Sequential Function Chart (SFC)
In Process Control SFC charts stand apart or, to be more precise, rank above other three languages. SFC charts are high-level graphic instruments. Thanks to SFC, the idea of turning system model into complete program became real. Unlike additional modeling tools, SFCs give the active program. SFCs are directed from up to down and displayed as straight lines. There's no way of assigning first steps in SFC, only one chart step is initial. SFC graphic chart consists of steps and transitions between them. Condition defines transition permission. Definite actions are connected with the step. Action descriptions are executed in ST language. SFCs do not contain any control commands.
Due to internal memory presence necessity, only function blocks and programs can be implemented in SFC, functions do not have such a possibility.
SFC application purpose is task division into simple stages with formally defined system operation logic. SFC allows fast system prototype creation without coding. Besides, there is no need in detailed action description for higher-level development.
SFC implementation in extensive components allows shortening execution time and, accordingly, system response time. With the help of steps, the monolithic program is divided into short fragments executed in various work cycles. In other languages, extensive and simultaneously fast program implementation needs additional coding of the step-wise execution engine. SFC stimulates equal processor computation power distribution without additional efforts. Function block and program implementation in SFC has a significant peculiarity. There are no first and last instructions. RETURN operator is not used as well. As if the program has no end. Every SFC component call equals one cycle execution. What is executed by the component depends on its previous condition.
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