• By Peter Lutz
  • Digital Transformation

OPC UA including APL, TSN, and 5G for the field: The Field Level Communications initiative reaches a major milestone.

Figure 1. FLC initiative developing extensions to the OPC UA framework for field exchange

A little over two years after its launch, the Field Level Communications (FLC) initiative of the OPC Foundation has completed the first release candidate (RC1) supporting horizontal communication between shop floor systems. It includes the exchange of real-time and safety-critical data between controllers (e.g., programmable logic controllers and distributed control systems) in a vendor-independent way. This is an important milestone to further develop OPC UA as a uniform and manufacturer-independent interoperability solution for industry that fully scales from field to cloud, including communication and information exchange at the control and field level (figure 1). For this, OPC UA is taking advantage of enabling communication technologies, such as Ethernet Time-Sensitive Networking (TSN), Ethernet Advanced Physical Layer (APL), and 5G mobile networks.

FLC initiative – Goals and achievements

In November 2018, the FLC initiative was founded under the umbrella of the OPC Foundation. A total of 27 companies, including the largest automation manufacturers in the world, have joined the initiative’s steering committee and support it financially as well as with labor and technical know-how (figure 2). The common goal is to expand the scope of OPC UA to the field level and to establish OPC UA as a uniform and consistent communication standard in factory and process automation. In the technical working groups, which are open to all members of the OPC Foundation, a total of over 300 experts from more than 60 companies are currently working to develop appropriate concepts and specifications.

Figure 2. The member companies of the steering committee of the OPC Foundation’s FLC initiative

Work on the first version of the specification has made good progress—despite COVID-19 and the associated restrictions. The basic concepts for the use case controller-to-controller (C2C) have been developed and have been incorporated into a first set of specifications. The initial release candidate has been completed. This so-called RC1 is now used to implement prototypes and to execute interoperability testing to validate the specifications. At the same time, test specifications are being generated. They will later be converted into corresponding test cases for the OPC UA Compliance Test Tool (CTT).

In a second version of the specification, the already developed concepts will be extended for the use cases controller-to-device and device-to-device, so that OPC UA can then be used as a uniform and consistent communication solution for vertical and horizontal integration. This includes field, edge, and cloud (figure 3). This opens up completely new possibilities, especially with regard to the different Industry 4.0 application scenarios and information technology/operational technology (IT/OT) convergence.

Figure 3. Evolution of OPC UA driving convergence and harmonization in industrial communication

Extending OPC UA to the field level

OPC UA framework

The field extensions specified by the FLC initiative are based on the OPC UA framework (IEC 62541), which enables a secure and reliable, manufacturer- and platform-independent information exchange (figure 5). Controllers and field devices support both the connection-oriented client/server communication model and the publish/subscribe extensions, which are indispensable for communication at the field level due to the corresponding requirements for flexibility, efficiency, and determinism. The security mechanisms specified in OPC UA are also used, which, among other things, support authentication, signing, and encryption of the data to be transported. They can be used for both client/server and publish/subscribe communication relationships.

OPC UA FX – Extensions for field exchange

The initial release candidate of the FLC initiative, completed in November 2020, consists of four specification parts (OPC UA Parts 80–83, figure 4) and focuses on C2C communication for the exchange of process and configuration data by means of peer-to-peer connections and a basic diagnosis. These parts are labelled with OPC UA FX (field exchange):

  • Part 80 (OPC UA FX 10000-80) includes an introduction and provides an overview of the basic concepts for expanding OPC UA for communication with and at the field level.
  • Part 81 (OPC UA FX 10000-81) specifies the basic information model for controllers and field devices (automation components) and the communication concepts to meet the various use cases and requirements of factory and process automation.
  • Part 82 (OPC UA FX 10000-82) describes network services such as topology detection and time synchronization. 
  • Part 83 (OPC UA FX 10000-83) describes the data structures for the exchange of information required for offline engineering using descriptors and descriptor packages.

In addition, a 40-page technical paper was published. It explains the overall vision and the technical approach.

OPC UA Safety – Fail-safe communication based on Profisafe

Work on the safety solution for OPC UA (OPC UA Safety) is also very advanced. A joint working group with Profibus & Profinet International (PI) developed a first OPC UA Safety specification based on client-server mechanisms. It was published in November 2019 (Part 15, OPC 10000-15). A revision of the OPC UA Safety specification will be available soon. It will describe the extensions for OPC UA pub/sub and the parameterization of safety devices, including controller-to-device (C2D). OPC UA Safety supports a maximum user data length of 1500 bytes, the creation of any network topology (e.g., star, line, grid), hierarchical safety IDs for simplified management of series machines, and dynamic connection setup with changing partners, such as modular machines, autonomous guided vehicles, autonomous moving robots, and tool changers.

OPC UA Motion based on Sercos and CIP Motion

Progress can also be reported with regard to OPC UA Motion. In mid-2020, a working group started to develop an OPC UA-based motion solution comprising motion control functions for various types of motion devices, such as controllers, standard drives, frequency converters, and servo drives. The FLC steering committee has agreed to base the work on the CIP Motion and Sercos specifications and to adapt them to the OPC UA information modeling and system architecture, considering the relevant Industry 4.0 and Industrial Internet of Things (IIoT) use cases. As with safety, existing concepts and specifications are being used, so the specification work can be significantly accelerated.

OPC UA with TSN, APL, and 5G

OPC UA is much more than a protocol. It is an industrial framework that is fundamentally transport-agnostic, and therefore it can be easily adapted to different transport layers depending on the application-specific requirements and use cases. Key technologies for bringing OPC UA to the field level are Ethernet Time-Sensitive Networking and the Ethernet Advanced Physical Layer. For OPC UA FX, two communication profiles are defined. One is mapping the OPC UA protocol (UADP) to UDP/IP. The other one is directly mapping UADP to Layer 2 of conventional Ethernet (non-TSN), but also to Ethernet TSN. The latter option is being used to reduce the protocol overhead and to increase the protocol efficiency for demanding automation applications, such as motion control or high-speed I/Os.

Figure 4. Initial specifications for OPC UA FX

By using a universal quality-of-service (QoS) modeling concept, which includes real-time communication capabilities with guaranteed bandwidth and low latencies, information and services can be easily mapped to different underlying transport protocols and physical media. But OPC UA applications using deterministic communication will not be bound to Ethernet TSN only. Wireless communication standards, such as 5G or Wi-Fi 6, support similar QoS guarantees and therefore will also be supported in the future.

The combination with TSN

Using Ethernet TSN facilitates deterministic data transmission via OPC UA, which is particularly indispensable for demanding automation applications. In addition, TSN allows different applications and protocols to be operated using a standardized hardware and a common network infrastructure. This enables the implementation of convergent industrial automation networks in which various IT and OT protocols can coexist. A working group of the FLC initiative is currently identifying which TSN substandards are mandatory for OPC UA–based end devices and infrastructure components to meet the specified requirements for performance, flexibility, and ease-of-use. The OPC Foundation has given a clear commitment to the TSN-IA (Industrial Automation) profile, which the IEC/IEEE 60802 working group is developing. For this reason, the OPC Foundation has entered into liaisons with the standardization bodies IEC SC65C and IEEE 802.1.

Figure 5. OPC UA framework with extensions for FX using QoS mapping to support different transport options

The combination with APL

Ethernet APL describes a physical layer for Ethernet that was specially developed for the requirements of the process industry. Ethernet APL enables data transmission at high speeds over long distances; the supply of energy and data via a common, twisted two-wire cable; and protective measures for safe use in hazardous areas. This makes Ethernet APL the enabling technology for the use of OPC UA and other Ethernet-based protocols in the process industry. 

Due to the special importance of this technology, the OPC Foundation joined the APL project group in June 2020 to develop and promote APL together with other nonprofit organizations and various industrial partners.

The combination with 5G

Data exchange via OPC UA is not limited to wired or wireless Ethernet communication. Support for the 5G mobile communications standard is also on the OPC Foundation’s roadmap. For this, the OPC Foundation has been working on concepts to include 5G in its quality of service modeling concept to enable the seamless integration of 5G into the existing OPC UA architecture. Furthermore, a cooperation with the 5G Alliance for Connected Industries and Automation (5G-ACIA) has recently been established. The entities will identify and leverage the synergies of combining OPC UA with 5G with the goal of supporting Industry 4.0 and IIoT applications with a reliable yet flexible communication solution.

Information at the data source

Figure 6. Semantic interoperability with OPC UA from the sensor to the cloud

The OPC UA (IEC 62541) framework with the extensions for field exchange (OPC UA FX) specified by the FLC initiative, in combination with underlying communication technologies such as APL, TSN, and 5G, offers a complete, open, standardized, and interoperable solution. It not only fulfills the requirements of industrial communication, but also enables consistency and semantic interoperability from the field level to the cloud and vice versa (figure 6). With this approach—and by adopting additional device companion specifications that numerous organizations all over the world develop—information is made available with standardized semantics directly at the data source, if possible. A flowmeter, for example, offers directly standardized “OPC UA flow measuring data” as soon as the APL cable is plugged in. And analogously, servo drives directly process standardized “OPC UA drive set points” and provide standardized “OPC UA actual drive values” as soon as they are integrated into a machine network with Ethernet TSN.


FLC initiative
FLC initiative technical paper
APL white paper

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About The Authors

Peter Lutz is director field level communications of the OPC Foundation. He has more than 25 years of experience in open control systems, industrial automation, and real-time communications. He has been engaged in several national and international standardization committees, including IEC SC65C (digital communication), IEC SC22G (adjustable speed electric drive systems), and IEC/IEEE 60802 WG (TSN Profile for industrial automation). Since April 2019, he has been managing the OPC Foundation’s FLC initiative with the goal of establishing OPC UA as a globally accepted standard for field level communications in the factory and process industries.