Tool for visual difference display of programs in IEC 61131-3 ladder diagrams

The IEC 61131 standard is commonly used for the programming of Programmable Logic Controllers (PLCs). One of the graphical languages of this standard is the ladder logic diagram based on the circuit diagrams of relay logic systems. This paper reports on a tool that enables the control system engineer to compare two ladder logic based programs developed in the Rockwell Automation RSLogix5000 environment and to display the differences in both textual and graphical form. The comparison employs modified Token Passing, Longest Common Subsequence and Longest Common Substring algorithms performed upon the XML representation of the RSLogix5000 project. This paper describes the steps in the whole comparison procedure and demonstrates this solution including visual comparison on examples shown in our implemented tool.