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Sequential Function Chart

A Sequential Function Chart is a graphical programming method for industrial automation that models control logic as ordered steps and transitions. It is used in programmable logic controller and process control systems to show state changes, branching and parallel execution, making machine behaviour easier to design, test and troubleshoot.

What Sequential Function Charts do in industrial control

Sequential Function Chart, often abbreviated SFC, is a structured way to express control logic as a sequence of steps, transitions and actions. In industrial automation, that gives engineers a clearer view of how a process moves from one state to the next, including decision points, branching and parallel execution.

The main value of the notation is that it separates the flow of the process from the details of the individual actions. That makes it easier to reason about state changes in PLC programs and process control systems, especially where timing, interlocks and handoffs matter.

Why Sequential Function Charts are used

SFC is useful when machine behaviour is naturally stateful rather than purely procedural. A packaging line, batch process or material-handling system often needs explicit start, hold, resume, fault and shutdown states, and the chart makes those transitions visible.

Because the sequence is explicit, teams can review logic more quickly than when the same behaviour is buried inside dense ladder logic or procedural code. It is also easier to communicate intended behaviour to operations, maintenance and safety stakeholders who need to understand the process flow, not just the code implementation.

How the model represents control behaviour

An SFC typically uses steps to represent active states and transitions to represent the conditions that allow movement to the next step. Actions are attached to steps, so the logic can describe what happens while a state is active instead of mixing that with the transition condition itself.

Parallel branches are especially important in automation, because many industrial processes do more than one thing at a time. The model can show when two activities proceed concurrently, when they join back together, and where one branch must complete before the process can continue.

Operational and troubleshooting value

Sequential Function Charts are often chosen because they make debugging and testing easier. When a process fails, engineers can inspect where the sequence stopped, which transition did not fire, or which branch is waiting for a condition that never arrives.

That visibility helps with commissioning, change review and incident analysis. It also supports safer modifications, because the impact of a logic change is usually easier to assess when the control sequence is described as ordered states rather than as scattered control conditions.

Risk and Threat Considerations

Sequential Function Charts reduce ambiguity, but they can also create a false sense of clarity if the underlying transitions are poorly designed, insufficiently tested, or overly dependent on hidden conditions. In industrial systems, a logic error in sequencing can produce unsafe state changes, unexpected concurrency, or a process that never reaches a safe stop.

Failure mechanism: Faulty transition logic, race conditions between parallel branches, or incomplete handling of exception states can let the controller advance at the wrong time or remain stuck in an unsafe step.

Impact: The result can be production interruption, equipment damage, degraded safety interlocks, or difficult recovery because the operator sees a sequence that appears orderly while the underlying control state is not.

Standards & Framework Alignment

This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.

NIST SP 800-53 Rev 5 and CIS Controls v8 set the governance and control requirements practitioners need to meet.

Framework Control / Reference Relevance
NIST SP 800-53 Rev 5 CM-2 — Baseline Configuration SFC logic is part of controlled system configuration for PLC and process control environments.
SI-2 — Flaw Remediation Control logic defects in SFCs require detection and remediation after testing or incident review.
SA-11 — Developer Testing and Evaluation SFC behaviour should be tested to confirm transitions, branches and exception paths work as intended.
Recommendation — Baseline SFC logic and related controller configurations before deployment. Remediate confirmed SFC logic flaws promptly and verify the corrected sequence. Test SFC transitions and branch conditions before putting control logic into operation.
CIS Controls v8 CIS-16 — Application Software Security SFC implementations are application logic in industrial control software and need secure design and testing.
CIS-4 — Secure Configuration of Enterprise Assets and Software PLC and process-control logic should be configured and changed under controlled baselines.
Recommendation — Review SFC logic for design flaws and unsafe state transitions before release. Manage SFC changes through controlled baselines and approved configuration workflows.

Practitioner Guidance

Why practitioners should care: SFC is most valuable when the process has clearly defined states, handoffs and exception paths. Use it to make those behaviours explicit, then validate that every transition has a deliberate condition and every branch has a known merge point.

What to watch for: Treat state explosion, hidden dependencies and poorly documented parallel paths as design warning signs. If troubleshooting depends on assumptions that are not visible in the chart, the model is no longer doing its job as an operational control aid.