1. US5610594 - Frequency monitoring device

A frequency to be monitored is taken to a counter for the continuous measurement of its cycle time. The counter is so set that, when this frequency drops, i.e. when its cycle time increases, it reaches a certain condition and issues error signals (flags) as long as the condition lasts. The first flag actuates a flip-flop which in turn actuates a timer (alarm counter) which, after a short time, triggers an alarm unless it has previously been stopped by the resetting of the flip-flop. The flags are also continuously sent to a second counter which is so arranged that it cannot reach a certain (high) condition in the interval between two successive flags for a continued condition of the first counter. If the frequency to be monitored becomes normal again, the flags disappear and the last flag occurring once again starts this second counter so that it rises to the preset (high) position and emits a reset signal. If the alarm has not yet been triggered, this reset signal can reset the flip-flop and thus cancel its actuated state. Thus the timer (alarm counter) is stopped and the triggering of an alarm is prevented.