Standards Help Ethernet Networks Stay in Sync
Its asynchronous characteristics make it difficult to ensure orderly delivery of packets in real time, so many users are adding IEEE 1588 and ODVA’s CIP Sync to synchronize all operations.
In high-performance fields like robotic control, test and measurement, and power plants, these specifications help users run many pieces of equipment at very high speeds. The two specifications share many traits.
“CIP Sync incorporates the IEEE 1588 services that measure network transmission latencies and corrects for infrastructure delays,” says Kevin Davenport, business development manager for Cisco Manufacturing (www.cisco.com). “The result is the ability to synchronize clocks in distributed devices and switches to within hundreds of nanoseconds of accuracy. When all the devices in a control system share a synchronized, common understanding of system time, real-time control can be accomplished by including time as a part of the motion information.”
This ability to transmit signals with known delivery times is extremely important in a broad range of applications. With IEEE 1588 and CIP Sync, packet delivery times can go into the nanosecond range.
“It is essential that when a control signal is sent to a control device, such as an intelligent automation device, the system needs to know in real time when the device will actuate,” says Steven Baird, a senior hardware engineer who installs Moxa (www.moxa.com) equipment. “IEEE 1588 gives Ethernet a more synchronous nature, and timing accuracies must be in the sub-microsecond range.”
IEEE 1588 has seen growing usage in recent years. That’s partially because equipment speeds and end user demands continue to rise. Usage has also soared since the completion of Precision Time Protocol (PTP) Version 2 in 2008.
“PTPv2 is a method that provides the high degree of accuracy for systems that require strict synchronization in their operations,” says Ken Austin, automation systems marketing specialist for Ethernet at Phoenix Contact (www.phoenixcontact.com). “It comes into play when extreme precision is a paramount requirement.”
Though many facets of Ethernet communications will remain the same when IEEE 1588 is used, some must be used carefully or ignored. For example, multicasting could create synchronization issues, particularly when signals travel long distances.
“Multicasting is great in information technology, but for controls, not so much,” says Jeff Smith from American Axle & Manufacturing (AAM, www.aam.com). “You don’t want a signal from the grand master clock going to something that’s a football field away.”
Expanding the distances for synchronized systems was one of the focal points during the development of IEEE 1588 Version 2. One of its mainstays is a transparent clock. It augments the boundary clock used in the initial version of the standard.
“A transparent clock does not have its own clock, but inserts its own delay so that the end slave devices downstream can take that into account in doing their calculations,” Baird says. “This is necessary when dealing with a very large network topology.”
Several Ethernet alternatives provide real-time capabilities, but most require some special hardware or mandate that all real-time equipment use the same software. With standards, there are fewer limitations.
“The benefit of a standard is that disparate systems can now use the same protocol, opening opportunities for multi-vendor designs and solutions,” says Matt Newton, regional sales manager at Opto 22 (www.opto22.com). “For applications such as motion control where timing is key, 1588 allows nodes to be kept on the same time down to the sub-microsecond range."