An approach for comparison of IEC 61131-3 graphical programs

IEC 61131-3 is an open standard that provides guidelines for programmable logic controllers and control systems. The standard defines specifications for control system programming languages, both textual and graphical. A control system application developed using these languages can go through several changes over the course of the project's life-cycle. While differences in textual programs can be detected using traditional text difference algorithms, there exist no effective means for tracking changes at a granular, semantic level for graphical control system programs. In this paper, we present an approach for comparison of graphical IEC 61131-3 programs, specifically those written using the Function Block Diagram and Sequential Function Chart notations. To perform this comparison, we represent the programs in an XML format, and use this to compare individual elements. The differences are computed and the programs are then rendered in a customized graphical viewer that highlights changes between programs. We implement a prototype comparison utility for graphical IEC 61131-3 programs and show how it can help reduce the effort involved in manual review and maintenance activities.