Ethernet with determinism aids industrial apps
Industrial protocols such as Profibus, Ethernet Powerlink, and IEEE 1588 provide real-time, precise operation
BY DONNA IMAM
Freescale Semiconductor
Austin, TX
http://freescale.com
Connectivity is fast becoming an underlying must in every aspect of our lives. It is no longer just an emerging trend to want every electronic device to connect quickly, easily, securely, with flexibility and to require seamless interoperability. The need to connect, communicate with and monitor industrial control systems, medical devices or any other industrial equipment is a fast growing and pressing one.
Ethernet is the most predominant choice for networking since its use became commonplace starting in the 1990s. Its familiarity as a world-wide easily accessible open standard, providing large bandwidth without added costs, is the top reason for its popularity. Ethernet alone does not allow communication; protocols such as TCP and IP provide network connectivity and allow us to send e-mail, move files, and download Web pages. However, these protocols do not address the special needs of an industrial network.
Whether the need is to access data in drive systems, operator workstations, or I/O devices, industrial applications require communication that is precise and in real time. Hence the need for industrial protocols such as Profibus, Ethernet Powerlink (EPL), and IEEE 1588 which provide that real time, precise operation referred to as “determinism.”
Legacy protocols or field buses
In the past many manufacturers have relied on proprietary home grown protocols to solve unique industrial application needs. Today legacy protocols or field buses such as Profibus and CAN are still popular and growing because they are tested and proven and the market offers a wide range of products that support these protocols.
Design kits such as the MPC8360E-RDK help reduce industrial networking development time.
Profibus (www.profibus.com), with greater than 15 million devices installed globally is the leading field bus. It was designed to eliminate point-to-point cabling, and helps reduce network implementation cost and complexity for a wide spectrum of applications. These include factory automation, fail-safe devices, motor drives, and motion control.
Field buses are more sensitive to wiring and layout faults and common transmission methods such as RS-485 limit the maximum number devices on one Profibus segment to 32.
Field buses such as Profibus as are being challenged by Ethernet-based protocols such as Ethernet Powerlink (EPL) because they can use all the traditional Ethernet hardware and software to configure, access and control industrial devices. Ethernet provides the avenue to leverage higher bandwidth communications.
Ethernet Powerlink and IEEE 1588 protocols
Ethernet Powerlink is a deterministic real-time protocol that is completely license-free and managed by the Ethernet Powerlink Standardization Group (EPSG) (www.ethernet-powerlink.org). Suitable for applications that need guaranteed transfer of time-critical data within microseconds, and time-synchronization of nodes with sub-microsecond precision, EPL is a software only implementation and hence is compatible with all Ethernet hardware and the TCP/IP protocol suite.
Current implementations have reached 100-μs cycle time with a timing deviation (jitter) below 1 μs, which meets the highest requirements of hard real-time and determinism. EPL’s seamless communication down to sensors and actuators is achieved by using IP-based protocols in real-time and non-real-time domains, with a clear separation of domains through a dedicated router that ensures security.
Although EPL is emerging and not as widely adopted as Profibus, one of its major advantages is that its application interface is based on mechanisms defined in the CANopen profile such that it supports easy migration and integration from/with CANopen. This offers users potential for more technology sources and more vendor-independent system design.
IEEE 1588-2002 (ieee1588.nist.gov) is a precision clock synchronization protocol for networked measurement and control systems and was developed to synchronize clocks of various inherent precision, resolution, and stability to achieve system-wide synchronization accuracy in the sub-microsecond range. IEEE 1588 is appropriate for, and graining significant acceptance for, applications such as telecommunication services, industrial network switches, power line networks, and test/measurement devices.
What sets the IEEE 1588 standard apart of many existing time synchronization protocols is it’s ability to synchronize time between nodes on a network, sensors and actuators, and power grid switches with minimal network and local clock computing resources—allowing simple systems to be installed and operated with minimal maintenance. Its low-cost implementation in Ethernet networks with smaller hardware footprint and lower CPU load to achieve nanosecond level accuracy is propelling the popularity of Version 1 over other time synchronization methods.
However, Version 1 lacks the support for sub-nanosecond accuracy, faster sync message rates, unicast messaging, SHA-1/2 authentication of PTP messages, and transparent clocks. These features are critical for many high-accuracy control, telecom, and residential Ethernet applications and are being addressed in Version 2.
A number of suppliers have design kits that help reduce industrial networking product time to market. Freescale Semiconductor has introduced an industrial development platform, the MPC8360E-RDK which uses the MPC8360E communications processor with a QUICC engine running at up to 500 MHz and the e300 core operating up to 667 MHz.
The platform is available with optimized evaluation binaries of Profibus provided by IndusRAD (www.indusrad.com), IEEE 1588 provided by IXXAT Automation GmbH (www.ixxat.de). Access to the EPL is available directly from IXXAT. The system ships with a standard Linux BSP, CodeWarrior development tools, and USB Tap, and is supported by Wind River Linux and VxWorks.
The card complies with the COM Express form factor and is available as a production-ready COTS SBC rated for industrial temperature range operation from LogicPD. ■
See http://electronicproducts-com-develop.go-vip.net/digital.asp for more information on multicore processors.
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