MicroLogix 1200 4-20mA Scaling

CSnerd

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• Jan 9, 2009

• #1

Hey all,
We have inherited a MicroLogix 1200-L4BWA with an old processor that does not support the SCP or the CPT blocks.

Does anyone out there have an example of how to implement the scaling of a 4-20mA signal to engineering units without the aid of these blocks?

Thanks,

 

Mickey

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• Jan 9, 2009

• #2

It may not support a CPT instruction but I think it does support the SCP instruction. But just incase I'm wrong see links below for scaling help.
What analog module do you have?

http://www.ronbeaufort.com/y=mx+b_part1.pdf

http://www.ronbeaufort.com/y=mx+b_part2.pdf

 

Roy Matson

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• Jan 10, 2009

• #3

You could always revert back to 4 function maths the way we did back in the 70s
Regards
Roy

 

nettogrisen

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• Jan 10, 2009

• #4

This is what the SCP instruction does

Out = (In-InRawMin)*((InEUMax-InEUMin)/(InRawMax-InRawmin))+InEUMin

 

Last edited: Jan 10, 2009

dahnuguy

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• Jan 10, 2009

• #5

just use math blocks.

I did it this way when I wanted a specific FB with scaling and alarm limits built in.

If I can do it, and in ladder, it can't be that difficult.

Remember to include the offset and to limit the upper and lower limit.

 

CSnerd

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• Jan 11, 2009

• #6

Thanks for all of the reply's,
After some digging i was going to go with what nettogrisen had suggested which is:
Implement this with math blocks:
Out = (In-InRawMin)*((InEUMax-InEUMin)/(InRawMax-InRawmin))+InEUMin

However, I found this and wondered if anyone had tried something similar:
First you can do DIV 32766 / Input = F:8.0
Next DIV 600 / F8.0 = F8.1
F8.1 = 0-600 psi
First divide the raw value by 4095, this will give you the percent of full scale, the since your
Engineering units are 0-600, multiply the percent by 600, the result will be the scaled result

Unfortunately the processor revision does not support the SCP or CPT so we have to go old school on this one, thanks all.