With programmable systems it is always possible to accidentally have two inputs finding their way to a single output. What confuses the issue is the update times with the output showing the correct value for some of the time and for the remainder not.
Take a system with a default update time of 10 minutes, and this is not changed in either of the two modules, the one that is meant to relay to the output being affected, and the one accidentally updating the output. When the intended module transmits the output shows the correct value but, some time later, the offending unit now updates the output and the value is now incorrect.
10 minutes later the correct module updates and, at the same delay as before, the offending module does its thing and so the output cycles good-bad every 10 minutes.
Please don't laugh, this very fault nearly had a complete system thrown out for being "unreliable" and "not capable of expansion as was promised" (as you can probably gather this 'fault' crept in when a module was added to the system - but try tell the customer this!).
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