728,382. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd. Feb. 15, 1952 [Feb. 23, 1951], No. 4373/51. Class 40 (4). [Also in Group XL (c)] In a telephone system in which a switching stage having access to a plurality of subscribers' lines comprises a plurality of sets of static switching devices, each set being adapted when conducting to extend conversational and control paths through the stage, pulsating current is caused to flow over a control path on the conduction of a set of switching devices in the stage and on the removal of a subscriber's instrument terminating one of the lines, and supervision of the connection through the stage is effected due to the flow of said pulsating current. General arrangement, Fig. 1. The exchange disclosed is a 100 line P.A.X. the common control and marking circuit CMC registers the digits of a calling line and sets one of the line finders LFDR allotted by allotter AL, which also resets CMC when necessary. Cold cathode tube ring counters LTMC, LUMC are continuously pulsed, and are stopped in positions to register the tens and units digits of a called line by control over TCL, UCL from the line circuit LCA. Sequence switch LSS is also a tube counter and is pulsed from LTMC, LUMC. The circuit-closing means in the speech and control paths are sets of tubes. The set to be fired is determined by markings from LTMC, LUMC for the line finder and from similar counters FTMC, FUMC (pulsed in response to the dialled digits) for the final selector. Each line finder is permanently associated with a final selector, preferably through an amplifier. Initiation of call and registration of calling line number. Pulse generator PG10 (Fig. 4), which is continuously operating, feeds pulses over ten leads each including elements such as C31, N50 to the ten tubes CCT of counter LTMC. With no line calling, the junctions such as that of R40 and R44 are at earth potential, so that tubes N50 strike to these pulses, and the tubes of the counter fire in succession, each in firing priming the next tube and dousing the preceding one. When a line, such as SL (Fig. 2), having the tens digit "1" calls, the top of " tens " resistor R40 (Fig. 4) goes to - 30 V. and prevents N50 from striking when PG10 pulses it. Thus CCT2 does not receive a pulse; and subsequent tubes CCT, although receiving a pulse, are not primed. The counter therefore stops with CCT conducting, this indicating the tens digit " 1." In the cathode circuit of each counting tube CCT is a pulse generator such as N51. When CCT1 conducts, N51 has striking voltage across it but C32 has to charge before striking occurs. This delay is longer than the free-running time of conduction of a counting tube, but when the counter is stopped, N51 strikes and a positive pulse is transmitted through T30 and T31. That from T30 is applied to the tubes CCL2 (Fig. 2) of lines having that tens digit. The calling line circuit applies - 30 V. to the cathode of its tube CCL2 so that that tube only will fire. The right-hand end of R11 goes positive; thus busying the calling line in the final selector multiple, and removing the negative voltage across R40 (Fig. 4). In the sequence circuit LSS (Fig. 4), CCSS1 is normally conducting, its positive cathode potential enabling PG10 and striking N54, thus disabling N53 and similar pulse generators. The pulse from T31 strikes CCSS2 and douses CCSS1, whereupon PG10 is stopped, N54 is extinguished, and PG11 is started. Counter LUMC operates similarly to LTMC; but with CCL2 (Fig. 2) fired, the voltage across R49 (Fig. 4) stops N52 from striking so that if the units digit of the calling line is "1" tube CCU1 remains conducting. After a delay, N53 strikes and the pulse developed across R47 strikes CCSS 3. Operation of line finder. Marking leads extend from the cathode resistors of the counting tubes (Fig. 4) to the similarly designated leads in a marking network MN (Fig. 2). Thus with CCT1 and CCU1 (Fig. 4) conducting, leads 1 - and - 1 are marked and the point M11 (Fig. 2) goes to + 50 V., which voltage is applied over leads such as ML (Fig. 2) to the control electrodes of the corresponding sets of tubes CCLF in all the line finders. The ring counter allotter AL (Fig. 5) preselects an idle line finder which is assumed to be LFDR1 (Fig. 2). Thus tube CCAL1 (Fig. 5) is conducting, and + 50 V. across R83 (Fig. 5) is applied to the junction of R24 and R27 (Fig. 2). When CCSS3 fires the voltage of the cathode of V10 then rises to + 50 V. This renders the normally cut-off V10 conducting, but does so slowly so that no pulse results. The continuously operating pulse generator N20 (Fig. 2) feeds negativegoing pulses to the grid of V10 and these now appear as positive-going pulses in its anode circuit and are fed to all the tubes in the associated line finder. However, only those tubes which are primed by the marking over MN are fired, and the calling line is extended to the final selector. When the " private " tube CCLF3 strikes, a negative pulse is fed to T20 (Fig. 3) which repeats it as a positive pulse to strike CCBU (Fig. 2). The consequent rise in its cathode potential cuts off N20, and raises the junction of R84 (Fig. 5) and R28 (Fig. 2) to + 50 V., so that MR55 (Fig. 6) conducts. V26 conducts and cuts off V27, whereupon pulse generator N42 operates. This drives the allotter AL until it reaches a line finder in which CCBU is not conducting. The conducting tube V28 (Fig. 6) is then cut off abruptly, and a positive pulse from its anode resets LSS by firing CCSS1. Counter LTMC then drives round until another call is initiated. The striking of CCLF3 also applies positive potential to the cathode of CCL2 (Fig. 2) which is thus extinguished. Registration of called number. Alternating current having a frequency considerably greater than the dialling frequency is fed over lead 10 (Fig. 2) and when the call is initiated the negative half cycles strike N10 so that pulses are fed to the private wire. When CCLF3 conducts these pulses are fed via T20 (Fig. 3) to limiteramplifier V20 (Fig. 5), rectified and applied as negative bias to V21. V21 ts cut off and N30 strikes, whereupon V22 (Fig. 6) conducts and extinguishes CCB. Dial tone is reverted as described below and the caller dials the wanted number. At each break the negative potential across L10 (Fig. 2) is removed so that the A.C. from lead 10 is without effect on N10 and no pulses are fed over the private wire. V21 again conducts, N30 is doused, and V22 cut off. Tubes N32, N33 strike, but N31 does not since the delay introduced by C41 is greater than the length of a break impulse. Tube V23 conducts and extinguishes CCC. At the end of each break impulse V21 is again cut off, N30 strikes, V22 conducts, N32 and N33 are extinguished, and V23 is again cut off. Tube N34 does not strike due to the provision of C43. At the end of the first digital train, V23 remains cut off and N34 strikes. The positive pulse from R72 strikes CCC (whereupon N34 is doused), and is also fed to the final selector sequence circuit FSS as described below. The circuits designated A, B, C, D, E, F, H in Figs. 5 and 6 correspond to the conventional final selector relays. When the final selector is taken into use, tube MA (Fig. 5) is conducting, so that N21 (Fig. 3) is fired, and the pulses from the junction of R62, R63 (Fig. 6), which follow the dial impulses, are fed to tens counter FTMC (Fig. 3) which is thus driven to a position corresponding to the tens digit dialled. When N34 fires at the end of the train, the positive pulse from R72 fires MB (Fig. 5) so that N22 (Fig. 3) is struck and N21 doused. The second train is thus repeated to FUMC, re-striking of N34 then firing MC. Control of final selector. In the cathode circuit of each of the units counter tubes CCU are two pulse generators " A " and " B." The time constant of the " A " generators is greater than a break period so that while the counter is being operated no " A " generator is operated. The " B " generators are controlled as described later. When the units counter stops with, say, CCU(1) conducting, the "A" generator N23 strikes after a delay and a positive pulse is fed over lead UMA1 to all UMA leads of the " private " tubes of the sets of tubes CCFS corresponding to units digit " 1." Assuming that the tens counter has stopped with CCT(2) conducting the positive cathode potential is fed over leads TM2 and TM to the control electrodes of all the sets of tubes CCFS corresponding to tens digit " 2." Hence only one " private " tube has the priming tens bias voltage and the striking units pulse and if the corresponding line is idle CCFS3 fires. A positive pulse is thus fed via T51 (Fig. 5) to strike HAA (Fig. 6), which is primed since N39 and N40 are normally conducting. Prior to this, with HAB conducting, the right-hand end of R82 (Fig. 6) is positive and via lead 27 this prevents the " B " generators, e.g. N24(Fig. 3), from operating. When HAA fires, lead 27 assumes the negative battery potential and the " B " generator whose anode is positive, in this case N24, operates applying a positive pulse over lead UMB1 to the remaining tubes CCFS1, CCFS2, CCFS4 of the selected set. When MC (Fig. 5) fires, as mentioned above, a positive pulse is applied to N41 (Fig. 6) which strikes after a delay sufficient to allow N23, HAA, and N24 to strike. N41 strikes CCE and its cathode potential is applied to leads 26 and 27 to douse N23 and N24. CCE in conducting douses N39 and N40 thus removing the priming voltage from HAA. Ringing called station. When CCFS4 strikes current flows in a resistor such as R86 (Fig. 2) in the called line circuit and the tube such as CCL1 is fired, thus applying ringing current from source RC (Fig. 2) to the wanted line. On the reply of the called party pulsating current is injected into the private wire and is extended via CCFS3 and T51 to V24 (Fig. 5). It is then rectified and applied as negati