Download Basic Principles of PLC Ladder Diagram

 PLC (Programmable Logic Controller) ladder diagrams are a commonly used method for programming PLCs. The ladder diagram is so named because it resembles a ladder, with two vertical rails (representing the power supply) and horizontal rungs (representing the logical connections). The basic principles of PLC ladder diagrams include:

  • Inputs: These are the signals that are received by the PLC from external devices, such as sensors or switches. In the ladder diagram, inputs are represented by contacts and are typically located on the left side of the ladder.
  • Outputs: These are the signals that are sent by the PLC to control external devices, such as actuators or lights. In the ladder diagram, outputs are represented by coils and are typically located on the right side of the ladder.
  • Rungs: Rungs are the horizontal lines that connect the inputs and outputs. Each rung represents a logical statement, such as "if input A is true and input B is false, then turn on output C." Rungs are read from left to right, and the order of the rungs can affect the overall program.
  • Logic: Logic is used to determine the state of the outputs based on the state of the inputs. Common logical operators used in PLC ladder diagrams include AND, OR, NOT, and XOR.
  • Timers and counters: Timers and counters are used to delay or count the number of times an event occurs. They are represented by special instructions in the ladder diagram and can be used to create more complex logic.

PLC ladder diagrams are widely used in the industrial automation field and are considered a simple, intuitive, and powerful method of programming PLCs. The ladder diagram is a graphical representation of the program, it's easy to understand and modify by technicians and engineers.

 This PDF is an introduction to the programming language Ladder Diagram (LD) used in Programmable Logic Controllers (PLC). The book provides a general introduction to Ladder programming with a focus on enabling learners without an electrical engineering education to learn Ladder programming. The book systematically describes basic ladder programming, which means that knowledge is built and deepened throughout the book. Therefore, it is recommended to read the book from the beginning, as the first chapters are a general introduction to PLC programming, which is the basis for being able to write a good PLC program. Afterwards, the book can be used as a reference to solve specific problems. 

There are several explanations to the program code examples, and there is a focus on the program code being clear, readable and structured. The book was developed for the full-time “Academy Profession (AP) Graduate in Automation Engineering” course and the part-time “AP Degree in Automation and Operation” course at the Dania Academy, Randers, Denmark. Please note that the book can easily be used for the many other educations and institutions that teach PLC programming including educations such as electrician, electrical engineer, automation control, PLC programmer, instrument technician and process electrician, automation technician and electrician. 

The PDF describes Ladder programming as described in the standard IEC 61131-3. PLC vendors understand this standard in different ways, and not all vendors follows the standard exactly. This will be clear through material from the vendor. This means that some of the program examples in this book may not work as intended in the PLC type you are using. In addition, there is a difference in how the individual PLC type shows graphic symbols and instructions used in Ladder programming. This is a book for beginners and therefore advanced techniques such as ARRAY, LOOPS, STRUCT, ENUM, STRING, PID and FIFO are not included. 

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