The Linux operating system, especially with the PREEMPT-RT real-time extension, is already used in many automation devices and its popularity shows no signs of waning. The latest Linux versions now include mechanisms for TSN. The open PLCnext Technology ecosystem from Phoenix Contact is based on RT Linux, and TSN is supported on suitable Ethernet hardware.

AXC F 3152 PLCnext Control: this controller will support TSN. It uses Ethernet hardware that has been validated with the TSN test tools, which made development considerably simpler. The next consideration is whether this hardware can be tested uniformly under Linux and how this can be done in order to facilitate easy and high-performance integration into automation systems, such as PLCnext Control. Every Linux-based automation device essentially consists of the following components:

Hardware

The hardware includes the processor (CPU), the TSN-capable network interface(MAC), and the Ethernet line driver (PHY). It is the chip manufacturer who is responsible for the hardware.

Kernel

The Linux core function is executed in the kernel of the device. This function provides the basis for the uniform and deterministic behavior of all applications running under Linux. Access to the Ethernet interface is also realized in the kernel. This special part of the kernel is referred to as the network driver.

Network interfaces

This area makes the functionality of the Ethernet TSN hardware available to higher-level applications.

Industrial middleware

There are industrial transmission protocols that can be used with and without TSN. Profinet and OPC UA are two such examples. In the concept of “network convergence”, this type of middleware can also be used simultaneously in a network.

Application

This term refers to the relevant automation application. On a controller, for example, this could be the program for a filling or assembly plant. While a field device, for example, is used to control digital and analog process signals. The control application communicates with the field devices via the respective middleware and the TSN network.

Synchronization

A key feature of TSN is the option for the highly accurate synchronization of applications. The acronym “Time-Sensitive Networking” clearly describes this feature. Synchronization allows distributed applications to run synchronously on controllers and field devices. The capability for synchronization is already integrated in Linux. However, the TSN-capable hardware must support the necessary mechanisms for this. The level of accuracy that can be achieved depends on the corresponding implementation, but it can be significantly better than one microsecond (µs).

The latest kernel versions now include the use of Ethernet TSN mechanisms in RT Linux. This provides significant advantages for device manufacturers, who are able to develop a neutral TSN solution without the need for special hardware or software. The test environment presented here can be used to qualify these implementations, thus enabling the easy and robust use of TSN in Linux-based devices – for every manufacturer. This will contribute significantly to the widespread adoption of Time-Sensitive Networks.

Internal structure of a Linux-based controller that supports Time-Sensitive Networks.

Testing on widely used TSN-capable network chips

As illustrated in the figure above, it is no longer device manufacturers like Phoenix Contact who are responsible for TSN implementation under Linux; that responsibility now lies with the chip manufacturers. Therefore, the performance and quality of an automation device are only as good as the underlying TSN hardware and software. As a result, device manufacturers are now writing their own network drivers in order to ensure the required level of quality. In some cases, specific hardware is also required and this is implemented in FPGAs (Field Programmable Gate Arrays), for example.

These manufacturer-specific solutions are expensive and make it more difficult to change the underlying hardware during the lifecycle of a product.

o the possibility of independently qualifying hardware and drivers from chip manufacturers before devices are developed and the hardware and application are set up is of particular interest to device manufacturers. The same is true for chip manufacturers who would like to test their TSN solutions without specific automation applications.

As all the necessary basic TSN functions have now been integrated into Linux, the need emerged for an independent test environment for TSN. With this in mind, Phoenix Contact has worked together with chip manufacturer Intel to develop appropriate test tools and test them out on widely used TSN-capable network chips.

In the future, these test tools will be made available as an open source solution in a suitable organization for all interested parties. Chip and device manufacturers will thus be able to qualify their respective TSN products. Furthermore, the test tools can be used to compare different solutions or to familiarize customers or research organizations with the topic of TSN.

Test tools for measuring the quality of Linux-based device platforms: a reference application and a test application verify the quality of the relevant TSN solution.

Highly accurate measurements of runtimes and deviations

For the independent qualification of a chip manufacturer’s TSN solution, the industrial middleware and the application are replaced by an emulation that uses the underlying hardware and its network drivers. The emulation behaves like an automation device and, following successful testing, it is later replaced by the manufacturer’s middleware and device application. The emulation consists of the following parts:

Reference

This test tool emulates cyclic real-time communication as well as acyclic network communication. Both can be configured in a wide range in a file. So, in addition to very short cycle times in the microsecond range, it is also possible to set the size and quantity of packets, for example. The reference part also checks received packets and measures the runtimes from application to application. In addition, the reference application uses the special “Hackbench” tool to stress test the CPU. All the results are recorded.

Test

This tool receives the cyclic and acyclic communication from the reference part and sends it back. Certain values are also recorded.
Both the reference and the test application are synchronized with each other via the network by means of TSN mechanisms. This makes it possible to perform highly accurate runtime and deviation measurements.

Through close collaboration between Phoenix Contact, as a controller and device manufacturer, and a major international chip manufacturer, it has been possible to verify whether the test tools are working correctly during the actual development phase and to ensure the error-free implementation of TSN hardware and software. Furthermore, the TSN test tools enabled performance measurements in the microsecond range as well as optimizations. The test environment has thus demonstrated its capabilities.

Release as open source

Phoenix Contact and the chip manufacturer are currently discussing the open source release of the test tools. This release would enable more manufacturers to qualify their existing and planned TSN solutions, thus making the porting of TSN device software even easier and more reliable. As the TSN tools essentially work independently of any TSN middleware, they should be accessible to all interested parties. However, a sustainable solution needs to be found for long-term maintenance. There are several options here, which will be announced publicly once they have been defined.

Following publication and widespread acceptance, Phoenix Contact anticipates a broad platform of TSN-capable solutions that can be used in its own products. Chip manufacturers will also benefit, as they will be able to use the test tools to qualify and improve their products. This in turn will lay the foundations for fair competition with regard to the best solution based on reproducible criteria. Open source TSN test tools ultimately provide scope for future extensions. For example, the possible integration of security encryption for TSN traffic or advanced test methods. In addition, it will be easy to accommodate any further development of the TSN standards.

Summary

Open source test tools for device and chip manufacturers offer numerous opportunities and possibilities for boosting the potential of real-time Linux and TSN. Phoenix Contact hopes that these test tools will achieve broad acceptance, as this will promote the widespread adoption of TSN in its own devices and devices from other manufacturers and will ultimately make this future technology more readily available to all users.

Gunnar Lessmann, Master Specialist Profinet and TSN, PLCnext Technology, Phoenix Contact Electronics GmbH