The message is only as good as the medium
September 22, 2005
By Ian Verhappen
Both Foundation Fieldbus and Profibus PA are based on the same physical layer and Electronic Device Description Language specifications. The committees developing the fieldbus standards had as two of the criteria for the new protocol that it needed to be able to reuse the existing installed cabling infrastructure and that it needed to support Intrinsically Safe applications so that the equipment could be “worked live” if required.
Of course if the cable being purchased or already installed exceeds all these criteria it should be well-suited for use in a fieldbus system. Further to this end the standards groups developed criteria for four different types of cables as defined in this second table.
The most commonly specified cable for new fieldbus installations is Type “A” cable, though it tends to be more expensive than conventional No. 18 AWG single twisted pair. The confusion in industry is related to multiple conductor cables as would be used on the home run cable of a chicken-foot configuration where multiple segments terminate in a single field junction box, much like today’s common practice. Type “B” cables have length restrictions and no mention of individual shields on the wire pairs, though it is mentioned elsewhere in the specification. Therefore, be sure that any cable used for digital communications, Fieldbus, Profibus, or HART has individually shielded cables.
The other good news on this front is that just like they did for power supplies, the Fieldbus Foundation is developing a specification for Fieldbus cables so that they too can get a “check mark”; certification as well.
Of course now that the Fieldbus Foundation has successfully demonstrated the use of High Speed Ethernet (HSE) in an industrial environment at the BP demonstration and that most industrial protocols also have an Ethernet version, there is a need to “harden” the physical layer for Ethernet, as well. Because all these protocols use commercial off the shelf Ethernet, they also use the RJ-45 connector, which certainly is not suited to the industrial environment. The Germans have specified connectors for their industrial Ethernet connections, while there are also a number of “Industrial” RJ-45 connectors on the market.
Unfortunately, these connectors are not standardized so users must be careful when selecting them. Then again, this may become somewhat of a moot point since Ethernet can also use wireless to send the message and there are standards being developed in this area by ISA. The ISA-100 committee is working on four reports in this area. If you would like to learn more about this committee check out www.isa.org.
I have heard through the grapevine that a number of new developments, products and services will be unveiled in the next six months to provide end users and manufacturers with tools to help them solve the challenges of data integrity. That way, you will be sure that the message and the medium are both of the highest quality.
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 email@example.com, or visit his website www.ice-pros.com.
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