| 摘要 |
A watch-dog type monitoring circuit for a communication device has a counter and a timer. The communication device outputs a communication signal including plural groups of pulse-string signals. The maximum number of pulses in each group of pulse-string signals and the minimum idle time between successive groups of pulse-string signals are previously prescribed by a communications protocol. The counter counts pulses and outputs an abnormal condition signal when a counted number exceeds a predetermined number. The timer inputs the communication signal, measures the idle time between successive groups of pulse-string signals, and resets the counter when the measured idle time exceeds a predetermined amount of time. Since the monitoring circuit monitors whether the communication device is in an abnormal condition by judging the normality of the communication signal, it an abnormal condition of not only the communication device to which the monitoring device is connected but also an abnormal condition of earlier-stages of the communication system.
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