US2722567A

US2722567A - Electronic tube switching system

Info

Publication number: US2722567A
Application number: US265486A
Authority: US
Inventors: Davison Alan; Threadgold Ronald
Original assignee: Automatic Telephone and Electric Co Ltd
Current assignee: Automatic Telephone and Electric Co Ltd
Priority date: 1951-02-23
Filing date: 1952-01-08
Publication date: 1955-11-01
Anticipated expiration: 1972-11-01

Description

Nov. 1, 1955 A. DAVISON ET AL. 2,722,567

ELECTRONIC TUBE SWITCHING SYSTEM Filed Jan. 8, 1952 6 Sheets-Sheet l LFDR F5 1 Z 3 4 1 2 l w l v v L A AMPi TCL. A

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A TTURNE Y5 United States Patent 2,722,567 ELECTRONIC TUBE SWITCHING SYSTEM Alan Davison and Ronald Threadgold, Liverpool, England, assigl ors to Automatic Telephone & Electric Company Limited, Liverpool, England, a British com- P y Application January 8, 1952, Serial No. 265,486 37 Claims. (Cl. 179-18) The present invention relates to telecommunication or like systems and is more particularly concerned with switching arrangements in which the usual electromagnetically-operated devices are replaced by their electronic equivalents.

The invention is alsoconcerned with arrangements for testing for an idle outlet and for determining whether a selected outlet is idle or busy.

According to one feature of the invention, in a switching device, a set of electronic tubes is provided as circuitclosing means for conversational and control paths.

According to another feature of the invention, in a coordinate switching device for use in a telecommunication or like system, a set of discharge tubes is provided as circuit-closing means at each coordinate intersection.

According to a further feature of the invention, in a switching arrangement comprising a plurality of sets of gas discharge paths arranged in groups, each set being adapted to be struck to complete connections between input and output paths two voltages are employed for controlling the striking of the discharge paths, one voltage being applied to all sets of discharge paths in a particular group while the second is applied to corresponding sets of discharge paths in all the groups whereby one set of discharge paths only is struck in response to the joint application of said two voltages.

According to another feature of the invention, in a switching arrangement comprising a plurality of sets of gas discharge tubes arranged in groups, each set being adapted to be struck to complete connections over conversational and control paths one voltage is applied to corresponding electrodes of all sets of discharge tubes in a particular group and a second voltage is applied to said corresponding electrodes of corresponding sets of discharge tubes in all the groups, one set of discharge tubes only striking in response to the joint application of said two voltages.

According to a further feature of the invention, in a switching arrangement for associating a calling line with subsequent equipment and comprising a plurality of sets of gas discharge tubes arranged in groups, each set being adapted to be struck to complete connections over conversational and control paths means for determining the number of the calling line serve to apply a biasing voltage to corresponding electrodes of corresponding sets of dis charge tubes in all the groups as determined by the number of the calling line while arrangements are provided for allotting each group of discharge tube sets to calling lines in turn by the application of a pulse to said corresponding electrodes of all the sets of discharge tubes in a group whereby one set of tubes in the allotted group only strikes to extend the calling line to subsequent equipment by the joint application to said corresponding electrodes of said biasing voltage and said pulse.

According to yet another feature of the invention, in a switching arrangement for completing connections to called lines and comprisinga plurality of sets of gas discharge tubes arranged in groups, each set being adapted to be struck to complete connections over conversational and control paths means are provided for registering the penultimate and final digits of a called partys number and for extending a biasing voltage to corresponding electrodes of all the sets of gas discharge tubes in one group as determined by one of said digits and for extending a pulse to said corresponding electrodes of corresponding sets of gas discharge tubes in all groups as determined by the other of said digits whereby one set of tubes only strikes to complete the connection to the called line by the joint application to said corresponding electrodes of said biasing voltage and said pulse.

According to a further feature of the invention, in a switching arrangement comprising a plurality of sets of gas discharge tubes arranged in groups, each set being adapted to be struck to complete connections over conversational and control paths means are provided for registering a digit necessary for extending a connection over a desired route and for extending a biasing voltage to corresponding electrodes of all the sets of gas discharge tubes in one group as determined by said digit in order to select said group while means are provided which subsequently become operative to deliver pulses successively to said corresponding electrodes of corresponding sets of gas discharge tubes in all groups until available conversational and control paths within the selected group are found whereupon the set of gas discharge tubes appertaining to said idle paths are struck by the joint application to said corresponding electrodes of said biasing voltage and said pulse.

According to still another feature of the invention, in a switching arrangement comprising a plurality of sets of gas discharge tubes, each set being adapted to be struck to complete connections between input and output circuits a voltage applied to an electrode of one tube of a set causes said tube to strike if the outlet appertaining to said set is available thereby causing a voltage to be applied to similar electrodes of the remaining tubes of said set while if the outlet is unavailable, the striking of said first tube together with the application of said second voltage to the remaining tubes is prevented.

According to a further feature of the invention, in an electronic relay, a thermionic tube to which an input signal is applied is arranged to control the operation of a gas discharge tube the electrodes between which the discharge takes place being connected between the anode of said thermionic tube and a source of negative potential in such a manner that said discharge tube strikes only when said thermionic tube is cut off.

The invention will be better understood from the following description of one embodiment taken in conjunction with the accompanying drawings comprising Figs. l-7.

In the drawings,

Fig. 1 shows a block schematic of the system.

Figs. 2-6 show the detailed circuits of the system of Fig. 1 and should be arranged as shown in Fig. 7.

It should be explained that the invention has been illustrated with reference to a P. A. X having a capacity of lines and employing the electronic equivalent of finder switches and final selector switches. In the embodiment shown in Fig. 1, four line finder switches LFDRI, LRDR2, LRDR3, LFDR4 are employed, each having access to all the lines of the system. One line only is shown with its accompanying line circuit LCA. A control and marking circuit CMC is provided in common to all lines for the purpose of registering the digits of a calling line and for extending a marking to the line finders in accordance with the registered digits to cause a connection to be completed between a preselected line finder and the calling line circuit. It will, of course, be understood that in a system having a greater capacity than 100 lines it may be necessary to provide more than one control and marking circuit in which case an allotter would probably have to be provided for allotting the circuits for use in turn. The line finders themselves are allotted for use in turn by the allotter AL which is provided in common to all the line finders and which serves to reset the control and marking circuit CMC at the appropriate time over the reset lead RS.

The control and marking circuit CMC comprises a tens marking counter LTMC, a units marking counter LUMC and a sequence switch LSS. The tens and units marking counters take the form of electronic counters employing cold cathode tubes arranged in a ring-often but obviously other forms of electronic counters could be used. The counters are continuously pulsed from a suitable source and when a line is in the calling condition the counters are stopped in the appropriate position to register the digits by a control over the leads TCL and UCL extending thereto from the line circuits. The sequence switch also consists of a cold cathode tube counter and is pulsed from the tens and units marking counters.

The circuit-closing means in the conversational and control paths consists sets of electronic tubes preferably cold cathode tubes and as regards the finder switches the selection of the tubes as determined by the tens and units digits registration is effected through a marking field to which potentials are applied from the tens and units marking counters.

The final selectors are controlled by a tens marking counter FTMC and a units marking counter FUMC which respond to the digits dialled by the calling party and are controlled in their operation by the sequence switch PS5. The marking counters FTMC and FUMC and the sequence switch FSS are similar to the corresponding equipment in the control and marking circuit CMC but it will, of course, be understood that FTMC, FUMC and PS8 are provided individual to each final selector. Similarly the circuit-closing means in the final selector conversational and control paths consist of cold cathode tubes and when fired complete the connection to the called line circuit,

Two final selectors PS1 and F52 only have been shown but it will be understood that if four line finders are used, four final selectors will be necessary, each line finder being permanently associated with a final selector preferably through an amplifier as shown in Fig. 1.

A description will now be given of the detailed circuit of the system shown in Figs. 2-6.

When the calling subscriber lifts off his handset, the line is looped and current flows through R and L10 in the line circuit LCA and negative potential of approximately 30 volts appears across L10. This potential is applied to the cathode of the gas discharge tube CCL2 for a purpose which will be described later and also causes a current flow from earth at the lower end of the tens resistor R40 (Fig. 4) in the line finder tens-marking counter LTMC, over the tens lead TCL, rectifier MR10 and resistor R11. There are ten resistors such as R40 each connected in parallel to lines having the same tens digit. The top end of each resistor, such as R40, is connected via a resistor and gas discharge tube, such as R44 and N50 to the control electrode of one of the gas discharge tubes in the counter. A continuously-operating pulse generator PG10 feeds pulses over ten leads each of which is connected over a capacitor such as C31 to the right-hand end of the resistor such as R44. If none of the lines are in a calling condition, the junction of the resistors, such as R40 and R44 will be at earth potential so that the discharge tubes, such as N50, will strike in response to pulses from PG10. It will be understood that each neon tube N50 is individual to a particular tens digit and is connected to that tube in the counting circuit which represent that digit. I i

The counter LTMC consists of a chain of three-electrode gas discharge tubes CCT 1, CCTZ, CCT3 and so so on up to CCTO (not shown). The counting circuit is of the type in which the striking of one tube extinguishes the preceding tube and primes the next succeeding tube to enable it to strike on the next pulse. At any particular instant, therefore, only one tube is conducting. Thus when tube CCT1 is conducting the voltage developed at the junction of R41 and R42 is applied over R43 to the priming electrode of CCTZ so that the next pulse from JG10 applied over C30 serves to strike the tube CCTZ. In the absence of the priming voltage, the application of a pulse from JG10 to the priming electrode is without effect. Thus when none of the lines are in a calling condition, each pulse from JG10 is applied to all the tubes of the counting circuit but only one tube strikes in response to a particular pulse and in striking extinguishes the preceding tube.

When a line is in a calling condition, for instance, the line shown in the drawing which has the tens digit, 1, the negative 30 volts across R40 applies a negative bias to the tube N50, thus preventing the striking of N50 and thus of the tube CCTZ when the appropriate pulse from JG10 arrives. The counter is thus stopped since subsequent pulses from P610 although applied to the tubes in the counter will not cause the tubes to strike owing to the absence of the priming voltage. Tube CCT1 thus remains conducting indicating that the tens digit of the calling line is l.

A discharge tube pulse generator is located in the cathode circuit of each counting tube, the pulse generator associated with counting tube CCT1 comprising resistor R46, tube N51, primary windings of T30 and T31 and capacitor C32. When CCT1 is non-conducting, the 50 volts between earth applied through R41, R42, R46 and battery applied through T31, T30 is insufficient to strike N51. When CCT1 is conducting, however, there is volts across R41 and R42 which is applied through R46 to the tube N51. The tube thus has 80-1-50 volts across it and strikes. Before it strikes, however, C32 has to charge through R46. This delay is arranged to be longer than the time during which CCT1 is conducting when the counter is running freely, so that no pulse generator will operate during this time. When the counter has been stopped with CCT1 conducting, C32 charges up and N51 strikes, discharging C32 and transmitting a positive pulse through T30 and T31.

The pulse from T30 is applied to the control electrodes of all tubes such as CCL2 associated with lines having the same units digit. As previously mentioned the negative 30 volts from the line circuit is applied to the cathode of CCLZ and assuming the line SL is the only one in a calling condition, this tube will be the only one having 30 volts on the cathode and will hence be the only one to strike.

The anode of CCL2 is connected to a supply of 80 volts through R49 and when the tube strikes, the righthand end of R11 goes positive. This voltage is applied to the P lead and serves to busy the calling line in the final selector multiple as will be described in detail later. In addition the negative voltage across R40 is removed.

The pulse from T31 is applied to the sequence control circuit LSS. In the normal condition, the tube CCSSI in this circuit is conducting so that its cathode is positive. This positive voltage is applied to P610 to enable the latter to operate and also to one electrode of N54, the other electrode being connected to a negative potential over R48. The tube N54 is thus conducting and the consequent positive voltage at the bottom end of R48 prevents the striking of N53 and similar pulse generators. The pulse from T31 extinguishes CCSSl and strikes CCSS2. When CCSSI is extinguished, the voltage developed across cathode resistors R54 and R55 falls from a positive value to zero so that earth is now applied to the pulse generator P610 to prevent further operation thereof and also to. N54 which is thus extinguished to enable the mass? pulse generators such as N53 subsequently to operate. The striking of CCSS2 causes a positive voltage to be developed across cathode resistors R56 and R57 and this is applied to pulse generator PG11 to cause it to start generating. The output from the pulse generator PG11 is connected over ten leads to the priming electrodes of each tube in the units counter LUMC, the connection including a capacitor C34 and discharge tube N52. The circuit is arranged to operate in a similar manner to the tens counter, that is to say the discharge tubes such as N52 normally conduct in response to pulses from PG11 but when tube CCL2 strikes the voltage developed across R49 reduces the bias applied to N52 to a value where the appropriate pulse from PG11 is insufiicient to strike N52. When the pulse generator starts up, therefore, the counter runs round from its random starting position until tube CCUO strikes, the units digit being After a suitable delay, the discharge tube N53 connected to the cathode of CCUO strikes and the pulse developed across R47 is applied to the control electrode of the tube CCSS3. This tube strikes while CCSS2 is extinguished.

The position now is that the subscriber on line 10 has lifted off his receiver as a result of which CCT1, CCL2, CCUl and CCSS3 have all struck.

Connections are taken from the junction of the pair of cathode resistors in each counting tube to a marking network in such a manner that a bias is applied in accordance with which tubes are conducting, through the network to the appropriate set of cold cathode tubes in an idle line finder which when struck connect the calling line through the line finder to the final selector. Thus referring to the marking network shown in Fig. 2, the leads marked 1-, 2-, 3- and 4- will be connected to the correspondingly marked leads of the tens marking counter. Similarly the leads marked -1, 2, 3, -4 in the marking network will be connected to the correspondingly marked leads in the units marking counter. Assume for example that the tubes CCT1 and CCUO are non-conducting. Then the voltage at the junction of R41 and R42 is zero and similarly for the junction of R52 and R53. The voltage at the point M10 in the marking network is thus zero. Now assume that CCT1 and CCUO are conducting, as they will be in the present case. The voltage at the junctions of the cathode resistors will now become positive and the circuit is so arranged in conjunction with the rectifiers in the marking network that the point M10 now takes up a voltage of +50. This voltage is applied to the control electrodes of the corresponding sets of discharge tubes in all the line finders. It will be noted that the positive voltage at the junction of the cathode resistors will also be applied to other rectifier pairs. For instance, the voltage between R41 and R42 will be applied to all the rectifiers connected to lead 1- while that between R52 and R53 will be applied to all rectifiers connected to lead -0. However the positive potential at the junction points such as 12, 13 and so on and 21, 31 and so on will be less than 50, approximately 25 and this is below the voltage necessary for priming the line finder discharge tubes.

The preselection of an idle line finder is effected by the allotter AL (Fig. which is in the form of a ring counter and it will be assumed that line finder LFDRI has been allotted for use and consequently tube CCALl in the allotter is conducting. The positive voltage developed across R83 is applied to the junction of R24 and R27 (Fig. 2). The voltage across R83 is 50. Before CCSS3 (Fig. 4) strikes, there is no voltage drop across R51 and the component values are selected so that the junction of R23 and R24 (Fig. 2) is at +25 volts and V remains non-conducting, the cathode voltage of V10 being greater than +25 volts, but less than +50 volts. When CCSS3 conducts as described above, this voltage is increased to +50 and V10 conducts. However, the buildup of the control gridvoltage of V10 is comparatively slow so that no pulse is obtained in the V10 output. As-

sociated with V10 is a continuously operating pulse generator comprising neon tube N20, resistors R21, R20 and capacitor C20. This pulse generator feeds negative-going pulses to the control grid of V10 through C21 and when V10 is rendered conducting as mentioned above, the pulses from the pulse generator appear in the anode circuit of V10 as positive-going pulses, the negative-going pulses on the grid being of sufficient amplitude to cut-off V10. These positive-going pulses are fed into the grid circuits of every tube in the associated line finder but are of insuificient amplitude to cause the tubes to strike unless a bias is applied from the marking network as described above. In the present case therefore only the tubes shown i. e. those leading to line 10 will strike to extend the line to the final selector via the two-way amplifier AMP.

When tube CCLF3 in the private lead strikes a negative pulse is developed across the primary winding of the transformer T30 which is common to all private anodes of the finder switch. This pulse is inverted and fed as a positive pulse from the secondary winding into the grid of CCBU. Since CCALl (Fig. 5) is conducting, CCBU is primed through R27 and therefore strikes. R25 and R26 now have volts dropped across them, and this cuts off the pulse generator due to the connection thereof to the cathode of CCBU. R26 has 50 volts across it and this is fed through R28 to the allotter step-on control ASC (Fig. 5). A connection is also made to the lead from the cathode of CCALl via R84. Thus, if CCBU is not conducting and CCALI is conducting, the junction of R84 and R28 is at +25 volts and MR55 and V26 are not affected. When however, CCBU conducts the junction of R84 and R28 will be at +25 volts and MR55 conducts. building up a positive voltage across R85. Valve V26 conducts and cuts off V27, which allows pulse generator N42 to operate. This drives the allotter to the next position. If the tube corresponding to CCBU in the next finder is conducting V26 conducts again, since R85 has a positive voltage across it due to MR54 conducting and N42 pulses again. N42 will drive the allotter until a free line finder is found, i. e. until a tube corresponding to CCBU which is not conducting is found. The allotter will then come to rest and that particular finder will be preselected.

While a free line finder is being found V28 (Fig. 5) will be conducting. When a free finder is located, V28 ceases to conduct abruptly since N43 and N44 are extinguished. A positive pulse will thus be transmitted from the anode of V28 to the sequence switch LSS, driving the counter from CCSS3 to CCSSl. This resets the control and marking circuit, and LTMC drives round until another call comes up. Further when CCLF3 in the private lead strikes, the right-hand end of R11 goes more positive, the value of R11 being high compared with the resistance of L10. This increased positive potential is applied to the cathode of CCLZ which is thus extinguished.

Consideration will now be given to the operation of the electronic equivalents of the A, B and C relays of the and V23 are normally non-conducting while CCC is conv ducting. Tube 34 is thus non-conducting.

Alternating current having a frequency considerably greater than the dial frequency is fed over lead 10 (Fig. 2) and condenser C10 to the tube N10 and to L10. Owing to the negative voltage developed across L10 when the subscriber lifts his receiver, the negative half-wave of the alternating current causes N10 to strike so that pulses are fed through C10 to the private lead. These pulses are, however, ineffective until the line finder has extended the subscribers line to the finalselector. When this occurs, tube CCLF3 in the finder conducts and the pulses on the private lead are transmitted through CCLF3 to the primary winding of the tuned transformer T30. The pulses are stepped up and filtered in the secondary winding of T30 and are fed to the input of the tube V20 (Fig. 5) which is arranged to operate as a limiter-amplifier and is provided with a transformer T50 in the anode circuit. The output from the secondary winding of T50 is fed to a voltage-doubler circuit of known type, the negative D. C. output of which is fed via R61 to the control grid of V21. It will be noted that one side of the voltagedoubler circuit is negatively biased for pulse-shaping purposes. The negative output from the voltage-doubler is applied to V21 and causes the tube to be cut oil, thus allowing N30 to strike. This causes V22 to conduct thus extinguishing CCB but N31 can still not strike nor can N32 or N33.

Dial tone is returned to the calling subscriber in a manner to be described later and he operates the dial switch in accordance with the required number. On the first break over the loop, the negative potential across L is removed and consequently the alternating current fed over lead 10 is without efiect on the tube N10. No pulses are therefore fed over the private lead to the limiter-amplifier and the output of the voltage-doubler goes positive to such an extent as to enable V21 to conduct again. Tube N30 is thus extinguished and V22 is cut off. It should be noted that CO3 does not strike at this time since the 25 volt priming bolts applied over R65 is not high enough. Tubes N32 and N33 strike but the striking of N31 is delayed by the provision of capacitor C41 and the duration of the delay is arranged to be greater than the duration of the longest break pulse likely to be encountered. Tubes N32 and N33 in striking cause V23 to conduct thus extinguishing CCC. At the end of the first break period, the alternating current will be again transmitted to the limiter-amplifier whereupon the negative-going D. C. output from the voltage doubler will again out ofi V21. Tube N30 again strikes, allowing V22 to conduct and extinguishing N32 and N33 so that V23 is again cut off. Tube N34 does not, however, strike at this time since, similarly to N31, its striking is delayed by condenser C43 for a period longer than the duration of the longest make.

A similar operation occurs for each break pulse in the train. At the end of the impulse train, as V23 is cut off for a longer period than the duration of the longest make, N34 will eventually strike, causing a positive pulse to be developed across R72 whereupon CCC again strikes and N34 is extinguished. This positive pulse is also fed to the final selector sequence switch FSS as described later.

It will be seen from this that tube V22 follows the impulses transmitted by the calling subscribers dial switch in that it is non-conducting during breaks. The positivegoing pulses developed in the anode circuit are taken from the junction of the anode resistors R62 and R63 to circuits which effect the setting of the final selector in a manner to be described later. It will also be noted that tube N31 does not strike during impulsing and it is this that gives the associated circuit the character of the B relay. This will be better appreciated from the description of the release of the connection when it will be seen that N31 strikes to initiate this release when the calling party hangs up. Finally it will be seen that the circuit associated with tube N34 has the characteristic of a C relay since tube N34 strikes at the end of an impulse train to perform a switching operation as described later.

When the final selector is taken into use, tube MA in the final selector sequence switch PS8 is conducting. The positive voltage across R74 is thus applied to N21 in the final selector tens marking circuit ,FIMC. This tube thus strikes and consequently the pulses from the junction of R62, R63 which follow the dial impulses are fed to FT.MC, and appear as positive pulses across R30. As previously mentioned FTMC operates in a similar manner to LTMC and thus takes up a position corresponding to the number of pulses in the first train. At the end of the first train N34 strikes as previously described and the positive pulse developed across R72 is applied via C44 to cause MB to strike in PS8 and consequently to extinguish MA. When MA is extinguished, N21 in FT MC is extinguished but the striking of MB causes N22 in FUMC to strike. The second train of pulses is thus applied to the units marking circuit. At the end of the second train N34 strikes again, extinguishes MB in FSS and strikesMC.

In each of the cathode circuits of the units counter are two pulse generators which will be referred to as the A and B generators. The time constant of A is greater than a break period so that while the counter is being operated, no A pulse generator is operated. The B pulse counters are controlled as described later. When the counter stops with, say, tube CCU(1) conducting, the associated A pulse generator comprising N23 strikes after a delay. A positive pulse is thus fed over lead UMAI to all the UMA leads of the private tubes of the sets of discharge tubes corresponding to the particular units digit. Now assume that the tens counter has been stopped with tube CCT(2) conducting, the positive voltage at the cathode will be applied over lead TM2 to the grid resistors of all the sets of tubes corresponding to the particular tens digit. Hence only one private tube has applied to it the priming voltage from the tens counter and the striking pulse from the units counter. Assuming that the line represented by the two digits is idle, the private tube, say, CCFS3 strikes and in striking passes a positive pulse through T51 to strike HAA, HAA being primed by the positive voltage developed across the cathode resistor of N39 and N40 which are normally conducting.

The tubes HAA and HAB are analogous to the H relay in the final selector. Together they form a bi-stable element of which HAB is normally conducting. In this condition a voltage is developed across R and accordingly MR50 conducts and the consequent current flow causes a voltage to bedeveloped across R82. This voltage will be positive with respect to negative battery and will be applied over lead 27 to all the B generators, of which two only, namely N24 and N26, are shown in the drawing. This positive bias applied to the B generators prevents their operation when the associated counting tube strikes. However, when HAA strikes, current ceases to fiow through R80 and R82 and the full voltage of the negative battery is applied to the B generator, in this case N24, which thereupon operates and over lead UMBI applies a positive pulse to the remaining tubes of the selected set. Tubes CCFSI, CCFS2 and CCFS4, therefore, strike, It will thus be seen that access to a required line is obtained by applying a pulse to the P tube of the set of tubes having access to the desired outlet and if this tube strikes, indicating that the line is available, a pulse is then applied to cause the remaining tubes of the set to strike.

When tube MC of the final selector sequence switch strikes as explained above, a positive pulse is applied to N41 (Fig. 5) which together with tubes CCE, N39 and N40 forms the electronic equivalent of the E relay. It is, however, arranged, by means of the parallelconnected capacitor, that the striking of N41 in response to the positive pulse is delayed for slightly longer than the time taken for N23 to strike, HAA to strike and N24 to strike. When N41 eventually strikes, it causes CCE to strike and the positive voltage developed across the cathode resistor is applied to lead 26 and through MRSl to lead 27. This positive voltage extinguishes 9th ubes N2 and N 4, while QCE in con q ns,

ca'u'sesN39 and to be extinguished thereby remov ing the priming voltage from HAA.

As a further result of the striking of HAA, the positive voltage across R81 is applied over R78 as a priming voltage to tube CCF which is analogous to the F relay in a final selector.

When tube CCFS4 strikes, current flows over the D lead and a resistor such as R86 (Fig. 2) in the wanted subscribers line circuit. The positive voltage developed across this resistor is applied via resistor R87 to strike the tube, such as CCLl, which is individual to the subscribers line circuit. When this tube strikes, ringing current is applied from source RC through transformer T and the tube to the wanted line.

When the called subscriber removes his receiver, alternating current is injected into the private lead as in the case of a calling subscriber. This is extended over the final selector multiple to the private lead in the final selector and thence through tube CCFS3 to T51. Here the alternating current is amplified in V24, limited and rectified to give a negative D. C. bias to V25. This tube is normally conducting but is cut off by the negative bias to allow N36 and N37 to strike. In striking N37 applies a positive pulse to CCF which, being primed when HAA strikes, now strikes and primes N38. The alternating current applied to N38 now passes through the tube and the positive voltage developed across R79 on positive half-waves serves to strike HAB and to ex tinguish HAA.

In addition when the called subscriber replies, a negative voltage is developed across the impedance corresponding to L10. This extinguishes the tube corresponding to CCLll to disconnect ringing. Finally when CCF conducts, a large negative potential is developed across L50 which is suflicient to extinguish tube CCFS4 to prevent further striking of the tube corresponding to CCLI.

The operation of the circuit in the case where the wanted line is busy will now be considered. The tube CCLF3 or a corresponding tube in another finder will be conducting at this time and the current flow through the resistor corresponding to R11 causes the right-hand end of R11 to go approximately 70 volts positive. This voltage is applied to the P lead in the final selector multiple and thus to the cathodes of all the tubes such as CCFS3. Consequently the tube CCFS3 in the present example, will fail to strike in response to the application of the biasing voltage and the operating pulse. Tube HAA is therefore not struck and HAB remains conducting so the full negative voltage is not applied to the B tubes which are therefore prevented from striking. Tube N41 however, conducts after its delay since it is controlled by tube MC of the sequence switch. Consequently CCE also strikes to exert the above-described control over leads 26 and 27 and to extinguish tubes N39 and N40. The tone control circuit TSC is arranged in such a manner that with HAA conducting (wanted line idle), ring back tone is fed to the calling subscriber when CCE strikes while with HAB conducting (wanted line busy), busy tone is fed to the calling subscriber. Referring to the tone circuit, the

leads

21, 22 and 23 are connected to a tapping on the cathode resistor of HAB, a tapping on the cathode resistor of HAA and the cathode of CCE respectively. Now CCE strikes whether the wanted line is busy or idle and thus a positive potential with respect to negative battery is applied to lead 23 in both cases. If the line is idle HAA strikes and a positive potential is applied to lead 22 while negative battery potential is applied to lead 21 since HAB is extinguished. Further lead 25 will be connected to earth potential since CCF does not strike until the called subscriber replies. Hence the junction between rectifiers MR30 and MR31 in the tone circuit will be at a positive potential and the positive side of MR34 will be at earth potential so that MR34 will conduct and ring-back tone over lead RT will be transmitted to the 10 calling party over transformer T21. subscriber replies, CCF conducts and a positive potential is applied to lead 25, this potential having a higher value than that at the junction of MR and MR31. Rectifier MR34 thus ceases to conduct and the transmission of ring-back tone to the calling subscriber is prevented. It will be noted that at this time the junction between MR32 and MR33 will be at a potential between that of the negative battery and the positive potential at the cathode of CCE while the positive side of MR35 is at earth potential. It is arranged that the potential between MR32 and MR33 is below earth so that MR35 does not conduct and the transmission of busy tone to the calling party is prevented. wanted line is busy, conditions in the tone circuit are reversed and MR35 conducts while MR34 is block. Busy tone is thus returned to the calling subscriber.

With regard to the transmission of dial tone, it should be explained that when a line finder switches to a calling line, the sequence switch in the associated final selector will have MA conducting. A positive potential having a value greater than volts will thus be applied to lead 24 so that MR36 will be conducting and dial tone will be fed to the calling line over T21 as soon as the line finder switches through. When the first train of impulses has been received, MA is extinguished and lead 24 is then earthed. Rectifier MR36 ceases to conduct and the further transmission of dial tone is prevented.

At the end of the call, the calling and called parties both replace their receivers and accordingly current is disconnected from the private lead extending from both line circuits. As regards the calling line this enables tube V21 to conduct again thus causing N30 to be extinguished. As regards the called line, V25 again conducts and N36 is extinguished. When both N30 and N36 are extinguished, V22 is cut off and N31 strikes. The consequent positive potential developed over R67 is applied to ZA which strikes and ceases the operation of relay Z. Relay Z at contacts Z1 disconnects the H. T. supply from all points marked X in the drawing. This extinguishes the tubes in the speaking and private leads and the tubes in the counting circuits FTMC and FUMC. It also extinguishes the conducting tube in the final selector sequence switch PS8 and the tube ZC, while is normally conducting. Further it extinguishes tube CCE (Fig. 6) in the final selector and tube CCBU in the line finder thus making the line finder available to the allotter. Finally it disconnects the H. T. supply from V20 and V24 and from tube CCE which is thus extinguished.

It will also be noted that the positive potential across R67 causes a pulse to be applied to CCB which strikes and extinguishes N31.

The tube ZA forms a bi-stable element with ZB which is normally conducting. When ZA strikes, ZB is extinguished thus enabling N35 to strike after a delay. When N35 strikes, ZB again strikes and ZA is extinguished. Relay Z thus releases to reconnect the H. T. supply. The reconnection of the H. T. supply causes ZC to strike and apply a pulse to the normal position tubes in the counters FUMC and FTMC and in the sequence switch FSS.

All the circuits are now in readiness to accept another call. Release in the case where the called line is busy takes place in a similar manner when the calling subscriber replaces his receiver and it is thought that no detailed description thereof is necessary.

It will be appreciated that various modifications may be made to the circuits described without departing from the scope of the invention as set forth in the claims. For instance, the sets of discharge tubes have been shown as each comprising individual tubes but it is possible that a number of tubes may be grouped together in one envelope. Further while the system shown is a two-coordinate arrangement only, the capacity of .the system could be enlarged by employing a three-coordinate arrange-' When the called If the.

ment. Again it will be appreciated that although no group selecting feature has been disclosed, it would be a simple matter to adapt the final selector to operate as a group selector. For this purpose the counter FUMC would be operated from a pulse source and a test for the idle or busy condition of the outlets of the selected group would be made after each step of the counter.

It will be noted that the discharge tubes shown in the drawing are either of the two or three electrode type cold cathode tube. While these are preferable it is to be understood that the invention is not limited thereto but other types may be used if desired.

We claim:

1. In a telecommunication system, a switching arrangement comprising a plurality of sets of gaseous discharge tubes arranged in groups, input paths to said switching arrangement, output paths from said switching arrangement, each input path communicating with an output path over the discharge paths of one set of gaseous discharge tubes, control means for applying a first potential to all the sets of gaseous discharge tubes in a group, and control means for applying a second potential to one set of gaseous discharge tubes in each group, each set of gaseous discharge tubes striking only in response to the concurrent application of both said first and second potentials thereto to complete a connection between an input path and an output path.

2. In a telecommunication system, the arrangement claimed in claim 1 in which the potential applied by one of said control means is a steady potential and the potential applied by the other of said control means is of pulse form.

3. In a telecommunication system, the arrangement claimed in claim 1 wherein the first mentioned control means is a non-numerical control means and wherein the second mentioned control means is a numerical control means.

4. In a telecommunication system, the arrangement as claimed in claim 1 wherein the first and second mentioned control means are each numerical control means.

5. In a telephone system, a plurality of subscribers lines, switching means comprising a plurality of sets of gaseous discharge tubes arranged in groups, each discharge tube having at least two electrodes, circuit means including the discharge paths of said tubes and constituting conversational and control signal paths between said subscribers lines and subsequent equipment, means for determining the number of a calling subscribers line and for applying a bias potential to corresponding electrodes of a set of discharge tubes in each group in accordance with the number of the calling line and control means for applying a potential of pulse form to corresponding electrodes of all the sets of discharge tubes in a group, said discharge tubes striking only upon the concurrent application of said bias potential and said pulse-form potential thereto to complete a connection between said calling line and said subsequent equipment.

6. In a telephone system, the arrangements claimed in claim 5 and comprising a plurality of counting circuits each comprising a ring of gaseous discharge tubes, means for applying pulses of potential to said counting circuits successively to cause the tubes of each ring to strike in sequence and means for stopping the striking sequence of the tubes in said counting circuit at points representative of the number of the calling line.

7. In a telephone system, the arrangements claimed in claim 6 and comprising, in addition, a pulse generator, means for applying pulses from said pulse generator to one of said counting circuits to effect the continuous operation thereof, leads extending from subscribers lines and multipled together in accordance with one digit of the numerical designations of the lines, means responsive to the calling condition of a line for applying a voltage to a particular oneofr" said leads and means responsive to the application of said voltage to said lead for controlling said first mentioned means whereby the application of further pulses to said counting circuit is prevented.

8. In a telephone system, the arrangements claimed in claim 7 comprising, in addition, an output circuit for each stage of said one counting circuit, means for delaying the transmission of a pulse through an output circuit whereby an output pulse is obtained only when the operation of said one counter is stopped further leads extending from the subscribers lines, which leads are multipled together in accordance with a further digit of the numerical designations of the lines, a second pulse generator for applying pulses to a second counting circuit, means responsive to an output pulse from said one counting circuit for extending said leads to control said second pulse generator and means for initiating the operation of said second pulse generator.

9. In a telephone system, the arrangements claimed in claim 8 comprising, in addition, a co-ordinate marking network and means responsive to the setting of the counting circuits in accordance with the calling subscribers number and operative to feed a potential through said co-ordinate marking network to corresponding electrodes of a set of discharge tubes in each group.

10. In a telephone system the arrangements claimed in claim 9 in which a sequence switching device comprising a plurality of serially arranged discharge tubes is provided for controlling the sequential operation of said counting circuits.

11. In a telephone system, the arrangements as claimed in claim 10 in which means are provided responsive to the sequence switching device reaching the final point in its switching sequence and operative to apply a voltage to a preselected group of sets of discharge tubes to cause a pulse to be applied to corresponding electrodes of all said sets of discharge tubes.

12. A switching arrangement as claimed in claim 11 including allotting arrangements comprising a plurality of serially-arranged discharge tubes operative to preselect ditferent groups in dependence upon which tube in the series is struck and responsive to the operation of one set of discharge tubes to transfer the conducting condition from one tube to the next tube in the series so as to pro-select another group and to cause said sequence switching device to switch to the first point in its switching sequence, said counting circuits being responsive to the switching of said sequence switch to reset to normal.

13. In a telephone system, a plurality of subscribers lines, switching means comprising a plurality of sets of gaseous discharge tubes arranged in groups, each discharge tube having at least two electrodes, circuit means including the discharge paths of said tubes and constituting conversational and control signal paths for completing connections to a called subscribers line, means for storing the penultimate and final digits of a called subscribers number and serving to apply a bias potential to corresponding electrodes of all the sets of discharge tubes in a particular group as determined by one of said digits and to apply a pulse potential to particular corresponding electrodes of corresponding sets of discharge tubes in all groups as determined by the other of said digits, said sets of discharge tubes striking only in response to the concurrent application of said bias potential and said pulse potential thereto to complete a connection to a called subscribers line.

14. In a telephone system, the arrangements claimed in claim 13 and comprising, in addition, first and second counting circuits for storing said digits of the called subscribers number each said counting circuit consisting of a ring of discharge tubes and a sequence switching device comprising a plurality of serially-arranged discharge tubes for routing the penultimate and final digits to the appropriate counting circuit.

15. In a telephone system the arrangements claimed in-claim .1 and c m risin i a d n. n mp s I3 sponsive device, parallel connected output circuits for said impulse-responsive device connected to said counting circuits, and a blocking discharge tube in each of said output circuits, said discharge tubes being rendered conducting sucessively under the control of said sequence switching device.

16. In a telephone system, the arrangements claimed in claim 14 wherein said impulse-responsive device comprises a thermionic tube, means to cause said tube to become conductive in response to the connection of a calling line through to the switching means and means are provided responsive to the break periods of an impulse train from the calling subscribers dial switch to render said tube non-conducting.

17. In a telephone system, the arrangements claimed in claim 16 including, in addition, a source of alternating current associated with each subscribers line circuit, means responsive to the removal of a calling subscribers handset for causing transmission of alternating current from said source over the private conductor of the calling subscribers line circuit, means responsive to the transmission of alternating current over said conductor to cause said thermionic tube to conduct and responsive to the operation of said dial switch to render said tube nonconducting.

18. In a telephone system, the arrangements claimed in claim 17 and comprising, in addition, a second thermionic tube, a trigger circuit between said first and second thermionic tubes whereby said thermionic tubes are always in complementary operating conditions, a three-electrode discharge tube connected between the anode and cathode of said second thermionic tube, a two-electrode discharge tube connected between the anode and priming electrode of said three-electrode tube and means for delaying the striking of said two-electrode tube whereby it is only struck in response to the non-conduction of said second thermionic tube during the interdigital pause while said three-electrode tube is non-conducting, said twoelectrode tube in striking serving to operate the sequence switching and to strike said three-electrode tube thereby being itself extinguished.

19. In a telephone system the arrangements claimed in claim 18 and comprising, in addition, a three-electrode discharge tube connected across said second thermionic tube, a two-electrode tube connected between the anode and priming electrode of said three-electrode tube, means for delaying the striking of said two-electrode tube whereby it is only struck in response to prolonged non-conduction of said second thermionic tube while said three-electrode tube is non-conducting, said two-electrode tube in striking serving to initiate the release of the connection and to strike said three-electrode tube whereby it is itself extinguished.

20. In a telephone system, the arrangements claimed in claim 17 in which said second thermionic tube is responsive to the flow of alternating current over the private leads extending from both the calling and called parties line circuits to remain conducting until the cessation of such flow.

21. In a telephone system, the arrangements claimed in claim 14 and comprising, in addition, means responsive to the registration of said two digits on said counting circuits for causing a test to be made of the idle or busy condition of the called line.

22. In a telephone system, the arrangements claimed in claim 21 and comprising, in addition, a bi-stable element consisting of a pair of discharge tubes responsive to the idle condition of the called line to change from one stable condition to the other stable condition.

23. In a telephone system, the arrangements claimed in claim 22 including, in addition, busy tone transmitting means, ring-back tone transmitting means and a discharge tube operative in conjunction with said bistable element in said one stable condition to connect said means for transmitting busy tone to the calling partys line and in conjunction with said bi-stabl e element in the other said stable condition to connect said means for transmitting ring-back tone to the calling partys line.

24. In a telephone system, the arrangements claimed in claim 23 wherein said discharge tube is operative in conjunction with said bi-stable element in said one stable condition positively to prevent the striking of the discharge tubes in said switching means for completing connections to the called party.

25. In a telephone system, the arrangements claimed in claim 23 including means responsive to the removal of the called partys handset to transmit alternating current over the private lead to cause the striking of a further discharge tube, said further discharge tube being operative to prevent transmission of ring-back tone to the calling party and to restore said bi-stable element to said one stable condition.

26. In a telephone system, the arrangements claimed in claim 23 and comprising a switching network consisting of a plurality of metal rectifiers and biasing means for said switching network dependent upon said bi-stable element.

27. In a telephone system, the arrangements claimed in claim 21 in which said bi-stable element is operative in the said other stable condition to control the application of potential to cause the striking of the discharge tubes in the switching means which complete the connection to the called party over conversational leads and the private lead.

28. A telephone system comprising a plurality of subscribers lines, a first plurality of sets of gaseous discharge tubes, a second plurality of sets of gaseous discharge tubes, means for successively triggering one set of said first plurality of sets and one set of said second plurality of sets to complete a connection between a calling and a called line, means responsive to the triggering of said one set of said first plurality of sets for preventing thereafter the triggering of any of the sets of said second plurality connected to said calling line and means responsive to the triggering of said one set of said second plurality of sets for preventing thereafter the triggering of any of the sets of said first plurality connected to said called line.

29. A telephone system, as claimed in claim 28 in which in each set of discharge tubes of said first and second plurality of sets one discharge tube is responsive to the operation of said triggering means and means are provided responsive to the striking of said one tube to apply a bias potential to an electrode of each of the remaining discharge tubes in said one set.

30. An electronic relay comprisinga thermionic tube, an anode circuit therefor, a gaseous discharge tube having at least two electrodes between which the discharge takes place, connecting means between said anode circuit and one of said electrodes, a source of negative potential and connecting means between said source of negative potential and the second of said electrodes whereby said discharge tube strikes only when said thermionic tube is cut oil.

31. An electronic relay as claimed in claim 30 in which said discharge tube has three electrodes and a second discharge tube is connected across the anode and priming electrode of said three electrode tube, said priming electrode being connected to a source of priming voltage.

32. An electronic relay as claimed in claim 31 in which a capacitor is connected across said second discharge tube to delay the striking thereof.

33. In a telecommunication or like system, a circuit arrangement employing two electronic relays as claimed in claim 31 in which the anode of the thermionic tube of the first electronic relay is connected to the control grid of the thermionic tube of the second electronic relay over a discharge tube and a source of negative potential is con- 15 nected to said control grid, the striking of the second gaseous discharge tubes of each relay being delayed by different amounts.

34. A telephone system comprising a plurality of subscribers lines, a plurality of line circuits each connected to one of said lines, a plurality of sets of gaseous discharge tubes through which conversational and control paths are extended on the striking of the tubes of a set, a source of alternating current in each line circuit, a digit storage device, means responsive to the striking of one of said sets of tubes for connecting one of said line circuits to said digit storage device and for completing a path from the source of alternating current in said one line circuit through one tube of said set to said digit storage device, means responsive to the transmission of impulses representing a digit over the subscribers line connected to said one line circuit for intermittently interrupting the transmission of alternating current over said path to enable the digit to be stored in said digit storage device and means responsive to prolonged interruption of said alternating current to release the connection set up from or to a o e ine c c it- 35. In a telephone system, a switching stage comprising a plurality of sets of gaseous discharge tubes, means for striking a set of tubes to complete conversational and control paths through the stage, a source of alternating current, a receiving device for said alternating current, means responsive to the striking of a set of tubes in the stage for transmitting alternating current from said source to said receiving device over a control path, means for interrupting the flow of alternating current over said control path and means in said receiving device operative in response to the interruption ofalternating current flow over said control ,path to effect control and supervisory functions.

36. 'In a telephone system, the arrangements claimed in claim 35 in which there is provided timing means responsive to interruptions in the flow of alternating current of greater than a predetermined duration to extinguish said set of tubes.

37. In a telephone system the arrangements claimed in claim 36 and including a calling line circuit, a called line circuit and at least one of said switching stages extending a call between said calling line and said called line, means in said calling line circuit responsive to the termination of said call at the calling end and jointly efiective with means in the called line circuit responsive to determination of said call at the called end to prevent the transmission of said alternating current over said control path whereby to render said timing means elfective to extinguish the set oftubes in said switching stage.

References Cited in the file of this patent UNITED STATES PATENTS