If you are new to PLC programming, ladder logic is the best place to start. It is the most widely used programming language in industrial automation, used in over 80% of PLCs worldwide. The reason it became so popular is simple: it looks like a drawing, not a code. Anyone who can read an electrical wiring diagram can learn to read ladder logic in a single day.

Ladder logic was designed by engineers who wanted technicians and electricians to be able to program a PLC without learning a computer language. So instead of writing text commands, you draw contacts and coils on a screen that look almost identical to the relay control circuits people were already familiar with.

This guide explains what ladder logic is, how it works, what every symbol means, and how to write basic programs step by step. By the end, you will be able to read any simple PLC ladder program and write your own logic for motors, valves, timers and counters. If you are not sure what a PLC is yet, start with our article on what is a PLC and how it works.

What Is Ladder Logic?

Ladder logic is a graphical programming language used to write programs for Programmable Logic Controllers (PLCs). Instead of writing lines of text code, you create a diagram made up of contacts (inputs) and coils (outputs) connected between two vertical lines called rails.

The diagram looks like a ladder lying on its side, which is exactly where the name comes from. The two vertical rails represent the power supply lines, and each horizontal row of logic between them is called a rung.

Where did ladder logic come from?

Ladder logic was invented in the late 1960s when engineers at Modicon built the first PLC to replace panel after panel of physical relays used in automotive factories. The goal was to create a programming language that maintenance electricians could already understand without any retraining. So they designed the new language to look exactly like the relay wiring diagrams technicians were already reading every day.

Today, ladder logic is defined by the international standard IEC 61131-3, which means it works consistently across all PLC brands including Siemens, Allen-Bradley, Mitsubishi, Omron and Schneider Electric.

Structure of a Ladder Logic Diagram

Before learning the symbols, understand the physical structure of every ladder diagram. It has three parts:

|                                                              |
|     --[ ]--          --[ ]--               --( )--           |
|    Input 1           Input 2              Output 1          |
|  (Start Button)    (Safety OK)           (Motor ON)         |
|                                                              |
|     --[/]--                                --( )--           |
|    Input 3                               Output 2           |
|  (Stop Button NC)                        (Alarm ON)         |
|                                                              |

Power is imagined to flow from the left rail to the right rail through each rung. If all the input conditions on a rung are satisfied, power flows through to the output on the right. If any condition is not satisfied, power cannot flow and the output stays OFF.

The PLC executes each rung from top to bottom and left to right, one complete pass at a time. This is called the PLC scan cycle and it repeats continuously, typically every 1 to 50 milliseconds.

Ladder Logic Basic Symbols Explained

There are only a handful of basic symbols to learn. Once you know these, you can read and write most industrial ladder programs.

AND, OR and NOT Logic in Ladder Logic

All logic in ladder diagrams comes down to three fundamental operations that you already know from everyday life.

AND Logic (Series contacts)

Place two contacts in series on the same rung. Both must be ON for the output to energise. This is exactly AND logic.

|   --[ ]--        --[ ]--               --( )--    |
|   Start_Btn     Safety_SW             Motor_Run  |
     Both must be ON for motor to run

OR Logic (Parallel contacts)

Place two contacts in parallel branches on the same rung. Either one being ON is enough for the output to energise. This is OR logic.

|   --[ ]--   +                       --( )--    |
|   Sensor_A  |                       Alarm      |
|             |                                  |
|   --[ ]--   +                                  |
|   Sensor_B                                     |
     Either sensor ON triggers the alarm

NOT Logic (NC contact)

A Normally Closed contact acts as a NOT. The output is ON only when the input is OFF. The NC contact inverts the signal.

|   --[/]--                             --( )--    |
|   Healthy                            Fault_Lamp |
     Lamp ON only when Healthy bit = 0 (OFF)

The Motor Start/Stop Circuit (The Most Important Ladder Example)

The motor start/stop circuit is the first program every PLC engineer learns. It is used in almost every industrial machine in the world. Understanding this one circuit teaches you about NO contacts, NC contacts, output coils and latching logic all at once.

What we want the circuit to do

• Press the Start button once: the motor starts and keeps running
• Press the Stop button: the motor stops
• The motor should NOT restart automatically when the Stop button is released

Wiring inputs to the PLC

The ladder logic program

|                                                          |
|   --[ ]--     --[/]--                    --( )--      |
|    I0.0         I0.1                      Q0.0         |
|   Start_PB    Stop_PB                   Motor_Run       |
|                                                          |
|   --[ ]--                                               |
|    Q0.0                                                  |
|   Motor_Run                                              |
|   (seal-in)                                              |
|                                                          |

The key to understanding this circuit is the seal-in contact (the second parallel NO contact of Q0.0). Here is what happens step by step:

1. Start button pressed (I0.0 = 1) The top branch is complete: I0.0 is ON AND I0.1 NC is passing (Stop not pressed). Power flows to Q0.0. Motor_Run output turns ON. Motor starts.
2. Start button released (I0.0 back to 0) The top contact opens. But now Q0.0 is ON, so the seal-in contact (Q0.0 on the parallel branch) is also ON. Power still flows through the bottom branch. Motor keeps running.
3. Stop button pressed (I0.1 NC contact opens) The NC contact of the Stop button opens. No path from left rail to Q0.0 coil. Power cannot flow. Q0.0 turns OFF. Motor stops.
4. Stop button released NC contact closes again. But now Q0.0 is OFF so its seal-in contact is also OFF. No power flow. Motor stays stopped until Start is pressed again.

Timers in Ladder Logic

Timers are one of the most commonly used ladder instructions after basic contacts and coils. They let you create time delays in your program. There are three types.

TON timer example: fan starts 5 seconds after motor starts

|   --[ ]--                    --[TON]--          |
|   Q0.0                       Timer_1            |
|   Motor_Run                  PT = 5s            |
     Rung 1: Start timer when motor is running
|                                                  |
|   --[ ]--                    --( )--            |
|   Timer_1.DN                 Fan_Output         |
     Rung 2: Fan ON when timer done bit is set

The timer instruction has three important bits to know:

• EN (Enable bit): ON when the timer input rung is true (timer is running)
• TT (Timer Timing bit): ON while the timer is counting but not yet complete
• DN (Done bit): ON when the accumulated time equals the preset time

Counters in Ladder Logic

Counters count the number of times an event happens. They are used everywhere in industry: counting bottles on a conveyor, counting machine cycles, tracking batch quantities.

|   --[ ]--                  --[CTU]--           |
|   Product_Sensor            Counter_1           |
|                             PV = 100            |
     Rung 1: Count products as each passes sensor
|                                                  |
|   --[ ]--                  --( )--             |
|   Counter_1.DN              Batch_Alarm         |
     Rung 2: Alarm when 100 products counted
|                                                  |
|   --[ ]--                  --[RES]--           |
|   Reset_Button              Counter_1           |
     Rung 3: Reset counter for next batch

How the PLC Executes Ladder Logic (The Scan Cycle)

Understanding how a PLC reads and executes your ladder program is essential for writing correct logic. The PLC does not run all rungs simultaneously. It follows a continuous repeating loop called the scan cycle.

The time to complete one full cycle is called the scan time, typically 1 to 50 milliseconds. Learn more about the PLC scan cycle explained step by step.

Ladder Logic Symbols Reference Table

Common Ladder Logic Mistakes Beginners Make

• Forgetting the seal-in contact on a start/stop circuit. Without the seal-in contact on the output coil, the motor will only run while the Start button is held down and stop the moment you release it. Always add the seal-in parallel contact.
• Confusing NO and NC contacts. Remember: NC contact in PLC ladder passes power when the input bit is 0 (the device is not activated). A Stop button wired NC to the PLC will have bit = 1 when not pressed, so use an NC contact instruction so it passes power normally and breaks when pressed.
• Using the same output coil address on two rungs. In most PLC brands, if the same output address (e.g. Q0.0) appears as an output coil on two different rungs, only the last rung evaluated (the lowest rung) will control the output. The first rung's result gets overwritten. Use Set/Reset pairs instead when you need multiple conditions controlling one output.
• Ignoring the scan cycle for fast signals. If a signal pulse is shorter than the PLC scan time, the PLC will miss it completely. Use high-speed interrupt inputs or edge detection contacts (OSR/P) for fast signals.
• Not adding comments to rungs. Always add a rung comment explaining what each rung does. Future engineers (and your future self) will be unable to understand the logic without comments after six months.
• Putting outputs on the left side of the rung. Outputs (coils) must always be on the rightmost position of the rung. Contacts (inputs) are always placed between the left rail and the output. Reversing this will cause a program error.

Further Reading and External Resources

Frequently Asked Questions: Ladder Logic for Beginners