Sequential Function Chart, SFC

Sequential function chart is the one of the five IEC 61131-3 controller languages built specifically for logic that advances through states, and it is the natural home for any control problem that reads as a flowchart. A program is drawn as steps, each a box that holds the actions active while that state runs, connected by transitions, each a condition that must become true before control moves to the next step. Parallel branches let several steps run at once and alternative branches let exactly one of several paths execute. That structure maps directly onto the sequences that dominate process automation. A batch phase order, a plant startup that purges then pressurizes then ramps, a clean-in-place cycle that steps through pre-rinse, caustic, intermediate rinse, acid, and final rinse. It is the wrong tool for continuous regulatory control, which belongs in function block, and for dense discrete machine logic, which often stays in ladder. The notation descends from Grafcet, the French sequential-control formalism standardized as IEC 60848, adapted into IEC 61131-3 as an executable controller language. The link back to the drawing is concrete. Each step corresponds to a defined set of valve positions and instrument setpoints, so a charge step might open a specific feed valve, arm a high-level limit, and set a temperature controller, and the tag names that step references are exactly the ones an I/O list carries, which is why commissioning verifies each step's device interactions before testing the full sequence.

How SFC reads.

Vertical chart. Each step is a box, each transition is a horizontal bar with a Boolean condition. The active step, state executes its associated actions until the transition condition becomes true, at which point control moves to the next step. Concurrent branches let multiple steps run in parallel. Alternative branches let exactly one of several paths execute based on conditions. Loops and jumps allow complex flow. The whole program reads top-to-bottom like a flowchart, which is exactly the point.

When SFC is the right tool.

Batch phases, charge then mix then react then transfer. Natural SFC. Plant startup sequences, purge, pressurize, ignite, ramp. Natural SFC. Equipment recipes, CIP cycle. Pre-rinse, alkali wash, intermediate rinse, acid wash, final rinse. Natural SFC. Continuous regulatory control. Not SFC. Use ladder or function block. Discrete machine control with tight cycle requirements. Usually ladder, sometimes SFC for the higher-level state machine with ladder for the per-step logic.

Frequently asked.