5. Conclusions and Related Work

This structure of the net makes it easy to interpret any results of the Petri net analysis in terms of the source code. The initial marking of the Petri net is also given by the result tuple from. The part describes the used variables with types (number of places) and initial values (marking of these places). The Petri net for the user program of the 2-hand switch PLC consists of 45 places and 126 transitions, the total net including the environment model comprises 58 places and 142 transitions. For more details, including the model of the environment and of the system program, see [Heiner 98b]. More challenging case studies (production cells comprising several machines) are under preparation. Computer-aided verification should rely on verified tools, or at least on tools developed as careful as possible. For that reason, a Petri net semantics has been introduced formally for a subset of the standardized Instruction List language [IEC 1131-3]. The subset's syntax as well as static and operational semantics have been specified strictly. Afterwards, the classical operational reference semantics has been substituted by an equivalent Petri net semantics. Due to this prudent practice, the equivalence proof of the substitution step is quite obvious, and the implementation of the translator has been proven to be straightforward. To stress the analyzability of the generated Petri nets, the net semantics has been described completely in terms of ordinary safe place transition Petri nets. The used graphical enhancements are just syntactical sugar. Due to the structural regularities, many net components might be folded to coloured Petri nets. Related work may be found in [Hanisch 97], where IL programs are modelled by Timed Net Condition/Event Systems (TNCES). The advantage of TNCES is their dedicated modelling power, which excludes however (at least up to now) most of the sophisticated analysis options. [Rausch 97] discusses generally the transformation between different model forms in discrete event systems. In the future we intend to weaken the IL 0 restrictions step-wise into the direction of the whole language IL. This will include multiprocessor and multitasking systems, where the Petri net based design and analysis methodology promises its best results. Moreover, the incorporation of other standardized PLC languages might be considered (see e. g. [Heiner 97a] for the transformation of Sequential Function Charts into Petri nets).