Troubleshooting FOUNDATION Fieldbus networks
By John Rezabek
End users and manufacturers have learned a lot about Fieldbus since its early prototypes were rolled out in the late 1990s. Fieldbus aims to be an open and interoperable standard, allowing two-way communication between devices and hosts from a diverse array of manufacturers. It invariably took a few iterations to minimize the gray area for bugs to lay in wait for pioneering end users. Fieldbus Foundation's "Interoperability Test Kit" for field devices is now in its 6th major release, and the level of certainty with which users can experience "freedom to choose" is excellent. Certifications exist not only for field devices, but also for hosts, power supplies, cables, and termination hardware. Users can now specify virtually every component of their fieldbus system to possess a certification that is "fit for purpose"-meeting or exceeding the user's minimum requirements.
Host vendors have been running interoperability test beds of their own, aiming to vet nearly any device's compatibility with their distributed control system. These are largely empirical tests where devices are installed, commissioned, and "burned in" for periods of weeks, months, or longer. There has generally been a laudable degree of cooperation between hosts and device vendors-sometimes fierce competitors-to assist one another in solving interoperability issues.
Host, device testing at the Fieldbus Foundation
Within the Fieldbus Foundation's automation infrastructure, interoperability is possible because devices and software must all conform to the same standard, and they are tested and registered to that standard. Products bearing the FOUNDATION product registration symbol have undergone a series of common tests administered by the Fieldbus Foundation. End users can select the best device for a specific measurement or control task, regardless of the manufacturer.
There are three basic paths for testing and registration within the Fieldbus Foundation: H1 Testing for devices residing on the H1 network; HSE Testing for devices residing on the high-speed HSE network; and Host Profile Testing for host systems. The Foundation has test kits for each level of testing. Host testing and registration is of particular importance to end users.
The host and device testing are continuously being refined to address more user requirements and to provide a higher degree of interoperability. As mentioned, the Fieldbus Foundation is now in Version 6 of its device Interoperability Test Kit, and the Foundation has completely revamped their Host Profile Registration process to include more mandatory features that will guarantee interoperability between devices and hosts from different vendors.
Despite the growing evolution of specifications and the thoroughness and efficacy of product testing, there is a chance an end user will encounter a malfunction on their fieldbus networks. Here is a synopsis of steps that could get you to a quick solution.
An ounce of prevention: Aim for a quality installation
The vast majority of fieldbus problems-more than 90% by most accounts-owe to deficiencies in the physical layer, that is, the wiring, terminations, and power supplies. Experienced fieldbus users are adamant that a little effort in training the installers pays immense dividends at commissioning time. At Reliance's Jamnagar Refinery in India, for example, hundreds of local electricians were trained in the rudiments of the installation. When it came time for commissioning, only three out of more than 3,000 segments exhibited any network issues.
Staying with fieldbus check-marked hosts, devices, and components is another "best practice" to deliver "certainty of outcome" and minimize unforeseen issues. Especially when you are under the gun to deliver a project on time and under budget, it is best to leave any "science projects" for the end user in the operational phase. Early adopters found a lot of variation in things like wire & cable-entire shipments sold as "Fieldbus" cable with nothing close to consistent impedance, let alone a nominal 100 Ohms at 31.25 KHz. Today, you can find at least 10 suppliers who can supply registered cable in a variety of constructions, including armored and multi-pair cable.
Finally, inspect the installation early and often. It is best to catch any hurried or errant craftsmanship in the early going and have a "teachable moment" if you will, or find a task (other than say, terminations) for those who continue to be challenged.
The "pound" of cure: Fieldbus troubleshooting
Maybe a pound is overkill, as the majority of fieldbus networks are either "working" or "not working"-much like OTA digital television. Here are a few categories of issues that have been encountered and some steps to analyze and remedy them.
What if you think all the wires are landed and device "X" will not show up in your host's engineering/commissioning tool like the previous 300 did? It is fairly quick and easy to check for the proper voltage at the field device; you can use a conventional multi-meter. The fieldbus specification calls for a minimum of about 9 volts for a device to function, but most installations will see more, depending on whether the design calls for IS or other energy-limiting circuitry. Many devices are now delivered polarity-insensitive, but you can check this too if you notice your device's terminals have "+" and "-" indicated.
Assuming all of the above checks out (or if, for example, there is no voltage at the device), you can start looking for physical layer errors. Are the shields properly landed and grounded, preferably (depending on where you are) in only one point at the host? If you are using quick connects, are the conductors landed correctly in the device? Are the terminals torqued to the recommended tightness?
Whatever the root cause, most experienced users prefer a shortcut to fieldbus network diagnosis. They choose test equipment that falls into a few distinct categories: Built-for-purpose portables like the Relcom FBT-6 and P+F ADM, multi-function hand-held that include some physical layer diagnostics like the 475 Communicator by Rosemount, built-in diagnostic modules like those offered by P+F, Relcom, and Turck, and generic test devices like the Fluke Scopemeter. Since fieldbus operates at a very specific frequency-31.25 KHz-training traditional instrument techs to dial up the right range on an oscilloscope that spans everything from a few Hertz to many Megahertz (and millivolts to tens of volts in the Y axis) can be daunting. This is further complicated if the demand to utilize the device is infrequent. To help, Fluke has incorporated some innovative "Bus Health" functions into recent products like the Scopemeter 125 that allow inexperienced or infrequent users to test for the same properties as the built-for-purpose devices.
On-line diagnostic tools
Several options exist for on-line diagnostic tools, which can be permanently attached or portable devices. Today's diagnostic tools offer measurements, such as voltage per segment, segment noise detection, maximum and minimum fieldbus signal level, low resistance between shield and negative or positive signal pole, and more. Tools are also available to measure fieldbus "jitter" on a segment.
If the on-line diagnostic tool is permanently installed as part of the FOUNDATION fieldbus power supply, additional information may be available, such as minimum, maximum, and real-time bulk voltage supply to the FOUNDATION fieldbus power supply, power supply operational status, and min/max and real-time current measurements.
Benefits of a permanently installed diagnostic tool may include the ability to historize the data, and provide real-time alarming and trending of the data. Portable diagnostic tools assist in troubleshooting specific problems and may present additional data not available with permanent diagnostic tools. Permanently installed diagnostic tools should be well integrated with the host system.
The biggest challenge facing end users and installers is often achieving the minimal level of craftsmanship across a large project. Once you have specified and procured check marked components and devices, the path to success consists mainly of correctly landing shielded twisted-pair cables-the same as it ever was.
ABOUT THE AUTHOR
John Rezabek is Process Control Specialist at International Specialty Products Inc. (ISP) Lima, Ohio, plant, which is part of Ashland Inc. He has site responsibility for capital projects and technical support of all process measurement and control systems. He is a graduate of Case Western Reserve University with a BS, Systems Engineering.