Block Structure of Fieldbus Devices

The physical standard and technical report produced by ISA SP50 were used as a starting point for much of the work done by the Fieldbus Foundation. In particular, the Fieldbus Foundation function block specification team adopted some basic concepts such as the definition of mode and status with minor changes from the definitions contained in the SP50 User Layer technical report. However, as work progressed on the Fieldbus Foundation function block specification, we found it necessary to expand the block types that were documented in SP50 and to change the way blocks were structured, defined, and implemented. Much of this change was driven by input from control system and field device manufacturers.

The Foundation function block specification team initially collected information on measurement, calculation and control functions and associated parameters that were common to the major suppliers of process control system. Through an analysis of this capability, we were able to identify functionality and parameters that were common to these manufacturers. Based on this work, the specification was broken into two parts. Part one of the specification contains a description of the architecture and formal model of the Function Block application process. In this part of the specification we addressed the components that make up the function block application process. To provide precise definitions that are sufficient to support implementation, the model is based on an object oriented design. Part 2 of the specification contains a basic set of ten function blocks that utilize the model and architecture defined in part 1 of the specification. This initial set of blocks addresses a variety of common measurement and control applications.

The function blocks defined for measurement and actuation were base on the parameters found in major process control systems. However, a fieldbus device must also contain many other parameters to support measurement and actuator diagnostics and calibration. We initially pursued the concept of the manufactures adding parameters to the basic IO blocks for device diagnostics and calibration. However, it soon became clear that this would lead to IO function blocks that were specific to each manufacture. If we took this approach then it would be necessary to know upfront what devices would be used in a particular installation before it would be possible to do basic control configuration. Our goal was to allow measurement, calculation and control to be configured independent of the device manufacturer. Thus, the concept of a transducer block was introduced into the architecture and block model. The transducer block serves as a container of calibration and diagnostic parameters associated with IO processing. By taking this approach, it was possible to design the IO function blocks that are the same for all device manufacturers.

A third type of block, the resource block, was defined to contain parameters associated with the physical device or are global to all function blocks. Some example of these typed of parameter are the manufacturer identification number and selections to enable or disable write lock protection. The specification requires that all fieldbus device support a resource block. Part 2 of the specification defines the resource block’s parameters. Only one resource block may be defined in a fieldbus device.

The IEC61804 international standard, Function Block for Process Control, includes the three blocks types defined by the Fieldbus Foundation function block specification. The transducer block is referred to in the standard as a Technology block. Thus, because of this consistency in definition, Foundation fieldbus devices comply with this standard. If you have an interest in learning more about the Fieldbus Foundation’s function block specification or the IEC61804 standard, then copies of these documents may be purchased at the following sites:

Fieldbus Foundation Specification

IEC61804 Standard