SE365685

1291469 Fail safe gating circuits WESTINGHOUSE ELECTRIC CORP 10 Nov 1969 [3 Dec 1968] 54840/69 Heading H3T [Also in Division G4] A fail-safe gating circuit comprises an amplifying element passing a pulse or alternating signal under the control of a periodic or like gating signal, the amplifier having an output circuit which blocks direct current and the power for the amplifying element being derived from a capacitor charged under the control of complementary transistors having their bases fed in parallel with the gating signal. Fig. 1 shows the channel selector for the twenty-fifth channel of a pulse multiplex system, for example for selecting the upper speed limit of a railway track block. The Specification describes how any component failure including the freezing of the counter in any condition, leads to a "safe" output. The selector comprises a 32-counter 20 and fail safe AND gates 30 for detecting the combination of "zeros" and "ones" corresponding to the count of 25, this count allowing the pulse input at 36 to pass to a tuned fail-safe detector 32. The gates comprise a transistor 45 to the base of which are applied pulses M via a D.C. restoring circuit 68, 74 and derived from a master oscillator and shaper (Fig. 3, not shown) or from a previous stage. The collector receives a negative supply derived from a capacitor and rectifier circuit 50, 51 charged by a switching circuit 46, 48 when the output Q from FF1 is a "one". Transistor 45 thus conducts during coincidence of the "one" and a pulse M, causing the output transformer 62 to ring for a half-cycle, the resonant frequency corresponding to the duration of the pulse in order to eliminate spurious pulses. The turns ratio is chosen to provide unity gain in the stage. If the Q input persists (e.g. due to freezing of the counter) the capacitor discharges and the gate closes. Modifications.-The gate output may be a tuned circuit with capacitive instead of inductive coupling to the output terminals. In Fig. 4 (not shown) the gate is rearranged to be opened by a "zero" input. The gate may also be controlled by a so called " comma-free code" having a predetermined minimum repetition frequency. Pulses below this frequency allow the collector supply to decay and thus fail to safe. Pulses M could be replaced by sine wave in which case the output would be turned to this wave and in Fig. 5 (not shown) the gate transistor 45 is replaced by a transformer coupled push-pull amplifier.