PLC State Transfer Diagrams in 2026: SFC, IEC 61131-3, and Grafcet Revisited
Share
AI Summary
- SFC and state transfer diagrams are the same family under IEC 61131-3 Edition 4, reaffirmed in 2024 and still in force through 2026.
- The classic conveyor example maps cleanly to a 3-step SFC (Idle → Start → Running), with sensor-driven divergence now standard in 2026 vendor tools.
- Vendor dialects in 2026: Siemens GRAPH on S7-1500, Mitsubishi GX Works3 FX SFC, Allen-Bradley Studio 5000 via the Sequential Function Chart add-in / PhaseManager.
- Grafcet (the 1980s French predecessor) is still taught in EU curricula but rarely shipped in new NA builds; IEC 61131-3 SFC is the modern spec.
- For troubleshooting in 2026, the active SFC step number is exposed as a runtime tag, so SCADA / HMI can show the stuck step without ladder tracing.
A state transfer diagram is the IEC 61131-3 SFC model in visual form: steps, transitions, and directed arcs. In 2026 every major PLC still ships an SFC dialect, but vendor editors diverge sharply.
Allen-Bradley · Siemens · Mitsubishi · Engineering Notes · 2026-06-30
By the KOEED engineering desk · 2026-06-30 · 8 min read · Engineering Notes
The state transfer diagram method is still the fastest way to specify and read a sequential control problem in 2026. Re-stated in modern terms, it is the graphical front end of the Sequential Function Chart (SFC) language defined by IEC 61131-3 — and every major PLC vendor still ships a flavor of it.
> TL;DR
- State transfer diagrams and SFC describe the same thing: step → transition → step with directed arcs.
- IEC 61131-3 Edition 4 (reaffirmed 2024) is the current spec; vendor dialects differ in step addressing and the SFC-to-ladder bridge.
- In 2026 every SFC editor can export the active step number as a runtime tag — no more ladder hunting on a fault.
What is the state transfer diagram method?
A state transfer diagram is a directed graph of steps (the boxes), transitions (the horizontal bars that guard the move), and directed arcs (the arrows). The PLC activates exactly one initial step at power-up. As the program scans, each transition is evaluated; when a transition's Boolean condition becomes true, the active step is cleared and the step downstream of the transition becomes active. Actions attached to a step run while the step is active, and IEC 61131-3 distinguishes pulse actions, level actions, and stored actions so the same step can drive latched outputs without extra ladder.
In practice the diagram is also the documentation. Maintenance technicians who are new to the line can read the chart on the cabinet door and find the active step in seconds, which is the reason the method has survived four decades of programming-language churn.
What changed since 2024
The 2024 article that originally accompanied this page predated the IEC 61131-3 Edition 4 reaffirmation and the wide availability of vendor SFC add-ins. The practical changes a maintenance engineer will see in 2026:
- IEC 61131-3 Edition 4 (reaffirmed 2024) is the active spec. The SFC language element set is unchanged but the formal semantics around simultaneous and alternative branches are tightened, which removes the "phantom step" bug seen on some 2018-vintage Mitsubishi projects.
-
Siemens GRAPH on S7-1500 is now the default SFC editor; TIA Portal V19+ (2025) exposes the active step as a system tag (
SFC_ACTIVE_STEP) that the HMI can read directly. - Mitsubishi GX Works3 ships the FX SFC dialect for FX5U / FX5UC and the structured-text SFC for iQ-R. The FX3U SFC is now legacy; for greenfield FX work move to FX5U SFC.
- Allen-Bradley Studio 5000 does not bundle SFC. The third-party Sequential Function Chart add-in (PhaseManager successor) targets ControlLogix 5380 / 5580 and exports the step number as a tag.
- Schneider EcoStruxure Control Expert (M340 / M580) supports SFC as a first-class language with the same step-tag visibility.
IEC 61131-3 SFC vs. Grafcet vs. vendor dialects (2026)
The terms are often used interchangeably in field documentation. They are not identical:
| Method | Origin | 2026 status | Notes |
|---|---|---|---|
| State transfer diagram | Generic term, 1980s Chinese PLC textbooks | Informal | Conceptually identical to SFC; useful for teaching. |
| Grafcet | French AFCET, 1977; precursor to IEC 848 / IEC 61131-3 SFC | Taught in EU, declining in NA new builds | No stored actions, no SFC-to-ladder bridge. |
| IEC 61131-3 SFC | IEC, 1993; current Edition 4 (reaffirmed 2024) | Active international standard | Steps, transitions, directed arcs, actions, qualifiers (N / P / S / R / D / L / SD / DS). |
| Siemens GRAPH | Siemens, 1995 | Active; default SFC on S7-1500 (TIA V19+) | Adds chain view, interlock / supervise, step comments. |
| Mitsubishi FX SFC / iQ-R SFC | Mitsubishi, 1990s | Active; FX3U SFC moved to legacy in 2024 | Block-level SFC, transition programs in ladder / ST. |
| AB Sequential Function Chart add-in | Rockwell third-party, 2022 (PhaseManager successor) | Active | Targets L8x ControlLogix 5380 / 5580. |
Walkthrough: the conveyor example, modernized
The classic 3-state conveyor (Idle → Start → Running) from the 2024 article still serves as the smallest useful example. In 2026 the same machine diagram is usually drawn inside the vendor's SFC editor instead of on paper, and the actions live as IEC step qualifiers:
-
Step S0 (Idle) — action
Motor N 0(no action, motor off). -
Transition t1 —
Start_PB AND NOT E_Stopadvances the chart to S1. -
Step S1 (Start) — action
Motor N 1, qualifierL 2s(motor on, time-limited). -
Transition t2 —
TON S1_Timer.Q OR Object_Sensoradvances to S2. -
Step S2 (Running) — action
Motor N 1; on Stop_PB or sensor trip, transition t3 returns to S0.
In 2026 vendor editors the SFC chart is a first-class tag in the project tree. To display the active step on an HMI, point a numeric display at the runtime tag for the chart; no ladder tracing required.
Note
The 2024 article suggested exporting the SFC to PDF by printing from the editor. In 2026 every major vendor exports the SFC chart as SVG or as a structured XML so it round-trips into a CMMS / asset record. Keep the export next to the program backup.
2026 troubleshooting: read the active step, not the ladder
When a sequence stalls, the fastest diagnosis in 2026 is to read the active step tag from the HMI / webserver, then check the transition guarding the next step. The active step is a single integer in vendor dialects and a structured SFCSymbol in TIA Portal. If the chart is stuck on step S1, the issue is almost always a transition that never becomes true — a missing interlock, a latched fault, or a sensor that is not wired to the expected input.
For deeper diagnostics, cross-check the active step against the PLC Error Code Database on KOEED, or paste the runtime tag dump into the AI Diagnostic Tool for a vendor-specific reading.
> Tip
When commissioning an SFC chart, capture a one-line printout of the active step tag at every transition during the dry run. Most "phantom" sequence faults in 2026 are explained by a transition that fires once on the very first scan because the engineer forgot to qualify it with the initial-step flag.
Watch out: where state transfer diagrams go wrong
Two failure modes still show up in 2026 service tickets:
- Unreachable steps. A transition is gated by a condition that can never be true (e.g. two normally-closed contacts in series). The SFC silently parks on the previous step. The fix is to draw the transition condition as a Boolean ladder stub and prove it true on the bench.
- Unbounded loops. A divergence that returns to the same step without a qualifier or a counter runs forever and starves the rest of the scan. Vendor editors flag this in offline analysis in 2026, but legacy FX3U SFC charts do not always warn.
! Warning
Do not hot-swap a S7-1500 CPU with the SFC chart open in TIA Portal — the active step pointer is a retained tag, but the on-line / off-line buffer can be out of date. Always do a stop / download / restart cycle, and back up the project as .zap17 first.
Related on KOEED
- Engineering Notes — full archive on the KOEED blog
- Allen-Bradley ControlLogix & CompactLogix modules (1756 / 1769 / 5069)
- Siemens SIMATIC S7-1200 / S7-1500 / ET200 spare parts
- Mitsubishi MELSEC iQ-R / FX5U / MR-J5 modules
- PLC Error Code Database — Siemens / Mitsubishi / Omron fault lookup
- PLC Analog Calculator — 4-20 mA / 0-10 V scaling utility
- AI Diagnostic Tool — 24/7 fault and obsolete-part triage
Frequently asked questions
What is the difference between a state transfer diagram and SFC?
A state transfer diagram is the generic, vendor-neutral name for the graphical form — boxes (steps) connected by directed arcs, with transition guards. SFC (Sequential Function Chart) is the same diagram made a programming language by IEC 61131-3. In 2026 the two terms are used interchangeably in field documentation.
Does Allen-Bradley ControlLogix support SFC in 2026?
ControlLogix does not bundle SFC in Studio 5000. The third-party Sequential Function Chart add-in (released 2022 as the PhaseManager successor) targets L8x ControlLogix 5380 and 5580 controllers and exports the active step as a tag. For legacy PhaseManager, KOEED still stocks the 1756 modules needed for in-place repair.
Is Grafcet still used in 2026?
Yes, mostly in EU university curricula and on long-lived French / Spanish lines. Grafcet lacks the IEC 61131-3 action qualifiers and the SFC-to-ladder bridge, so new builds in 2026 normally pick IEC SFC unless the customer is locked to a Grafcet-based tool. KOEED can source both.
How do I expose the active SFC step to an HMI?
Every major vendor publishes the active step as a runtime tag in 2026: Siemens SFC_ACTIVE_STEP on S7-1500, Mitsubishi SFC_STEP on iQ-R / FX5U, Schneider %MW_active on M580. Point an HMI numeric display at the tag; no ladder code needed.
When should I avoid state transfer diagrams?
Skip SFC for continuous-control loops (PID, motion), for high-speed event chains that need sub-scan resolution, and for processes with more than ~200 steps. The chart becomes unreadable and vendor editors slow down. Use structured text or ladder for those cases.
Working on a step-sequence retrofit?
Send your BOM to Moritta@KOEED.COM. Active stock, EOL stock, and cross-references — all in one quote within 24 hours.
Send My BOM →The KOEED engineering desk
Industrial automation editors at KOEED. Writes about PLC sourcing, cross-reference, and legacy system support. Reach the team at Moritta@KOEED.COM.