Analogue Scaling

Hello there

I am having trouble scaling my analogue input signals.
If I use engineering units and use the RAW scale as 4000-20000 everything is fine. The problem is I need to use the scale -32768 -32767. So when there is 4mA the RAW data is showing roughly 29800 and when I have 20mA the Raw data is showing as 28500.

I am using the SCL function block. My question is how do I work out the maths so I can input the correct RAW data into the function block

CallumMacEwen said:

Hello there

So when there is 4mA the RAW data is showing roughly 29800 and when I have 20mA the Raw data is showing as 28500.

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If that is what you are seeing then something is very wrong.

Can you tell us the PLC, analog card, and how you have it configured?

Also tell us the engineering unit values you want for your 4 to 20ma signal.

4ma =??
20ma = ??

Post its program if you can ( zip it first)

The file is too big to send.
Engineering unit values are 4mA - 0 20mA - 16

I need to send the RAW data via MODBUS. So I need to modify the incoming RAW data I think

We need to know how your analog card is set up.

Can you at least tell us the analog card you are using, its complete catalog number and the PLC you are using, its complete catalog number.

See below for a scaling formula and some links that may help you.

I need to send the RAW data via MODBUS. So I need to modify the incoming RAW data I think

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What are you sending the info to? Why raw data? Why not engineering units (your 0-16)

I am using the Compact Logix Processor 1769-L32E
I am using the 8 channel Analogue card 1769-IF8

It is configured to 4-20mA and I am using RAW/Proportional.

I need to send the RAW value so the DCS can process the value at there end. They don't want me to send the Engineering value because they don't want to scale at there end.

The Formula you gave me is the straight line formula isnt it? I am already using that on my SCL function block.

In the image I sent. If you look at SCL_03

Currently that reading should be zero. As when I am online with the processor and i change the card config to engineering units. The raw value is 4000

I am not a RSLogix5000 user. So hopefully those that are will jump in and offer some help on the "SCL" instruction as it is used in RSLogix5000.

What I call engineering units is your 0-16. The 4000 to 20000 is raw data that the PLC is calling engineering units ( they call it that because it matchs the 4 to20ma) as opposed to RAW/Proportional. You need to scale the raw data 4000-20000 to 0-16. You can use the formula I posted ( with math instructions) or the "SCL" instruction. ( hopefully someone helps with that).

Yes I know the method your talking about and that works. But my issue is I need to use the -32768 32767 scale because the DCS I am sending the readings to require it to be in this format. So when I use this scale 4mA = 28000 roughly. So that means the output value is obviously not 0 its 0.74.

But my issue is I need to use the -32768 32767 scale because the DCS I am sending the readings to require it to be in this format

Click to expand...

Then the DCS will do the scaling to engineering units (0-16)?


See pdf for the data format for your card.

Raw/Proportional is..

3.2ma = -32767
21ma = 32767

Roughly

4ma = -29822
20ma = 29085

notice the negative sign

Thanks for your help. I had a stupid moment haha. I thought in the scaling function the output data always had to be the engineering values of the instrument. So basically I am an idiot. I resolved the issue by making 2 scaling instructions for each instrument. First instruction I have. InRaw min - 4000
InRaw max - 20000

Out min - 0
Out max - 16

So the output tag of this instruction will control my logic

The second scaling instruction I have the same 4000 - 20000 input. But on the output I have -32768 32767 and the output tag I send to the modbus.

Just a daft moment I had haha.

Thanks for posting your solution.

And good to hear all is well.