Runnable simulator field guide
Manual traffic-light PLC scenario: test checklist
Direct answer
The learner can ensure only the permitted color is active, explain simultaneous-command policy and test every state from a known initial condition.
Written for pLC beginners learning Boolean selection, output exclusivity, command priority and observable lamp state.
Scenario setup
Manual red, yellow and green commands, enable state, priority, mutual exclusion, final outputs, lamp feedback and reset state.
I/O and state map
Operator selection through input image, arbitration logic, final output owner, modeled lamp state and independent observed color.
Normal run
Each single command producing the intended exclusive lamp state with off and reset conditions verified.
Boundary run
Two or three commands together, held command, command change during scan, enable loss, stuck output, restart and undefined selection.
Injected fault
A command, arbitration, priority, final-writer, output, lamp-model or observed-state mismatch.
Target check
The learning pattern retained as a regression truth table and any real application designed to its own approved requirements.
How do I prevent multiple traffic lights turning on in PLC logic?
Give the lamp state one owner and define explicit arbitration or a state variable so simultaneous commands resolve deterministically instead of relying on incidental rung order.
What should happen if red and green are selected together?
The requirement must state a safe deterministic result—commonly red priority or an invalid-command state. Test that case directly.