PLC Ladder Logic — Interactive Simulator, Contacts, Coils & Interlocks

Interactive motor seal-in circuit

Toggle the simulated field states. This demonstrates logic only; it does not represent a complete safety circuit.

How to read ladder

Read each rung left-to-right. A series path behaves like logical AND. Parallel branches behave like logical OR. An output coil becomes true when the logical path to it is true.

Seal-in / holding logic

A momentary START contact turns on the motor bit. An auxiliary copy of the motor command is placed in parallel with START so the rung remains true after the button is released. STOP or overload breaks the permissive path and drops the latch.

Timers

What you should understand first

Ladder logic visually resembles relay circuits, but PLC instructions operate on bits and values in controller memory. Understanding the difference prevents errors when translating a schematic into software.

Signal and energy boundaries

A PLC is a controller, not the power source for a motor. Separate the low-energy logic side from the field power circuit. Digital outputs often command an interposing relay, contactor coil, valve solenoid or VFD input. The motor power still passes through the MCC/MCCB/contactor/VFD power circuit.

Fail-safe thinking

For critical shutdowns, do not rely only on software logic. Emergency-stop, trip and safety functions may require dedicated safety-rated hardware and a risk assessment. A normal PLC output should not be assumed to provide a safety function unless the architecture is specifically designed and certified for it.

Common mistakes

• Assuming an NC ladder instruction always means a physically NC field contact
• Latching a motor without a clear unlatch path
• Placing safety logic only in a normal PLC
• No debounce/filter consideration for noisy field signals
• Using timer values without considering process dynamics

Commissioning checklist

1. Create an I/O list and cause/effect table.
2. Write normal and fail conditions before coding.
3. Test each rung with simulated input combinations.
4. Verify field contact philosophy: energized-to-run vs energized-to-trip.
5. Document timer setpoints and why they exist.

Frequently asked questions