The Ladder Logic programming language is the leading and most widely used programming language in PLC programming. It is a very intuitive, easy-to-learn, and easy-to-program graphical programming language.
The Ladder Logic programming language is very similar to electrical relay circuits. So if you know the relay control circuit, you will learn to program the Ladder Logic quickly and easily.
Ladder Basics:
Ladder Logic is a programming language used to program a PLC (Programmable Logic Controller). It is a graphical PLC programming language that expresses logical operations using symbolic notation using ladder diagrams, much like the bars and rungs of a traditional relay logic circuit.
Ladder Logic is a fast and simple way to generate PLC Boolean expressions to automate repetitive machine tasks and sequences. It is used in many industrial automation applications. Some examples of industrial automation applications where PLC ladder logic is used include.
Learn the basics of Ladder Logic:
It is relatively easy to learn the basic concepts of ladder logic programming, even if you have no experience with electrical circuits. Take comfort in knowing that ladder logic is the fastest and easiest PLC programming language to learn.
What is Ladder Chart in PLC?
The ladder diagram is the symbolic representation of the control logic used to program the ladder logic for a PLC. Ladder diagrams have horizontal lines of control logic called steps and vertical lines at the beginning and end of each rung called rails. In fact, the structure of the ladder diagram looks like a ladder, hence the name “ladder diagram”.
There are two differences between electrical layout and ladder diagram. The first difference is that the control logic in an electrical diagram is represented using components while ladder diagram symbols are used. The second difference is the implementation of the control logic in the electrical layout as per the operation of the electrical circuit while in the ladder diagram it depends on the systematic nature of PLC scanning.
Why is a ladder diagram used for PLC programming?
The reason ladder diagrams were used for PLC programming is that early control system designers were accustomed to relay logic control circuits and ladder diagrams that closely simulated these circuits. They preferred to use ladder diagrams rather than using text-based programming languages such as C, BASIC, Pascal, and FORTRON. Another reason to use ladder diagrams is that factory maintenance personnel already understand how to read relay control circuits, so using ladder diagrams for PLC programming meant they were easily able to troubleshoot control system problems.
ladder diagrams help you to formulate Boolean expressions into diagrams required for PLC programming. They represent conditional expressions and the input and output as symbols. So writing a PLC program using ladder diagrams is like drawing a relay control circuit.
Ladder diagram (LD) is the official name given in the international programming standard PLC IEC-61131. But, these days the terms ladder diagram, ladder logic diagram, ladder diagram, ladder control, ladder circuit, control logic diagram and logic diagram (to name a few) are used to describe relay logic circuits and ladder logic programming.
How to Draw Ladder Diagrams
Ladder logic diagrams are drawn in a similar way to a relay logic circuit. However, ladder logic diagrams express logical operations using symbolic notation rather than circuit components.
The busbars in the relay logic circuit represent the supply wires to the relay logic control circuit. However, in ladder diagrams, rails represent the beginning and end of each line of symbolic code.
The degrees in a relay logic circuit represent the wires that connect the components together. However, in ladder diagrams, the scores represent the logical flow through the symbolic code.
When implementing a ladder logic program in a PLC, there are seven basic parts of a ladder diagram that you should know. They are bars, degrees, inputs and outputs, logical expressions, markup/names tags and comments. Some of these items are essential and others are optional.
To help understand how to draw a ladder diagram, the seven basic parts of a ladder diagram are detailed below:
Rails:There are two rails in a ladder diagram that are drawn as vertical lines that run down the far edges of the page. If they are in a relay logic circuit, they will represent the active and zero volt connections of the power supply as the power flow goes from the left side to the right side.
Rungs:The scores are plotted as horizontal lines and the bars are connected with logical expressions. If they are in a relay logic circuit, they will represent the wires connecting the power supply to the switching and relay components. Each score is numbered in ascending sequential order.
Inputs : External control actions such as pressing a pushbutton or operating a limit switch. The inputs are physically connected to the terminals of the PLC and are represented in the ladder diagram by a normally open (NO) or normally closed (NC) connection symbol.
Outputs :Outputs are external devices that are turned on and off such as an electric motor or solenoid valve. The outputs are also connected to the terminals of the PLC and are represented in the ladder diagram by the relay coil symbol.
Logic Expressions :Logic expressions are used with inputs and outputs to formulate the required control operations.
Address Notation: The address notation describes the input and output memory processing structure and logical expression of the PLC. Tag names are descriptions assigned to headings.
Comments :Comments are a very important part of a ladder diagram. Comments are displayed at the beginning of each score and are used to describe the Boolean expressions and controls implemented by the score or groups of scores. Understanding ladder diagrams has become a lot easier with comments.
For more information about LADDER Logic we have put together the LADDER Logic Learning Book for Beginners. You can download it from here:
