Sequential function chart (SFC) is a kind of representation form which has been defined in the international standard IEC 1131-3 as a common element of languages of programmable controllers (PLC). SFC is derived from Petri net (PN) and, recently, it is available in many PLC. Because of many modifications from PN, a lot of theoretical results of PN can not be directly used for SFC. This paper aims to solve such problems and proposes a method which translates SFC to a PN model and verifies the behavior of SFC based on its PN model. In this paper, it is pointed out that there exists a class of SFC which corresponds to a unique PN model. According to the PN model, a method describing a sequential control program by SFC and verifying its behavior by Structured Text (ST), which is also a language of PLC and is defined in IEC 1131-3, is provided. A whole PN model, not only of SFC but also all of a given sequential control system, will be constructed by ST inside of a real PLC, so that the behavior of SFC will be easily validated by this PN model.
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