Emerson Rosemount OCX 8800, Instruction Manual

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Section 7: Foundation
Fieldbus
April 2017
Instruction Manual
IM-106-880C, OI
NOTE:
The following descriptions and definitions are not intended as a training guide for Foundation
Fieldbus technology but are presented as an overview for those not familiar with Fieldbus and to
define device specific attributes for the Fieldbus system engineer. Anyone attempting to
implement Fieldbus communications and control with this analyzer must be well versed in
Fieldbus technology and protocol and must be competent in programming using available tools
such as DeltaV. See “References” below for additional sources for Fieldbus technology and
methodology.
7.2.2 Introduction
A Fieldbus system is a distributed system composed of field devices and control and monitoring
equipment integrated into the physical environment of a plant or factory. Fieldbus devices work
together to provide I/O and control for automated processes and operations. The Fieldbus
Foundation provides a framework for describing these systems as a collection of physical
devices interconnected by a Fieldbus network. One of the ways that the physical devices are
used is to perform their portion of the total system operation by implementing one or more
function blocks.
7.2.3 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 combining 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 F
OUNDATION
has established the function blocks by defining a small set of
parameters used in all function blocks called universal parameters. The F
OUNDATION
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 F
OUNDATION
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 7-2 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.
Once the inputs are snapped, the algorithm operates on them, generating outputs as it
progresses. Algorithm executions are controlled through the setting of contained parameters.
90 Introduction