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Dec
11

Process Control Function Blocks

As the Fieldbus Foundation (FF) function block specification was being developed, I became aware of the work by International Electrotechnical Commission, IEC, on function block modeling. The IEC TC65C/WG6 (now SC65B/WG15) committee had produced some draft documents that would eventually become the IEC61499 Function Block Standard. This standard defines abstract models that may be used by other IEC standard committees to write function block standards that are specific to industry segments. The WG6 committee’s work was of interest since these early drafts contained terminology and architectural concepts that may be used to precisely describe the distributed environment of fieldbus networks. Thus, in editing the function block specifications, we were able to adopt many of the WG6 definitions and architectural concepts into part 1 of Fieldbus Foundation (FF) Function Block speciation. This part of the specification describes the architecture and formal model of the function block application process. On several occasions, I met with Jim Christensen, Chairman of WG6, to discuss and review different aspects of the function block specification. Eventually, I joined WG6 as one of the US Experts and actively participated in the committee meeting for a few years. As a result of this cooperation, the terminology and even many of the architecture drawing included in the Fieldbus Foundation function block specifications are well aligned with those in the final IEC61499 standard.

Soon after the Fieldbus Foundation specifications were published, IEC formed the SC65C/WG7 (now SC65E/WG7) committee to standardize Function blocks (FB) for process control. I joined this committee as one of the US Experts and have actively contributed to this effort. The IEC61804 standard was produced by the WG7 committee. One of the primary tasks of the committee was to creating a function block standard that addresses the requirements of the process industry. The abstract model defined by the IEC 61499 preliminary standard and the work by ISO 15745-1 helped set a foundation for this work, as illustrated in the introduction of the IEC 61804 standard. Also, the function block specification work by the Fieldbus Foundation, Profibus International, and the Noah European project influenced the IEC 61804 standard. In particular, some key technical requirements addressed this standard are:

Deterministic block execution

Block types for resource management, measurement processing and function

block classes for measurement, calculation and control.

Mode parameter

Function block input/output parameter status

Contained parameters of a block for the support of configuration and plant operation

e.g. tuning parameters, setpoint, and mode.

In addition, the IEC 61804 standard addresses a means for a manufacturer to precisely describe the application within a field device through the use of an Electronic Device Description Language, EDDL. The constructs defined by this language allow the manufacturer define calculations and interactions needed to support device calibration and diagnostics. The EDDL defined in the IEC61804 standard is a superset of the device description language utilized by the Hart Communication Foundation, Fieldbus Foundation, and Profibus international. Thus, IEC 61804 is an important standard for the process industry. EDDL is utilized by handheld devices and engineering stations that support HART, Fieldbus Foundation and Profibus devices. The function block application process utilized by Foundation and Profibus fieldbus devices is consistent with the IEC 61804 standard.