The reality of real-time ethernet

'Seamless connectivity' is undoubtedly one of the catch-cries of the Australian automation industry today. It brings to mind 'integrated communications', 'real-time data', 'e-manufacturing' and 'internet-enabled control'. For many larger companies, the day is already here when senior executives and managers expect the latest production figures on their computer screens at any time of the day, from anywhere in the world.

This makes information transparency from the enterprise layer (or corporate IT world) right down to the factory floor, and back again, more important than ever. One of the primary challenges is bridging the gap between the enterprise 'IT' communications layer (almost universally ethernet) and the industrially hardened control networks linking PLCs and plant floor devices. The latter are traditionally a disparate mix of sophisticated, proprietary and dedicated networks that require advanced software and hardware to interface with the ethernet-based enterprise system.

This is one reason why ethernet has been identified as the future of industrial communications networks. If complete systems were to utilise a single communications protocol from top to bottom, it would facilitate total information transparency throughout the enterprise. Moreover, ethernet has other advantages to be leveraged in an industrial environment: high speed and bandwidth to provide overall high throughput, an existing user base spanning the globe, and complete openness, with no licensing fees.

Until recently, the single catch has been the lack of determinism and environmental robustness available with standard ethernet. Control communications have more demanding requirements of data speed, complexity, stability and tolerance of electromagnetic influences. According to the IEEE 802.3 standard, ethernet is not designed to meet these requirements as it cannot handle real-time data transfer, flexible topologies or guarantee determinism. This is where Ethernet Powerlink comes in.

Real-time and deterministic

Just as the IT world has flourished via the openness of ethernet, industry now demands the same of its automation technologies. Gone are the days when proprietary networks were deemed acceptable. Like its parent, ethernet, Ethernet Powerlink is a truly open communications protocol, managed by the Ethernet Powerlink Standardisation Group (EPSG). This independent organisation ensures the development of Ethernet Powerlink meets the needs of any automation application, anywhere in the world.

Ethernet Powerlink is the first real-time deterministic ethernet. Industrially hardened for the most demanding control applications, it allows deterministic data transfer with implemented cycle times as low as 200 µs, and timing precision better than 1 µs. This means ethernet communications may now be deployed at all levels of an automation network, providing information transparency from the plant floor right up to the corporate level and beyond.

The distinguishing feature of Ethernet Powerlink is the mixed polling and time-slicing mechanism that guarantees real-time and deterministic data transfer where it is required in the network. The network actually distinguishes between real-time and non-real-time domains: time-critical data is transferred within precise isochronous cycles with configurable timing, whereas less time-critical data is routed transparently between domains using standard IP frames. This also allows for optimal use of bandwidth, since multiple nodes are able to share common time slots over different cycles at the discretion of the managing node (see breakout story).

Ethernet Powerlink is based on the International Organisation of Standardisation/Open System Interconnection (ISO/OSI) layer model and supports client/server and producer/consumer communication relationships. It adheres to the Institute of Electrical and Electronics Engineers (IEEE) 802.3 standards. The current physical layer is 100Base-X fast ethernet (100 Mbps), but in the future this could be extended to faster variants such as GBit ethernet (1000 Mbps).

Totally ethernet

Industry groups have recognised the importance of extending ethernet to the plant floor. Alternative versions of industrially hardened ethernet incorporate time-based switching mechanisms, bit-stream decoding with application-specific integrated circuits (ASIC), or shortened ethernet frames to lower transfer times. However, these approaches each have major drawbacks, either through limiting the usable ethernet bandwidth, violating established global standards, or instilling a dependence on proprietary components. We believe Ethernet Powerlink is the only industrial ethernet protocol that provides the total package: total performance and total adherence to global ethernet standards.

Two additional areas of increasing importance are security and safety. In terms of security, the division between the real-time and non-real-time domain is key to ensuring that non-authorised communications from the outside don't impact the machine level. The protocol provides clear dividing lines and access controls on the machine level from the very beginning.

This alone has safety implications. But to meet the international safety standard for control networks (IEC 61508), the EPSG has developed the next-generation safety protocol for real-time industrial ethernet: Ethernet Powerlink Safety (EPL Safety). Fulfilling the requirements of Safety Integrity Level (SIL) 3, EPL Safety transmits safety relevant data via an embedded data frame inside standard communication messages. This protocol is capable of handling data transfer times as low as 100 µs.

Deployed in over 80,000 nodes around the world since its inception in late 2001, Ethernet Powerlink is already an industry-accepted network for control applications. Its inherent compatibility with enterprise-level ethernet TCP/IP protocols provides the seamless connectivity demanded by modern industry. E-manufacturing and internet-enabled control are one step closer with Ethernet Powerlink. Industrial ethernet is no longer the goal, it's the reality.

Andreas Pfeiffer, Ethernet Powerlink Standardisation Group