Emerson Rosemount OCX8800A Manual page 114

F Fieldbus
™
OUNDATION
December 2022
portion of the total system operation by implementing one or more function
blocks.
8.1.3 Function blocks
Function blocks within the Fieldbus device perform the various functions required
for process control. Because each system is different, the mix and configuration
of functions are different. Therefore, the Fieldbus Foundation has designed a
range of function blocks, each addressing a different need.
Function blocks perform process control functions, such as analog input (AI) and
analog output (AO) functions as well as proportional-integral-derivative (PID)
functions. The standard function blocks provide a common structure for defining
function block inputs, outputs, control parameters, events, alarms, and modes
and combine them into a process that can be implemented within a single device
or over the Fieldbus network. This simplifies the identification of characteristics
that are common to function blocks.
The Fieldbus Foundation has established the function blocks by defining a small
set of parameters used in a function blocks called universal parameters. The
Foundation has also defined a standard set of function block classes, such as
input, output, control, and calculation blocks. Each of these classes also has a
small set of parameters established for it. They have also published definitions for
transducer blocks commonly used with standard function blocks. Examples
include temperature, pressure, level, and flow transducer blocks.
The Foundation specifications and definitions allow vendors to add their own
parameters by importing and subclassing specified classes. This approach
permits extending function block definitions as new requirements are discovered
and as technology advances.
Figure 8-1
begins, input parameter values from other blocks are snapped-in by the block.
The input snap process ensures that these values do not change during the block
execution. New values received for these parameters do not affect the snapped
values and will not be used by the function block during the current execution.
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illustrates the internal structure of a function block. When execution
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