| 摘要 |
A digital alarm system of the type having first and second electrically conductive loops connected by an alarm switch. The alarm switch is normally open, but can be closed when an alarm state exists to electrically short one loop to the other. Currents are generated in each of the loops for use in detecting either the closing of the alarm switch or the occurrence of a fault in one of the loops. Resistors are connected to each of the loops for use in detecting the presence or absence of the currents. A first pair of transistor switches alternatingly short-circuit one resistor then the other; and a second pair of transistor switches, synchronized with the first pair, alternatingly short the ends of one loop then those of the other so that when the alarm switch is closed the first pair of transistor switches simultaneously short-circuit both resistors even if a fault occurs anywhere in one of the loops. Recording devices, such as flip-flops, coupled to the resistors, record signals indicative of a magnitude of the resistor voltage drops during intervals in which the resistors are not short-circuited. Logic circuitry is then used to process this stored information to generate signals indicating whether the alarm switch has been tripped or whether a fault has occurred.
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