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By Dick Caro, CMC Associates, Acton, MA
FOR MANY YEARS, the User Layer working group of ISA standards committee SP50--the creator of the function block concept now used in Foundation Fieldbus--characterized the function block as a software object resident on any network node. As in any software object, the internal algorithm of the function block is not visible or programmable by the user, although the supplier typically documents it. All the function block’s communication with the outside world is through its parameters, called “attributes” in software object language. The attributes are defined by the programming of the function block as specified in Foundation Fieldbus specifications or the supplier’s proprietary specifications, but their values are supplied from one of four sources:
Following this design, the name, data type, and any initial default values for function block attributes in Foundation Fieldbus were defined in the Device Descriptions or DDs that were exactly modeled on the equivalent definitions of HART. Profibus-PA also had similar definitions called EDDs, but were less well-defined, since there were no specified computations specified in the Profibus-PA standard for field devices. All of this has now changed since there has been an agreement by the Fieldbus Foundation, HART Communications Foundation, and Profibus International to use the common EDDL being defined in a draft international standard: IEC 61804-2 (2004-05), “Function Blocks (FB) For Process Control - Part 2: Specification of FB Concept and Electronic Device Description Language (EDDL).” All three consortiums are presently in the process of adapting the attributes specified in the draft standard to the same functions previously expressed by their proprietary data definitions. Therefore, all suppliers conforming to any of these three fieldbus specifications will accept the same terminology for any one attribute.
|FIGURE 1: FIELDBUS AT WORK|
More than 20 different pressure transducers connected via Foundation Fieldbus, saving wiring cost and making operation easier.
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