AiVibeSequential Function Chart (SFC)
Sequential function chart is an IEC 61131-3 graphical element for structuring control programs as steps linked by transitions, with actions attached to each step, making sequential and parallel machine or batch operations explicit.
An SFC is built from steps, transitions and directed links. Only active steps execute their associated actions, which can be written in any IEC 61131-3 language and carry qualifiers such as N (non-stored), S (set), R (reset), P (pulse) and time-limited or time-delayed variants. A transition holds a Boolean condition; when it becomes true and the preceding step is active, control passes to the next step. Divergent branches allow alternative paths or simultaneous sequences that later converge.
SFC derives from the French GRAFCET method, standardised as IEC 60848. It suits machine cycles, start-up and shutdown sequences, material handling and batch phases, because the current step of a machine is visible at a glance. That visibility shortens diagnosis when a sequence stalls while waiting for a transition condition, such as a sensor that never switched.
Good SFC design keeps steps meaningful, avoids ambiguous parallel branches and defines how sequences behave after a stop, fault or power loss. Vendors differ in how they handle step timers, forcing of steps and chart resets, so behaviour must be confirmed in the platform documentation. In batch control, SFC-like structures commonly implement ISA-88 procedures and phases.
Key points
- Organises programs into steps, transitions and actions
- Supports alternative and parallel branches that later converge
- Derived from GRAFCET, which is standardised as IEC 60848
- Shows the active step, which speeds diagnosis of stalled sequences
- Common for machine cycles and ISA-88 batch phases
Where AiVibe comes in
AiVibe designs and manufactures the AiAmbA AI Factory, whose edge devices and AI agents let people talk to PLC, CNC and robot controllers in plain language. Agents only propose changes, a trained operator confirms each one, and the AI Factory never replaces a machine's control or safety systems.