Capturing transitions (Positive and Negative)

[quadPLC]

Hey guys,

I am trying to set up something that seems so simple in theory, but is making tons of headaches in reality.

I have a value that I want to watch both positive and negative transitions on. We will call it "saving". If saving goes high, I want to fire "saving_state_change" for one scan. But if it goes low, I want to fire the "saving_state_change" for one scan. These scans are used to reset a reference count.

I have tried most permutations but, from what I see, you can only get one or the other to work (whatever evaluates last). Is the solution only to use two separate "save_state_change" variables?

I am doing this in SIEMENS Total Integrated Automation v11 with an S7-1200 PLC.

Thanks in advance, guys!

 

[L D[AR2P#0.0]]

Yes.

 

[TurpoUrpo]

Yes.

No.

To be honest, im not sure if i understood correctly. Are those variables bool? Is fired variable same, positive and negative will give same variable high for one scan?

 

[harryting]

If saving goes high, I want to fire "saving_state_change" for one scan. But if it goes low, I want to fire the "saving_state_change" for one scan

If if the value change regardless of direction your do "A" once?

What's hard about that, I must be missing something.

 

[Tharon]

Compare the current state to the previous state. If they are not equal, then it has changed, either high or low, it doesn't matter. So one scan, move the current value into a storage bit. The next scan, compare the current value with the stored value, then once done comparing and setting a bit true if they are not equal, move the current value into the storage bit to be ready for the next scan.

It works with INT, REAL, BOOL, BITS, etc.

 

[Liam Moran]

you need 2 variables, 1 for the 'value' and a second for a 'value image'
Compare the 2 values for not equal if true then fire your 'saving state change'
immediately afterwards move the 'value' to 'value image'
the comparison will thus be true for 1 scan only on change of 'value'.

 

[Liam Moran]

Sorry Tharon, I must remember to 'refresh'. I did not see your post before I concocted mine.