CODEWITHSUNDEEP
iec61131-3sfc
Taehoon Kim
Taehoon Kim

Reputation: 41

SFC Steps in IEC 61131-3 Programming

So I have a problem where in my SFC it jumps to an inital step but the commands written in the step would not register.

At the end of the SFC a step inputs 5 into A_Status(INT). The very next transition checks if the value of A_Status is 5. No problems so far, but after the transition when it jumps to the start of the SFC, where the first step is supposed to input 0 into A_Status, A_Status stays at 5.

The cycle time of my program is 100ms. I have tried slowing the cycle but it didn't work. What seems to be the problem here? Maybe the same variable used in such a sequence just doesn't work?

Reply would be greatly appreciated.

Upvotes: 2

Views: 236

Answers (1)

pboedker
pboedker

Reputation: 533

You don't mention if you write the values during Entry/Exit or in the SFC step actions. But beware, that on some occasions code from a previous step can be executed later than code in the new step.

Here is a link that explains the call order and why sometimes parts of the code is executed twice: https://infosys.beckhoff.com/english.php?content=../content/1033/tc3_plc_intro/45035999420423563.html

I've had succes with adding the following code in all the actions to prevent this from happening.

IF STEP_NAME.x THEN // Only execute this while the step is active.
  // Insert code here.
END_IF

Upvotes: 0

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