How to monitor a variable increaseing or decreasing?

S7PP · Jan 25, 2012

Dear experts, I think my brain is not working now. I'm using siemens Step 7, I think of this for 2 days now without an answer, please help~~

If there is a variable (Real value), it's going up and down all the time. I want to create two BOOLs to indicate this variable value is increasing or decreasing. How should I do that? I'm planing to scan this variable every 100ms.

Thanks a lot.

TurpoUrpo · Jan 25, 2012

What if its equal?

Code:

Up := Value > OldValue;
Down := Value < OldValue;

OldValue := Value;

S7PP · Jan 25, 2012

Sorry I forgot to tell that I'm using Step 7 with LAD, FBD, STL; but I'm not using SCL.
I'm not able to do this now.

If it's equal then both BOOLs come to FALSE.

TurpoUrpo · Jan 25, 2012

Im sure you understand what that means. But here is for stl:

Code:

L OldValue;
L Value;
<R ;
= Up;
>R;
= Down;
T OldValue;

Jeebs · Jan 25, 2012

Code:

L "OldValue"
L "NewValue"
<R
= "Bigger"
>R
= "Smaller"
T "OldValue"

Replace Bigger and Smaller with Increase, decrease if you'd like.

EDIT: Lol turpo sniped me with the exact same code.

S7PP · Jan 25, 2012

Well, I don't really understand,but I will try it.

I think you Load the value twice in the same scan, the "OldValue' and the "NewValue" should be the same.

It looks like won't work to me.

I will try and see the result. Thanks a lot Turpo and Jeeds. I'll post the result.

Jeebs · Jan 25, 2012

Well a lot of it depends on how you are implementing the 100ms scan of the variable.
This part of the code needs to be done right after each scan. If you let it run at every PLC scan cycle, then no, it won't work.

S7PP · Jan 25, 2012

Thanks guys, it works. I made a piece of LAD code, which is working too, but much bigger than yours.

I like STL better now.
Thanks~~

How to monitor a variable increaseing or decreasing?

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