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2013-01-30T23:21:37.413
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Control Logix PLCs do not have to execute on a cyclic sweep. I don't have RSLogix 5000 in front of me right now, but when defining the project, you are required to create one Program that executes on a cyclic sweep. But you can create other programs that do not. You can also run them off a trigger (not useful for regular input scanning) or off a fixed timer (very useful for input scanning). Keep in mind that there is no point in setting the input scan timer faster than your instrumentation updates-modern PLCs can frequently execute a scan much faster than a meter can update the data. One good technique I have used for this is to create a program called one-second or something similar. This program will scan all your inputs, and perform all your signal processing, and then write to buffered memory locations. The rest of your program looks at those buffered memory locations, and never monitors the inputs directly. You can set the input-buffering program to execute as fast as needed for your process, up to whatever the PLC can handle before it faults. It would also be a good idea to write your signal processing functions them selves as Add On Instructions, and then call them, with whatever parameters you need. So you could have an AOI with a call interface like this: <pre><code>input-1_buffered := input_smooth (low_pass, input-1); </code></pre> This would call your input_smooth function, using input-1 as the value and input-1_buffered as the final location. low_pass would be used within the input_smooth function to jump to the appropriate logic. Then you can write your actual smoothing logic in structured text, without anyone needing to understand it, because it will only exist in that one AOI.
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1. POSignal processing or algorithmic programming for a PLC
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1. USJason Kennaly
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