The Fieldbus Jungle

Fieldbus Can Be Good. Fieldbus Can Be Bad. An Experienced Integrator Examines the Benefits Claimed and Their Relationship to Industrial Reality

By Hunter Vegas, Avid Solutions

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This is Part I of a two-part article. Part I discusses fieldbuses in general and outlines the true costs of field installations. "The Fieldbus Jungle — Part II" discusses advantages/disadvantages of individual fieldbus technologies.

There have been countless articles that tout substantial savings generated by fieldbus networks. Most claim greatly reduced wiring, marshalling panel and I/O space, reduced commissioning time, and extensive savings from putting control out in the field devices.

SEE ALSO: Trouble-Free Fieldbus Tips

These all are mostly true statements. However, when you consider the larger picture, those benefits and savings can be offset by other factors rarely mentioned. It is important that the automation professional considers all of the relevant factors before choosing a technology.

In this article the word "fieldbus" refers to digital networks in general. The scope of this discussion is limited to the instrument-level fieldbuses: Profibus-PA, Foundation fieldbus, AS-i and DeviceNet.

The Real Economics
Does fieldbus save wiring cost and reduce installation cost? It depends.

Fieldbuses replace multiple I/O cards and individual instrument wires with a single card and a single wire running to the area and then split off to the instruments. The amount of reduction is dependent on the fieldbus used, how it is installed, and the area electrical classification. On average, it is fair to say that fieldbuses will reduce wires and/or terminations by at least a factor of 4, and some fieldbuses can save a great deal more than that.

It is also true that there can be some engineering reduction because the marshalling panel sizes are reduced or eliminated and field junction boxes are no longer used. However, those savings are often offset — even overwhelmed — by other factors.

Consider that there is a reason very few people employ this technology.

Fieldbus I/O hardware is significantly more expensive than a standard I/O card, and the final hardware cost is at best break even. On average, the control hardware of a single, non-redundant AS-i port costs about the same as the DI and DO cards associated with 20-25 on/off valves with two limit switches each. Some vendors make dual-port cards, others single. These values are on a per-port basis. If you install fewer than 20-25 valves per port, the AS-i hardware likely will cost more on a per-instrument basis. An AS-i card also requires a power supply and might require power conditioners and repeaters.

A single, non-redundant, non-IS Foundation fieldbus (FF) network card with power supply costs about the same as the analog input I/O associated with 15 transmitters or valves. Most segments will not carry that many devices, so the Foundation fieldbus hardware actually costs more than standard analog I/O hardware for the same number of points.

Price comparisons with Profibus-PA are more difficult because the cost of the segment depends on how the DP/PA couplers and DP communication cards are implemented. However, in a typical installation, the cost of a single, non-redundant, non-IS channel of Profibus-PA is about the same as the I/O cards associated with 5-10 valves or transmitters. This puts the cost of Profibus-PA hardware on par with standard analog I/O.

Fieldbus wire is much more expensive than regular 4-20 mA wire. Although theoretically almost any shielded pair can be used, the specific fieldbus wire will allow maximum network lengths with minimum communication problems. Prices vary, but fieldbus wire is typically 3-5 times more expensive. That price difference can jump significantly if it includes the cost of special connectors, whip cords, etc., which are required in most fieldbus installations.

Fieldbus field devices are almost always more expensive than traditional I/O devices. The differential is shrinking, but still exists. Lead times can be longer as well.

Foundation fieldbus and Profibus-PA can eliminate field junction boxes, but they often replace them with some kind of segment coupler or block that must be installed in the field to allow the local devices to connect to the network. These blocks are vital because they usually protect the network from shorts in individual transmitters. The total cost of these segment blocks as well as the fittings, terminators, etc., can approach or eclipse the cost of the original junction box.

Thumbs Down

Fieldbuses eliminate engineering associated with junction boxes and marshalling panels, and replace it with the engineering of the network itself. This cost can be very significant, since it often requires knowing — within a few feet — where the devices are located before the facility even exists. As the process design changes and equipment is relocated, the network design must be re-checked. A 4-20 mA wiring scheme is very forgiving. It doesn't matter if the cable is 10 ft or 1,000 ft, it still will work. Few fieldbus installations allow changes of that magnitude without a revisit to the design.

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  • <p>End-users of fieldbus responded with a completely different view. See their responses here: <a href=""></a> They responded problems were experienced with the very first Foundation fieldbus-based installations in the early years. However, the last few years their installations are very straightforward and implemented efficiently. Fieldbus does make a difference in cost, schedule and overall lifecycle savings. There are significant benefits regarding the digitalization of the process control, playing a significant role in keeping the plant running reliably. It is really reliable. Everything is going digital in this world.</p> <p>Products are continuously improving. For instance, DeltaV has been continuously improved versions 9, 10, 11, and 12 with many new enhancements that make fieldbus easy to use, and lately also with native integration of WirelessHART:</p> <p>DeltaV version 9 (2006) • Function blocks in the H1 card for complex time-synchronized loops • Greater function block and VCR capacity so it doesn’t have to be engineered • Enhanced EDDL (graphical device display) for easier to use devices</p> <p>DeltaV version 10 (2008) • Device replacement and commissioning using only a screwdriver without touching software like 4-20 mA for easy commissioning and maintenance/replacement • Express download for faster commissioning • HPR 61a registration</p> <p>DeltaV version 11 (2010) • Integrated power in the H1 Card (eliminating the marshalling cabinet) for smaller footprint and cost • EDDL device level access (dashboards) for devices as easy to use as 4-20 mA • HPR61b registration</p> <p>DeltaV version 12 (2013) • NAMUR NE107 (prioritized device diagnostic alarms) eliminating device diagnostic alarm flooding • DD Update Manager (automatic DD download) to keep the system up to date with new device types and versions without manual DD downloads <a href=""></a> </p>


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