PLC / Digital I/O, Sensors & Actuators
Distinguish a contact's physical state from its PLC logic meaning
Normally open, normally closed and software-inverted signals can make a healthy contact appear logically unexpected.
Environment and inputs: Industrial PLC / automation guidance · Verify the exact controller, module, firmware, engineering tool and machine documentation. Linked vendor examples apply to their stated products and library versions; observation, state changes and physical work require distinct authorization.
Observe and record
Compare the documented normal mechanical condition, contact type, mapped raw bit and application-normalized tag during an authorized observation.
Interpret the evidence
Normally closed does not mean every logical condition should be TRUE. The intended fail indication and machine sequence determine how the signal is interpreted.
Resolve and verify
Correct misleading mapping, naming or approved documentation through the controls owner and validate both normal and fault cases using the machine's test procedure.
Machine and data safeguards
Do not invert a protective signal or alter its failure behavior merely to make a screen show the expected color.