Formalization of PLC programs to sustain reliability

The complexity of existing PLC programs precludes dealing with or modifying them. The interest in the formalization of PLC programs has increased in recent years to overcome this problem. Formal methods can provide for the development of PLC programs, since they serve as a medium to analyze and verify them due to increased awareness of safety and quality. This is done by performing what is known as verification and validation (V&V). Moreover formal methods support reuse of already implemented PLC programs to meet new production demands. This paper examines different possibilities to formalize PLC programs. It explains the pros and cons of these different alternatives. The paper outlines a formal description of timers and counters used in PLC programs according to the alternative that fulfills the criteria best.