Lessons · Industrial control · one false scan throws it away
One false scan throws it away
A TON is not retentive. Any break in its input, however brief, sets the accumulated time back to zero.
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
It is the reason a three-second timer never finishes on a machine where the condition looks on every time you glance at it.
How to think about it
Watch the accumulated value, not the done bit. If it keeps falling back to zero, the input is not steady, and that is a field problem wearing a software costume.
Worked example
[Run] [TON T1 300ms] [T1.DN] (Ready)Run on, dropped for one scan, then held. Ready: 0 0 0 0 1 1 -- two scans later than the version that was never interrupted.
Your turn
What does a break in the input do to the accumulated time?
sets it to
Run a rung and watch the scan
The trap
Fix the input, not the timer. Shortening the preset only makes the machine wrong faster.
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