Synchronized clocks are critical in a broad range of environments. In utility power grids, clocks must be synched so the many switches run by different providers interconnect smoothly. In factories, production speeds can be increased when all the system components are synchronized to a single clock.
A number of different solutions let users synchronize multiple clocks to ensure that all actions happen at the right time. These precisely timed networks go beyond determinism, helping operators set up time-sensitive tasks like motion control.
In some applications, where all the equipment that needs to operate with tight precision is physically close together, synchronization can be managed with a backplane synchronization scheme such as PXI. When PCI Express and the PXI backplane are connected, users can employ a 100 MHz differential clock, according to the PXI Systems Alliance.
For those who need to link equipment that’s located farther apart, there are a handful of networking schemes. The Network Time Protocol lets users whose requirements aren’t too stringent synchronize devices attached to most industrial Ethernet networks. It offers millisecond synchronization appropriate for low-speed steps that are not time-critical.
For those with more demanding needs, some deterministic variants of Ethernet provide techniques that let system integrators set up synchronized systems. For example, EtherCat uses a Distributed Clock (DC) mechanism that it claims is easy to employ and robust during operations. SERCOS III uses internal clocks to precisely synchronize execution times.
Over the past couple years, IEEE 1588 promulgated by the Institute of Electrical and Electronic Engineers has gained much support. This Precision Time Protocol standard provides submicrosecond synchronization over long distances with standard cabling. It doesn’t require a lot of overhead from microcontrollers or network controllers, so it’s used by a number of companies.
Network and automation vendors Rockwell Automation, Cisco Systems, Moxa Americas and Yaskawa America are a few of those who support IEEE 1588. The standard has gained enough momentum that suppliers that don’t use the scheme in their systems have devised software that meshes with IEEE 1588 elements outside their deterministic networks. For example, EtherCat has its own synchronization technique, but users sometimes connect it to nodes that use IEEE 1588.
“For EtherCat networks, the only use of IEEE 1588 is for boundary clocks, which enable the EtherCat network to be synchronized to standard IEEE 1588 external timing sources such as to other networks, GPS pulses,” says Joey Stubbs, the North American representative of the EtherCat Technology Group.
Many chipmakers, including Intel and Freescale Semiconductor, have integrated IEEE 1588 on many of their industrial Ethernet chips. Some system makers feel that the technology should be considered almost any time real-time networking is employed.
“If a deterministic outcome is what you want, you need to have IEEE 1588 in your toolbox,” says Steve Zuponcic, applications engineering manager at Rockwell Automation in Mayfield Heights, Ohio.
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