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Abstract

An environmental monitoring system in which a controller is arranged to test the workability and reliability of a plurality of detector units on a cyclic basis. Each unit has an analog sensor and an analog to digital converter for converting the analog sensor voltage to a digital value in a range of 0 to N. Each unit responds to a test instruction to cause the sensor analog output to assume an alarm value. The sensor output is limited to a maximum value that under a no drift condition of the analog to digital converter, its corresponding digital signal value will be intermediate a tolerance range of values, the upper limit of such range being less than N. The controller includes a test routine program for both issuing the test instruction and for determining if the digital signal test value is within the tolerance range and issuing a trouble output indicative of the test value being outside the range. The controller also includes a polling program which directs the polling of the detector units such that a test instruction is sent sequentially to the detector units on successive loop polls, where a loop poll is one poll of all the units coupled to the line.

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First Claim