Technical Article

Understanding the OPC Unified Architecture (OPC UA) Protocol

April 02, 2023 by Anthony King Ho

Learn about the history, framework, and current uses of the Open Platform Communications United Architecture (OPC UA) communication protocol in industrial automation.

What Is OPC UA?

The Open Platform Communications United Architecture (OPC UA) is a data exchange standard used in industrial automation and communication. OPC UA is an independent standard with no relation to a specific system or manufacturer. It communicates by PC-to-machine communication or machine-to-machine communication. 

Back in 1994, a group of software and hardware vendors in the industrial automation sector and other engineering disciplines formed what is now known as the OPC Foundation

The OPC Foundation set out to develop a single client/server specification that would allow any vendor to develop software and applications that share data in a fast and robust way and, at the same time, eliminate the proprietary schemes that made these vendors duplicate their development efforts.

As a result, the OPC Foundation developed the first specification for OPC DA, Data Access Specification 1.0a. This was released shortly after, in early 1996. The Data Access Specification defines how both the client and the server application interface must be constructed. Using this specification, vendors could quickly develop client/server software.


Open Platform Communications United Architecture (OPC UA) is a data exchange standard used in communication and industrial automation

Figure 1. OPC UA is a data exchange standard used in communication and industrial automation. Image used courtesy of ABB


How Does OPC UA Work?

However, since OPC DA relies heavily on Windows Distributed Component Object Model (DCOM), many vendors recognize that OPC DA is not a truly open standard, behaves poorly in a disconnected state, does not work well with firewalls, and only works in Windows.

To overcome the drawbacks of OPC DA, the OPC Foundation developed OPC UA, which was significantly different from its predecessor. The goal was to move away from mainly using Windows DCOM to meet industrial automation's evolving needs better.

The first specification of OPC UA was published in 2006, and the latest version, 1.04, was released in November 2017, adding publish/subscribe communications infrastructure and new security policies.

Some of the improvements that were introduced to OPC UA include:

  • Open—available for anyone to use and implement under the GPL 2.0 license.
  • Cross-platform—it was not tied to one single operating system or programming language.
  • Increased protocol security—provides users access to authentication, authorization, integrity, and confidentiality.
  • Introduction of Method, which represents the function call of an object—the method is called (invoked) and returns after the function is completed, whether it is successful or not.
  • Integration of the Information Model in IEC 62541—this specification is the basis of the infrastructure necessary for vendors to integrate their information and model their complex data into an OPC UA namespace. It takes advantage of the rich service-oriented architecture of OPC UA.


OPC UA Framework (IEC 62541)

The field extensions specified by the Field Level Communication (FLC) initiative are based on the OPC UA Framework (IEC 62541). This framework provides vendors with an independent platform that allows secure and reliable information exchange.


OPC United Architecture Protocol FLC system architecture

Figure 2. OPC UA FLC system architecture (click to enlarge). Image used courtesy of OPC Foundation


The OPC UA framework supports the client/server services and protocols and publish/subscribe (PubSub) models and protocols. OPC UA can run on a dedicated client and server relationship. In the PubSub scenario, the server sends (publishes) data to the network, and the client (who subscribed) will receive the data.

It is important to note that in the OPC UA specification, authentication, signing, and encryption of data are heavily emphasized for both the client/server and PubSub models.


The Role of OPC UA in Industrial Automation

Besides being a machine-to-machine communication protocol for industrial automation, OPC UA is also a perfect candidate to connect the machine and business networks. OPC UA not only transfers machine information such as set-points, measured values, and process parameters, but it also defines and describes the data. This is done through mappings in the OPC UA specification.

With the OPC UA information model, new processes between a PLC and any higher-level, business-oriented software layer can be established very efficiently.


Figure 3. OPC UA in the automation pyramid. Image used courtesy of OPC Connect


In an industrial process, set points and control variables can be easily and centrally maintained and controlled as part of the material master data. Even customer order-specific information can be directly exchanged with the PLC instead of replicating the data across different software layers.

Furthermore, providing measurement and process details as enhancement of business documents for comprehensive analytics is also an easy task since the connectivity is standardized.

With Industry 4.0, the separation of the levels and the top-down approach of the information flow started to mix, which means that in an intelligent network, each device or service can autonomously initiate communication with other services.

PLCopen (an association of IEC 61131-3-based controller manufacturers) has collaborated with the OPC Foundation to define corresponding OPC UA client function blocks. It created a way for PLC to exchange complex data structures horizontally with other controllers or vertically through an OPC UA server in a Manufacturing Execution System (MES) or Enterprise Resource Planning (ERP) system to retrieve new production orders or write data to the cloud. The effort enabled the production line to act autonomously in combination with integrated OPC UA security.

Industries around the world have implemented vertical integrations using OPC UA. Each component in the industrial process, such as the controller, sensor, robot, camera, and measuring device, serves as an independent machine unit, which each itself serves as OPC UA Server and OPC UA client at the same time.

Consequently, each machine unit can make use of the OPC UA methods, events, or data points that publishes its modes, attributes, and functionalities and offers itself as a service.


Industry 4.0 and OPC UA

As mentioned earlier, with Industry 4.0 and the Industrial Internet of Things (IIoT), information can flow freely among different devices in an intelligent network. This posed a serious challenge for secure and standardized exchange of data and information.

In 2015, the Reference Architecture Model for Industry 4.0 (RAMI 4.0) recommended only the IEC standard 62541 OPC UA for implementing the communication layer. As a result, any product being advertised as “Industry 4.0-enabled” must be OPC UA-capable, either integrated or through a gateway.

In the client-server model, TCP and HTTPS are typically used. In PubSub model, UDP, AMQP, and MQTT are used.


OPC United Architecture Communication Protocol diagram

Figure 4. A diagram showing how OPC UA works (click to enlarge). Image used courtesy of Emerson


It is worth mentioning that OPC UA is also implemented in chips, small devices, and sensors. Besides being used on the factory floor, OPC UA applications are already deployed in other areas, like commercial kitchen equipment such as fryers, ovens, coffee machines, and dishwashers.


OPC UA Future Road Map 


With the increasing popularity of OPC UA in numerous industries, OPC UA is a good candidate for configuration. Simple configuration tasks can be solved using methods, for more complex processes, transactions will be needed.


MetaData in the Cloud

When data is published to cloud applications such as Amazon Web Services (AWS) and Google Cloud, the data usually does not include the meta information in the server’s address space. The MetaData will help address this issue in the future.


Cloud Relay

The cloud-relay capability allows for connectivity between different OPC UA applications even when both server and client are behind separate firewalls.


Deterministic Communication

In the current and past communication generations, communications are not deterministic. With 5G, the 5th generation of wireless systems, it will provide better performance and determinism. It will be similar to Time Sensitive Networking (TSN), mapping the PubSub model to 5G protocol will make the OPC UA more deterministic.


Additional Protocol Mappings for Deterministic Communication

In addition to 5G, mappings to WiFi 6/7 can make the protocol become deterministic for wireless and mobile industrial applications. Furthermore, mapping to the Layer 3 network technologies with QoS (Quality of Service) support is expected to enable deterministic OPC UA communication seamlessly routed over wired and wireless network segments.


OPC UA Communication Protocol

This information should help users understand the basics of OPC UA communication. The exact future of the communication protocol is not certain, but it seems there is a lot of potential for where this protocol could go. 

*Article was originally published June 12, 2021, revisions have since been made.