Modeling of PLC behavior by means of timed net condition/event systems

Net condition/event systems (NCES) are a particular representation of the modeling paradigm of condition/event systems introduced by Sreenivas and Krogh (1991). This paradigm allows modular modeling of discrete event systems. In this paper, we use NCES for modeling of PLC programs and their execution. Since the internal behavior of a PLC is time-driven and since we intend to model timers as well, we extend the strictly causal, untimed model of NCES to a timed NCES (TNCES). We demonstrate how this model is used to describe in detail the behavior of a PLC. An automatic translator which uses instruction list as input and which generates a TNCES model of the PLC behavior has been developed. As a result, we achieve very realistic, mathematical models of a PLC which can be used either to simulate the PLC behavior or-together with models of the plant-to verify the control code