Lessons · Industrial control · safety is not the PLC
The PLC is not the safety system
Safety-rated functions (light curtains, e-stops) use dedicated safety relays or safety PLCs; the standard PLC only sees their status.
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
A standard PLC can hang, be reprogrammed, or lose an output card. Standards require the safety path not to depend on it.
How to think about it
Is this a safety function? In the program, treat safety inputs as read-only status: block motion when they say so, require a manual reset after a trip, never bypass.
Worked example
[/CurtainOK] (L Blocked)Trip latches.
[CurtainOK] [Reset] (U Blocked)Manual reset once clear.
[Run] [/Blocked] (Motor)Motion only while not blocked.
Your turn
Require reset after a trip.
[CurtainOK] [] (U Blocked)
Run a rung and watch the scan
The trap
Auto-resetting when the curtain clears. Someone may still be inside the cell.
Now you try
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