peculiar logic

[rejoe.koshy]

hi guys,

Can you help me to understand the logic rung 10 (pls find the attachment).It has rather contradicting instructions , but it works when downloaded in to the controller!

 

[rdrast]

What is contradictory?
It appears to be a basic ramp generator. If the ramped output is less then a setpoint, it adds a small increment to the ramped output. Then it checks to see if by adding the increment, the output exceeds the setpoint. If it does, it just moves the setpoint to the output.

 

[rejoe.koshy]

In the rung 10, it checks whether A is greater than B & proceeds to check whether B is greater than A.
Nw if A is greater than B , obviously B cant be greater than A .So either of them should work ,not both!
I want to know in which order the controller scans the logic?

 

[HJTRBO]

rdcast has the correct reply. Please try extra to understand it.
That entire rung is scanned on one scan and the order of rung / branch processing is like reading a book.

 

[hagos]

No Contradiction

No contradiction, the rung is written in a correct manner.

The whole idea is to limit the reference value if it exceeds certain boundaries, read it again you might figure out the idea.

 

[rdrast]

In the rung 10, it checks whether A is greater than B & proceeds to check whether B is greater than A.
Nw if A is greater than B , obviously B cant be greater than A .So either of them should work ,not both!
I want to know in which order the controller scans the logic?

You aren't reading that correctly.

Okay, yes, it checks if A is greater than B.
Then it ADDS a value to B.
Then it checks if B is greater than A (which, after the ADD, it can be). If it is, it limits the value of B.

Scan for LAD is Left to Right, then Top to bottom. In your example, the scan would be GRT, ADD, GRT, MOV