S7-300 Math Question

agarb · Feb 16, 2016

As you will soon be able to tell from my question, I am not a Simens expert!

I want to take PIW300 (from a connected RTD) and divide it by 100 and put the result in a real.

I'm trying to figure out what conversion instructions I need to use. Step 7 keeps complaining of a type conflict.

Thanks!

DeKemp · Feb 16, 2016

L PIW300
L 100
/I
ITD (interger to double)
DTR (double to real)
T <real variable>

agarb · Feb 16, 2016

I can follow your statement list with info from the help file...

But, if I may indulge you, how would you do it in ladder? A contractor used FC105 scale block and now all the RTDs read wrong. So I am just trying to replace a single network within the ladder.

agarb · Feb 16, 2016

Wait, maybe I figured it out. Didn't realize I had to divide then convert to double then to real. I'll try it...

DeKemp · Feb 16, 2016

You can both use ladder, stl, and fbd in a single block. But only 1 type per network.

its probably easier to do in STl as you wont need temps for the in-between values.

Peter Nachtwey · Feb 16, 2016

I would convert the analog value to a REAL and divide by a REAL to avoid round off errors.

agarb · Feb 16, 2016

Peter Nachtwey said:
I would convert the analog value to a REAL and divide by a REAL to avoid round off errors.

I was wondering about that but, as the Siemens newbie, too afraid to ask!

DeKemp · Feb 16, 2016

All the RTD's are reading wrong you said, are the min/max values set correct on the scaling blocks?

Lare · Feb 16, 2016

DeKemp said:
All the RTD's are reading wrong you said, are the min/max values set correct on the scaling blocks?

RTD's are showed with one or two decimals on integer variable. FC105 scaling block don't work, because it assumes that raw data is between 0-27648 (or -27648 and 27648)

DeKemp · Feb 16, 2016

Lare said:
RTD's are showed with one or two decimals on integer variable. FC105 scaling block don't work, because it assumes that raw data is between 0-27648 (or -27648 and 27648)

My bad, i'm used to our software generator so we hardly have to do anything with analogue inputs. I was asuming that thermocouples also where. -27648<>27648

Lare · Feb 16, 2016

DeKemp said:
My bad, i'm used to our software generator so we hardly have to do anything with analogue inputs. I was asuming that thermocouples also where. -27648<>27648

Quick fix is FC105 block (bipolar setting) and scaling to -276.48 to 276.48, if temp range is enought for application (I think that 1 or 2 decimals for input is selectable from RTD card's configuration so range -2764.8 to 2764.8 is also possible?)

But converting int -> dint -> real -> divide with 100 gives same result and full temp range

agarb · Feb 16, 2016

Lare said:
Quick fix is FC105 block (bipolar setting) and scaling to -276.48 to 276.48, if temp range is enought for application (I think that 1 or 2 decimals for input is selectable from RTD card's configuration so range -2764.8 to 2764.8 is also possible?)

But converting int -> dint -> real -> divide with 100 gives same result and full temp range

Just now saw your response. I actually did both of those in my playing around today. I find all the type conversions a bit clunky as I'm used to A/B handling that automatically. But, I'm getting better with the Siemens stuff... just need more practice.

Though I still can't figure out why one of my RTD inputs is always at 32767 counts with no RTD connected. All the other (5) channels downscale except this one. It does read correctly when I hook up the RTD, though. After double-checking wiring, I've concluded it is in the card. I have another identical system that I'll try tomorrow and see if it behaves the same way.

Lare · Feb 17, 2016

check manual for the card, at least some Siemens RTD cards have wire break setting for channels.

"Wire break" diagnostics enabled, 32767 / 7fff = "Open circuit" diagnostics message
Wire break" diagnostics disabled, -32767/ 8000 (hex) ="Lower limit value undershot" diagnostics message

Henny · Feb 17, 2016

DeKemp said:
L PIW300
L 100
/I
ITD (interger to double)
DTR (double to real)
T <real variable>

L PIW300
L 100
/R
T <real variabele>

Should also work directly without the conversion.

Kind regards

Henny

S7-300 Math Question

agarb

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Peter Nachtwey

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