Patent GB-822141-A (GB)
IPC: H04Q3/545
Abstract: 822,141. Automatic exchange systems. WESTERN ELECTRIC CO. Inc. Nov. 15, 1957 [Nov. 19, 1956], No. 35646/57. Class 40 (4). In an electronic exchange the condition of subscribers' lines is detected by scanning while a cathode-ray tube is used to store surface charges indicating the detected condition of the subscribers' lines. As shown in Figs. 1, 2 and 3, the exchange uses a two-stage cross-point network 2-19, the elements of which may be transistors or Zener diodes, and a call is set up by breaking down a path across the network from one subscriber's line to one side of a transformer in a trunk circuit such as 3-11, forming a link between two terminals of the network 2-19, while the other side of the transformer is similarly connected to the other subscriber. Ringing and busy tones are connected to subscribers by breaking down paths across the network to one side of transformers in trunk circuits such as 3-13 which have 800 or 1000 cycles, continuous or interrupted, applied to their other sides. Cathode-ray tubes 1-10, 1-11, and 1-20, 1-21, scan photo-transistors individual to subscribers' lines to detect changes in their condition and also store charges to indicate whether the line has become a closed loop or is being rung. Cathode-ray tubes 2-24, 2-25 and 2-26, 2-27 store discrete charges to register the identities of the called and calling lines and also provide a timing function during the establishment of a call. Cathode-ray tubes 3-20, 3-21, and 3-22, 3-23 scan phototransistors individual to each side of a trunk such as 3-11 and the cross-point network terminals of the ringing and busy tone trunks. Cathode-ray tubes 3-24, 3-25, act as timing stores to disconnect trunk circuits on release of a call. The cathode-ray tube storage systems are of the kind described in Specifications 682,156, 693,424 and 705,498 [all in Group XIX], and utilize the fact that current flowing to a capacitor plate covering the end of the tube in response to a lack of balance between direct beam current and secondary emission from the tube face, is greater if a charge has been earlier deposited on the tube face by the beam in a diffuse way, as by writing a circle, than if the charge were initially deposited as a spot. Where photo-transistors are to be scanned as well as storage spots, apertures are made in the capacitor plate or the plate may be made of mesh. The cathoderay tubes are arranged in pairs connected in parallel so that one tube may be disconnected for servicing without disabling the exchange. Tubes with photo-transistors are placed face-toface with the photo-transistors sandwiched in between. A circuit 2-10 comprising a master oscillator stepping cascade connections of binary quinary and decade counters acts as a source of timing pulses for the whole exchange. Subscribers' sets are provided with transistor amplifiers for speech and ringing. The exchange makes extensive use of junction or point contact diodes and transistors, which latter may be of either P-N-P or N-P-N type. Gate circuits, bi-stable circuits, pulse lengtheners, amplifiers, and binary counters, employing transistors are described with reference to Figs. 56 to 67 (not shown), and a detailed description of the arrangement of Figs. 1, 2 and 3, using these circuits is given with reference to Figs. 6 to 49 (not shown). Scanning subscribers' lines.-Lines 1-14, 1-15, 1-22 and 1-18; the line 1-22 providing a party line; are connected to photo-transistors 1-34, 1-35, 1-37 and 1-38, such that when a subscriber closes his loop a voltage is applied across his photo-transistor. Storage spaces are provided, as shown in Figs. 50 and 51, above and below each photo-transistor. Outputs from the photo-transistors and storage capacitor plates go to detector circuits 1-50, 1-51. Line scanning is controlled by deflection circuits 1-30 and a triode gate 1-53 and gates 1-31, 1-32, which connect two-phase oscillator 2-36 to move the cathode-ray beams in small circles. Pulses from the timing circuit 2-10 cause the deflection circuits 1-30 to step the beams of tubes 1-10, 1-11, and 1-20, 1-21, at the same time and at the same rate, on to the phototransistors of each of their groups of subscribers in turn. When positioned on a phototransistor, the beam is deflected upwards to one storage space and downwards to the other space, and in all cases the beam is only switched on after it has been accurately positioned. In the position of illuminating photo-transistors, the switch 1-53 causes the beams to have added brightness. If a line is scanned and found to have a closed loop, a circle is recorded in the store above its transistor. If the line is in receipt of ringing current a circle is recorded in the lower store. Subscriber calls.-When subscriber 14 lifts his receiver transistor 1-34 conducts but no output is obtained from the interrogation of either upper or lower store. Detector 1-50 therefore records a transition from an open to closed condition of the line and causes a circle to be written in the upper store, sufficient time for this purpose being obtained by applying a pulse of fixed length to inhibit the output of the master oscillator in timing circuit 2-10 and thereby slowing down the operation of the whole exchange. This loop closure has to be distinguished from a closure at the end of a dialling pulse or at the end of a flashing signal made during an established call. Since the tubes 1-10, 1-11 and 1-20, 1-21 step in unison the outputs from the detector circuits 1-50, 1-51 are delayed with respect to one another so as to separate in time the information of each group of subscribers and apply this information in sequence to a decimal to binary translator 1-52 which, as determined, from the position in the timing cycle and the position in the sequence, transmits the subscriber's identity as markings on eight wires and the transition of his loop form open to closed as a marking on two wires. The sequence of outputs from the translator 1-51 occurring during the time that any one phototransistor is being scanned is registered, one output after the other, in a chain of line stores in store 2-12; one store of the chain in being seized locking itself out and marking the next store idle. When the scanning tubes are stepped to the next photo-transistors the line stores carrying information from the previous scan are disconnected from the translator 1-52, and are replaced by an alternate chain of line stores in store 2-12, so that their information may be made available for interrogation. So long as the loop of subscriber 14 remains closed, scanning of his line produces no further output from the detector circuit 1-50 on account of the circle read out of his upper storage space and automatically re-written therein. Since these circumstances are incompatible with ringing of subscriber 14 the lower storage spot is not interrogated. The full significance of the subscriber's loop condition is determined and recorded by the call register. The call register.-This comprises two sets of two cathode-ray storage tubes 2-24, 2-25, and 2-26, 2-27, with detector, control, and matching circuits 2-34, 2-35. The control circuits are such as to divide the tube face into two sets of two columns of fourteen spots each, each set of two columns being sufficient to record information necessary to set up one call. The arrangement of columns and functions of each spot are indicated in Fig. 55. The call register is controlled by the timing circuit to pursue four functions in sequence during the period of scanning a subscriber's line. Firstly, calls registered in the tubes 2-24, 2-25, and 2-26, 2-27, are compared with the identity of subscribers' lines stored in the line stores of 2-12. Secondly, those calls which are identified as new calls are registered in the tubes. Thirdly, elapsed time between signals is registered for each recorded call. Fourthly, calls for which complete information has been accumulated are established over the cross-point network. In the first function the tubes direct their beams to the first spots of the first columns of each pair. These spots indicate if a pair is engaged in registering a call and are consequently termed activity spots. If the activity spots in both sets of tubes read idle for one pair of columns then the tubes step on to the next activity spots and when one of the activity spots is found busy the identity of the calling subscriber recorded in the columns is read out to a matching circuit, of which each pair of call register tubes has one, and over a commutating switch 2-14 is compared with the registrations in the line stores of store 2-12. In the case of subscriber 14 no match will be found and it is deduced from this that the detected closure of his loop does not represent the end of a dialling pulse; but it remains to be determined if it represents a new call or a flashing signal during an established call. To this purpose the identity of the calling subscriber 14 is transferred from its line store in 2-12 to a common store 2-16 by means of a commutating switch 2-13 which connects up all such new calls in line stores of 2-12 in sequence to store 2-16. The identity of subscriber 14 in binary form is now applied from store 2-16 to a translator 2-17 the decimal output of which marks the row and column wires peculiar to subscriber 14 of a line member group diode matrix and thereby applies a voltage to that calling line. If the line is in use the voltage causes increased conduction over the cross-point switching network 2-19 and a pulse will be generated in a pick-off transformer in the line. If the line is free, as will be true for subscriber 14, no effect follows the application of the voltage so that after a delay the register tubes hunt for an idle activity spot and when one is found the contents of the common store 2-16 is read into the spaces 2 to 9 of the first column to record the calling line number and