Real-time Ethernet/TSN
Sercos via TSN
Ethernet TSN allows deterministic transmission of real-time critical messages via standard hardware. How can sercos and Ethernet TSN be combined to transmit real-time and normal Ethernet communication via a uniform network standard?
Although today's real-time Ethernet systems are all based on Ethernet, they require special hardware support in order to function correctly and achieve the corresponding transmission and real-time performance. This means that these systems are not compliant with the IEEE 802.1 and 802.3 standards, which means that vertical and horizontal integration with Ethernet cannot be implemented optimally.
To make matters worse, most real-time Ethernet protocols cannot coexist in a shared network infrastructure without affecting performance and real-time characteristics. Various real-time Ethernet solutions even use the network infrastructure exclusively, so that other protocols have to be tunneled via the underlying real-time protocol (Fig. 1a). However, this requires a running or functioning real-time protocol in order to be able to communicate with the devices at all.
Real-time Ethernet solutions that explicitly support coexistence with other Ethernet protocols and are therefore multi-protocol capable take a different approach. Other protocols can be used both with and without the respective real-time protocol (Fig. 1b). Representatives of these real-time Ethernet solutions include Sercos III and Profinet IRT.
Ethernet TSN - An IEEE standard
Ethernet TSN heralds a new generation of industrial communication: for the first time in the 40-year history of Ethernet, time-controlled and deterministic transmission of real-time critical messages via standard Ethernet hardware is now possible (Fig. 1c). Ethernet TSN uses the principle of a time slot method, which sercos has been using for real-time communication for over 25 years. Since Ethernet TSN allows real-time communication and normal Ethernet communication to be transmitted via a uniform network standard, there are numerous advantages:
On the one hand, standard hardware with integrated real-time capability can be used, resulting in low hardware costs and a wide range of manufacturers and products. In addition, the TSN standard enables the convergence of production and IT networks: real-time and normal Ethernet communication can be transmitted via a uniform network standard. - This is an ideal basis for the implementation of Industry 4.0 and IIoT concepts, as it enables end-to-end communication - both vertically and horizontally.
Ethernet TSN is based on the IEEE 802.1Q standard, which specifies the division of physical networks into several logically separate, prioritized virtual networks. Various sub-standards are used to specify additional features based on this:
■ Time synchronization: All network participants have a common understanding of time. Ethernet TSN uses mechanisms from IEEE 802.1ASrev and IEEE 1588 for this purpose. The time synchronization protocol described therein defines how spatially distributed real-time clocks are synchronized with each other.
■ Time slot method: Synchronous time slots allow the transmission of different traffic classes and time-controlled data transmission. Ethernet TSN uses the IEEE sub-standard IEEE 802.1Qbv (time-division multiplexing) for this.
■ Scheduling and traffic shaping: All devices work according to the same rules when processing and forwarding network packets. Ethernet TSN uses the IEEE sub-standard IEEE 802.1Qcc (stream reservation) for this.
■ Frame preemption: Telegrams can be interrupted and continued later. Ethernet uses the IEEE sub-standard IEEE 802.1Qbu (Frame Preemption) for this.
Sercos over TSN 'Proof of Concept'
But how can existing real-time Ethernet solutions migrate to Ethernet TSN? To what extent can conventional real-time Ethernet devices be operated together with other Ethernet nodes in an Ethernet TSN network infrastructure? These questions were the starting point for a sercos TSN demonstrator, which premiered as a proof of concept at SPS IPC Drives 2016.
The sercos TSN demonstrator was realized by the ISW of the University of Stuttgart with the support of several industry partners. It shows the possibilities of providing a real-time and multi-protocol-capable network infrastructure based on TSN for automation technology and beyond (Fig. 3).
Figure 4: Sercos SoftMaster with Ethernet TSN interface and PTP support in accordance with IEEE1588.
© Sercos InternationalA TSN-based Sercos III SoftMaster with a Soft CNC from Industriellen Steuerungstechnik GmbH (ISG) is used. The Precision Time Protocol (PTP) according to IEEE 1588 was integrated into this master so that all network participants communicating via TSN switches from Hirschmann Automation & Control use a standardized time base (Figure 4).
Parallel to the sercos real-time communication via TSN, video streams from a webcam are transmitted to a remote display without impairing the characteristics and functionality of the real-time transmission. The investigation of the real-time behaviour showed that the errors in the time synchronization are limited to a two-digit nanosecond range. This means that neither the functionality nor the real-time characteristics of sercos are restricted. Sercos III devices can be integrated into a TSN network unchanged and also together with other Ethernet devices and communicate with each other via TSN. Existing tools, such as the sercos monitor as a diagnostic and analysis tool, can also continue to be used to their full extent.
Sercos technology benefits from TSN in several ways: On the one hand, standard Ethernet components with integrated real-time capability can be used and thus flexible network topologies can be realized. Secondly, higher transmission bandwidths are available with Ethernet TSN. This means that sercos devices based on Fast Ethernet with 100 Mbit/s full duplex can also be operated in a 1 Gb/s or 10 Gb/s-based network infrastructure. In addition, the sercos master can be operated remotely from the sercos slaves, allowing edge controller concepts with centralized control of decentralized peripheral devices to be implemented. Ethernet TSN thus makes a decisive contribution to bringing about the convergence of conventional real-time Ethernet solutions into a uniform, standardized and consistent network infrastructure.
Author: Peter Lutz is Managing Director of Sercos International
The Sercos SoftMaster
With a Sercos SoftMaster, a standard Ethernet controller is used instead of specific Sercos III hardware. With the Sercos SoftMaster, the hardware functions required for the correct functioning of Sercos are transferred to the software and the Sercos functions emulated in software are calculated on the host CPU. By dispensing with proprietary hardware controllers, the hardware costs as well as the acquisition and runtime costs of the industrial PCs can be significantly reduced.
It should be noted that the use of conventional Ethernet controllers results in a telegram jitter of ~20...60 μs, depending on the available CPU performance and the real-time characteristics of the operating system, which results in a device synchronization of ~1 μs.
With new generation Ethernet controllers (e.g. Intel I210), which already support various TSN features such as queues and telegram scheduling, it is even possible to achieve synchronization and maximum availability in the range of a hardware-based master. At the same time, the Sercos SoftMaster can be integrated into an Ethernet TSN network in a fully compatible manner without restricting its functionality with regard to Sercos III real-time communication.
The Sercos SoftMaster is available for download as open source software via SourceForge. The ANSI-C implementation is completely platform-independent and can be adapted to any operating system and platform. The following operating systems are currently supported: Linux PREEMPT_RT (OSADL Linux), QNX Neutrino, Windows Embedded Compact, TenAsys INtime, IntervalZero RTX/RTX64, Windriver VxWorks, Linux (Standard Ubuntu 14 LTS), Windows Embedded Standard.
Link: sourceforge.net/projects/sercos-softmaster-core
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