PLC software modularity and co-operative development

The paper introduces a software design method to pursue modularity of PLC-based system control code. Modularity is a critical condition to support co-operative development of complex systems, where components supplied by a number of subcontractors are integrated to realize the envisaged plant. The method proposes a disciplined use of the SFC, FBD and ST languages of the IEC 61131-3 standard.