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Not sure but it could be a way of letting one sensor work for either folks that want a 0 or folks that want a 1 when they see a desired signal.
It may sound like a trivial thing but there is a lot of legacy code in industrial automation. In order to keep from making 2 different models of sensors (and from plants around the country from having to stock 2 different sensors in their parts cribs), they could offer one an have it work with either normally ON or normally OFF controllers.
Just a thought.
Joe J.
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