| 摘要 |
A protective arrangement operating in conjunction with a multiplexing system adapted to convey data from a plurality of low-impedance analog sensors to a central digital receiving terminal. Each sensor is coupled by an input line through a noise-rejection filter that includes a capacitor connected across the line to a commutator serving to sequentially sample analog data derived from the sensors and to apply the samples to an analog-to-digital converter. The protective arrangement functions to detect the occurrence of an open circuit in any one of the sensors and to produce an indication of this abnormal condition, the arrangement including a like plurality of photon-couplers, one for each sensor. The light-emitting diode of each photon-coupler is energized by a power source common to all sensor circuits. The light emitted by the diode is intercepted by a photo-transistor connected through resistors of high value across the related sensor input line to define a network generating a small offset-current. Under normal conditions, the low-impedance sensor shunted across the high-impedance network renders the offset current ineffective; but should an open-circuit occur, the offset current then serves to charge the capacitor of the filter to a high level with a polarity opposed to the normal polarity established thereacross by the sensor. As a consequence, the digital value produced by the converter in response to the sample taken from the open-circuited sensor has an abnormal level well outside the valid range, thereby providing an indication of the abnormal condition.
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