Sequential Function Chart (SFC)

Sequential Function Chart

SFC is the IEC-blessed way to express sequential machine logic without inventing a step counter by hand. The program is drawn as a graph of:

• Steps — numbered states the program can be in. Each step has zero or more actions attached, each action written in one of the four “real” languages (ST, IL, LD, FBD) and run while the step is active.
• Transitions — boolean conditions between steps. When the condition fires, the source step deactivates and the target step activates.
• Branches — divergence (one step → many possible next steps) and convergence (many possible previous steps → one next step), both in alternative (“OR”) and parallel (“AND”) flavour.

SFC is sometimes called the “fifth language” but it is not on the same level as the other four. ST/IL/LD/FBD are general- purpose programming languages; SFC is a structuring layer that uses one of those four to describe what each step does.

Sub-pages

• Elements — steps, transitions, action qualifiers, divergence / convergence patterns.