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F
Fieldbus
™
OUNDATION
April 2023
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
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.
Figure 8-1: Function Block Internal Structure
A. Input events
B. Execution control
C. Output events
D. Input parameter linkages
E. Input snap
F. Processing algorithm
G. Output snap
H. Output parameter linkages
I. Status
96
illustrates the internal structure of a function block. When execution begins,
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