PackML, ISA-88 and OPC UA for PLC programmers

PackML state model (ISA-TR88.00.02-2022 is current)

How states are left:

• An acting state leaves on its own when its work is done: State Complete (SC).
• A wait state leaves only on a command.
• OPC 30050 adds the superstates 18 = Running and 19 = Cleared.

Command.CntrlCmd values: 1 Reset, 2 Start, 3 Stop, 4 Hold, 5 Unhold, 6 Suspend, 7 Unsuspend, 8 Abort, 9 Clear.

• Execute leaves on SC through Completing → Complete. A "Complete" command, and Hold from Suspended, exist only as vendor or OPC UA extensions.
• Unit modes: 1 Production (formerly "Producing"), 2 Maintenance, 3 Manual, and 4–31 user-defined. Each mode can disable states. Stopped, Execute and Aborted are mandatory in every mode (sources conflict on Aborted).
• PackTags:
  • Command: UnitMode, UnitModeChangeRequest, MachSpeed, CntrlCmd, CmdChangeRequest, Parameter[], Product[], …
  • Status: StateCurrent, UnitModeCurrent, CurMachSpeed, EquipmentInterlock, …
  • Admin: StopReason, Alarm[], Warning[], ProdProcessedCount[], ProdDefectiveCount[], …
• No PLCopen FB library exists. PLCopen has only a short paper that maps PackML to SFC. Use the vendor libraries instead:
  • Schneider: the Machine Expert PackML library (FB_UnitModeManager2).
  • Beckhoff: Tc3_PackML.
  • Rockwell: PhaseManager is ISA-88-style.

Implementation pattern (ST)

// One unit: the state machine owns transitions only; outputs derive from the state.
CASE eState OF
    E_PackML#Stopped:
        IF xCmdReset THEN eState := E_PackML#Resetting; END_IF;
    E_PackML#Resetting:
        IF xResetDone THEN eState := E_PackML#Idle; END_IF;                 // SC
    E_PackML#Idle:
        IF xCmdStart THEN eState := E_PackML#Starting; END_IF;
    E_PackML#Starting:
        IF xStartDone THEN eState := E_PackML#Execute; END_IF;              // SC
    E_PackML#Execute:
        IF xCmdHold THEN eState := E_PackML#Holding;
        ELSIF xCmdSuspend THEN eState := E_PackML#Suspending;
        ELSIF xBatchDone THEN eState := E_PackML#Completing; END_IF;
    // … Holding/Held/Unholding, Suspending/Suspended/Unsuspending, Completing/Complete …
    E_PackML#Clearing:
        IF xClearDone THEN eState := E_PackML#Stopped; END_IF;              // SC
END_CASE;
// Stop and Abort are global: evaluate them after the CASE so that they win.
IF xCmdAbort AND eState <> E_PackML#Aborting AND eState <> E_PackML#Aborted THEN
    eState := E_PackML#Aborting;
ELSIF xCmdStop AND eState <> E_PackML#Stopping AND eState <> E_PackML#Stopped
      AND eState <> E_PackML#Aborting AND eState <> E_PackML#Aborted
      AND eState <> E_PackML#Clearing THEN
    eState := E_PackML#Stopping;
END_IF;

Record the first stop or abort cause in Admin.StopReason. Mode-dependent disabled states and the remaining branches are omitted here.

ISA-88 / IEC 61512

• IEC 61512-1:2026 (Ed.2.0, 2026-02-20) replaced the 1997 edition. It adds UML state diagrams, an expanded procedural state model (Annex B) and conformance clauses. The ISA documents are 88.00.01-2010, .02-2001, .03-2003 and .04-2006.
• Physical model: Enterprise → Site → Area → Process cell → Unit → Equipment module → Control module. Levels may be omitted, except the Unit.
• Procedural model: Procedure → Unit procedure → Operation → Phase.
• Recipes: general → site → master → control.
• Phase states (classic example model): Idle, Running, Complete, Pausing, Paused, Holding, Held, Restarting, Stopping, Stopped, Aborting, Aborted. The commands are start, stop, hold, restart, abort, reset, pause and resume. There is no Resetting state and no "complete" command; phases self-complete. The exact 2010 and 2026 lists are paywalled, so verify them before claiming compliance.
• How it maps to code:
  • Control module: one FB or AOI instance per device (valve, motor, transmitter), with modes (Auto/Manual/Out-of-service), interlocks and alarms.
  • Equipment module: coordinates several control modules for a minor processing activity.
  • Phase: a state machine with one routine or action per acting state.
  • ISA-88 does not link procedural elements to control modules directly.

OPC UA

• OPC 30000 v1.02 (2020-11-25), the PLCopen information model:
  • Namespace: http://PLCopen.org/OpcUa/IEC61131-3/.
  • Types:
    • CtrlConfigurationType (derives from TopologyElementType)
    • CtrlResourceType (derives from DeviceType)
    • CtrlProgramType and CtrlFunctionBlockType (both under CtrlProgramOrganizationUnitType)
    • CtrlTaskType (Priority, Interval, Single)
    • SFCType
  • Variables hang off HasInputVar, HasOutputVar, HasLocalVar and related references.
• OPC 30001 v1.02 (2023-11-07), the client FBs:
  • Current blocks: UA_Connect, UA_NamespaceGetIndexList, UA_NodeGetHandleList, UA_ReadList/UA_WriteList, UA_MethodCall, subscriptions and monitored items, UA_NodeReleaseHandleList and UA_Disconnect.
  • Every block follows the Execute/Done/Busy/Error/ErrorID pattern, with a Timeout.
  • The single-item UA_Read, UA_Write and UA_NodeGetHandle are phased out.
  • Recommended: WSTRING.
  • Sequence: connect → get namespace index → get handles → read/write loop → release handles → disconnect. Release the handles on error paths too.
• OPC 30050 v1.01 (2020-11-11) is the PackML companion specification.
• OPC UA FX: the C2C maintenance release V1.00.04 came out 2026-07-27. Controller to device was still at release-candidate stage in June 2026, so do not promise it.

Exposing PLC variables

Design rules:

• Expose a curated interface struct per unit, not every internal variable.
• Make it read-only by default. Every write path is a command that the PLC validates (range, state and permission) before acting.
• Never let OPC UA write directly to an output or setpoint that bypasses interlocks.

Sources

• ISA-88 standards list: https://www.isa.org/standards-and-publications/isa-standards/isa-88-standards
• IEC 61512-1:2026: https://webstore.iec.ch/en/publication/75287
• OPC references: https://reference.opcfoundation.org/PLCopen/v102/docs/, https://reference.opcfoundation.org/PLCopen-CFB/v102/docs/, https://reference.opcfoundation.org/PackML/v101/docs/
• Schneider EcoStruxure Machine Expert PackML Library Guide, EIO0000002809.03 (12/2025).
• Beckhoff TF6100 pragma documentation: https://infosys.beckhoff.com/content/1033/tf6100_tc3_opcua_server/15563857163.html