By Ian Verhappen
By Ian Verhappen
The physical layer is the lowest of seven layers in the Open System Interconnect (OSI) model. It is also the most important layer because everything above it depends on signals transmitted from point A to point B with no deterioration. Foundation Fieldbus and Profibus PA use the same wiring and termination equipment-or physical layers-to transmit power and signals.
Fieldbus systems, which are wired in parallel, have unique requirements. Research shows that 80 percent of fieldbus installation problems are due to the physical layer, which is the signal transportation medium. We often blame the technology when our systems don’t work, but end users can prevent many problems through good engineering and installation practices.
In certain circumstances, fieldbus technologies are better than analog ones. Fieldbuses are less susceptible to noise because they determine signals-in this case, “ones” and “zeros” from the transition-from one state to the other. Conversely, analog signals infer signals from the magnitude of the output.
Another advantage with fieldbus is that the host system becomes aware immediately when a signal fails. That’s not the case with analog technologies. When a signal fails in an analog system, output could show a “zero shift” due reasons such as lower-than-necessary voltage and higher current drop, and result in a “true” system reading of 37.5 percent, or 10 mA, when in fact it’s 50 percent, or 12 mA. The host system would not be aware, and everything would operate under these incorrect conditions until the next scheduled maintenance cycle.
The first step in minimizing physical layer problems is to ensure that everyone associated with the automation project receives training. Training can be as simple as spending two to four hours with electricians and installers. Everyone on the project, especially the electricians and installers, needs to know that fieldbus is a parallel wiring situation and that two terminators must be installed on each segment. Other items that the electricians and installers should know about are related to grounding and the need to be careful with the shield foil as it can cause sporadic problems that are the most difficult to resolve.
The design team should also ensure that all instructions sent out to the field are correct so the likelihood of difficulties later on is small.
Fortunately, many manufacturers of physical layer devices are making their equipment more modular with fewer connections, which means there’s less room for error. Using a preconfigured field junction box to collect the spurs from each device minimizes the number of connections in the field, provides a consistent quality of termination, and reduces the field construction time so the net effects are project savings and a more reliable system.
Ian Verhappen is an ISA fellow and director of ICE-Pros, Inc., an independent instrument and control engineering consulting firm specializing in fieldbus, process analyzer sample systems and oil sands instrumentation and control. E-mail him at Ian.Verhappen@ICE-Pros.com, or visit his website www.ICE-Pros.com.