A sequential function chart, or SFC, is the IEC 61131-3 language for describing processes that happen in ordered steps. Instead of expressing everything as parallel logic, SFC lays a process out as a flowchart of steps and transitions - do this, then when a condition is met, move to the next step. It is the natural language for start-up sequences, batch operations, and any process with a clear beginning, middle, and end.
Sequential Function Chart in one line: A sequential function chart (SFC) is a graphical PLC programming language in IEC 61131-3 that structures a process as a series of steps and transitions, where each step performs actions and the program advances to the next step only when its transition condition becomes true.
An SFC is built from two core elements: steps and transitions. A step represents a stage of the process and holds the actions active during it. A transition is the condition that must be true to move from one step to the next. Control flows down the chart, dwelling in each step until its transition fires, then advancing. SFC also supports branches - divergences for choosing between alternative paths and parallel branches for running steps simultaneously.
Importantly, SFC usually does not do the detailed work itself. The actions inside each step are typically written in one of the other IEC 61131-3 languages - ladder, function block diagram, or structured text. SFC provides the skeleton of the sequence; the other languages fill in what each step actually does. This separation makes complex sequences far easier to design, read, and troubleshoot.
SFC is ideal for any process that is inherently sequential: equipment start-up and shutdown routines, batch recipes, cleaning cycles, and multi-stage operations where order matters. When you can describe a process as "first do A, then wait for B, then do C," SFC captures that structure directly, and operators and engineers can see exactly which step the process is in at any moment.
In oil and gas, sequential logic appears in compressor start-up sequences, well start-up and shut-in routines, and automated proving cycles. Because the current step is visible, SFC also makes troubleshooting intuitive - a stalled sequence points immediately to the transition that failed to satisfy. A SCADA platform reads the step status and process values as tags, letting operators watch a sequence progress remotely; Merobix reads such tags over Modbus, OPC UA, and other protocols without altering the underlying logic.
A step is a stage of the process that holds the actions active during it; a transition is the condition that must become true to advance to the next step. Control dwells in a step until its transition fires, then moves on, giving the process a clear ordered flow.
Yes, and it usually is. SFC provides the overall sequence, while the detailed actions inside each step are written in ladder, function block diagram, or structured text. SFC is the skeleton; the other IEC 61131-3 languages do the work.
Use SFC when a process is inherently step-by-step - start-up and shutdown sequences, batch recipes, cleaning cycles, or automated proving. It makes the order explicit and shows which step is active, which makes both design and troubleshooting far clearer.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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