After years of programming SLCs i've been pitted against a CompactLogix and a 1769-IF8 analog input card. Ive been playing with configuring the inputs from the card and have come to the conclusion that you can either have it presenting percentage values or engineering/raw values showing the milliamp input values or raw values which arent much use to me either way. Percentage would be good if it were a tank level analog but its a flow meter and I need to scale it to some meaning full range in m/sec.
And so my question. if you do set to use engineering units scaling on the card configuration is there anywhere you can put your own engineering units in to it?
If you can't what do you use (instead of the SCL & SCP commands that were so damn helpfull in the SLC) to convert the analog value to some meaning full engineering value
Im sure it must be me thats being dumb and not the incapable software but any comments either way are gratefully appreciated.
SCP and SCL functions can be easily translated to a mathematical formula, so you can have your one unit of measure.
I, too, remember being surprised that the "Engineering Unit" option on 1769 I/O didn't mean process engineering units but the actual electrical input parameter to the card. Before I figured out what they'd done, I thought "Boy this is cool, we can just configure the engineering unit conversion right into the input module". The option the way they have it is useless. If I have to convert from raw input to engineering units (real ones) I can just as easily do it from 0-32,767 as from 0-10.00.
Steve Myres, PE
It's not very hard to use math functions to duplicate the function of scale and scale with parameters. It's basic slope and offset stuff. It often helps to plot what you want to do. Doing the math with RLL is rather inelegant compared to any other programming language but it should be possible to do it once, perhaps as a subroutine and reuse it. My offhand explanation would probably confuse more than illuminate, but the process is exactly the same as would be used to calibrate an analog sensor with a 4-20 ma. output. You can probably find good examples of that if you google 4-20 ma transmitter. You are mapping one range to another.
Try looking these over. They were written for SLC users, and are not specifically aimed at converting the function to the CLX platform. But
they will give you a lot of insight into what's actually happen. Particularly the sections about using the SLCs *WITHOUT* the SCP instruction. You can extrapolate from them to do what you want on the CLX.
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