Over years of visiting end user sites, talking to system integrators, and internal conversations with others within our own company, tribal knowledge begins to accumulate of what works, what doesn't and what can be improved. When working in a Foundation fieldbus (FF) context, we find some user companies swear by the technology and won't consider anything else, while others cannot claim such positive experiences.
When considering what differentiates enthusiastic users from those who gave up, many of the same top 10 points emerge. Some of these points are specific to Foundation fieldbus and others are more universal, so even if you aren't a Foundation fieldbus user you might find some familiar topics addressed. Here are the Top 10, plus one:
Top Ten Fieldbus Issues Plus One
- Wiring Practice Pitfalls
- Terminators
- Power Supplies
- Stick with Tested and Registered Products
- Incorrect DD/CFF Files
- Using the Link Active Scheduler
- Device Addressing
- Choosing Between Publisher-Subscriber and Client-Server Communication
- Traditional Project Management Techniques May Not Apply
- Mismatch of Work Processes
- Misunderstanding the Value Proposition
1. Wiring Practice Pitfalls
Good wiring practices apply to all sorts of field devices, but are particularly important with Foundation fieldbus because the digital communication running through the cables is sensitive to electrical noise. Terminations need to be executed properly and Foundation fieldbus cabling should be installed in appropriate cable trays or conduit just like any other field device wiring, so why is this a potential problem area?
Many companies have encountered issues with simple termination and routing rules. In one instance we found high-voltage cables in the same trays with cables from process sensors and actuators. As a result, individual field devices might fall off a segment for a while and then mysteriously reappear.
Basic troubleshooting may show a correlation between operating a specific piece of equipment, say a pump controlled by a variable frequency drive (VFD), and device segment drop-offs. If the cables to that VFD are too close to the Foundation fieldbus cables, electrical noise generated by the VFD can disrupt the digital signal. Putting an oscilloscope on the segment can prove the point by showing the offending electrical noise and consequent distorted waveforms.
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Figure 1-1: The terminations in this cabinet follow |
Other wiring pitfalls relate to device connections. Many field devices are insensitive to polarity, but others are not, and technicians need to pay particular attention.
Here's a quick quiz: Most Foundation fieldbus cables are brown, blue, and green colored wire coverings. Which wire should be positive? The brown wire or the blue one? How is it connected at the H1 module or power isolator? Trying to figure it out by intuition is a good way to get into trouble. Perhaps you say to yourself: "Brown represents earth or ground and is therefore negative. That means blue is for the sky and positive."
There is a certain logic to that idea but it's wrong. With Foundation fieldbus, brown is positive and blue is negative, which provides a guide for hooking up transmitters, positioners and H1 interfaces. However, the key is consistency in which wire is connected to which pole.
Device manufacturers may not help much. Some terminal blocks are more clearly marked than others, and just because polarity is not obvious does not mean the device is not sensitive to polarity.
To complicate matters further, Foundation fieldbus cable has three conductors: blue, brown and a shield, often green or striped green. A given device may also have three terminals. Since a Foundation fieldbus cable provides power to the instrument, the normal assumption is that it must follow the same practice as normal power wiring. Standard 120 Vac power has a hot (usually black or some other color), neutral (white) and ground (green or conduit). Everything needs to be thoroughly grounded.
But Foundation fieldbus is first and foremost a communication medium so a different rule applies: signal wires shall not be grounded. Unfortunately, what sometimes happens is the installer sees three conductors and terminates three conductors. The result is noise on the segment. Hopefully, the installer will know to check the number of Fieldbus terminators (more on that later). If that count is correct, a look at the terminal block will show that the shield is screwed down at the wrong termination. Shields should only be connected to a single point. Most installers now do this at the H1 host or power isolator interface end only.
2. Terminators
Since Foundation fieldbus is a bus system, terminators are placed at each end of the segment to avoid communication reflections which can add noise to the segment and therefore inhibit communication. Technicians typically expect that two terminators on a segment, one near the H1 card and one on the last device on the segment, solves the problem, but it doesn't. There can be either too few or too many terminators on the segment, as sometimes one terminator can be too few and three too many.
Here's what can happen: If the segment length is short, there is probably very little noise on the segment whether it has a terminator or not, and this situation may have existed in the field for years.
But the day comes when you need to add more devices, and you decide to use a short segment because there is plenty of room, which is perfectly sensible. You extend the trunk cable from one junction box/tee to another and increase the overall segment length. Now noise shows up where you didn't have it before. Why?
The problem has to be solved through consistent use of terminators. The best approach isn't always intuitive, and vendors may not be able to help as even something as simple as a terminator can vary from company to company.
Manufacturer X uses a jumper, manufacturer Y uses a switch and manufacturer Z has to be mounted in a junction box. Fortunately, this whole issue is becoming less of a problem as Foundation fieldbus ancillary product suppliers have offered more self-terminating junction boxes and power isolators. A wider range of devices is now available that avoids setting jumpers, selecting DIP switches or screwing down connectors in a termination device.
3. Power Supplies
Beware the temptation of using generic, non-Foundation fieldbus rated 24 Vdc bulk power supplies, as
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Figure 1-2: Fieldbus cables are more susceptible |
Foundation fieldbus devices have stringent power requirements.
The Foundation fieldbus specification allows for 9 to 32 Vdc power. Applying Ohm's Law, voltage on a segment will dip slightly as each new device is added and consumes current, but that's just milliamps and doesn't affect the voltage all that much.
The voltage should still remain comfortably within the requirements, well above 9 V, but there is more to it. There are many factors affecting voltage drop along the segment, so power isolators from many vendors are designed to ensure that power is maintained in the 19 to 28 Vdc range with little variation regardless of the number of devices drawing current.
That opens a new set of questions: What capacity (amperage rating) power supply is needed? Should it be redundant? Should the power isolator be redundant? Must there be one power supply per segment, or is it appropriate to use a single large unit to power multiple segments? Again, there are no simple answers, but here are many guidelines for segment design to deal with voltage and current consumption in order to answer these questions.
Suppliers are helping as many power isolators now provide segment diagnostics that can be sent to the host or asset management systems. Some can even determine where noise is originating down to individual devices, simplifying troubleshooting.
4. Stick with Tested and Registered Products
The Fieldbus Foundation has instituted a series of testing processes and procedures for devices, host systems and ancillary products to ensure interoperability among products from different suppliers. These tests go through regular reviews and revisions to help users ensure products from different suppliers will work with each other and the host. Suppliers also do their own internal testing with different devices and hosts to minimize risk during product development and ensure interoperability.
While this sounds very sensible, it has not always been the case, and problems may surface when using older devices. In the early days of Foundation fieldbus technology, suppliers often found areas where the specifications had to be interpreted, and this caused some interoperability issues. Reviews and revisions over the years have eliminated most of these ambiguities. Foundation fieldbus ancillary products like power isolators, junction boxes, cables and terminators are also tested with the same test criteria and registered at the Foundation.