Runnable simulator field guide
Timer-not-resetting PLC fault scenario: test checklist
Direct answer
The learner can identify timer type and instance, trace enable and reset conditions, test scan order and distinguish retained behavior from a logic error.
Written for pLC learners diagnosing a timer whose accumulated or done state does not return to the expected initial value.
Scenario setup
Timer type, instance owner, enable condition, preset, accumulator, done output, reset instruction, program order, task state and retention rule.
I/O and state map
Input condition through Boolean enable, timer execution, instance memory, done-state consumer, reset path and operator or machine result.
Normal run
Enable, time progression, completion, disable and reset produce the declared values over repeated clean cycles.
Boundary run
Held enable, simultaneous enable and reset, duplicate instance use, conditional routine, retentive timer, task stop, download and power cycle.
Injected fault
An input, Boolean condition, execution path, instance, instruction type, reset, task, retention or display mismatch.
Target check
The reduced case verified in the intended controller, firmware and task with current official documentation.
Why does a PLC timer not reset when its rung goes false?
It may be a retentive timer, its instance may be written elsewhere, the instruction may not execute, or the observed field may not mean what the programmer assumes.
Should timer reset have priority over timing?
That is a behavior requirement. Express and test the simultaneous condition explicitly because scan order and instruction semantics can change the outcome.