Deterministic Networks: Synchronizing Standard Gains Steam

Deterministic networks can ensure that messages arrive within time limits, but their delivery guarantees aren’t enough when large, complex systems have many elements that must move in tightly synchronized timeframes. For motion control and other complex functions, clock synchronization is necessary.

While IEEE 1588 gains acceptance, alternatives such as SERCOS are also retaining support.
While IEEE 1588 gains acceptance, alternatives such as SERCOS are also retaining support.

IEEE 1588, promulgated by the Institute of Electrical and Electronics Engineers (IEEE), is gaining acceptance in these applications. The standard’s descriptive name stands in marked contrast to its role in systems that require quick actions; it is known as the Precision Clock Synchronization Protocol for Networked Measurement and Control Systems.

The protocol, applicable to Ethernet and other networks, lets designers synchronize equipment in heterogeneous systems even when device clocks have various levels of precision and resolution. It supports system-wide synchronization accuracy in the sub-microsecond range while using minimal network and local clock computing resources.

The specification, developed by both the IEEE and the National Institute of Standards and Technology, is also used in test and measurement, communication systems and electrical power systems. Though it has been around for a few years, the specification is only now gathering solid momentum. For example, test and automation vendor National Instruments Corp., Austin, Texas, only completed its efforts to support the standard on all of its target products during the last half of 2010.

“We’ve added native support for 1588, which makes it very easy to implement. The applications where 1588 is most promising is for scheduling tasks, telling multiple devices to start or stop at the same time,” says Nate Holmes, Product Manager for Motion and EtherCat at National Instruments. He notes that the standard is used when large systems with many pieces of equipment are linked together, not when smaller systems are integrated.

IEEE 1588 has garnered support from a broad range of suppliers. Rockwell Automation Inc., the Milwaukee-based automation supplier, and networking equipment vendor Cisco Systems Inc., of San Jose, Calif., support the standard’s implementation in a range of products developed independently and through their partnership.

“We put information out to the devices so they can respond at exactly the right time. In motion control, we can tell every drive and motor to be at a certain point at a certain time,” says Steve Zuponcic, application engineering manager for Rockwell Automation, in Mayfield Heights, Ohio

Chip maker Intel Corp., Santa Clara, Calif., supports it, and Freescale Semiconductor Inc., based in Austin, Texas, puts timestamp support in many of its microprocessors. Moxa Americas, of Brea, Calif., offers hardware that employs the spec. Other suppliers are bullish on the standard’s role.

“We both use and promote IEEE 1588.  This is a major selling point since many multi-axis machines these days greatly benefit from coordinated precision motion that is guaranteed by utilizing IEEE 1588,” says Derek Lee, motion product engineer at vendor Yaskawa America Inc., in Waukegan, Ill.

Though support is growing, alternatives will hold 1588’s acceptance down. Suppliers and other standards groups have also addressed the challenges of linking many pieces of equipment. Supporters of these alternatives feel their solutions are more attractive.

“You need a lot of extra things if you use 1588,” says Scott Hibbard, vice president of technology at automation supplier Bosch Rexroth Corp., in Hoffman Estates, Ill. “SERCOS (serial real-time communications system) gives us determinism and synchronized signals that are sent out at specific times. This approach provides a more efficient use of bandwidth.”

Other alternatives provide many of the same features as 1588, making them suitable to many potential users. “It’s similar to the Network Time Protocol, which makes sure all devices are running without deviation,” says Martins Jansons, network consultant at automation supplier Siemens Industry Inc., Alpharetta, Ga. “If there is deviation, you want to shut down the line before time-critical processes get messed up.”

While most of these alternatives will be used independently, there will be some cases in which high-performance networks employ the standard in certain instances. For example, the EtherCat Technology Group (ETG) has endorsed it in one environment.

“The ETG only promotes IEEE 1588 as a mechanism for coordinating an EtherCat network to an external timing source or to another fieldbus network. This is done through a ‘boundary clock,’ which is an EtherCat slave device with an external IEEE 1588 connection. The ETG promotes distributed clocks as the internal synchronizing mechanism for EtherCat networks,” says Joey Stubbs, a spokesman for the ETG, based in Austin, Texas.

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