Scaling in Compact Logix

Please see the attached screen shots.

Under the properties of the analog card it is set up to 4-20 mA and and the data range is percent value.

The value for Local:11:I:Ch2 is usually right around 9970. The analog value going into the card is 18.8 mA.

The scaling is set for 0-12 on the instrument but 0-12 on the PLC. Can someone please explain how they are scaling the analog?

I do not understand the CPT block and the equation.

RS Logix 5000 20.04v

Thank you

JPG resolution is too small to clearly make out.

The pictures are a little to low res like the above post mentions to actually read.
When you say scaling on the instrument and PLC is 0-12 what does that mean?
Lets start with the question I can answer, the CPT block.
This block runs an equation and outputs the value into the destination field
To me the CPT instruction looks like it says
" (TRN ( Local:11:I.Ch2Data / 1000 ) ) something 4 "

Help file says TRN is Truncate so its removing your decimal point then I believe that other symbol before the 4 I cannot make out is a + so it is adding 4
so your input is 9970 / 1000 = 9.97 then decimal point is truncated so you get 9 + 4 = 13

However going back to the logic, that value is not stored in Local:11:I.Ch2Data, it is being stored in " Pre_LST_" I cannot read the rest.

As far as why is your input card reading 9970
4 - 20 mA and you are getting 18.8mA in?
I believe 18.8mA in would read (if your scale was 0-12) 11.1
I get that by 4-20 mA range
your input is 18.8 mA
your actual range is 16mA and your offset is 4mA
your Input - Y offset (4mA) / 16ma (Range) = .925 or 92.5% of range
so .925 * 12 (for your 0-12 range) = 11.1

To know how its setup we would need to see screen shots of the input card to see the range and scaling it has.

The CPT equation is ((TRN(Local:11:I:Ch2/1000)+4)

Local11:I:Ch2 is scaled 4 - 20 mA with data range selected as percent value. In Monitor Tags it reads in binary. I was expecting a float. I only have experience on L-7 control logics.

The integrator made the rung comment 0-12 ft. I don't see how he is getting that. The instrument is 0-10ft. My original post has a typo.

The math doesn't add up. I'm thinking the instrument was supposed to be 4 ft off the deck and that's were the +4 comes in. But that would make the scale 4 - 14 ft.

jstolaruk said:

JPG resolution is too small to clearly make out.

Click to expand...

Sorry about that. I uploaded some new pictures that should be clearer.

If you look at http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1769-um002_-en-p.pdf
page 3-11, you will see that the card gives 0 at 4mA and 10000 at 20mA
9970 lines up better with 19.8mA than 18.8mA

So if you divide 10,000 by 1,000, you get 10.
If you truncate you round down to the nearest whole number as per page 696 of this manual http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1756-rm003_-en-p.pdf
This is completely useless for Integer values though, as it will already be truncated as per page 343 of the same manual.

The CPT block performs the calculation in the expression field and stores it in the dest field. it is detailed in page 335 of the same manual.

Hope this helps. Let us know if you need further assistance.

I rarely use CPT blocks for input scaling. It is much easier to use a SCP block for your type of logic.

The SCP is not available on some CompactLogix, forcing people to either roll their own with an add on instruction, or use a CPT.

AustralIan said:

If you look at http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1769-um002_-en-p.pdf
page 3-11, you will see that the card gives 0 at 4mA and 10000 at 20mA
9970 lines up better with 19.8mA than 18.8mA

So if you divide 10,000 by 1,000, you get 10.
If you truncate you round down to the nearest whole number as per page 696 of this manual http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1756-rm003_-en-p.pdf
This is completely useless for Integer values though, as it will already be truncated as per page 343 of the same manual.

The CPT block performs the calculation in the expression field and stores it in the dest field. it is detailed in page 335 of the same manual.

Hope this helps. Let us know if you need further assistance.

Click to expand...

Thank you! Exactly what I needed

Saturn_Europa:
Just for the sake of oportunity.
You can use SCP with ladder, download it from here:
http://search.rockwellautomation.com/search?client=samplecode&oe=UTF-8&ie=UTF-8&output=xml_no_dtd&proxystylesheet=samplecode&site=sample_code&getfields=*&lang=en&hl=en&sort=date:L:d1&wc=200&wc_mc=1&ud=1&filter=0&q=scp%20aoi