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scadatech

Analog input module question

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Being new to programming, I have an issue with a Compact Logics PLC with a handful of DIs, DO, AIs and AOs in a municipal water plant. One particular quirk I have noticed is that the analog input module set up for a 4-20 mA scaled 0-5000 EUs will display 5000 if the analog input drops below 4.000 mA - or in the case of open input loop it will also go to full scale. This is being used on a 4-20 mA flow meter, specifically - and the trending we see on the scada when the flows get to zero or below is displaying 5,000 GPM on the ladder and in our scada trending. Is there a reason for this, or a way to correct this? Any help would be appreciated.

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The answer to your question will depend on the Processor CPU Model, Software Version, and Card Type. A 1769-IF4 is described in this manual http://literature.rockwellautomation.com/i...um002_-en-p.pdf. Do you have a specific card given you this problem?

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Thanks for the response, Bob - I also forgot to include that I also have two other applications where they use the Micrologix 1500s that exhibit the very same condition on the analog inputs. The original programmer who set these up a few years ago was the same person. Co-incidence?

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A lot of times people will set these up that way so you know you are getting an over-range. Typically in the program, under the alarming code, they will force the high value into the register if it goes over range. This value should be above the scaled max in the hmi, so that way it will display as **** on the screen (AB Panelviews use *) or something similar and the operator knows he has an error. You don't want to just show 0000 when you are over or under range because the operator may think something is wrong in the process rather than the instrumentation. So it seems to me that maybe he put in the max value instead of max value +1? Or maybe the process should never reach 5000, so that is an indication of an error? This is based on Clx, but I think it would be the same in Cmlx. Edited by IamJon

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I don't have experience with Micrologix analog signals, but in ControlLogix, CompactLogix and FlexIO modules that I have used there are status bits that will indicate out of range faults for each channel. You would monitor these bits and ignore the reported signal if they are high. Does something similar exist in the Micrologix?

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^I don't believe they have that in Mlx. If I remember correctly, that bit is used within the PLC to monitor over range, but as far as the HMI goes, they use an over range forced value as I described before. That way, you only have one numerical display connected to one tag for that data, and don't need another tag reading the over range bit, which would make the HMI programming more complex. Edited by IamJon

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The PDF that BobLfoot linked to very clearly shows that there are over/under bits in the input table for the input modules it covers (IF4/IF8).

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Most likely not. Again to give you a specific answer we'll need to know the model of analog card and possibly have you do a little sleuthing in the Micrologix code. Many of the members here will help if you can supply the info we need.

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I find that analog inputs commonly drop below 4ma (0). I work with Omron and with these a negitive value of an unsigned integer will be scaled to the high end. What I do is move the input data, then put limits on it (<0=0 and >5000=5000). Logix should have an instruction like Omrons LMT. PS: Someone get this Omron guy out of here!

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