Sequential Function Chart (SFC)

About sequential function charts

Sequential Function Chart (SFC) is the state-machine view of a cyclic PLC program — what's happening now and what triggers the next phase. It's the fifth language defined by IEC 61131-3:2013 §6.5 and the engineering subset of IEC 60848 GRAFCET. Where ladder and fbd describe per-scan combinational logic, SFC describes the temporal sequencing across them: which step is active, when it hands off, what runs in parallel.

In production code, ~10% of networks are written in SFC, but ~100% of nontrivial sequential machines have at least one SFC chart — batch reactors, robotic cells, packaging lines, assembly stations. Until now there was no good open-source SFC DSL: vendor IDEs (Studio 5000, TIA Portal, CODESYS) were the only option. Schematex ships an IEC 61131-3 SFC subset designed for AI generation.

Distinct from state (UML statechart for reactive UIs and lifecycle FSMs): SFC has cyclic-scan semantics, double-bordered initial steps, action-block qualifiers (N/S/R/L/D/P), and bar-based branches (single bar = OR, double bar = AND).

Unrecognized SFC line: N FillValve_Closed

1. Your first chart

Two steps, one transition, an initial marker:

sfc
step S0 [initial]
step S1
transition from: S0 to: S1: Trigger

S0 renders as a double-bordered rectangle (the IEC initial-step convention); S1 as a single-border rectangle. Between them is a horizontal transition bar with the condition text Trigger to its right.

If you forget [initial], the first declared step is auto-promoted to initial.

2. Steps

step S_Filling [label: "Filling tank"]
  N FillValve_Open
  D Mixer_Run T#30s
  P StartChime

A step has:

• An id (unique across the chart) — used in transitions and jumps.
• An optional [label: "..."] for display.
• An optional [initial] (one allowed) or [final] (vendor stop step, three borders).
• Zero or more action blocks, indented one level, each with a qualifier letter.

3. Transitions

A transition declares a directed link between two steps with a boolean condition:

transition from: S0 to: S1: StartBtn
transition from: S1 to: S2: TankLevel >= 80.0 AND NOT EmergencyStop
transition T_Reset from: S5 to: S0: ResetBtn

The condition text is opaque — Schematex stores it verbatim and renders it next to the bar. Every transition must have a non-empty condition; use TRUE for unconditional links.

Transitions whose from and to are linearly adjacent in the body render as inline bars between the steps. Transitions whose pair is not linearly adjacent (e.g. a jump back to an earlier step) render as margin arrows on the left or right side of the chart.

4. Action qualifiers

Actions attach to the right side of a step and run according to their qualifier letter:

Time-parameterized qualifiers (L, D, SD, DS, SL) take a duration literal:

step S1
  L LimitedRun T#5s
  D DelayedRun T#2s

5. Alternative branches (single bar — OR)

Only one branch fires per scan, picked by transition condition:

step S0 [initial]
step S_Pick

alt from: S_Pick:
  branch [priority: 1]:
    transition: IsExpressShipping
    step S_Express
      N PrepExpressBox
    transition: TRUE
  branch [priority: 2]:
    transition: IsStandardShipping
    step S_Standard
      N PrepStandardBox
    transition: TRUE
merge_to: S_Ship

step S_Ship

transition from: S0 to: S_Pick: ProductOrdered
transition from: S_Ship to: S0: Shipped

The single horizontal lines above and below the branches are the divergence and convergence bars. Each branch starts with its entry transition (between div bar and first step) and ends with an exit transition (between last step and conv bar).

6. Simultaneous branches (double bar — AND)

All branches run concurrently; the chart waits at the convergence until every branch finishes:

sim from: S_Heat: TRUE
  branch:
    step S_Bake
      D Oven_Run T#15m
  branch:
    step S_Cool
      L Cooler_On T#5m
merge_to: S_Done: Bake_Done AND Cool_Done

The two parallel horizontal lines (gap 4px) above and below the branches are the simultaneous bars. The shared transition above (TRUE here) triggers the divergence; the shared transition below (Bake_Done AND Cool_Done) is checked before convergence fires.

7. Jumps (loops)

A transition whose target is an earlier step renders as a margin arrow:

step S0 [initial]
step S1
step S2
transition from: S0 to: S1: A
transition from: S1 to: S2: B
transition T_Reset from: S2 to: S0: ResetBtn
transition from: S2 to: S1: NOT ResetBtn

The forward S2 → S1 (back-edge) gets the margin arrow on the right; the T_Reset jump back to S0 goes on the left. Each margin arrow shows its target id and condition.

8. Variables

Reused from ladder and fbd:

var StartBtn: bool
var TankLevel: real
var BakeReady: bool
var Counter: counter
var T1: timer

Variables declared in conditions and actions are not validated — Schematex treats condition / action body text as opaque strings, matching how state handles guards and actions.

9. v0.1 limitations

• Nested branches (alt-in-sim, sim-in-alt) parse but layout collapse heuristics are basic; deep nests may overlap.
• S/R action-pair dashed connectors (visually link an S action with its matching R elsewhere) are deferred.
• Active-step runtime indicator (yellow fill on the currently active step) is deferred — useful for debugging integrations that surface PLC runtime state.
• GRAFCET forcing orders (out-of-scope per IEC 60848-only feature).
• Final step parses with [final] but renders with the same double border as initial; the IEC triple-border convention is deferred.