Runnable simulator field guide
Normally closed ladder-logic scenario: test checklist
Direct answer
The learner can predict the rung for both tag states, explain the physical-to-software mapping and prove deliberate stop and restart behavior.
Written for pLC beginners learning how a normally closed examine instruction responds to a Boolean tag and how that differs from a physical contact symbol.
Scenario setup
Input tag, normal physical state, logical polarity, instruction truth, output owner, stopping condition and reset expectation.
I/O and state map
Physical or simulated switch through input value, normally closed examination, rung result, output coil and visible response.
Normal run
Both input states toggled repeatedly with the rung and output matching the written truth table.
Boundary run
Startup value, held input, simultaneous conditions, inverted mapping, open wire assumption, restart and repeated output writes.
Injected fault
A physical-contact, input-polarity, instruction-truth, mapping, output-owner or restart misunderstanding.
Target check
The pattern rebuilt with the exact target instruction and tested against real input wiring and safe machine behavior.
When is a normally closed PLC instruction true?
It is generally true when the examined Boolean value is false; distinguish that software truth from the mechanical normal state of a field contact.
Does a normally closed instruction make a circuit fail safe?
No. Fail-safe behavior depends on the complete device, wiring, diagnostics, architecture, output elements and validated safety function.