Patent GB-1455273-A (GB)
Title: Electrical switching circuitzry
IPC: H04Q3/52
Filed: 1973-12-06
Abstract: 1455273 Automatic exchange systems CONTROL NETWORKS CORP 6 Dec 1973 [21 June 1973] 56538/73 Heading H4K In a multi stage crosspoint switching network, an individual control circuit is associated with each matrix outlet and a path is switched through on a semi-self-seeking basis by successively enabling the control circuits of the first stage until a crosspoint operates and then successively enabling the control circuits of the second stage and so on until either a complete path is cut-through or a dead-end is found in which case the process recommences. In the final and penultimate stages, those outlets having access to a preallotted junctor are marked by the junctor so that all the control circuits of these stages may be pulsed simultaneously since only one crosspoint in each stage can be operated. The crosspoints may be SCRs or reedrelays. The control circuits allow unwanted crosspoints to turn-off very quickly and ensure low current drain from the exchange battery. They each incorporate a constant current source to minimize attenuation caused by connection of the control circuits to the crosspoint matrices. General operation (Fig. 1). In the 3-stage network shown, the crosspoints 27 are SCRs and each matrix vertical, e.g. 13, 14 has an individual control circuit 10. The latter are enabled in turn by a scanner 17 which permits square wave pulses from a generator 18 to reach the control circuits via gates 22, 19. A calling line circuit 11 applies negative potential to its matrix appearance 28 whereby if vertical 13 is free the pulse from gate 22 is passed by the control circuit 10 so as to enable the crosspoint 27. If the vertical 13 is busy, as indicated by positive potential thereon, the crosspoint 27 is unaffected and scanning of the next vertical, e.g. 14, occurs. Assuming a free crosspoint, e.g. 27, is formed, the negative potential is passed on to the next stage. Here gates 34, 32, pertaining respectively to outlets extending to the lower and upper matrices of the final stage, are enabled in turn by the pulse generator. In general a link circuit 12 is preallotted so that only one of these gates can be opened by the pulses. If outlet vertical 39 is free therefore, crosspoint 41 fires and extends the negative potential to the final stage. Therein, a particular outlet, e.g. 49, and a control circuit, e.g. 44, are usually marked so that the final crosspoint, e.g. 48 may fire to complete the path. Thereafter positive battery in the link 36 is used to hold the path. Control circuit, Fig. 2 comprises a level detector 57 which is coupled on the one hand via a decoupling diode D11 to a vertical 13 and on the other hand to a reference voltage source 59. A constant current source 62 is also coupled to the vertical 13 and via a diode D8 is clamped to the reference voltage. If the vertical 13 is free, then detector 57 provides an output to gate 56 which is enabled in its turn by a gated pulse from the generator 18. The amplified output of the gate serves as an enable signal for the associated column of crosspoints via decoupling diode D9. A diode D10 clamps the control wire 67 such as to further prevent and SCR from firing unless a negative potential is being extended from the associated line circuit. In the Fig. 3 embodiment, the control circuits 10 are shown with constant current sources, i.e. transistors Q2 that are each connected to a Zener diode regulated source 74. The level detection is effected by transistor Q1 which is effective to turn-off amplifier transistor Q3 whenever the associated vertical 13 is free so that a gated pulse over lead 24 may turn-on the SCRs associated with this control circuit. Filter network R6, C1 reduces the noise sensitivity of the crosspoint.