Algorithm of transformation from PLC ladder diagram to structured text

An algorithm of transformation from PLC ladder Diagram (LD) to structured text (ST) based on treating LD components separately in accordance with their type after Analysis of the IEC 61131-3 standard PLC LD and ST. In this algorithm virtual node is introduced, and then it is combined to establish activity on vertex (AOV) diagraph. Nodes are treated according to their type and the topological sorting of the AOV diagraph, then the transformation from LD to ST is achieved. The rules of treating the nodes and a transformation are present. The algorithm is a general transformation algorithm for transformation any complex LD to ST. It is applied to Lantian series numerical control system successfully in National Engineering Research Center for High-End CNC.