Emerson Rosemount OCX 8800 - Function Blocks

7.1.3 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 7-1 illustrates the internal structure of a function block. When execution 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.
Function block internal structureFigure 7-1:
Foundation Fieldbus
96 OCX 8800