PLCopen Announces SFC Guidelines for Software Creation

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  PLCopen Announces SFC Software Design Guidelines

6. July 2018 – Although there are guidelines for many programming languages, they are important for the industrial control domain, such as: IEC 61131-3 and its PLCopen extensions. Nevertheless, the software is becoming more and more important in the industrial environment, the software projects are larger and the error costs increase. Software now accounts for half of the initial project cost, and between 40 and 80% is spent on software lifecycle cost maintenance. To handle the complexity of larger programs requires modern software development processes that support a structured approach and create reusable code. We also need to increase coding efficiency by reusing pre-defined functionalities and helping to better understand the program throughout the lifecycle.

Therefore, PLCopen launched its initiative for Software Construction Guidelines in 2015, led to the release of two documents:

  • PLCopen Coding Guidelines, published in 2016, and
  • Creation of PLCopen-compliant libraries, released in 2017 .

A Third Document Structuring with SFC: Do's and Don'ts is Now Published as Version 1.0

This document explains the benefits of Sequential Function Chart, SFC. This is a very meaningful graphic formalism of the IEC 61131-3 standard. It is not considered a programming language because it requires other languages ​​to express transitional conditions and actions.

SFC provides a means for partitioning a programmable controller program unit into a set of steps and transitions interconnected by directed connections. Each step is assigned a set of actions, and each transition is assigned a transition condition. Since SFC elements require the storage of state information, only building blocks that can be structured with these elements are function blocks and programs (not functions).

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When part of a program organization unit is partitioned into AS elements, the entire program organization unit should be split. If no SFC partitioning is specified for a program organization unit, the entire program organization unit is considered a single action that is executed under the control of the calling entity.

If a sequential process is to be controlled, SFC must be taken into account as best suited for structuring the internal organization of a POU, in particular in the functional description:

  • If the process consists of several steps to be executed sequentially in a time frame, for example one Assembly process, SFC can be used to map the different stages of assembly as steps and structuring the process as a step sequence.
  • If the process can be modeled as a state machine, these states can be mapped to steps and the transition from one state to another can be structured by transitions] SFC structures the internal organization of a program, helping to resolve a control problem into manageable parts to disassemble without losing track.

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