Patent GB-925464-A (GB)
IPC: H04Q3/52
Abstract: 925,464. Automatic exchange systems. WESTERN ELECTRIC CO. Inc. June 30, 1959 [July 3, 1958 (2)], No. 22363/59. Class 40 (4). In an electronic switching network for providing transmission paths between selected pairs of network terminals, comprising a plurality of bi-stable devices interconnected to form paths, bi-stable marking circuits are connected in shunt directly to the nodes of the paths, and control circuits apply potentials to switch selected bi-stable marking circuits, thereby marking the nodes to control the impedance of the transmission paths. As shown, Fig. 7, connections between subscribers are effected by breaking down cross-point diodes of the PNPN type. Thus when connection between subscribers 713b, 713d is to be effected, the cross-point diodes 701a . . . 701c, 701d . . . 701f are broken down. To this effect +30 v. pulses are applied successively in directions outwards from the centre from the priming control circuit 706, 707 to the lower ends of the PNPN bi-stable circuits 705a . . . 705c, 705h . . . 705f whose upper ends are biased at - 30 v. via field effect varistors 710. The increased current through the latter increases the voltage drop across them so that the voltages at the upper end of the diodes 705 increases to + 30 v. The junctor priming control operates similarly to raise the central point of the transmission path to +30 v. The circuit M then applies a - 30 v. pulse to diode 705e which now breaks down to lower the central point to- 30 v. Diode 701c now has 60 v. across it and breaks down so applying the - 30 v. to diode 701b and so on. The diodes 701a . . . 701c thus break down and diodes 701d . . . 701f also break down in a similar manner. With the connection thus completed detection of the increased holding current (from 716 to 717) is used to effect the cessation of the - 30 v. in D and M, the latter causing the potential of the mid-point to rise towards +30 v. to break down diode 705d whereupon rectifier 719 conducts and holding current is supplied from the - 10 v. source 722. The whole transmission line is now near earth potential, thereby preventing the breaking down of any diodes 705 connected thereto for the purpose of establishing another connection over any part of the line. To break down the connection, diode 705a (or 705h) may be broken down by a pulse from the priming control circuit because the field effect varistor 710a holds rectifier 726a back-biased until the diode breaks down. The transmission line as far as the centre point is thus raised to +30 v. and diode 705e, which is simultaneously pulsed with - 30 v. pulse 724 from the circuit M, breaks down to lower the centre point to - 30 v. A positive-going pulse 725 applied to rectifier 721 at this time back-biases it, to switch diode 705d to its high impedance state. Pulses 724, 725 are now removed in succession. With holding source 722 thus disconnected the connection is broken down. The use of the field effect varistors in the biasing circuits for the diodes ensures that these sources do not affect the transmission characteristics of the circuit since they are normally on the high A.C. impedance part of their characteristics. The pulses from the priming control circuits have gradually rising leading edges, thereby enabling diodes 705 to be used which have poor transient characteristics, i.e. are liable to be switched on by transients even though the maximum voltage across them does not exceed their normal breakdown value. Steep pulses may, however, be used with such diodes if the resistances 703 are replaced by field effect varistors, Fig. 6. Initially the field effect varistors having about 30 v. across them are in their low resistance state and apply the full - 30 v. bias to their associated nodes, and when +30 v. is applied from the priming control source the diodes 621 break down. Varistors 630 are designed to reach their high resistance state at lower current than varistors 631, so that when diodes 621 are switched to their low impedance state the varistors 630 drop the whole of the 60 v. and the node attains a potential of +30 v. When - 30 v. is then applied to break down diode 621d and so lower the said point to - 30 v., the diodes 620 break down successively in an outward direction to complete the connection. Break-down of a circuit may be achieved by interrupting the hold current at either end of the connection. The connection may also be established with the use of series diodes all poled in the same direction and held by a hold current from positive at one end to negative at the other (instead of having one terminal of the hold source in the middle), Fig. 2 (not shown). Specifications 704,108 and 896,246 are referred to.