Runnable simulator field guide
Start-stop without latch scenario: test checklist
Direct answer
The learner can predict why an output follows the start input only while held and can state when maintained state is or is not part of the requirement.
Written for pLC beginners comparing direct momentary control with seal-in or latched behavior while learning contact truth, scan execution, stop priority and feedback.
Scenario setup
Initial input states, momentary start and stop semantics, output ownership, scan order, request duration, independent feedback and restart expectation.
I/O and state map
Start and stop input values through contact instructions and rung truth to output command, modeled response and observed feedback.
Normal run
The output energizes only while the permitted start condition is true and stop retains explicit priority.
Boundary run
One-scan pulse, simultaneous buttons, input chatter, held start, lost feedback, program restart and power return.
Injected fault
An input-state, contact-semantics, rung-order, output-owner, request-duration, feedback or restart mismatch.
Target check
The learned pattern compared with the actual machine requirement and then tested on the intended controller and interface.
Why does a PLC output turn off when I release Start?
Without seal-in, set/reset or another state mechanism, the rung becomes false as soon as the momentary start input becomes false.
When should a start command not be latched?
Use non-maintained behavior when the requirement calls for hold-to-run or jog operation and after the risk and stop behavior are properly designed.