An 'open' discussion on the future of fieldbus

InTech, Oct 2001 by Fussell, Ellen

Industry asks the age-old question:

'What does open really mean?'

Open fieldbus-is it still a conundrum to proponents of the technology? Control.com plans to operate an independent testing facility to review interoperability, testing and integrating open control products from different suppliers to see if they'll work together adequately. But other issues continue to swirl around the vortex of "open" definitions. Is the openness of fieldbus a matter of having corporatelevel commitment to ensure interoperability, assuaging political differences on standards compliance, or purely answering technical needs of interoperability?

"You can't have something that's completely open, but you need something that will give you confidence you can maintain interoperability and accessibility to the technology," said Erwin Icayan, president of Aces Inc. in Richland, Wash.

The success of an open fieldbus is a political issue for Icayan, who said it is critical to have corporate-level commitment to Foundation fieldbus. In the past few months he has participated in evaluating control system vendors to implement an I/O system and has asked vendors to show how they implemented Foundation fieldbus within their system. "We received various degrees of implementation, from not integrated at all to completely integrated," he said. "This shows me that, no matter how open the technology is, the degree of implementation is dependent on the commitment of support from the vendor and how much the vendor is willing to convince the customer that this is the best thing since sliced bread."

"I don't see that politics play a big role, as far as interoperability goes," said Ted Schnaare, engineering manager at Rosemount Corp. in Chanhassen, Minn. A lot of those political issues come about through the standards, but when we're talking about interoperability, it's a technical exercise to determine what the interoperability issues are. Then it becomes a matter of preference and subjective things."

Standards compliance

Yet the issue of standard compliance is still strong. Other labs-HART and Foundation fieldbus-test equipment and confirm whether a product complies with their respective standards.

"But that doesn't guarantee all the features of the standard are met," said Mark Coppler, engineering manager at Ametek in McKees Rocks, Pa. "In my experience, there are all sorts of things that HART could be, but you may implement only a subset of the features that can be used with HART. It doesn't mean you don't conform to HART, but some of the features you haven't implemented," he said.

Ron Helson, executive director of HART Communication Foundation, agrees. "One thing you can say about HART is that every device implements all the universal commands, which guarantees a basis of functionality that's in every device," he said. "It gives universal commands that access most of the information people are looking for pertaining to an operational device."

Examples include upper/lower range values, process variables, engineering units, percent range, and sensor limits. It's all standard and universal common access, Helson said. Other commands are optional, such as common practice. "But if you choose to implement it, you have to follow the specifications," he said. Other commands such as device specific pertain to device setup information. "Those are accessed through host applications that use device descriptions, but you just have to be sure you understand what the issues are," he said.

Likewise, Schnaare said the Foundation fieldbus standard is like a lot of others: "It doesn't cover 100% of every issue that you might encounter in the field. There are possible 'gotchas' that would make two devices not operate together well," he said.

There may be some software implementations; a manufacturer envisions the software function block working one way, and another manufacturer envisions the software function block working another way. "You won't know until you get them working together if they will work at the optimal level," he said.

"No standard or specification could conceivably cover 100% of the situations that may be encountered in the field," said Fieldbus Foundation chief executive officer Richard Timoney. "Foundation-registered devices are interoperable; that's been proven over and over again, and the reason is simple," he said. "The extensive testing process a device must pass in order to receive the foundation's registered test mark assures compliance with our specifications."

Timoney reviewed the steps involved in testing, which include conformance testing, a physical layer test, and finally interoperability testing, which consists of more than 300 test cases. "Each test case is a standalone test that exercises the functionality defined by our specifications," he said. "The tests ensure every device tested by the foundation meets the specifications. Once our check mark goes on a device, the end user of that device can feel confident it will interoperate with other registered devices."