The Fieldbus Jungle

By P. Hunter Vegas, Avid Solutions

This is Part I of a two-part article. You can read both parts now at www.industrialnetworking.net/fieldbus11q1. Part I discusses fieldbuses in general and outlines the true costs of field installations. 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.

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.

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.

Fieldbus installations often require additional software and/or software licenses that are not required for standard I/O. These costs can be very significant, especially if they are priced on a per-tag basis. This additional software often is overlooked during the initial cost evaluation phase.

Be wary of savings calculators provided by automation vendors. These calculations can make unrealistic installation assumptions and use labor and material costs very different from those of your plant.  Take the time to do your own calculations.

Reduced Commissioning Time?

Can fieldbus reduce instrument commissioning time dramatically, as many claim? Yes. It also can make a startup a complete nightmare. You just don't hear much about the nightmares.