Sequential Function Chart (SFC): Steps, Transitions and Examples

Direct answer: what Sequential Function Chart is

A Sequential Function Chart (SFC) organizes control behavior as active steps, Boolean transitions, associated actions and directed links. When the preceding step or steps are active and the next transition is satisfied, the chart changes its active-step state according to the target platform's execution rules. SFC is useful when the process is naturally described as phases: initialize, fill, heat, mix, inspect, transfer, clean and recover.

The current IEC 61131-3:2025 Edition 4 publication record describes Structured Text, Ladder Diagram and Function Block Diagram as the programming-language suite, with additional graphical and equivalent textual structuring elements for Sequential Function Chart. Older teaching material often calls SFC “one of five IEC languages.” That shorthand is not the current edition's formulation, and Instruction List has been removed from the current suite.

SFC does not eliminate the logic inside each phase. Step actions and transition conditions still need implementation, and vendors allow different action languages and editor behavior. SFC owns the coordination structure; reusable function blocks, ladder routines or Structured Text can own the detailed equipment behavior.

Decide whether the problem is actually sequential

SFC is strongest when operators and engineers already describe the process with named phases and allowed changes between them. A batch vessel has clear phases. A packaging machine has an index, clamp, process, inspect and discharge cycle. A utility plant has startup, warm-up, online, controlled shutdown and trip-recovery states.

Do not force every Boolean interlock into SFC. A motor seal-in, direct permissive chain or small three-state device may be clearer in ladder or a reusable equipment function block. Likewise, a mathematical algorithm remains a poor fit for boxes and transitions simply because SFC is available.

Convert the operating narrative into a state model first

Before opening the editor, write the normal phases, entry conditions, completion conditions, aborts, holds, timeouts and recovery decisions. Identify which operations may occur simultaneously and which must remain mutually exclusive. Ask operators what evidence distinguishes “waiting normally” from “stuck.”

Keep equipment behavior below the sequence layer

The SFC should command a valve module to open and wait for OpenProven; it should not duplicate the valve's travel timer, output arbitration and feedback alarm in every step. This separation makes the sequence readable and lets the equipment block be tested independently.

Build steps and transitions that explain themselves

A step name should describe a stable phase, not an output coil. FillToTarget, HeatSoak and AwaitDischargeClear are more informative than Step12. Give every step an explicit purpose, active actions, expected completion evidence, maximum useful dwell time and possible exit paths.

Write transition conditions as proof of readiness to leave

“Time elapsed” is rarely enough. A transition from Fill to Mix might require target mass reached, measurement valid, inlet valves proven closed and no active abort. Separate process completion from permission to continue when both matter.

Good transition design is:

• necessary: the next phase must not begin without it;
• sufficient: all required conditions are present, not just one convenient signal;
• stable: noise or a one-scan flicker will not create an unintended transition;
• observable: a technician can tell which term prevented progress;
• bounded: unreasonable dwell time creates a useful diagnostic or controlled response;
• owned: one clear part of the program determines the result.

Avoid opaque transition expressions

A single expression containing twenty tags may compile, but it is difficult to commission. Build named intermediate conditions such as FillComplete, OutletReady, QualityAccepted and AbortRequired, then expose the false terms in diagnostics. Names are not enough by themselves; each must have a documented meaning and test.

Understand SFC scan execution

SFC editors animate active steps, which can create the illusion that the diagram flows continuously. A PLC still evaluates behavior in cycles or tasks. The precise ordering of step actions, IEC actions, transitions, step activation and deactivation is platform-defined.

CODESYS documents an exact SFC processing sequence, including initialization, reset handling, action processing, transition evaluation and step activation. It also warns that the same action can execute multiple times in a cycle if it is associated through multiple active paths. Beckhoff TwinCAT documents similar editor and action considerations in its platform context. Rockwell provides a dedicated Logix 5000 SFC manual covering step, transition, action, pause and reset behavior.

Do not turn those examples into a universal IEC execution algorithm. The standard structures the language; the compiler and runtime documentation define the concrete behavior that your test must match.

Simple scan trace

Assume Fill is active, TargetReached changes from false to true, and the platform deactivates Fill and activates Mix according to its documented cycle semantics:

The table intentionally avoids pretending that all systems switch at the same point inside the scan. Run a small target-specific experiment with trace tags for active steps, transition values, action calls and physical outputs.

Entry, active and exit behavior are not interchangeable

An entry action typically executes once when a step becomes active, an active action while it remains active, and an exit action once as it deactivates. Exact support and timing vary. Use entry for one-time initialization only when repeating that initialization after pause, reset or online edit is understood. Use an active action for behavior that must remain serviced. Use exit for deliberate cleanup that will still run in every relevant abnormal path.

Design alternative and parallel branches correctly

Alternative and parallel structures solve different problems. Confusing them can start two mutually exclusive paths or wait forever for a path that was never activated.

Alternative branches need explicit priority

In an alternative branch, more than one transition may be true on the same evaluation. CODESYS documents left-to-right evaluation for its alternative branches. Other products may use a configured order or different rule. Design conditions to be mutually exclusive where practical and document priority where simultaneous truth is possible.

For an inspection result, prefer named decisions such as:

1. AbortRequired — highest priority;
2. InspectionInvalid — stop and request evidence;
3. PartRejected — divert;
4. PartAccepted — continue.

Do not place broad TRUE or timeout exits before more specific branches unless that priority is intentional and tested.

Parallel branches need synchronization and timeout policy

A parallel divergence activates several paths. A convergence should not proceed until the required paths reach their synchronization condition. Decide what happens if one branch completes early, one remains busy, one faults or an operator requests hold.

Worked SFC example: batch mixer

Consider a jacketed batch mixer with two ingredients, a heater, agitator and drain valve. The example is educational and omits the detailed safety instrumented functions, electrical design, recipe management and validated process controls required by a real plant.

Step and transition specification

The two ingredient feeds might run in parallel if the process design permits them, while heating may start only after a minimum level or may run in a third parallel branch. That choice belongs to the process hazard and quality design, not to a generic SFC tutorial.

Example transition definitions

PrecheckComplete := RecipeValid
    AND VesselAvailable
    AND Drain.ClosedProven
    AND Weight.Valid
    AND Temperature.Valid
    AND SequencePermit;

FillAComplete := FeedA.TargetReached
    AND FeedA.FlowStopped
    AND FeedA.OutletClosedProven;

HeatComplete := Temperature.InBand
    AND HeatSoakTimer.Q
    AND Temperature.Valid;

AbortRequired := EmergencyProcessCondition
    OR FeedA.UncontrolledFlow
    OR FeedB.UncontrolledFlow
    OR Temperature.HighHigh;

Names such as EmergencyProcessCondition are not a substitute for a safety design. A standard application SFC must not be treated as the sole risk-reduction layer where a safety function is required.

Commission the worked example in layers

First prove each equipment module with simulated or isolated feedback. Then prove each transition expression without changing physical outputs. Next run one path at a time, including timeout and invalid-instrument cases. Prove parallel completion and a fault in each branch. Finally run integrated recipes under the authorized commissioning plan and capture the actual sequence trace, alarms, operator messages and product acceptance results.

Action models and qualifiers require product documentation

SFC environments can associate actions with steps and offer qualifiers such as non-stored, set, reset, pulse, delayed or time-limited behavior. The exact qualifier set, notation, timing and interaction with entry/exit actions varies. CODESYS distinguishes IEC actions from step actions and documents how associations and qualifiers execute. TwinCAT provides entry, active, exit and action-association features. Schneider Machine Expert documents qualifiers, implicit variables and SFC flags.

Stored actions are easy to start and easy to forget to reset. For critical outputs, explicit equipment commands and a visible ownership model are often easier to audit than widely separated set/reset associations. Never allow several active steps to write the same physical command without a documented arbitration layer.

Hold, pause, abort, reset and restart are separate requirements

These words are often used interchangeably in operating conversations, but they imply different state changes.

• Hold preserves the production context while actions move to a defined hold response. Some actuators may stop; others may need to continue maintaining temperature, pressure or position.
• Pause may mean freezing chart advancement, but freezing software does not freeze physics. The platform's pause behavior must be understood.
• Abort deliberately leaves the normal path and coordinates equipment toward a controlled process state.
• Reset clears specified latched conditions or reinitializes chart state only after its prerequisites are met.
• Restart begins a new cycle or a validated recovery sequence. It should not blindly re-energize outputs from retained step bits.
• Resume continues from a preserved phase after physical-state reconciliation.

Reconcile physical state before software state

After power loss or a long hold, the vessel may have cooled, a pneumatic cylinder may have drifted, a valve may have moved manually, material may have settled and downstream equipment may no longer be ready. Retaining the active step does not prove that the phase can continue.

Build a recovery assessment using trustworthy feedback: actual positions, valid instruments, device health, remaining material, recipe identity, elapsed hold time and operator authorization. Route ambiguous conditions to a recovery mode rather than forcing the normal transition true.

Define action response for every interruption

Compare SFC with ST CASE and ladder step sequences

Many sequences can be implemented in more than one language. Choose the representation that makes state, transition priority, abnormal paths and maintenance evidence easiest to review.

Rockwell's September 2025 design considerations recommend SFC for high-level management, repetitive sequences, batch, motion sequencing and state machines in its Logix context. The same manual recommends ladder for maintenance-visible interlocking and FBD for continuous process/drive and loop control. That is a useful architecture prompt: use different languages for different responsibilities while keeping output ownership clear.

Vendor implementation map

Do not claim that “all major PLCs support SFC” without qualifying product, license and version. Support differs by controller family, engineering package and edition.

Siemens product families outside the supported GRAPH context, small controllers or limited software editions may not offer the same surface. Rockwell Micro800/Connected Components Workbench is not interchangeable with ControlLogix/Studio 5000. CODESYS-based products can restrict enabled languages. Select the controller and toolchain first, then confirm the exact SFC capability.

Test and troubleshoot an SFC

Test the chart as a state-transition system. For every step, prove normal completion, no-completion timeout, abort, hold, restart, invalid input and impossible-state detection. For every branch, prove each choice and simultaneous conditions. For every parallel region, fault and delay each path.

Build a transition coverage table

Diagnose the first false or unexpected boundary

If a chart is stuck, identify the active step or steps, elapsed dwell, next transition, each named term in that transition and equipment-module state. If it jumped unexpectedly, capture which competing transitions were true and the platform priority. If two steps are active unexpectedly, distinguish a valid parallel region from a corrupted or forced state.

Schneider documents that its SFC online editor does not provide the same breakpoint and stepping behavior as other IEC languages; it instead offers monitoring, writing/forcing and SFC flags subject to its rules. CODESYS exposes implicit variables such as step active time and error flags. Use these facilities carefully. Forcing step state can create uncontrolled chart combinations and must not become a casual production diagnostic technique.

Add impossible-state and dwell supervision

Detect mutually exclusive steps active together, no active step when one is required, parallel paths that never converge, and a step active beyond its credible duration. Diagnostics should identify the step, elapsed time and first missing transition term. A generic SequenceFault bit is not enough for a technician under outage pressure.

Safety and quality boundary

An SFC is an application organization method, not proof of functional safety or process safety. Emergency stop, guard monitoring, burner management, high-integrity shutdown and other required safety functions need the specified architecture, equipment, development lifecycle and validation. A standard chart can request a controlled stop and display status, but it must not be represented as the independent risk-reduction layer unless the complete safety design establishes that role.

Likewise, a sequence animation does not prove product quality. Batch genealogy, recipe authority, measurement validity, material reconciliation, electronic records and validated process requirements may need independent controls and evidence.

Frequently asked questions

What is a Sequential Function Chart in PLC programming?

SFC is a step-transition structuring method for PLC sequences. Active steps represent phases, actions implement behavior, and transitions determine when the chart changes active state. It is especially useful for batch, machine-cycle, startup, shutdown and other phase-oriented coordination.

Is SFC one of five IEC 61131-3 languages?

That is common older shorthand, but it is not the current IEC 61131-3:2025 Edition 4 formulation. The current publication record lists Structured Text, Ladder Diagram and Function Block Diagram as the suite and describes additional SFC structuring elements. Instruction List was removed from the current suite.

What is the difference between SFC and GRAFCET?

They share step-transition concepts but are governed by different standards and use contexts. GRAFCET is a specification language standardized in IEC 60848, while SFC is defined within IEC 61131-3 for programmable controllers. Do not assume every GRAFCET interpretation, action notation or evolution rule maps directly to a vendor SFC runtime.

When should I use SFC instead of ladder logic?

Use SFC when named phases, alternative paths, parallel paths and online sequence state are the clearest representation. Use ladder for short discrete sequences or maintenance-visible Boolean interlocks when the SFC structure would add little. The architecture can use both while keeping one owner for each output.

How are SFC transitions evaluated?

The preceding step configuration must be active and the transition condition must be satisfied, but the exact evaluation and activation order is platform-specific. Alternative-branch priority also differs. Confirm the runtime manual and verify a scan trace on the exact software/controller version.

What is the difference between an alternative and a parallel branch?

An alternative branch selects one path according to transition conditions and priority. A parallel branch activates multiple paths together and later synchronizes them. Tests must cover simultaneous alternative conditions and delayed or failed parallel paths.

What are SFC action qualifiers?

Qualifiers describe how an associated action behaves, such as while a step is active, stored until reset, pulsed, delayed or time limited. The available letters and exact timing differ by product. Use the target editor's documentation and test re-entry, reset and interruption behavior.

What should happen to an SFC after a power failure?

There is no safe universal answer. The design may initialize, retain selected context or enter recovery, but it must reconcile physical equipment and process state before commands resume. Verify positions, instruments, material, permissions and operator authority rather than trusting retained step state alone.

How do I troubleshoot an SFC stuck on one step?

Record active steps, dwell time, the next transition, every named term in that transition, equipment-block status and any timeout. Find the first false prerequisite or invalid signal. Avoid forcing the transition true until the underlying physical and logic conditions are understood.

Can an SFC replace safety logic?

Not by itself. A normal application SFC does not provide certified risk reduction merely because it includes abort or permissive transitions. Required safety functions need the appropriate safety architecture, components, programming environment, verification and validation.

Primary sources and further verification

This page owns sequence structure, SFC execution, branching and recovery intent. Use PLC function blocks and FBD for reusable equipment behavior and signal-flow logic, Structured Text programming for a text-based state-machine implementation, and the IEC 61131-3 standards pillar for the broader language model.