Patent GB-850214-A (GB)
Title: Improvements in or relating to communication switching networks
IPC: H04Q3/52
Filed: 1957-10-11
Abstract: 850,214. Automatic exchange systems. WESTERN ELECTRIC CO. Inc. Oct. 11, 1957 [Dot. 19,1956], No. 31826/57. Class 40 (4). [Also in Group XL (c)] In a switching system in which connections are set up over a series of cross-points, e.g. gas discharge diodes, " bisector " circuits are interposed between cross-points of certain stages to control the setting up maintenance and release of a connection in accordance with markings applied at the inputs and outputs of the switching system. As described connections are set up between input and output terminals over six stages of gas diode cross-points the " bisectors " occurring between the third and fourth stages, the arrangement being symmetrical about the bisectors. Application of a marking to an input terminal fires successively tubes of the first, second and third stages, the markings being extended from one stage to the next by means of propagator circuits which may be of the types described in Specifications 850,211 and 850,213. A marking on the output terminal is similarly extended backwards over the sixth, fifth and fourth stages. A bisector circuit between two stages thus fired then establishes and controls the maintenance of the connection. In the bisector circuit, shown in the upper part of Fig. 3, terminals 41, 43 are connected to third and fourth cross-point stages. Incoming markings are subject to limitation by diodes 91, 92 biased by battery 77. When the markings are first applied to the first and sixth stages a bisector enabling scanner 42 is set into operation, and when the positive going markings reach the terminals 41, 43 of the bisector, diodes 60, 61 are back-biased so that when the scanner applies simultaneous pulses to leads 72, 73 the pulses are applied to the starter electrode of the partially preionized tube 70 which now fires over its main gap. Condenser 65 sharpens the applied pulses. Condensers 53, 54 absorb the peaks of the pulses to eliminate noise. Resistance 64 is made high for the same reason. Ionization of the main gap causes rectifiers 82, 86 to conduct and complete a communication path via condenser 88. Decoupling from the low impedance source 77 is effected by coil 87 tuned by parallel condenser 85 to the middle frequencies of the communication band. Large voltage surges across the coil during setting up and disconnecting operations are prevented by the Zener diode 74. The conduction of tube 70 also lowers via diodes 76, 81 the potentials of points 41, 43 to hold the connection. Protection against positive going surges on one of the leads, e.g. 43 is provided as follows. Such a pulse back-biases diode 82 and would extinguish the cross-points connected to lead 41 were it not for the presence of diode 76 and resistor 75, the latter being dimensioned so as to take two-thirds of the holding current as compared with one-third taken by the resistor 80. This circuit is also effective, as is also the Zener diode 74, for alternating current limiting thereby keeping the cross-point diodes on suitable parts of their characteristics. When disconnection is to be effected a mark pulse appears on one of the input or output terminals of the first or sixth cross-point stage. This enables a common pulsing circuit 52 which may be arranged to respond only when one such marking is present in the absence of the other. The marking when reaching terminal 41 or 43 passes via rectifier 91 or 92 to enable tube 94 to fire on its main gap when its cathode is pulsed from the release pulse source 52. The consequent drop in potential is communicated via condenser 101 to the anode of tube 70 which is thus extinguished to remove the hold current for all the cross-point tubes. The clamping action of diode 104 and source 71 restrict a single incoming marking potential to such a value that it cannot effect the firing of tube 70. Circuits are provided to switch off the scanner 42 and the release pulses 52 immediately their functions have been performed. Thus the increase in potential over coil 106 when tube 70 is fired enables the normally non-conducting transistor 117 to produce a pulse to stop the scanner 42. Likewise the decrease in potential over coil 106 when the tube 70 is extinguished enables transistor 115 to stop the release pulses 52. Specifications 686,427, 704,108, 770,936, 789,875, 789,876, 814,952, and 850,212 are referred to.