A Basic Consideration on Safety of the Programs with Ladder Diagram

Ladder Diagram of Programmable Logic Controllers are widely used in Industrial Automation Systems even its low readability, because normally closed contacts used in it are expected to stop all of the systems easily when an abnormal situation has occurred. However, the low readability is a high risk in safety programs. We propose a method to represent a program of Ladder Diagram by state transition equations and validate and verify the program of Ladder Diagram based on the obtained state transition equations. Finally, we explain that some standard Function Blocks appeared in IEC 61131–3 included the definition of Ladder Diagram can be also represented by their corresponding state transition equations.