Eric Nelson
Lifetime Supporting Member + Moderator
I wouldn't be comfortable with it ONLY in logic. There may be an unforeseen scenario that is overlooked. A future programmer may make changes to the logic. He might not realize that the logic is providing personnel safety.* If light curtain is tripped: air remains on, PLC outputs remain on (or off), PLC logic has input for light curtain and prevents valves from shifting in logic. Essentially, safety is handled in the PLC.
How about this?... For the cylinders in question, use double-solenoid valves, wire these cylinders to outputs that share the same common, and hardwire that common supply through the light curtain safety relays. This way, even if the PLC tries to fire the output, there will be no voltage to energize the solenoid.
This is safer than just using logic, but I would STILL prefer that all air got dumped.
-Eric