Worst-case timing prediction of relay ladder logic by constraint analysis

In this paper, we propose a technique of predicting the execution time of PLC application programs written in relay ladder logic. The proposed technique divides an RLL program into several RLL blocks and then transforms them into boolean logic equations. The internal states of an RLL program is analyzed and their dependencies are used as constraints when we solve the boolean logic equations. The plant information represented by exclusiveness of input contacts is also used as constraints. The constraint analysis reduces the complexity of logic programming as well as the worst-case timing prediction. The algorithm for partitioning variables in the logic equations is developed to reduce the complexity further