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Introduction and Basic Parts of Ladder Diagram in PLC Programming

Today, I will explain to you about the Ladder Diagram in PLC programming and different parts of the Ladder Diagram.

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There are different standards programming languages defined for Programmable Logic Controller (PLC). Some of the internationally accepted standards are…

  • Ladder diagram (LD)
  • Structured Text (ST)
  • Sequential Function Charts (SFC)
  • Instruction List (IL)
  • Function Block Diagram (FBD)

Related Read: The explanation of the 5 different types of Programming Languages

Generally, the Ladder diagram is most popular all over the world (including India). This language is easy to learn and it recognizes and looks similar to the electrical circuit diagram.

So I will be focusing only on the Ladder Diagram for PLC.

In this article, I will explain to you more about the Ladder Diagram in PLC programming.

Let’s start with some basics introduction.

What is the Ladder Diagram?

The ladder diagram is the universal programming language of PLC. It has a short abbreviation as ‘LD’ and also known as ‘Ladder Logic’.

It is one of the oldest programming languages for PLC.

In the ladder diagram, the programming language that used to create the program to control the PLC system is known as ‘Ladder Diagram Language’ or ‘Ladder Logic Language’.

It has signified by the graphical representation, just like electrical wiring for logic control. (At the end of this article, I will share LD examples, that will get you a clear understanding.)

Different Symbols used in Ladder Diagram:

This programming uses different graphic elements. These graphic elements are also called as ‘Symbols’.

Basic Important Parts of Ladder Diagram in PLC Programming:

  • Rungs
  • Branches
  • Inputs and Outputs for PLC programming
  • Addressing Inputs and Outputs
  • Instructions

So let us see one by one.


What is Rung Ladder?

In Ladder diagram, the horizontal lines called as ‘Rungs’. You can put as many numbers of rungs as per your project requirements.

The above diagram is shown with the ‘N’ number of rungs.


There are three types of branches. They are as follows.

1) Series Branch:

In the series branch, inputs or outputs are connected in the series.

2) Parallel Branch:

In the parallel branch, inputs or outputs are connected parallelly.

3) Nest Branch:

The combination of series and parallel branches in the same or different rungs are called as Nest Branch.

Inputs and Outputs for PLC Programming

For writing the program, Inputs and outputs are play most important role.

  • Inputs refer to the switch or Push Button (PB).
  • Output referred to the Coil or Lamp or Load.

Input is Normally Open (NO) Contact or Normally Closed (NC) Contact.

Addressing Inputs and Outputs

Another next most important thing in PLC programming is addressing of inputs and output for writing the program. Various addressing schemes are used for the different brands of PLC.

Let us take examples from different Brands of PLC

Different Brands of PLC and their Addressing

I explain brands of PLC with their instruction.


The format for the digital I/O address is as below,

  • Input address: I0, I1, I2,…………, In.
  • Output Address: Q0, Q1, Q2, Q3, ………….., Qn.

2) Siemens PLC:

The format for the digital I/O address is as below,

File types Byte Number.Bit Number

Consider, I/O address for the 1 Byte-

  • Input address: I0.0, I0.1, I0.2, I0.3,……. I0.7.
  • Output Address: Q0.0, Q0.1, Q0.2, Q0.3,……. Q0.7.

3) AB PLC:

The format for the digital I/O address is as below,

File type: Slot Number. Word Number/ Bit Number

Consider, I/O address for the 1 Byte-

  • Input address: I:0.0/1, I:0.0/2, I:0.0/3, ……….., I:0.0/7.
  • Output Address: O:0.0/1, O:0.0/2, O:0.0/3, ……….., O:0.0/7.

4) Delta PLC & Mitsubishi PLC:

In both PLC, the function’s addresses for input, output, and memory are the same. The format for the digital I/O address is as below,

  • Input address: X0, X1, X2,…………, Xn.
  • Output Address: Y0, Y1, Y2, Y3, ……….., Yn.


The types of instruction used for PLC’s controlling purpose.

I explain the list of instruction –

  • Basic Instruction: Input, Output, Timer (T), Counter (C), Set & Reset, Latch & Unlatch
  • Comparison Instruction: Greater than (GET), Lesser than (LET), Equal (EQU), Not Equal (NEQ), Greater than or equal(GEQ), Lesser than or equal(LEQ), Increment & Decrement
  • Mathematical Instruction: Addition, Subtraction, Multiplication
  • Bit Instruction: Positive H Trigger  (P) & Negative H Trigger (N), Flip-Flop, Not, Midline
  • Sequencer Instruction: Bit Shift Left (BSL), Bit Shift Right (BSR), Sequencer output (SQO), Sequencer compare (SQC), Sequencer load (SQL)
  • Program Control Instruction: Master Control Relay (MCR), Jump (JMP), Lable (LBL), Move
  • Advance Instruction: One Shot Rising (OSR) & One Shot Falling (OSF)
  • ASCII Instruction: ABL, ACB, CAN

What’s Next?

To understand these aspects more clear, check out the PLC programming example for Running Motor.   I have explained it in detail.

Next, go through the rules for writing the PLC program using Ladder Diagram.

This is all from this tutorial about the introduction and different parts of the Ladder Diagram in PLC. I will explain further details in upcoming articles about PLC programming. Till now, if you have any question, you can ask in the comment section.

12 thoughts on “Introduction and Basic Parts of Ladder Diagram in PLC Programming”

  1. Hi, Dipali
    For Freshers you have taken very good step so that he can learn basics.You write a topic in a soft learning language. Because six years ago, I was learn PLC programming through internet and reading PLC books. warm welcome to you. Thanks
    Arun Siwach
    Automation Engineer

    • Thanks to appreciating me,

      My motive to share my knowledge as well as learning more things through this blog.

      I’m so glad for shares the blog.

      I hope that this blog will be helpful to the learner.

  2. Hello dipaliI
    I downloaded software Delta WPLsoft, but I did not know how to work with this program as you wrote these led diagrams that you are presenting in this lesson.

    Please which program are you explain this lesson
    Thank you

  3. I must thank you for the efforts you have put in writing this website. I am hoping to view the same high-grade content by you in the future as well. In fact, your creative writing abilities has encouraged me to get my very own website now. 🙂


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