Page review information
Edition covered
OPC UA Parts 1/3/4/5/9/18; MQTT Sparkplug; NIST SP 800-82r3 — OPC UA parts retrieved 2026-07 (Phase 49a); NIST SP 800-82 Rev. 3
Content status
Reviewed
Coverage
Edge-inference rationale, transport semantics, write-authority separation, result contract, OT placement. Version-dependent details flagged verify-against-your-version.
Last reviewed
August 2026
Primary reference
The official published standard — required for final decisions

Confirm the governing edition, jurisdiction, and project-specific requirements before applying anything on this page.

Authority-first design

AI Interfaces & Handshakes

Analyse the fast signal where it is measured; send the verdict, not the waveform. Neither OPC UA nor Sparkplug assigns an AI result a standard meaning or authority — you define and test the contract, and the inference service stays publish-only.

This page is part of the AI & ML Integration section; read the authority gate first, and the Safety & Security Boundaries page for why the service stays read-only. Review pending.

The rule

Perform high-rate acquisition and inference at the source, then publish the lower-rate result with its class or estimate, confidence or uncertainty, model version, timestamp, quality, and freshness. OPC UA PubSub/TSN can support many architectures; the limitation below is the chosen acquisition path, not OPC UA as a whole.

flowchart LR
    subgraph BAD["✗ This acquisition path cannot reconstruct a kHz waveform"]
      direction LR
      V1["Vibration sensor
sampled at kHz"] --> P1["Conventional PLC scan
(ms-class)"] --> O1["Embedded OPC UA server
sampling floor · no knowledge of the
source's update logic, so the waveform
cannot be recovered by oversampling"] --> H1["Historian polling
+ compression by design"]
    end
    subgraph GOOD["✓ Acquire and infer where the signal is sampled"]
      direction LR
      V2["Vibration sensor
sampled at kHz"] --> EDGE["Edge acquisition
+ inference at the source"] --> R["Result contract
class · confidence · model version
timestamp · quality · freshness"] --> O2["OPC UA / Sparkplug
publish at process rate"] --> C2["PLC / SCADA /
historian"]
    end
    classDef bad fill:#faeeec,stroke:#a33327,color:#1e1e1e
    classDef good fill:#eef4ed,stroke:#3a6b35,color:#1e1e1e
    classDef neutral fill:#f2f2ef,stroke:#a0a09a,color:#1e1e1e
    class V1,P1,O1,H1 bad
    class EDGE,R,O2 good
    class V2,C2 neutral

Why the edge, in four constraints

“kHz signals cannot go through OPC UA” is false as a protocol claim — OPC UA PubSub/UADP over TSN has been measured at a 250 µs (4 kHz) cycle. The edge-inference conclusion rests on four different constraints:

  1. Server sampling floor. OPC UA Part 4 makes sampling best-effort (the server returns a revisedSamplingInterval it can meet); Part 5 defines MinSupportedSampleRate. Some embedded OPC UA servers impose a floor of tens to hundreds of milliseconds — orders of magnitude short of kHz. Verify the floor for your device.
  2. Decoupled source and aliasing — the decisive one. Part 4 states the server “has no knowledge of the data update logic” of the decoupled source. You cannot reconstruct a waveform by oversampling a tag that updates once per PLC scan; you alias it.
  3. The historian is a compressor, not a recorder. Deadband and swinging-door compression decimate by design; a pipeline reading archives gets decimated data.
  4. Segmentation. A kHz stream across the IT/OT boundary widens a conduit that NIST SP 800-82 Rev. 3 guidance says to keep narrow.

Neither transport gives an AI result a standard meaning

Define and test the application contract explicitly — the transport carries bytes, not semantics.

Transport What it does not give What you can build on
OPC UA No AI/ML companion specification; DataValue quality is a coarse StatusCode, not a confidence number Custom Structured DataTypes to define a ModelResult; Uncertain_* status for “model degraded”; SourceTimestamp for freshness; Part 9 Alarms & Conditions for an advisory needing human acknowledgement
MQTT Sparkplug B No standard AI-result meaning or authority Arbitrary per-metric properties (carry confidence / model version / freshness); birth/death certificates (NBIRTH/NDEATH) so a subscriber detects a dead publisher

(Companion specifications and payload conventions evolve — verify against the OPC UA companion-spec list and the Sparkplug version you deploy.)

Write-authority separation

The strongest, fully spec-backed part of the interface story. OPC UA separates read from write, and its well-known Observer role — browse / read / subscribe, no write — maps exactly onto an edge inference service that publishes results and must never write setpoints. Sparkplug B has no equivalent role construct: the same separation is a convention you enforce with broker ACLs and topic namespacing, not a guarantee you inherit.

So the inference service is publish-only with respect to control. It emits a result; a separate, verifiable layer — the envelope — decides whether to act on it.

The result contract

Publish a compact, self-describing result, not a raw stream:

{
  "result":        "bearing_outer_race_fault",
  "confidence":    0.87,
  "model_version": "vib-cnn 2.3.1",
  "timestamp_utc": "2026-07-16T09:41:07Z",
  "quality":       "GOOD",
  "freshness_ms":  250
}

Two optional fields strengthen it: a training-set identifier makes drift auditable, and an explicit authority tag lets the consumer enforce the register’s ceiling at the point of use. Publishing a compact result also crosses zone boundaries on terms a raw waveform stream does not.

The cybersecurity boundary

Place the AI service inside the existing OT security zones and conduits. NIST SP 800-82 Rev. 3 supports edge-local computation and read-only monitoring as a distinct, lower-impact class; keep the service read-only by default and least-privilege, and do not widen a conduit to carry a raw stream. (This page cites NIST SP 800-82r3 for OT placement; specific IEC 62443 clause numbers are not asserted — see the IEC 62443 page.)

Review status

Distilled from a corpus-owned Draft note and Review pending. Version-dependent details (server sampling floors, companion specifications, Sparkplug payload conventions) are flagged verify-against-your-version. Return to the authority gate, the method register, or Safety & Security Boundaries.

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