How To Read Ladder Logic Diagram

A Comprehensive Guide to Reading Ladder Logic Diagrams

Ladder logic diagrams are used in industrial automation systems to represent automated processes performed on machinery. Whether you are a technician, engineer, or just a curious bystander, understanding how these diagrams work and how to read them can be incredibly helpful and rewarding. This guide will provide a comprehensive overview of the components and elements of a ladder logic diagram, as well as step-by-step instructions for interpreting and understanding them.

The development of ladder logic diagrams began in the 1950s when it became necessary to standardize the wiring methods of industrial control systems. By the early 1960s, this type of diagram had become widely used in the industry and is still in use today. While it may seem intimidating at first, with a little bit of practice and understanding, you can learn how to read ladder logic diagrams quickly and efficiently.

Identifying Components in a Ladder Logic Diagram

Before you can begin to read a ladder logic diagram, you must first understand the components and elements that make up the diagram. This can be broken down into four main components: rungs, contacts, coils, and wires.

Rungs are vertical lines of a ladder logic diagram which contain contacts and coils. The contacts act like switches that control the flow of electricity, and the coils control the power to the other components. Wires are also present, and they are used to connect one component to another.

Understanding How Contacts Operate

Contacts are the basic building block of a ladder logic diagram and are responsible for controlling the flow of electricity. They can be either normally open (NO) or normally closed (NC). A normally open contact will allow electricity to flow through the system when it is closed, while a normally closed contact will block the flow of electricity until opened.

The contacts in a ladder logic diagram are usually represented by squares with either an “X” or an “O” inside. An “X” indicates a normally open contact, and an “O” indicates a normally closed contact. It is important to remember that the contacts themselves do not have any power, but that they act as switches that control the power flowing through the system.

Interpreting Coil Symbols

Coils are responsible for providing power to the contacts and other components in a ladder logic diagram. Coils are represented by rectangles with a triangle inside. The coils themselves do not have power, but rather they act as a source of power for the system.

Coils can be used to energize normally open contacts, de-energize normally closed contacts, or to maintain a power supply to a specific component. In order to interpret a coil symbol, you must look at the adjacent contact symbols and determine how it is affecting the flow of electricity.

Reading Wires and Connections

Wires are used to connect one component to another in a ladder logic diagram. They are usually represented by thin lines which indicate the direction of the flow of electricity. In some cases, dashed lines may be used to indicate a break in the circuit.

It is important to pay attention to the direction of the wires, as this will help you understand the flow of power in the system. Additionally, you should also take note of any connections between components, as this will help you understand how the components are interacting with each other.

Step-by-Step Guide to Reading a Ladder Logic Diagram

Now that you know the basics of interpreting the components of a ladder logic diagram, it is time to put your knowledge to the test. Here is a step-by-step guide to help you understand and interpret a ladder logic diagram:

  • Step 1: Identify the components of the ladder logic diagram.

  • Step 2: Pay attention to the contacts and coils, as they are the primary components that control the flow of electricity.

  • Step 3: Note any wires and connections between components, and make sure to pay attention to the direction of the flow of electricity.

  • Step 4: Start at the top of the diagram and follow the flow of electricity from left to right, taking note of the contacts and coils that are energized.

  • Step 5: Once you have reached the end of the diagram, take a few moments to review what you have learned and make sure you understand how the components interact with each other.


Reading a ladder logic diagram can be a daunting task at first, but with a little bit of practice and understanding of the components, you can quickly become an expert. By following the steps outlined in this guide, you can easily interpret and understand these diagrams and gain a better understanding of the way industrial automation systems operate.

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