813,338. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO. Ltd. Aug. 17, 1955 [Nov. 27, 1954], No. 34453/54. Class 40 (4). Line circuits are divided into 100-line groups in accordance with the directory numbers of the lines connected thereto, and scanning equipment scans corresponding line circuits in all the groups simultaneously, the scanning operation stopping when a line circuit is encountered in calling condition. The scanning equipment thus indicates the tens and units digits of the calling line circuit which also signals further equipment to indicate the thousands and hundreds digits of the calling line circuit. Trunking arrangement, Fig. 10. Subscribers' lines are arranged in groups of 400 served by line finder/final selector units of the cross-bar 12 x 10 type arranged for double-group working. Each unit has two primary switches PSA, PSB connected by 20 links to a secondary switch SSA with incoming and outgoing trunks AIT, AOT. Five such units make up a 2000-line line finder/final selector stage. The line circuits are connected to the line finder/ final selectors through distribution frames. The 2000 line circuits are divided into 20 blocks of 100 for scanning and marking purposes. Line finding. The 20 blocks of 100 line circuits are scanned simultaneously on a 10 x 10 co-ordinate basis until a calling condition is found. Two counters apply (1) tens bias potential, and (2) units pulse potential, to the line circuits grouped according to the tens and units digits, respectively, of their directory numbers. The tens bias counter is advanced one stage for each cycle of the units pulse counter. When a calling line is scanned the pulse and bias are coincident in that line circuit to send a pulse to the appropriate units digit pulse control circuit to stop the scan and to define the pulse control circuit serving the block containing the calling line. A high-speed hunting circuit is then started and when the first marked block is found the units digit pulse control circuit applies a marking pulse to the particular line circuit. This in combination with the tens marking bias selects the calling line which then marks a circuit indicating the class of service and signals an equipment position indicator which defines the point of termination of the line circuit in the line finder/final selector arrangement. Terminating calls. A control circuit is provided for each 2000 lines and includes a 400 p.p.s. generating source. During the intervals between driving pulses the control circuit tests an exchange controller concerned with all call terminations to determine whether the exchange controller requires the services of the scanning and marking arrangement for selecting a wanted line circuit. If as the scanning is stopped and the calling line selected according to digital information displayed by the exchange controller. Detailed description. Control circuit, Figs. 1-3. The control circuit includes a generator supplying pulsed H.T. over lead PHT at the rate of 400 p.p.s. It also provides a switched H.T. over lead SHT. Pulsed H.T. is inhibited when switched H.T. is cut-off from tube VPG, Fig. 1, or when VKOS, Fig. 3, or VKTS, Fig. 2, conducts. VKOS fires when a signal over lead OS indicates that the scanning equipment has found a calling line. VKTS fires to stop line scanning when the exchange controlled requires the scanning equipment to complete a connection to a called subscriber. Pulsed H.T. fires VKUP, Fig. 3, to supply pulses over lead UP to the units digit pulse distributer UDD. The control circuit tests the exchange controller between pulses to see if it wants to complete a call. During these pauses VCF.sends pulses to VKTCS which fires only if the exchange controller marks lead C. If VKTCS fires VKTS follows to bias off VPG, so disconnecting, pulsed H.T. When the line-scanning arrangements and an originating controller have completed their functions with reference to an originating call bias and pulse potentials on leads RB, OR fire VKOR, Fig. 1, to fire VKORM, VKTR, VKTRM in succession. The last tube reverses trigger pair TRC, Fig. 3, for 3 m.secs. to cause VSC to conduct, so cutting-off switched H.T. for a like period. This enables those circuits supplied by lead SHT to be restored. The switched H.T. is also interrupted by a pulse on lead RA which occurs if the high speed hunting circuit, having been started by the line-scanning circuit, fails to find the block containing the calling line. The pulse fires VKRA which reverses TRC and so on as before. Forced release. If a calling line is found as indicated by a pulse on lead - OS and this is not followed by pulse and bias on leads OR, RB to indicate that the originating controller has completed its functions, forced release takes place 230 m.secs. after the pulse on lead OS. The pulse on OS fires VKOS to inhibit the pulsed H.T. and strikes VKOC. VKOC fires VKORC, Fig. 2, to strike VKRDA which then charges the capacitor associated with VKRDB. If leads OR, RB remain inactive the potential on SHT remains and after 230 m.secs. VKRDB fires followed by VKRC which reverses TRC to disconnect switched H.T. as before. Operation of control circuit when calling line is found by scanning circuit. A pulse on lead OS fires VKOC to disconnect switched H.T. and mark lead OC. VKOC also fires VKMPC after a 2 ¢ m.sec. delay to pulse lead MPA. VKMPC fires VKGSC after a further delay of 7¢ m.secs. to pulse lead GS S to start the high-speed hunting circuit. Terminating calls. As already described a demand from the exchange controller fires, VKTCS to suppress pulsed H.T. VKTCS also fires VKTSZ to pulse lead SZ, and primes VKF so that when the exchange controller is ready to be released it signals over lead F to fire VKF to strike VKFM, VKTR, VKTRM in that order. The last causes a 3 m.sec. interruption of switched H.T. to reset the circuit. Under fault conditions, if there is no pulse on lead F, VKTCS, VKTRC, VKRDA fire in succession to interrupt switched H.T. after a delay of 230 m.secs. Under normal conditions VKTCS also fires VKBA after a delay of 2 ¢ m.secs. to condition access equipment for the transfer of digital information from the exchange controller to the marking tubes associated with the scanning circuit. VKBA fires VKBBD after 10 m.secs. to pulse lead BB extending to all the units digit pulse control circuits in the 2000-line group. VKBBD also pulses VKBR so that if VKTOB has fired over lead TOB due to an operator establishing a call to a busy subscriber, VKBR fires after a delay of 12¢ m.secs. to extinguish VKTOB and release relay TOB. Scanning arrangements, Fig. 4-7. Line circuit. Each subscriber's line circuit SLC has multiple appearances upon primary switches PS forming part of a line finder/final selector switching arrangement for a group of 400 lines. The GB leads of the 200 line circuits in a set of 2000 having the same tens digit are commoned and the GP leads of the 10 line circuits in each 100-line group having the same units digit are also commoned. The GB and GP leads of the 100 line circuits of each group are arranged on a 10 x 10 co-ordinate basis. Further leads MB, MP of each 100-line group are arranged to supply tens digit bias and units digit pulse potentials respectively are also arranged on a 10 x 10 basis and are commonedin the same manner as the GB, GP leads. Closing the loop raises the potential across resistor R1. When the scan reaches the calling line potential on lead GB from the tens digit distributer TDD and a pulse on lead GP from the units digit pulse control circuit UDPC overcome the backing-off potential applied to rectifier MR1 and the pulse is extended over lead SC to amplifier PA, Fig. 7, to stop the scanning operation. At the same time as bias is applied to GB another bias is applied to lead MB to prime VKEB which fires to a pulse on lead MP from the units digit pulse control circuit as a result of the pulse applied to lead SC when the calling line was encountered. VKEB then fires VKLPT which remains conducting until switched H.T. is interrupted. VKLPT marks lead AM extending to a co-ordinate marking field, as described later. VKLPT also fires VNI to pulse class-ofservice lead CS. VKEB blocks gate rectifier MR3 to busy the line. When a connection to the originating trunk relay set and a register is eventually established the particular primary switch PS earths lead A to extinguish VKEB but marks lead B to maintain MR3 backed off. The line circuit includes a subscriber's meter MR responding to pulses on lead M. Units digit distributers. Triggering pulses on lead UP and anode pulses PHT step on units digit distributer UDD which has one of its 14 stages US1-US14 operative at a time. Each stage comprises a count tube such as VKUC1 and pulse repeater tube such as VKUR1. The 11th stage has an additional tube VKSTC fired by a pulse on lead IP to start the distributer initially. Each count tube of stages US2-US11 when conducting primes a corresponding tube in a second series VKUB1- VKUB10. Each time repeater tube VKUR11 conducts it sends a driving pulse over lead DP to the tens digit bias distributer TDD. Stages US12-US14 are included to ensure that this driving pulse is effective before US1 is again operative. The repeater tubes VKUR1- VKUR10 pulse leads U1-U10 in succession. These leads together with leads UM1-UM10 from the cathode loads of VKUB1-VKUB10 extending to each of the 20 units digit pulse control circuits such as UDPC, Fig. 7. Pulses on leads U1-U10 cause VKGP1-VKGP10 to pulse in turn to pulse the corresponding GP leads of the associated 100 line circuits ten at a time in accordance with the units digits of their directory numbers. Tens digit distributer. Each of the ten stages DS1-DS10 of the tens digit bias distributer TDD comprisestwo count tubes such as VKCA1 and VKCB1 and an allied repeater tube such as VKDR1, the count tubes of each set being mutually extinguishing. Each stage has