Trouble-Free Fieldbus Tips

July 27, 2005
The Most Important Measure to Take in Order to Prevent Difficulties With a Given Fieldbus, Digital Networking Project is Protocol Communication
About the Author
Ian Verhappen is an ISA Fellow, Certified Automation Professional, and a recognized authority on industrial communications technologies with 25+ years experience predominantly in the process control field.As the number of fieldbus projects continues to increase around the world, many have yet to take advantage of this technology and truly reap the benefits of their investment. Many engineers and designers believe the various fieldbus devices are not much different from what they've worked with in the past, and they are right — about the sensor or controller anyway. What has changed significantly is the way in which a fieldbus device connects to the remainder of the control system and communicates with the host and other devices on the network.Depending on the type of fieldbus installed, and the configuration of the network, the impact of the addition of deletion of devices will be different. Take, for example, a Foundation fieldbus chicken-foot or tree configuration, in which multiple individual field devices terminate in a field junction box, where they connect to the "home run" cable to the host computer. This configuration is very similar to what the industry has used for many years. Multiple devices connect to a "central" field terminal box from which a multi-conductor cable runs to a host computer and where the individual signals terminate on the appropriate I/O card (See figure below).

Because the fieldbus is a network, it also will require terminators at each end of the network to balance the impedance and avoid reflections and signal degradation. Termination details for each fieldbus network are different and must be given careful consideration during design. Similarly, calculations on network loading, voltage, and current at each device, as well as cycle time, jitter and cable layout must be considered in the context of network design.

If the engineering and design process has been done properly and documented, then field construction should not be significantly different than a conventional system. Special tools are required to monitor how the infrastructure supports digital communications, although conventional analog devices can be used to monitor voltages, currents and continuity. Most fieldbuses have simple hand-held troubleshooting tools to conduct preliminary analyses.

Because fieldbuses expand the network to the device and these devices communicate digitally with the host, a much richer data environment exists. Almost all fieldbus systems have some form of device diagnostic or signal quality measurement capability.

[pullquote]Further, operations and maintenance people must have a clear understanding of how the various alarms will be handled by the control system. In the early days of Foundation fieldbus, one control system supplier was unable to differentiate between an alarm because of a device failure on the network, and an alarm because of the failure of a system controller. Both were treated as system-level alarms, which they were, since both occurred on the network and, hence, at system level. However, because of the severity of the alarm, it was not possible to disable the device from alarming and, as a result, any other alarms at the system level would have been masked by this alarm because a flow transmitter had failed. The operators were under the impression that fieldbus didn't work because they always got this alarm. In fact, it was working as it was supposed to by annunciating the device failure. A most significant practical change for network maintenance personnel is they have to work on the device and network while it is "live." This has implications on the types of tools required and on the engineering design, in that the field devices might be live and hot with covers and enclosures open.Another major shift in work procedure is if a change is made to a device in the field, it is now automatically propagated to the host and a mismatch will be identified and flagged. Depending on the configuration, the change might be disallowed.Fortunately, fieldbus systems offset increased operational complexity by providing a variety of different diagnostic capabilities. Properly used, such diagnostic capabilities can support a predictive maintenance-based system. Each of the fieldbus-supporting organizations offer training courses that provide, at minimum, an introduction to the technology along with online mini-tutorials delivered via their websites. In addition, there are a number of institutions offering training in fieldbus technologies, some of them certified by the supporting fieldbus organization.Consultants, though often expensive, can be valuable if their experience and expertise are applied at critical points along a project timeline. Generally, it is not the most economical choice to hire a consultant full-time. It is more cost-effective to have this person's skill and knowledge applied only at points where his or her advice can have the largest impact. In the case of engineers, this is likely during the project's initial scoping, then again at significant engineering review milestones. The risk of hiring consultants, of course, is that they do not effectively transfer the core knowledge required for local staff to proceed on their own.As the incident with the flow transmitter described above indicates, the most important measure that can be taken to prevent difficulties with a given fieldbus project is communication. Fieldbus and digital networking are fast becoming an integral part of the automation infrastructure of many of today's facilities — both as greenfield installations and as part of a retrofit or upgrade process. It is important that automation professionals obtain high-level understanding of how these systems work, see the importance that communicating operational changes networks brings to the organization, and recognize when it is time to bring expert help on board.