relay DR. Relay SF in the primary control set is operated in series with relay FK in the secondary distributor, and at armature sf2 completes an operating circuit for relay 00 which becomes immediately effective since it is assumed that the overflow finder has already found the calling line. Relay SF also at armature sfi opens the circuit to the start relay DS in the secondary start group and at the make contacts of this armature connects earth by way of the metal rectifier MRB to hold operated the changeover relay OFC and CPR for the purpose already described. Relay CO serves to release the primary control set and allow the overflow finder to switch through in the manner already described and when this is done the holding circuit to relay FK in the secondary distributor is opened and it releases. A circuit is thereupon completed at armature flci for operating the switching relay SK in the secondary finder which looks by way of its armature ski to earth connected to the release trunk conductor P from the associated group selector, at armature s7c2 connects guarding earth to the bank of the secondary distributor, and at armatures slc3 to skfi extends the negative, positive, M and P leads through to the associated group selector.

The calling subscriber now receives dialling tone from the group selector and proceeds to set up the connection by operating his dial switch in the usual manner.

Relay iG is operated from earth extended over the front contact of armature ski, wiper SE2 and armatures frd and fr6; and at armature i gi energizes the driving magnet SEM to prepare for the advancement of the wipers of the distributor switch on to the next set of contacts. Relay DR is short-circuited by earth connected up by armature i gi whereupon it releases and is followed shortly afterwards by relay FR which in turn opens the circuit to relay IG. All relays in the secondary control group are therefore released and upon the de-energization of the magnet SEM the wipers are automatically stepped on to the next set of contacts.

Considering now the condition when the last overflow finder in the group is taken into use, negative battery will be removed from the common conductor 28 extending to the valve VAB 10 (Fig. 5) whereupon the grid of this valve then becomes positive with respect to the filament and the increase in the plate current brought about by this condition results in the operation of relay OFB. Relay OFB at armature ofbl com- 15 pletes a circuit for relay OFZ which becomes effective when the last overflow finder has been fully set and the primary control set restores to normal. When this happens relay VR in the control set releases and at armature v'rl' removes 20 a short-circuit from relay OFZ which immediately operates and at armatures ofzl, ofzZ and ofz3 transfers the start circuit for the various primary control sets to conductors 29, 30 and 3| extending to overflow meters (not shown) in- 25 dividual to each set. Further calls during this condition are therefore registered on the particular overflow meter concerned. In case an overflow finder should become free meanwhile, the grid of the valve VAB will again become 30 negative and as a result of the decrease in the plate current, relay OFBwill release to open up access to the primary control sets.

Similarly, in case a regular finder should become free meanwhile, the grid of the valve VAA will become negative whereupon relay RFB releases to open up the primary control sets for access to the regular finder group.

If all secondary finders under the control of the start group SSI (Fig. 9) become busy, relay 40 OB will release due to the removal of all idle battery connections from the common conductor 32. Relay F3 is operated from armature obl and at armatures fbl and fb-Z opens the driving circuit to the respective secondary distributors to 45 prevent useless hunting, and at armatures fb3 and b4 extends earth to the access points SI and S2 and disconnects the start relay B8.

In case the secondary start group shown should be the last in use prior to the busy condition, it

50 will be seen that the release of armature ob2 bridges both access points SI and S2 to ensure that overflow finders connected to one access point are immediately busied in the banks of the distributor switches OPD, from the overflow find- 55 er in use connected to the other access point. It is conceivable that the condition may arise when all secondary finders in the exchange are in use at the same instant. This will have the effect of restoring all the OB relays in the exchange and 60 consequently negative potential will be removed from the grid circuits of all valves VAB in the exchange whereupon the respective relays are operated to connect up the overflow meters and prevent access to the overflow finders.

5 As mentioned in the early part of the description relay ST (Fig. 6) in operating is arranged to connect up the delayed alarm changeover set which functions in case the primary control set is not freed within a predetermined period of 7 time.

Considering now the operation of the changeover set in detail, relay ST in operating at armature st extends earth over common conductor 33 which sets in operation a motor arranged to 75 drive the cam assembly 34 which applies an earth impulse to the S and Z conductors having a definite period of time between each application. When earth is applied to the S pulse conductor a circuit is completed for operating relay TA over its left-hand winding whereupon the relay locks on its right-hand winding by way ofarmature tal, at armature m2 connects relay TB in circuit with the Z pulse conductor, and at armature m3 prepares a short-circuit to the lefthand winding of the start relay ST.

If the control equipment fails to release after the predetermined period has elapsed an impulse will be delivered overthe Z pulse conductor which operates relay TB over its left-hand winding so that this relay then looks over its right-hand winding at armature tbl, to earth at resting armatures similar to tb'Z in primary control sets P02 and PC3. cuit an individual fault alarm lamp LP is lighted, at armature tb2 earth is extended over conductor 35 to actuate an alarm associated with the particular rack upon which the equipment is mount ed, at armature we earth is extended over another common conductor 36 to set in operation a main alarm which is common to the exchange, at armature tb l the short-circuit is removed from relay TC (Fig. 8) at armature tb5 relay LK is released,- at armature tbfi the left-hand winding of the start relay ST is short-circuited and released and at armature tb'l the start circuit is transferred to primary control set PC3. The primary control set PC! is thus released and switched out of service and further calls thereto are handled by primary control set PC3.

If, however, the primary control set has become associated with a finder, one or other of the relays VR, BS or PA will be operated and therefore when the short-circuit is removed from. relay TC (Fig. 8) it immediately operates in series with relay HA in the finder. Relay TC locks by way of its armature tcl, at amature tc2 extends earth to the test wiper R! of the distributor if relay RS has not operated to busy the finder against further selection, at armature tc3 opens the circuit between relay PA and the test wiper PI of the finder since otherwise the operation of armature rs? would guard the line selected and prevent the subscriber from being served by other finders, at armature ted releases relay VR. and maintains relay RS if operated, at armature 1505 (Fig. 8) opens the circuit to the rotary magnet of the finder, and at armature tot (Fig. 6) provides a circuit for maintaining relay OFR. in case a fault has occurred during overflow working. In this manner, therefore, the faulty circuit is looked as busy until the fault is cleared, whereupon the attendant will momentarily open the fault release link FRL (Fig. 6) to open the locking circuit of relay TB and restore the circuit to normal.

In case the regular first group selector is artificially busied for test purposes by the connection of earth to the release trunk conductor P, this earth will find a circuit by way of the resting oif-norma1 springs N! in the finder to the testing bank RA of the distributor and therefore marks the associated finder as busy. i

If the first group selector associated with secondary finder is artificially busied for test purposes earth connected to the release trunk conductor P serves to busy the secondary finder concerned and also shunts the idle marking battery insofar as its function in holding relay OB is concerned, although it will be appreciated that owing to the presence of resistance YA in the cir- In parallel with this locking circuit, relay OB will remain held from idle marking batteries of other idle secondary finders in the same group.

We claim:

1. In a telephone system, calling lines having terminals arranged in groups, a finder switch having access to said terminals, said finder switch operated responsive to a call over one of said lines to step vertically to a group of said terminals and then operated in a rotary direction to find the terminal of the calling line, and means for continuing the rotary movement in the same direction to initiate the release of the switch.

2. In a telephone system, calling lines having terminals arranged in groups, a finder switch having access to said terminals, a vertical and a rotary magnet for operating said switch responsive to a call over one of said lines, said vertical magnet operating the switch vertically to the group of terminals in which the calling line is located, and said rotary magnet then operating the switch in a rotary direction to find the terminals of the line, and means for operating the rotary magnet again to initiate the release of the switch, and a self-interrupted circuit for said rotary magnet over which the rotary magnet is operated in both movements.

3. In a telephone system, an automatic hunting switch, lines accessible thereto, said switch having movement in two directions to find one of said lines, a common apparatus for controlling the movement of said switch, magnets in the switch for operating the switch in said two directions, and means for operating one of the said magnets to release the switch independent of the control by the common apparatus.

4. In a telephone system, a switch as claimed in claim 3 in which there is a self-interrupted circuit for said one magnet and means for operating said one magnet over said circuit both in the setting and release operations.

5. In a telephone system, a plurality of regular trunk circuits, a plurality of overflow trunk circuits, an electric discharge tube having a grid circuit, a source of potential in each of said regular circuits connected to said grid circuit, means in each of said regular circuits for varying the potential connected to the said grid circuit, and apparatus in the output circuit of said tube controlled bychanges in all of said circuits connected to the input of the tube for rendering the overfiow circuits effective.

6. In a telephone system, a plurality of circuits, an electric discharge tube, an input circuit for said tube connected to all of said circuits, means controlled over each circuit for disconnecting that circuit from the input of said tube, an output circuit for said tube, and apparatus in said output circuit operated when all of the circuits are disconnected from the input circuit of the tube.

7. In a telephone system, a plurality of switching devices, distributor switches operating to find an idle switching device, an electric discharge tube, means in each device for maintaining a potential on the grid of said tube as long as the devices are idle and for removing said potential when the devices are all busy, and means in the output circuit of said tube responsive to the removal of said potential for preventing the operation of said distributor switches.

8. In a telephone system, a group of regular finder switches and a group of overflow finder switches, an electric discharge tube, a lead from each regular finder to the grid of said tube, and means in the output circuit of said tube responsive to a change in potential in each of said leads for making the overflow finders available I when all the regular finders are busy.

REGINALD TAYLOR. GEORGE THOMAS BAKER,

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