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Old January 11th, 2018, 10:06 AM   #10
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cardosocea is offline
Join Date: Nov 2016
Location: Fields of corn
Posts: 920
Originally Posted by Mispeld View Post
While the OP question was about the accuracy of timing capability of the L71 processor, it is in the context of accurately totalizing (integrating) a sampled input signal. I would say that the error contribution of the software technique to the totalized result is negligible compared to instrumentation, wiring, analog/digital conversion, filtering, etc.

Using a totalizer pulse avoids some of these error contributions, though accuracy still depends on the instrument capability to accurately generate pulses. So, as pointed out, this is a preferred method if it is available and the continuous signal is not needed. Or use both as inputs to the PLC if desired.

If going down the route of totalizing an analog input, you will want to consider how that signal is filtered by the instrument and/or PLC input in determining the timer/task period. There is not much point in sampling at 50 ms if there is a filter time constant of 5 seconds, unless you need the totalized value to update that often. Likewise, a really slow sample period on a noisy input could produce less stable result.
I think your comment is assuming that the analog signal isn't changing. Not considering filtering, if the signal barely changes, then the integration error will be near zero. If the signal is changing constantly, the integration error may very well be worse than all the contributors combined.
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