To fieldbus or not to fieldbus

InTech, Oct 1997 by Ochsner, Marjorie, Schrier, Matthew

In addition to these variables, fieldbus provides access to even more information, including calibration date, local trends, and local alarms. Parameters can be grouped into view objects where certain information needed by the operator interface can be made available. For example, one view may contain all the information required by an operator to run a process and another view can contain static information.

The most significant advantage of fieldbus over other digital protocols is the variety of instrumentation that will be available. Most other protocols are unable to provide much information on the valve; a fieldbus can. Interoperability also enables information on numerous vendor devices to be accessible by a host.

Other considerations

In greenfield applications (i.e., nonretrofitted applications), fieldbus has advantages over other digital protocols that can make it more costeffective. The wiring options, control options, and field information mentioned previously, coupled with interoperability of multiple vendors' equipment, make fieldbus a viable choice.

There are several other factors to consider before changing over to fieldbus, especially where devices and control systems that use a digital protocol exist. In addition to the wiring, control, and information considerations already described, there is the actual cost of the transition. One method would be to upgrade the existing field devices to fieldbus and retrofit the existing valves. Many vendors offer such upgrade packages, but the upgrade costs are more than simply purchasing the hardware. The actual implementation usually means at least replacing the electronics board in the transmitter. Depending on plant practices, this usually cannot be done in the field. Instead, the transmitter must be removed from service, brought back to the shop, upgraded, tested, then reinstalled. Depending on how quickly this can be done and the plant labor rates, it could be a costly transition.

Another strategy is to replace transmitters that fail with fieldbus transmitters. Again, this is not simply a plug-and-play situation. The host that the transmitter is talking to must be considered. If the DCS is upgradeable, the upgrade still needs to be made. If the transition is not made all at once, then the appropriate input/output (I/O) modules need to be made available. There will probably be some host controllers that cannot be upgraded. In that case, this scenario turns into more than just a simple transmitter replacement.

Carefully analyze transition

Fieldbus does have advantages over existing digital field protocols; however, the cost and initial risk of transitioning to fieldbus must be carefully analyzed. If a plant currently uses a digital communications protocol to communicate between transmitters and the control system, many of the installation, start-up, and maintenance advantages have already been realized. The benefits of using fieldbus must be quantified to determine if the additional information, wiring options, control options, and interoperability can offset the cost of its implementation. In greenfield applications, the benefits are obvious. In retrofit applications, particularly those with digital communications, the benefits may not outweigh the costs.