Detecting a change in an integer value Step7

[rpop91]

Hey again guys,

Having all kinds of issues this week. Is there a way to detect when an integer value changes? I need to set a BOOL every time the value of an integer changes. Is there a function block already for this?

Any help would be greatly appreciated.

Thanks guys!

 

[bernie_carlton]

I don't know how to express it in Step 7 but my way is like this:

If 'current integer' is not equal to 'previous integer' then turn on bit.

Move 'current integer' to 'previous integer'.

 

[jdbrandt]

I do this with two rungs.
Rung A:
NEQ PRESENT_VALUE STORED_VALUE, XIO FIRST_SCAN, OTE THEY_CHANGED

Rung B:
*CREATE A HISTORICAL VALUE OF THE PRESENT_VALUE, FOR EVALUTATION NEXT SCAN*
MOV PRESENT_VALUE STORED_VALUE

 

[L D[AR2P#0.0]]

I don't know how to express it in Step 7 but my way is like this:

If 'current integer' is not equal to 'previous integer' then turn on bit.

Move 'current integer' to 'previous integer'.

.. and here is the source code for a function block

FUNCTION_BLOCK FB 2
TITLE =
VERSION : 0.1


VAR_INPUT
  iValue : INT ;    
END_VAR
VAR_IN_OUT
  bChanged : BOOL ;    
END_VAR
VAR
  iStoredValue : INT ;    
END_VAR
BEGIN
NETWORK
TITLE =

      L     #iStoredValue; 
      L     #iValue; 
      <>I   ; 
      S     #bChanged; 
      T     #iStoredValue; 
      SET   ; 
      SAVE  ; 

END_FUNCTION_BLOCK

 

[Addohm]

What they're saying in Ladder Logic...

On rung 5 (the rung you're looking for the change on), the only relevant instructions to you are the EQU and the MOV.

 

[Paul_Pereira]

What they're saying in Ladder Logic...

On rung 5 (the rung you're looking for the change on), the only relevant instructions to you are the EQU and the MOV.

The pic is gone. Please post it again

 

[AlfredoQuintero]

Paul, that post is from 2014.

 

[Paul_Pereira]

Paul, that post is from 2014.

I understand that. I'm asking if someone can repost it or recreate the answer.

 

[parky]

I do not have S7 here at the moment but it would be like this, you need to either reset M10.0 at some point or just make it an out M10.0 and it will be on for one scan of the PLC
So you just compare an integer with the last saved one & if it changes then it either sets a bit or equals a bit for one scan (if using an OUT coil)and then updates the saved integer so it is the same it will not trigger again until the value in DB10.DBW10 changes.
You can use any integer tags i.e. symbolic, DB.DBWxx or MWxx