As more and more process manufacturers incorporate digital busses into their automation architectures, debates often occur whether to limit the plant to a single bus or to incorporate multiple busses.
You can see both sides of this argument on a Foundation fieldbus forum in a Mixing fieldbuses thread. The original question posed:
In a new plant scenario, lets say an 800 point I/O count with about a 50-50 mix of analog and discretes. I think it’s a no-brainer to go FF [Foundation fieldbus] where applicable for the analog control/monitoring. The question is, the remaining 400 or so points where we need shut-off valves, motor controls etc. what do we do?
The early responses from some of the suppliers and consultants gave advice such as:
Multiple buses should not be mixed in a system, not because it can’t be done, because it can be done using most control systems in the market today, but because the system gets messy and expensive to operate and maintain.
You can see the thread in its entirety here.
Emerson’s position in this debate differed greatly and was well articulated by Dewey Kuchle in our Hydrocarbon and Energy Industry Center in Calgary.
His key points in this forum thread include:
1. Remember the objective is to have projects on budget, on schedule, and operate efficiently. The selected technology is a means to this end.
2. No one digital bus technology is best for all the types of transmitters, actuators, motors, analyzers and other process instrumentation. Each technology and how it is implemented in the various process automation systems has its strengths and weaknesses. Pick the right one for the job.
3. Nothing beats “hands on” to compare the various systems and how they implement these digital busses like Foundation Fieldbus, Profibus DP, DeviceNet, and AS-bus. This is a great way to see what it takes to engineer, install, commission, operate and maintain them.
Digital busses like Foundation fieldbus are great for much of the process measurement and actuator instrumentation. It can be used, but is not as good a fit for motor control centers as DeviceNet or Profibus DP. The ideal situation is to have a single architecture that can support all of the most common busses and integrates them seamlessly, like our DeltaV system.
Conversely, these more discrete-oriented busses cannot pass back to the engineers, operators, and maintenance personnel the predictive diagnostics that intelligent measurement and actuator instrumentation contain that help run the process more efficiently and with fewer upsets.
Most of the reasons for the differences are the analog or discrete starting points of the technologies. Without delving into longer explanations, I will point to PlantWeb University’s Engineering School which goes into the choices and tradeoffs in the “Choosing the right bus” section.