Glossary · PLC and control programming
Sequential function chart (SFC)
Also known as: SFC, Grafcet-based chart
German: Ablaufsprache (AS)
In IEC 61131-3, the sequential function chart (SFC) is a graphical language for structuring programs as steps and transitions, where actions are associated with steps and transitions define the conditions for moving from one step to the next.
- PLC programming
- Standards
In one sentence
SFC is a graphical IEC 61131-3 language that structures a program as steps with actions and transitions with conditions.
Example
A batch reactor program in SFC has the steps Charge, Heat, React and Discharge, with transitions such as Temperature >= 80 °C.
How it applies
- Engineering: SFC is derived from GRAFCET (IEC 60848). It supports alternative branches (one of several paths) and simultaneous branches (parallel paths). Actions in steps and conditions in transitions are written in other IEC 61131-3 languages, often ST or LD. Action qualifiers such as N, S, R and P define how long an action is active.
- Operation: The online chart shows the active step, so operators and technicians see at a glance where a sequence waits.
- Maintenance: Well-designed SFCs keep interlocks and fault handling visible. Hiding them inside step actions makes diagnosis harder.
- Documentation: SFC charts can be reused almost directly as sequence descriptions in specifications and manuals. Use the same step names on the HMI, in the chart and in the documentation.
SFC vs. ECC
SFC structures a whole program into steps. The Execution control chart (ECC) of IEC 61499 controls when the algorithms of one basic function block execute in response to events. Both are state-based, but at different levels.