MicroLogix 1100 SCP instruction Raw data not matching scaled data

Brad Frayo

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Join Date
Nov 2010
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Bothell, WA
Posts
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I have a few installations of MicroLogix 1100 PLC's in which my raw data (100-500 which is 1-5 volts for the analog input) is spitting out an incorrect scaled data for some reason using an SCP instruction.

I have confirmed this using a trending screen via SCADA. I found a tech note about some bad PLC's that were released that were exhibiting "anomalous behavior" and checked to make sure I didn't have any coflicting addresses as well.

Can anyone shed any light on this for me? Has anyone had this happen?
 
Can you post your parameters for the SCP instruction and the trending data? Would be easier if we can see what you're getting and how it's setup.
 
Here ya go

Here is what the scale looks like. I don't have the trend with me, but I can explain to you what is happening after spending hours trying to make sense of it.

For example. The raw input will be 400 (Which is 3/4 scale from 100-500 signifying 1-5VDC input at 4.00 volts or 16 mA with 250 ohm resistor across input.) On a 20 foot water tank that means it should be reading 15 feet. It will begin to read right, but my trend has both the raw input and the scaled output on it. The raw data will stay consistent, however the the scaled output will not be consistent and sometimes go off as much as 8-10%. I did this trending to verify that the level transmitter wasn't drifting itself.

Thanks for the reply,

Brad

ML Error.jpg
 
Try changing your InMin to 0 and your InMax to 32760 and Scaled Min to 0 and Scaled Max to 20.
Also, unless they have changed them the base analog is 0 - 10VDC

HTH
 
Last edited:
apologies, I mis-spoke. Yes it is 0-5 vdc, however, with 4-20 mA inputs being the standard for analogs, I refer them as 1-5 vdc. 4-20mA = 1-5 volts across 250 ohm resistor on PLC
 
From the manual see picture below.
The base analog input is 0 to 10 volts= 0 to 1023 raw value.
So at 1 volt = 102.3 raw value
5 volt = 511.5 raw value
Not sure if this is your problem but I post it for clarification.

baseanalog.jpg
 
Those aren't for the base input. The issue is something with the math in the PLC, but I can't figure it out. Or the PLC is just garbage :). I've done this same thing with many other installations, that's why I can't figure out what this weird issue is.
 
My post #9 is for the base unit for a ML1100. Your SCP instructions ( in post#3)
input min and input max setting are incorrect ( raw data) for a 1 to 5 volt input.

You mentioned 4 to 20ma across a 250 ohm resistor, what does this represent in engineering units?
4ma ( 1volt) = ?
20ma (5 volts) = ?
 
Ok, I agree with your scaling, but that doesn't compensate for the interpretation of non-linear data it has flucuations of about 8% of error from raw in to raw out.
 
Brad Frayo said:
Ok, I agree with your scaling, but that doesn't compensate for the interpretation of non-linear data it has flucuations of about 8% of error from raw in to raw out.
Click to expand...


Are you saying the input data is not linear or that the output of the SCP instruction is not linear and should be?
 
HMMM? I have not seen that kind of behavior from a SCP instruction.

Have you tried replacing the live input (I:0.4) with a unused integer and then vary the unused integer to see if you get the correct results from the SCP instruction?
This would eliminate any errors coming from the live input,scan time issues and just test the SCP instruction itself. ( using the correct scaling numbers of course)
 
Last edited:

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