The Ethernet vs. Fieldbus Cage Match

Industrial Ethernet Is Faster, Can Use Standard Wireless and Has More Topology Options

By Dan Hebert

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There's a battle raging in the process automaton networking arena, and this time it's between fieldbus networks and Ethernet-based systems. On the fieldbus side, the main players are Foundation H1, HART and Profibus PA. The primary leaders on the Ethernet front are EtherNet/IP, Foundation HSE and Profinet.

Larry O'Brien, global marketing manager at Fieldbus Foundation, doesn't see Ethernet encroaching on process automation networks at the fieldbus level. "Process field devices require power, and Foundation H1 provides digital communications and power over standard twisted-pair wiring. Ethernet does support power over Ethernet (PoE), but the primary use of PoE is for phones, panels, access points and cameras—not field instrumentation."

Process industries have other requirements, such as operation in hazardous areas, that are not met by simply implementing Ethernet at the physical layer. "Foundation H1 is a two-wire, twisted-pair, field-device-level network that can safely be installed in a hazardous area. It uses simple screw terminations, which are comfortable to a device installer," O'Brien says.

Diagnostics specific to process automation and control in the field are other key advantages of fieldbus networks. "Foundation H1 provides sophisticated diagnostic data management capabilities and a block structure that allows end users to implement function blocks in control valves or field devices for the purpose of implementing control in the field," he explains. "There is evidence that control in the field has an 80% increase in mean time between failures compared to traditional DCS control."

The 100-meter distance limitation of Ethernet-based systems is also a factor. With Ethernet, says O'Brien, "distances are significantly shorter, it has no multi-drop capabilities, and it's more susceptible to noise. You can use fiber-optic cable with Ethernet, but that's even more challenging to install."

Nevertheless, O'Brien sees the value of Ethernet in process automation. "Running at 100 Mbit/s, Foundation HSE is designed for device, subsystem and enterprise integration," he explains. "It supports the entire range of fieldbus capabilities, including standard function blocks and device descriptions, and application-specific flexible function blocks for advanced process and discrete/hybrid/batch applications."

The Profibus organization sees things a bit differently. "If devices are available with industrial Ethernet connections, use them," says Carl Henning, deputy director of PI North America. "Industrial Ethernet is faster, can use standard wireless and has more topology options. Compared to fieldbus, it also has greater bandwidth, unlimited node counts, improved diagnostics, easier upward integration and additional capabilities such as energy management.

"We actually spend some time in both our Profibus and Profinet one-day training classes on the advantages of Profinet over Profibus, while acknowledging that Profibus continues to grow. If someone is adding devices to an existing line with Profibus, keep using it. If adding a line, use Profinet. If additional capabilities are needed in an existing Profibus line, add Profinet, as existing Profibus segments can be connected to Profinet using proxies."

Rockwell Automation is one of the leading proponents of EtherNet/IP. "EtherNet/IP is designed to connect across applications from the instrumentation level all the way up to the end customer's IT infrastructure, offering the best pathway to a single network architecture," observes Mike Hannah, the product business manager for networks at Rockwell Automation. "The future will be based on standard, unmodified IP technologies such as EtherNet/IP. As the cost to deploy devices on Ethernet continues to go down, the value of having a device on Ethernet will outweigh the cost of integrating the device."

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