Lessons · Industrial control · when outputs update
Outputs update at the end of the scan
The PLC reads all inputs, solves all rungs, then writes all outputs. Nothing physical changes mid-scan.
Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.
What it is for
An output cannot flicker within a scan; a coil set then reset in one scan is never seen outside.
How to think about it
Think in scans, not lines. What is each coil at the END of the scan? That is what the world sees.
Worked example
[A] (Out)Sets Out.
[B] (Out)If B is false, clears it. The world sees the last write: off.
Your turn
Which write does the output show?
the rung that writes the coil
Run a rung and watch the scan
The trap
Pulsing an output for one rung to 'reset' something. The pulse never leaves the PLC.
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