Emerson Rosemount OCX 8800, Instruction Manual

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Instruction Manual
IM-106-880C, OI
Section 7: Foundation
Fieldbus
April 2017
Device
Descriptions
91
Contained parameters are internal to function blocks and do not appear as normal input and
output parameters. However, they may be accessed and modified remotely, as specified by the
function block.
Input events may affect the operation of the algorithm. An execution control function regulates
the receipt of input events and the generation of output events during execution of the
algorithm. Upon completion of the algorithm, the data internal to the block is saved for use in
the next execution, and the output data is snapped, releasing it for use by other function blocks.
Figure 7-2 Function Block Internal Structure
A block is a tagged logical processing unit. The tag is the name of the block. System
management services locate a block by its tag. Thus the service personnel need only know the
tag of the block to access or change the appropriate block parameters.
Function blocks are also capable of performing short-term data collection and storage for
reviewing their behavior.
7.2.4 Device Descriptions
Device Descriptions are specified tool definitions that are associated with the function blocks.
Device descriptions provide for the definition and description of the function blocks and their
parameters.
To promote consistency of definition and understanding, descriptive information, such as data
type and length, is maintained in the device description. Device Descriptions are written using
an open language called the Device Description Language (DDL). Parameter transfers between
function blocks can be easily verified because all parameters are described using the same
language. Once written, the device description can be stored on an external medium, such as a
CD-ROM or diskette. Users can then read the device description from the external medium. The
use of an open language in the device description permits interoperability of function blocks
within devices from various vendors. Additionally, human interface devices, such as operator
consoles and computers, do not have to be programmed specifically for each type of device on