01 March 2003
Quick pick at 7-Eleven?
San Ramon, Calif.—With all the digital buses available on the marketplace, it is not as easy to select the right one for your application as it is to go into your local 7-Eleven store and buy a Quick Pick lottery ticket.
The International Electrotechnical Commission (IEC) under IEC 61158 has made this selection process for your control system more difficult with the classification of a bus standard that includes eight varieties.
Another point of confusion is the area of discrete buses, which function in both manufacturing and process plants. These discrete buses, which are mostly on/off with some modulation capability, have also grown in numbers and options to further make the selection process more difficult.
Getting the selection process back on track will require a good working knowledge of one's process, I/O mix, control system capability, engineering resources, and naturally what field devices you plan to use.
One must address each of these topics independently before integrating into a solution that talks the same language. This ultimately narrows the bus selection.
Plant needs and the specific industry drive the applications and are the first criteria in the selection process. The process industry, which is primarily modulating control, has been the big supporter of fieldbus—any variety.
The biggest industry segments are chemical, refining, power, pulp and paper, and to some extent food and pharmaceutical.
These segments are driven to digital buses by the economics of reduced wiring costs, operational benefits, and predictive maintenance. Pharmaceutical has the unique requirement for validation and historical data in order to comply with government regulatory agencies.
The food industry is loath to invest in new technologies such as fieldbus due to its seasonal production, which reduces the payback on investments.
Manufacturing industries are primarily discrete because they are assembly oriented.
Industry sectors such as semiconductor, computer and electronics, water treatment, automotive, and specialty chemicals utilize a high degree of discrete signals.
Industries such as pharmaceutical leverage both modulating and discrete applications because they have a packaging component. Just about all of the modulating industry segments have discrete applications in some form.
SING A COMMON SONG WIRE
The driving factors among either modulating or discrete buses are very common themes. Cost reduction due to reduced wiring in new installations and some retrofits tops the list.
Next comes the benefit of diagnostics from the sensor/transmitter/positioner in ascertaining the health of the device and what might be failing: electronics, sensor, mechanical linkage, responsiveness, or the connection to process.
The digital buses that support modulating sensors seem to have the most options and the most contention among them.
With the reactivation of ISA's SP50 committee in 1984 to define a standard digital protocol, the evolution of the Fieldbus Foundation in 1994, the movement of Profibus-PA support to the Profibus Trade Organization, the continuation of WorldFIP after death, the niche market for LonWorks, plus the continued support of HART by almost every major vendor, the choices are many.
There seems to be no clear winner. To further complicate matters, there is very strong support to move everything to Ethernet. The final choice is probably application dependent.
There are differences among each bus for consideration. While they are all two wire, and most conform to the basics when it comes to the data link and application layers as defined by the IEC, there are differences in their attributes that a user must review before making a selection. These differences relate not only to the bus itself but also to the control system that must support it.
Selecting the control system is first because it is capital intensive and must support an I/O structure that will read and write to the selected process bus or buses.
Further, the configuration of a smart device on a digital bus needs software that is on an independent platform such as National Instruments' NI-FBUS configurator or one integral to the control systems that has been supplied by the control system vendor, typically in a Windows environment.
While HART may be referred to behind closed doors as the "poor man's fieldbus," it has grown up to take its place among the big guys due to its shear momentum. Worldwide HART is now supplied on 60% of the transmitters shipped by all suppliers, and some make their transmitters only in HART.
The differences in the buses, which we can classify as fieldbuses in the process industry, are in the speed of throughput, control algorithms at the device level, and the higher layers of the protocol stack in the vendor-specific software blocks that allow for advanced diagnostics.
The use of control in the field is the biggest advantage that Foundation fieldbus (FF) brings to the party. However, control-in-the-field implementations are few and generally fall into the area of overkill because processes just don't need such speed.
But for those that do require the fast response, control at the valve is great, and in fact the fast response of FF is so popular that it appears as a line item in most requests for quotation.
This is the area in which Profibus-PA and FF H-1 differ most. IT
James Noel is at the Foxboro Instrument division, Invensys Process Systems. His e-mail is firstname.lastname@example.org. Read his complete article on digital bus selection at www.isa.org/intech/digitalbus.
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