IEC 61499

Status (verified 2026-09)

• IEC 61499-1 and -2 are Ed.2.0 (2012-11-07); -4 is Ed.2.0 (2013-01-30). All three have stability date 2028. No Ed.3 has been published, so treat any claim of one as wrong.
• Eclipse 4diac: 3.0 was released 2025-12-10 and 3.3 on 2026-09-09. It now releases every three months, has a new ST editor and interpreter, and has all 61131-3 standard functions in FORTE.
• Schneider EcoStruxure Automation Expert is 61499-based and adds CATs (Composite Automation Types). A CAT bundles logic, HMI faceplate, I/O, simulation and documentation under a type/instance relation. Its current version was not verified; the newest source read was the v22.1 catalog from Jan 2023.

Execution model: why it is not a PLC scan

• FBs have event inputs and outputs as well as data. Data is sampled only for the variables tied to the arriving event by WITH, and only when that event arrives.
• A resource delivers one input event at a time to an FB. An input event is valid only on the first evaluation of the transitions. After that, only eventless transitions (1 or a pure guard) can fire, which prevents infinite loops.
• Transition syntax is EI_name[guard], a bare [guard], or 1. Evaluation follows the transitions' priority order.
• Runtimes differ. FORTE, FBRT, ISaGRAF (cyclic underneath) and others can run the same application differently. The consensus reading is sequential run-to-completion. Before relying on timing, state which runtime the application targets.

Design rules that prevent the usual bugs

1. Every data input you read in an algorithm must be WITH-associated with the event that triggered it. Otherwise you read a stale sample. This is the single most common 61499 bug, and validator rule FB005 checks the WITH targets exist.
2. Every output event must carry (WITH) the data it announces.
3. The ECC's first state is the initial state, conventionally START. Every other state needs an incoming transition, which rule FB016 checks.
4. Name algorithms after what they do, and keep them short. Put state logic in the ECC, not in IF ladders inside one algorithm.
5. Composite FBs are for reuse; subapplications are for distribution. An FB cannot be split across devices, but a subapplication can.
6. Use an adapter when a pair of FBs exchanges several events and data in both directions. The provider side is the plug, and the side that accepts it is the socket.

Minimal basic FB

This is trimmed from 4diac's E_SR.fbt. Modern 4diac writes no DOCTYPE and puts ST in CDATA.

<FBType Name="E_SR" Comment="Event-driven bistable">
  <Identification Standard="61499-1 Annex A"/>
  <VersionInfo Version="3.0" Author="…" Date="2025-04-14"/>
  <InterfaceList>
    <EventInputs><Event Name="S" Type="Event"/><Event Name="R" Type="Event"/></EventInputs>
    <EventOutputs><Event Name="EO" Type="Event"><With Var="Q"/></Event></EventOutputs>
    <OutputVars><VarDeclaration Name="Q" Type="BOOL"/></OutputVars>
  </InterfaceList>
  <BasicFB>
    <ECC>
      <ECState Name="START"/>
      <ECState Name="SET"><ECAction Algorithm="SET" Output="EO"/></ECState>
      <ECState Name="RESET"><ECAction Algorithm="RESET" Output="EO"/></ECState>
      <ECTransition Source="START" Destination="SET" Condition="S"/>
      <ECTransition Source="SET" Destination="RESET" Condition="R"/>
      <ECTransition Source="RESET" Destination="SET" Condition="S"/>
    </ECC>
    <Algorithm Name="SET"><ST><![CDATA[Q := TRUE;]]></ST></Algorithm>
    <Algorithm Name="RESET"><ST><![CDATA[Q := FALSE;]]></ST></Algorithm>
  </BasicFB>
</FBType>

• Older FBDK/Holobloc files carry <!DOCTYPE FBType SYSTEM "http://www.holobloc.com/xml/LibraryElement.dtd"> and write <ST Text="Q:=TRUE;"/>. Both forms are valid under the DTD.
• A composite FB has an <FBNetwork> with <FB Name Type>, <EventConnections>, <DataConnections> and <AdapterConnections>. Each <Connection Source Destination> names either an interface pin (START) or Instance.Pin.
• A system file (.sys) holds Application, Device → Resource, and <Mapping From="App.FB" To="Device.Resource"/>.
• 4diac file extensions: .fbt FB, .adp adapter, .sub subapp type, .sys system, .dev device, .res resource, .seg segment, .dtp data type, .atp attribute type, .fct function and .gcf global constants. The last two are 4diac extensions.

Validate with python3 ${CLAUDE_PLUGIN_ROOT}/scripts/plc_validate.py file.fbt, which runs rules FB0xx and lints the ST in each algorithm.

Porting 61131-3 logic to 61499

• ST algorithms and the elementary types carry over. VAR_TEMP exists since Ed.2.
• Scan-based idioms do not. An R_TRIG becomes an event, and a TON becomes an E_DELAY/E_CYCLE service FB driven by events.
• Each PROGRAM becomes a network. Split it into control-module FBs that fire on events, and map them to resources.
• Interlocks that must be evaluated continuously need a cyclic event source, such as E_CYCLE, or a resource that runs cyclically. Say which one you chose.

Sources

• IEC webstore: publications 5506, 5507 and 5508.
• Christensen et al., "The IEC 61499 Function Block Standard: Overview of the Second Edition" (2012): https://www.holobloc.com/papers/iec61499/61499_ED2_OVW.pdf
• Eclipse 4diac documentation and type library: https://eclipse.dev/4diac/