Abstract
Programmable logic controller (PLC) is widely used for sequence control in industrial automation system, and its program is mostly written in ladder diagram. In recent years, as processing performance of PLC has improved remarkably, ladder diagram becomes larger and more complex so that it is not easy to analyze and find the bugs or cause of unexpected control. To solve this problem, this paper describes how to apply a technique of program slicing to ladder diagram. In ladder diagram, data dependence almost represents control dependence, and we define slicing of ladder diagram as extracting directed graph of data dependence. But normally it tends to be divergent because ladder diagram is large and complex. So we focus on the directed graph composed only by frequently referenced data, which expresses the core of control structure, and we divide ladder slicing into three phases; extracting directed graph of data dependence only among frequently referenced data, slicing from start data to core structure, and slicing from core structure to the end destination data. The efficiency of this proposed method is shown through applying to actual ladder diagram analysis.
