Precise Timing For Demanding Jobs

Aug. 4, 2010
For many high-performance networks, delivering critical data packets on time can be more important than simply sending them through cables at high data rates. 
When complex tasks such as motion control are being run, ensuring that all elements of the system are synchronized with precise timing is the key to success. As Ethernet has gained popularity, deterministic field buses such as Modbus, DeviceNet and Profibus have all been adapted to provide Ethernet compatibility. These upgrades, along with the emergence of high-speed alternatives such as EtherCat and Ethernet Powerlink, have in turn made Ethernet more attractive for factory floor applications. With all networking schemes, the drive to provide ever more performance while also making installations simpler is continuing. For example, CC-Link is one of the networks that was designed to provide determinism. It’s providing more speed, more compatibility and improved determinism. CC-Link’s Industrial Ethernet has adopted the 1 gigabit (Gbit) version of Ethernet, adding this speed enhancement to its deterministic architecture. “With CC-Link IE running at 1 gigabit, one controller sends a token to the next device, which receives it on one port and sends it from another port,” says John Wozniak, automation networking specialist for the CC-Link Partner Association, which now manages the technology that Mitsubishi created. “Some devices may transmit simultaneously, but they will be sending data out of another port so there won’t be collisions.” Standards bodies are also stepping in to improve performance and compatibility. The Institute of Electrical and Electronics Engineers (IEEE) developed the 1588 Precision Time Protocol standard to synchronize clocks. “IEEE 1588 Precision Time Protocol ensures that there are not any synchronization issues when you’re using standard Ethernet,” says Joe Lee, product manager for EtherNet/IP, at supplier Rockwell Automation Inc, Milwaukee. “You can do motion control without it, but if you want tight synchronization, we recommend using 1588.” Another standards body, ODVA (formerly the Open DeviceNet Vendors Association), is using 1588 as the basis of its Common Industrial Protocol Sync extension. CIP Sync is the basis of EtherNet/IP’s “Time Synchronized Distributed Control” for motion applications. Its time synchronization services provide a distributed time reference for the packet time stamping used in Time Synchronized Distributed Control, and allow synchronization of services across distributed nodes. As these advances are made, there’s a strong focus on reducing complexity. Ethernet compatibility is a key factor. Rockwell stresses the benefits of using unaltered Ethernet. Using standard Ethernet lets developers use one network for peripherals and control systems, Lee notes. When versions of Ethernet that have been altered for industrial applications are used, separate networks are needed for different equipment. However, others say that it’s not a big deal to make changes. When engineers develop solutions for demanding jobs such as motion control, they typically have a good understanding of the intricate issues of electronic control. It’s not a complicated task to adapt standards like transmission control protocol/Internet protocol (TCP/IP) to work with. “Companies can take products that have TCP/IP connections and move them to CCLink-IE-Field just by altering the application layer, adding the protocol stacks to get fast deterministic performance,” Wozniak says.Related Feature - Peripheral VisionTo read the feature article relating to this story, go

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