Design recovery for relay ladder logic

Abstract

An algorithm for converting relay ladder logic (RLL) programs for programmable logic controllers (PLCs) into sequential function chart (SFC) programs is introduced. Since an SFC (a standardization of Grafcet) better represents the sequential flow of control logic, it is considerably easier to understand and modify SFC programs. The practical motivation for this study is to provide a tool for converting existing RLL programs into SFC programs by recovering the underlying sequential control logic. A basic design recovery algorithm is presented and applied to a simple RLL program for one stage of a batch chemical process. Several fundamental issues in discrete control logic representation and analysis are identified as fertile areas for future research.