Plugging into Network Certification
No man is an island, but then neither is control equipment in today’s world. Devices that at one time might have operated on their own are now networked together, a change that’s enabled more nimble control and increased the flow of information in both directions. Those are the benefits, but gaining them does require the network to work.
No man is an island, but then neither is control equipment in today’s world. Devices that at one time might have operated on their own are now networked together, a change that’s enabled more nimble control and increased the flow of information in both directions.
Those are the benefits, but gaining them does require the network to work. In theory, that’s simple, but just plugging a new device into an existing structure--though it won’t cause problems for the device itself, the network, or the other devices already present--does require adherence to network-relevant specifications.
“The true benefit of any open standard is the assurance it provides to users that the products they buy will interoperate seamlessly,” notes Lenore Tracey, executive director of Modbus-IDA, the North Grafton, MA-based trade association that maintains the standards for the Modbus communication protocol suite. “Unfortunately, any specification, no matter how carefully written, is subject to interpretation and occasional misunderstanding,” she says.
One method used to avoid such misunderstandings is the testing of devices to ensure they adhere to networking specifications, followed by some sort of visible seal of approval by a governing body. Referred to by some as 'certification’ and others 'registration’, this validation can help ensure a network functions as intended.
Such testing is done by Modbus-IDA and ODVA of Ann Arbor, MI, which maintains the standards for the Common Industrial Protocol (CIP) networking technologies which includes the HART Communication Foundation of Austin, TX, which deals with HART technology; the Austin, TX-based Fieldbus Foundation, which handles Foundation Fieldbus technology; and Profibus and Profinet International, which maintain namesake networking technologies though testing labs in Johnson City, TN, and elsewhere.
Ed Ladd, Jr., director of technology programs for the HART Communication Foundation, notes that end users have a strong say in this testing process. They can demand registered products, and vendors, he contends, will respond. Ladd sees the demand for this testing growing, driven in part by networking changes. “As wireless technologies proliferate and system integration improves, registration is going to be more and more important,” he says.
A look at some of the biggest industrial automation networks reveals their certification strategies and practices, along with hints about where things are headed.
Who tests the testers?
Broadly speaking, the groups supporting automation networks follow the same scheme when it comes to specifications and certifications. Specifications are hammered out through consensus, with a typical evolution including both a draft and final specification. Tests are developed from the specifications, with certification of a product coming only after successful passing of the test suite.
While the specifications are backwards compatible, the tests may not be. Thus, a product in the field may or may not pass the current test set.
Vendors pay for the cost of certification. They usually don’t have to be members of the trade organization, but being a member often pays dividends in terms of costs and licensing agreements.
If a device fails testing for some reason, mechanisms exist to help the vendor achieve certification. There are also forums and other feedback methods for end users to effect changes in specifications and test methods, although these may be indirect and through a vendor.
What does differ among the organizations is who does the testing and what is tested. The HART Communication Foundation, for example, does all of its own evaluations, although that may change in the future. Modbus-IDA offers two paths: testing by a third-party lab or by member companies in a self-testing program. ODVA maintains a master testing lab at its headquarters and handles certification of third-party test providers.
Testing for Profibus and Profinet is done by one of 10 labs around the world. Carl Henning, deputy director of the PTO, the North American Profibus/Profinet member organization, notes that the older of the two standards, Profibus, started out with the stance that certification was optional. When Profinet appeared in 2001, certification was a requirement of the standard.
The test labs are audited yearly to make sure they are compliant and capable of administering the test suite. However, Henning says that only a device’s networking capabilities are tested.
“If we’re testing a temperature controller that has a Profinet interface, we’re not testing that it does temperature control. We’re testing that the parameters are correctly transferred in conformance with the standard,” he says.
For the most part, that is true for all certification testing. The parameters checked correspond to the seven layers found in the OSI model, ranging from the physical layer that connects the device to the network on up to the application layer that interfaces to and performs application services.
The Fieldbus Foundation follows a slightly different approach. There’s a mixture of independent and internal test labs, with information available for the vendors who want to run their own tests to make sure of a device’s performance before submitting it for evaluation. Communication stack testing is done by the Fraunhofer Institute for Systems and Innovative Research in Karlsruhe, Germany. There’s also testing done at a level above that of the standard OSI model. This involves checking function blocks to make sure they’re interoperable, and it also validates the systems that host the applications.
In addition to the testing done for conformance to current specifications, there are new requirements on the horizon. The Fieldbus Foundation, for instance, plans to come out with two new registration marks fairly soon. One will involve cable and the associated jitter, the unwanted variation in the communication signal. If large enough, jitter can cause devices to misinterpret a communication packet. The result is data loss and network problems, such as degraded data rates as packets are retransmitted.
This new registration mark and the associated tests might allow end users to resolve a potential area of concern, says Foundation Fieldbus product manager Stephen Mitschke. “One could apply these test requirements to existing cable to get an idea of quality,” he says. “At the same time our registration process would encompass any vendor who could show they’re compliant.”
The second upcoming change concerns couplers. There the evaluation will involve checking for correct behavior from the device. If too much current is being drawn, for example, does the coupler then disconnect the segment?
At ODVA, upcoming conformance tests will also look beyond what’s currently being done. Katherine Voss, the association’s executive director, noted that ODVA’s current specifications don’t have much in them with regard to performance. Parameters like latency, quality of service, and data throughput are all known to impact real-time or deterministic automation applications. What isn’t completely clear is exactly what the network standards should be to ensure needed performance. Another issue that isn’t established is what the test methodology should be in order to best guarantee the standards will be met.
What’s needed is an extension of the specifications and tests to include performance, and ODVA has plans to develop that extension. “We will be doing that in the future for EtherNet/IP devices,” says Voss. She added that this work will be complete this year.
ODVA has an agreement on performance metrics with the U.S. Council for Automotive Research (USCAR) and NIST, the National Institute of Standards and Technology. NIST helped to research and develop metrics, methodologies and reporting standards, as well as helped to establish the performance testing facility and certification. USCAR is committed to the adoption of the standard in automotive assembly.
Given the increasingly networked nature of automation applications and the changing nature of the networks, it’s likely that other extensions of standards into new areas will be forthcoming. As that happens, certification and testing will have to evolve as well.
Hank Hogan is a contributing writer for Control Engineering.
Instrumentation vendors tackle testing, certification issues
Network organizations often use independent testing laboratories to ensure that devices work with their networks. The Measurement Control and Automation Association (MCAA) represents companies who manufacture and distribute process controls, field measurement and analysis instrumentation. According to association president Cynthia Esher, “MCAA leadership sees an increasing disconnect between the Nationally Recognized Testing Laboratories (NRTLs) which serve our industry and the testing and certification needs of instrumentation companies and their customers.” To address the issues, on May 20 MCAA will hold a Product Approval Process Summit in Phoenix where representatives of seven testing laboratories will discuss with MCAA members how to improve the product testing and certification process for everyone.
“In ranking their concerns, most MCAA members have put timeliness first, followed by internal processes (communication and accounting included) and cost,” says Esher. “MCAA members have told the association that time delays and price increases place them at a competitive disadvantage around the world.”
One significant issue is the lack of experienced engineering staff within the major laboratories affecting the certification timeline as inspectors get up to speed on the technologies, says Esher. This affects the whole project’s timeline and cost, since certification projects then require review by a senior engineer throughout the project.
Another issue is that the laboratories quote completion time and cost, then add requirements for additional testing during the certification process. This delays the approval and increases the cost. “Members frequently said that the NRTLs need to streamline their internal processes and improve their communication with those seeking approvals,” says Esher.
The inclusion of several NTRLs on the panel will help members gain an appreciation for the abilities of each of the laboratories to test and certify their products. Invited panelists include representatives from CSA-International, FM Approvals LLC, Intertek Testing Services, MET Laboratories, SGS US Testing Co., TUV Rheinland of North America, and UL Conformity Assessment Services.
Renee Robbins is senior editor of Control Engineering. She can be reached email@example.com.