Runnable simulator field guide
Stuck PLC input fault scenario: test checklist
Direct answer
The learner can preserve the symptom, compare the signal at successive boundaries and prove whether the disagreement belongs to the device, conductor, channel, input image, force or logic.
Written for pLC learners and maintenance technicians investigating an input that remains true or false after the physical condition changes.
Scenario setup
Physical stimulus, sensor indication, supply and common, conductor, terminal, channel LED, input image, force status and program reference.
I/O and state map
Field condition through sensor, wiring, input electronics, controller image, alias or tag and every program consumer.
Normal run
A deliberate open-to-closed and closed-to-open transition appearing once and coherently at every declared boundary.
Boundary run
Intermittent conductor, wrong common, welded contact, forcing, duplicate tag, stale display, controller restart and a state that changes only under vibration.
Injected fault
The first device, supply, conductor, channel, input-image, tag, force or program boundary that fails to follow the controlled stimulus.
Target check
The fault removed, forces cleared and normal plus changed-input cases repeated under current drawings and energy-control procedures.
Why can a PLC input stay on after the sensor turns off?
Possible causes include a welded or shorted field path, wrong common, leakage, failed input channel, active force, stale indication or logic that copied and retained the state. Trace each boundary rather than assuming the program is wrong.
How do I prove a stuck input safely?
Define the safe state, create an approved stimulus, predict each indication and measurement, then retain the first boundary that does not change. Use the exact drawing, module manual and employer procedure.