Rosemount OCX 8800, Instruction Manual

Page: 102 / 266

Instruction Manual
IM-106-880, Rev 2.1
April 2017
OCX 8800
7-2
NOTE:
The following descriptions and definitions are not intended as a training guide
for F
OUNDATION
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.
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.
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 7-1 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.