Most Asked PLC Ladder Logic Question and Why

What is the most asked PLC Ladder Logic Question? One of the most common and foundational examples of ladder logic programming involves designing a motor start-stop circuit that utilizes latching or seal-in logic.

This specific scenario not only serves as a practical exercise but also encapsulates several critical principles of Programmable Logic Controller (PLC) programming as well as the broader realm of industrial control systems. By exploring this example, we can gain insights into circuit design, relay actuation, and the mechanisms that enable machinery to operate efficiently and safely.

Why is this question so common?

Shows the cyclical scan of the PLC. The reason that the inputs and outputs are activated or deactivated in the program is because of the PLC scan’s cyclical nature, which continuously updates the inputs and outputs.

It demonstrates basic logic, utilizing a simple combination of AND and OR operations, which are fundamental to most ladder diagrams.


Introducing latching: The “seal-in” circuit utilizes the motor’s output contact to maintain the circuit flow after the start button is released. This concept is fundamental in differentiating PLC logic from a simple momentary switch.

Tests the knowledge of contacts, requiring both normally open (for the start button and seal-in) and normally closed (for the stop and/or emergency button) contacts.

The safety component is vital: A normally closed button, which is wired in series with the start logic, serves as an essential safety feature by breaking the circuit when the button is pressed. This design also highlights the importance of “fail-safe” principles; if a wire connected to the normally closed stop button breaks, the motor will shut down automatically.

The question can be made more complex by adding additional requirements, such as including a timer, a counter, or an alarm light. Still, the fundamental start-stop circuit remains the most frequent starting point for ladder logic examples.

To see how we incorporate a jog into this circuit, Click Here. Click here to learn how you can learn PLC programming easily with the Machine Simulator by Nirtec. 

Download this Do-More Start Stop Ladder Logic here.