| 摘要 |
A circuit for distinguishing between two classes of alarms on a single loop. An alarm of a first class causes an impedance of a first predetermined value to be added in the loop; and an alarm of a second class causes a much larger additional impedance, or an open condition, to be inserted in the loop. The loop is bridged across an element of a voltage divider and, therefore, the loop characteristics, including the changed characteristics due to the impedance change in response to an alarm of either class, controls the potential of a test point of the voltage divider. Voltage comparators monitor the potential of the test point relative to control potentials, and cause relays to provide a no alarm signal, a first alarm signal, or a second alarm signal when there is no alarm condition on the loop, an alarm of the first class, or an alarm of the second class, respectively, on the loop.
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