SEAMLESS connectivity is undoubtedly one of the catchcries 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--and from anywhere in the world.
This makes information transparency from the enterprise layer 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; plus 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--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 requirement of its automation technologies.
Like its parent, Ethernet, Powerlink is a truly open communications protocol, managed by the Ethernet Powerlink Standardistion Group (EPSG). This independent organisation ensures the development of Powerlink meets the needs of any automation application, anywhere in the world.
Powerlink is the first real-time deterministic Ethernet. Industrially hardened for the most demanding control applications, Powerlink allows deterministic data transfer with implemented cycle times as low as 200 microseconds, and timing precision better than 1 microsecond. 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 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.
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 IEEE 802.3 standards: the current physical layer is 100Base-X Fast Ethernet (100 Mbit per second), but in the future this could be extended to faster variants such as Gbit Ethernet (1000 Mbit per second).
Totally Ethernet
Other industry groups have recognised the importance of extending Ethernet to the plant floor.
Alternative versions of 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 have drawbacks, either through limiting the usable Ethernet bandwidth, violating established global standards, or instilling a dependence on proprietary components.
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 Powerlink 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 (EPLsafety). Fulfilling the requirements of Safety Integrity Level (SIL) 3, plus EPLsafety transmits safety relevant data via an embedded data frame inside standard communication messages.
*Andreas Pfeiffer is a member of EPSG, e-mail Andreas.Pfeiffer@ethernet-powerlink.org or visit www.ethernet-powerlink.org.