PLC scan cycle with Ladder Logic Part I

Ladder Logic Basics I

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In this post, you will learn the Ladder Logic Basics, but… what does the Ladder Logic do? Ladder Logic automates monotonous machine sequences and tasks. For making logic expressions for a PLC, it is a practical and quick method. Some examples of the Ladder Logic Basics are:

  • Hopper and Tank Level Control.
  • Logistics Package Conveying and Sorting.
  • Material Handling Conveyor System.
  • Beverage Bottling and Labeling.
  • Pallet Packing and Strapping.
  • Air and Liquid Flow and Pressure Control.
  • Ball Mill Lubrication System.
  • Cement Batching.

The Relay Logic is a hard-wired controller, used for process automation and machine. Even so, soon programming languages replace it. The Ladder Logic is a great example, because of the invention of the PLC and the appearance of microprocessors.

Why is Ladder Logic Popular?

Ladder Logic is similar to an electrical schematic drawing because it is a programming language based on graphics. For electricians, engineers and students, switching from an electric circuit to Ladder Logic were surprisingly simple.

The drag and drop of Ladder Diagrams while programming Ladder Logic in a PLC, lets you develop code efficiently and speedily. Moreover, Ladder Logic enables you to simply solve the problems with your code, so you can see, from the LHS start rail, the logic flow. 

Learning the Ladder Logic Basics

The fundamental principles of Ladder Logic programming are relatively simple to understand. We are going to explain them in the following order:

  • Show the Ladder Diagram.
  • Analyze the seven main sections of a Ladder Diagram.
  • Describe the logic and binary concepts involved in Ladder Logic.
  • Know the functions integrated into the structure.
  • Learn the five crucial logic functions.

What is a Ladder Diagram in a PLC?

The symbolic representation of the Control Logic used for Ladder Logic programming of a PLC is a Ladder Diagram. These have vertical and horizontal lines. The rungs are the horizontal lines because they are at the beginning and the rails are the vertical ones because they are at the end of each rung.

Between a Ladder Diagram and an electrical schematic, there are two distinctions. The principal difference between them is the Control Logic. Symbols express the Ladder Logic, while components express the Electrical Schematic. The execution is the second distinction. The PLC scan is the execution for the Ladder Diagram, while in an Electrical Schematic it is according to an electrical circuit.

Why is a Ladder Diagram used for a PLC programming?

The main reason is that the pioneers’ control system programmers were familiar with Relay Logic Control Circuits and if you realize, Ladder Diagrams and they are so similar. They opted to use Ladder Diagrams to program a PLC graphically because these allow you to articulate the Logic expressions. Another reason is that maintenance workers learned how to read Relay Control Circuits and with Ladder Diagrams, the troubleshooting problems were even simpler to solve.

The official name assigned in the international PLC programming standard IEC-61131 is Ladder Diagram (LD). However, other names for Relay Logic Circuits and Ladder Logic programming are Ladder Control, Control Logic Diagram, Ladder Logic Diagram, Ladder Circuit, Logic Diagram and Ladder drawing.

How to Draw Ladder Logic Diagrams

The drawings of the Relay Logic Circuits and the Ladder Logic Diagrams are resemblant, but the only thing that differentiates them is that the first one uses circuit elements and the other one (Ladder Logic) uses the symbolic notation.

On one hand, the rails in a Relay Logic Circuit express the Relay Logic Circuit’s supply cables. On the other hand, they indicate the beginning and end of each line of symbolic code in Ladder Logic Diagrams.

The rungs symbolize the cables that link the elements in a Relay Logic Circuit. Although, the rungs indicate the logic flow through the symbolic code in Ladder Diagrams.

There are seven fundamental sections of a Ladder Diagram, but keep in mind that some of them are essential and others are not.

7 fundamental sections of a Ladder Diagram

  1. Rails 

There are vertical lines that extend from the top of the page to the bottom that are part of the two rails of a Ladder Diagram. They would reflect the active and zero volt contacts of the power supply in a Relay Logic Circuit, where the power flow is from the left to the right.

  1. Rungs 

The horizontal lines represent the rungs. The enumerated rungs link the rails to the logic expressions. They would be the cables linking the power supply to the switching and relay elements of a Relay Logic Circuit.

  1. Inputs 

External control actions, such as tapping a push-button or activating a maximum switch, are used as inputs.  In fact, the inputs are programmed to the PLC terminals and are expressed by a Normally Open (NO) or Normally Closed (NC) symbol in the Ladder Diagram. 

  1. Outputs

External instruments that are switched on and off, such a solenoid valve or an electric motor, are examples of outputs. The outputs are both connected to the PLC terminals and are expressed by a Relay Coil icon in the Ladder Diagram. 

  1. Logic expressions

To develop the necessary control operations, the logic expressions are combined with the inputs and outputs.

  1. Address Notation & Tag Names

The address notation refers to the PLC’s input, output and logic memory addressing system. The details given to the addresses are the tag names. 


On a Ladder Diagram they are highly significant. The comments appear at the beginning of each rung to explain the control processes and the logical expressions that the rung or a group of them are carrying out. The use of feedback makes it much simpler to understand Ladder Diagrams. 


How to read Ladder Logic

From the left-hand rail to the right-hand rail, and from the first rung to the last rung, Ladder Logic is read (left to right and top to bottom). The input symbols in the rungs either pass or interrupt the logic flow. Each rung ends with an output symbol, which indicates the product of the Logic expressions found within that rung. 

To begin reading Ladder Logic, we must first understand certain fundamental binary principles and how they relate to Ladder Logic, as well as how Ladder Logic is performed and the simple logic functions incorporated into each rung.