Ladder logic (PLC)

Five rungs of a direct-on-line starter solved with the start button already released: the parallel Start_PB/Motor_Run branch seals the coil in through the healthy stop and overload contacts, the run-hours CTU sits at 412 of its 500-hour preset so the done bit stays low, and the engine reports the service lamp as the one output never energised in this state.

Eight rungs freeze a junction two seconds into its four-second amber: the 30 s green TON is done, the amber TON is still timing, the 25 s red TON has never been enabled, so only the amber lamp is lit and the engine names the green lamp, the red lamp and the unstarted red timer as never energised at this instant.

Six rungs where the photo-eye has been blocked for 8 s against a 6 s TON preset, so the jam latches on rung 4 after the belt coil was already solved true on rung 2 -- the one-scan lag that makes a real controller keep the belt running for one more pass -- with the beacon and horn coils drawn in series off the same latched fault.

Seven rungs latch the fill valve on the low-level switch and leave the unlatch rung unpowered because the high-level switch has not made, run the 180 s overfill watchdog out to 96 s, and end on a deliberate interlock chain with no coil, so the engine reports both the never-energised overfill alarm and the rung whose logic is solved and thrown away.

Five rungs of two-hand control where both buttons are made inside the 500 ms concurrency window (a TP pulse at 180 ms) and the ram is correctly permitted on rung 4 -- then a maintenance mute rung writes Ram_Solenoid a second time and silently overrides it, the duplicate destination flagged alongside a light-curtain tag examined by a contact that nothing in the program writes.

Eight rungs of a loading-bay roller door caught mid-travel with the close command sealed in: the open rung is held off by its own reverse interlock rather than by the buttons, the safety beam is clear so no reverse is latched, and the 180 s courtesy-light TOF is enabled by the moving door so its done bit keeps the bay lamp lit.