Inside NAMUR Open Architecture: Unlocking Field Data with a Second Channel

flowchart LR
  A["Field Devices"]:::blue --> B["Second Channel"]:::green
  B --> C["NOA Gateway"]:::red
  C --> D["M+O Domain"]:::blue
  style A fill:#2563eb,color:#ffffff
  style B fill:#16a34a,color:#ffffff
  style C fill:#dc2626,color:#ffffff
  style D fill:#2563eb,color:#ffffff

Process plants generate vast amounts of diagnostic and secondary process data at the field level. Most of this data never leaves the instrument. HART-enabled devices, for example, transmit up to 35 additional variables beyond the primary process value, yet legacy DCS architectures typically ignore them. NAMUR Open Architecture (NOA) provides a standardized method to access that stranded data through a dedicated second communication channel, without modifying or risking the availability of the core control system.

What Problem Does NOA Solve?

Traditional process automation follows the ISA-95 hierarchy: field instruments communicate upward through controllers, which feed SCADA and MES layers. This architecture was designed for deterministic control, not for data analytics. The result is a structural bottleneck: diagnostic data from smart instruments remains locked inside the control loop, invisible to maintenance and optimization systems.

NAMUR, the international user association for automation technology in process industries (with over 150 member companies), published the NOA concept to solve this problem. The core idea is straightforward: create a second communication channel that runs alongside the existing control infrastructure, extracting data for monitoring purposes while guaranteeing zero interference with the primary control loop.

NOA Architecture: CPC vs. M+O Domains

The NAMUR Open Architecture divides the plant into two distinct operational domains. Understanding this separation is the foundation of any NOA implementation.

flowchart TD
  subgraph cpc_domain ["Core Process Control (CPC)"]
    direction TB
    A["Field Instruments"]:::blue --> B["Controllers / DCS"]:::blue
    B --> C["SCADA / HMI"]:::blue
  end
  subgraph noa_channel ["NOA Second Channel"]
    direction TB
    D["HART Gateway / Edge Device"]:::green --> E["OPC UA Server"]:::green
    E --> F["NOA Security Gateway"]:::red
  end
  subgraph mo_domain ["Monitoring + Optimization (M+O)"]
    direction TB
    G["Asset Management"]:::purple --> H["Predictive Analytics"]:::purple
    H --> I["Cloud / ERP"]:::purple
  end
  A -.->|Side Channel| D
  F --> G
  style A fill:#2563eb,color:#ffffff
  style B fill:#2563eb,color:#ffffff
  style C fill:#2563eb,color:#ffffff
  style D fill:#16a34a,color:#ffffff
  style E fill:#16a34a,color:#ffffff
  style F fill:#dc2626,color:#ffffff
  style G fill:#7c3aed,color:#ffffff
  style H fill:#7c3aed,color:#ffffff
  style I fill:#7c3aed,color:#ffffff

Core Process Control (CPC)

The CPC domain encompasses the existing automation pyramid: field instruments, PLCs or DCS controllers, and SCADA/HMI systems. NOA treats this domain as inviolable. No NOA component is permitted to alter control logic, inject commands, or degrade the real-time performance of this layer. This principle of non-interference is what makes NOA acceptable to plant operators and safety engineers.

Monitoring and Optimization (M+O)

The M+O domain handles everything outside of real-time control: condition monitoring, predictive maintenance, energy optimization, and regulatory reporting. This domain consumes data from the second channel but has no direct write access to the CPC domain. It can be deployed on-premises (edge servers) or off-premises (cloud platforms), depending on security policy and latency requirements.

The NAMUR Recommendation Stack

NOA is not a single specification. It is a layered set of NAMUR Recommendations (NE), each addressing a specific architectural concern. The table below summarizes the four core documents.

Key Technology Building Blocks

OPC UA as the Transport Layer

OPC UA (IEC 62541) serves as the primary communication protocol for the NOA second channel. It provides built-in security (authentication, encryption, access control), a platform-independent architecture, and support for both client-server and publish-subscribe communication patterns. The publish-subscribe model is particularly relevant for high-volume diagnostic data streaming from multiple field devices to centralized analytics platforms.

PA-DIM: The Semantic Data Model

The Process Automation Device Information Model (PA-DIM) is an OPC UA Companion Specification managed by the FieldComm Group in collaboration with NAMUR, the OPC Foundation, and PROFIBUS & PROFINET International. PA-DIM standardizes how field device data (manufacturer, device type, diagnostics, calibration status) is represented, regardless of the underlying fieldbus protocol. This means a pressure transmitter from Endress+Hauser and one from Emerson will expose their diagnostic data in the same semantic structure when accessed through a PA-DIM-compliant gateway.

Ethernet-APL for Greenfield Installations

For new plant construction, Ethernet-APL (Advanced Physical Layer, IEEE 802.3cg 10BASE-T1L) provides a native Ethernet connection directly to field instruments. This eliminates the need for protocol gateways entirely: instruments can run OPC UA natively over a two-wire, intrinsically safe, 1 km cable run. When combined with NOA, Ethernet-APL transforms the second channel from a retrofit add-on into a built-in architectural capability, delivering bandwidths up to 10 Mbit/s to Zone 0 hazardous areas.

Brownfield Deployment: Extracting Data from Existing Plants

The majority of NOA deployments today target brownfield installations, where thousands of HART-enabled instruments are already installed but their diagnostic data is inaccessible. The deployment follows a structured approach:

Step 1: Identify the HART Side Channel

Many modern signal isolators and safety barriers provide a secondary HART port. This port allows a second HART master to communicate with the field device without interfering with the primary master (the DCS). If your barriers do not support this, HART-transparent multiplexers can be installed in the marshalling cabinet.

Step 2: Deploy an Edge Gateway

An industrial edge device (such as a Beckhoff embedded PC with EtherCAT HART terminals, or a Softing smartLink gateway) connects to the HART side channel. The gateway polls each instrument for secondary variables, device diagnostics, and calibration data, then converts this information into OPC UA using the PA-DIM information model.

Step 3: Enforce the Security Boundary

The NOA Security Gateway (as specified in NE 177) sits between the edge device and the M+O network. It enforces unidirectional data flow, functioning as a data diode. No traffic from the M+O domain can reach the CPC domain through this path. This aligns with IEC 62443 zone and conduit segmentation principles, ensuring that the side channel does not introduce a new attack surface.

Step 4: Consume Data in the M+O Domain

Once data reaches the M+O domain via OPC UA, it can be consumed by asset management software, predictive maintenance platforms, or cloud-based analytics. Because the data follows the PA-DIM model, integration is vendor-neutral: any OPC UA client that understands PA-DIM can interpret the data without custom mapping.

Verification of Request (VoR): Controlled Write-Back

NOA is primarily a read-only architecture. However, NE 178 acknowledges that some optimization scenarios require feedback to the control system, such as adjusting a PID setpoint based on analytics. The Verification of Request mechanism provides a secure, multi-step process for this:

1. Request Submission: The M+O application submits a change request through the VoR interface.
2. Validation: The request is checked for syntactic and semantic correctness.
3. Authentication: The identity and authorization level of the requester are verified.
4. Authorization: A human operator or automated policy engine approves or rejects the request.
5. Execution: Only after passing all checks does the request reach the CPC domain for execution.

This multi-barrier approach ensures that the NOA principle of non-interference is maintained even when write-back is permitted.

NOA vs. Traditional SCADA Integration

Frequently Asked Questions

Does NOA replace my existing DCS or PLC system?

No. NOA is explicitly designed to work alongside existing control systems. The Core Process Control domain remains untouched. NOA adds a parallel data path for monitoring and optimization without modifying any control logic or wiring.

What is the difference between NOA and the Purdue Model?

The Purdue Model (ISA-95) defines hierarchical levels for manufacturing operations. NOA does not replace this hierarchy; instead, it adds a lateral data channel that bypasses it. Data flows from the field level directly to the M+O domain without traversing every intermediate layer of the Purdue Model.

Can I implement NOA without Ethernet-APL?

Yes. Brownfield NOA deployments rely on existing 4-20 mA HART wiring with side-channel gateways. Ethernet-APL is the preferred physical layer for greenfield installations, but it is not a prerequisite for NOA.

How does NOA handle cybersecurity?

NE 177 defines three security zones (CPC, on-premises M+O, off-premises M+O) and mandates a Security Gateway between them. The gateway enforces unidirectional data flow and aligns with IEC 62443 zone and conduit segmentation requirements. The Verification of Request (NE 178) adds controlled, authenticated write-back for scenarios that require it.

Which vendors support NOA today?

Major process automation vendors including Beckhoff, Softing, Endress+Hauser, Pepperl+Fuchs, Siemens, and ABB offer NOA-compatible gateways, edge devices, or field instruments. The FieldComm Group and PROFIBUS & PROFINET International maintain the PA-DIM specification used for semantic data interoperability.

For a deeper understanding of the physical layer enabling NOA in new installations, read our guide on Ethernet-APL for Process Automation. To explore the OPC UA protocol stack that underpins the second channel, see our technical article on OPC UA over TSN architecture.

NAMUR publishes the full NOA recommendation series through their official NOA documentation portal. The PA-DIM specification is maintained by the PROFIBUS & PROFINET International organization in collaboration with the OPC Foundation and FieldComm Group.

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