Ladder logic is the universal language of PLC programming—and it’s not going anywhere. Whether you're working with an Allen-Bradley MicroLogix or a Siemens S7-1200, ladder logic is often the first—and sometimes only—language used to control machines and automate industrial processes.

If you’re just getting started with PLCs, understanding ladder logic is a must. In this guide, we’ll explain how it works, walk through a basic motor control example, and explore its most common instructions. You’ll learn how to visualize your control logic like a circuit diagram—and why ladder logic remains the industry’s favorite.

What Is Ladder Logic?

Ladder logic is a graphical programming language that represents electrical circuits using symbolic logic. It originated in the early days of industrial automation to help electricians transition from relay-based control systems to programmable logic controllers.

Why the name “ladder logic”? Because your program looks like a ladder:

• Vertical power rails (left and right) represent electrical flow.
• Horizontal rungs represent logic conditions that must be met for the rung to “energize” an output.

Each rung acts like a rule: if these input conditions are met, then do this output action.

Ladder Logic Basics

At its core, ladder logic is about evaluating true/false conditions from left to right across each rung. The main building blocks include:

• Power Rails: Left rail is the virtual “hot” side; right rail is the return.
• Rungs: Rows of logic between the rails.
• Contacts (XIC, XIO): Represent input conditions.
• Coils (OTE): Represent output actions.

Simple Example: Start/Stop Motor Control

This classic example controls a motor using a start and stop button.

|----[ ]----[ ]--------------------( )----|
|   Start   Motor                Motor   |
|         (seal-in contact)             |
|                                       |
|----[/]-------------------------------| 
|   Stop                                |
  

Common Instructions in Ladder Logic

• XIC (Examine If Closed) – True when input is ON
• XIO (Examine If Open) – True when input is OFF
• OTE (Output Energize) – Activates output
• TON (Timer On Delay) – Delays ON action
• CTU (Count Up) – Increments counter
• RES (Reset) – Resets timer or counter

Benefits of Using Ladder Logic

• ✅ Easy to visualize
• ✅ Widely supported across brands
• ✅ Reliable for discrete control
• ✅ Great for troubleshooting and documentation

Limitations of Ladder Logic

• ❌ Not ideal for complex math or motion
• ❌ Can get hard to maintain with long logic chains
• ❌ Alternatives like Structured Text may be better for advanced control

Real-World Use Cases

• 🏭 Conveyor line control
• 🧃 Filling and bottling machines
• 🚨 Alarm systems and safety interlocks
• 🚪 Gate and access systems

Key Takeaway

Ladder logic is still essential in modern automation. It’s easy to learn, visually intuitive, and trusted across industries for a reason. Whether you’re a beginner or looking to sharpen your skills, ladder logic is the foundation of programmable control.

Explore PLCs That Support Ladder Logic

Both CompactLogix and ControlLogix PLCs from Allen-Bradley fully support ladder logic programming via Studio 5000.

• ✅ Browse CompactLogix PLCs
• ✅ Explore ControlLogix Systems

Need help choosing? Contact our expert sales team and we’ll help match your application to the right PLC—ladder logic ready.