Chapter 8: Sequential Function Chart

Overview

This chapter reviews the IEC Sequential Function Chart (SFC) graphical language; this is the final language of the IEC languages in the set:

Structured Text, Function Block Diagram, Ladder Diagram, Instruction List and Sequential Function Chart.

SFC is a powerful graphical technique for describing the sequential behaviour of a control program. This chapter includes:

• The basic concepts of the Sequential Function Chart language;

• The definition of sequences in terms of steps and transitions;

• The use of the other various IEC languages for defining the behaviour of SFC elements, i.e. steps, transitions and actions;

• How to define when actions are executed;

• The use of SFC for top down design;

• Safe and unsafe chart designs.

8.1 SFC background

The definition of the IEC Sequential Function Chart language has been derived from current techniques that are used for depicting sequential behaviour. Many PLC manufacturers have offered some form of graphical language for describing sequences for a number of years. The majority of the European based PLC manufacturers now provide Grafcet, a graphical language based on a French national standard.

Describing the behaviour of a system in terms of states and transitions, depicted as circles and arcs, was originally defined in a methodology called Petri-net. Petri-net is now a well established technique, particularly in computer systems design, for formally describing the behaviour of programs that have multiple states.

Grafcet was evolved from Petri-net as an industrial form of the methodology. In 1988, the IEC published the standard IEC 848, Preparation of function...