Device Descriptions; Instrument-Specific Function Blocks - Emerson Rosemount OCX 8800 Reference Manual

Once the inputs are snapped, the alorithm operates on them, generating outputs as it
progresses. Algorithm executions are controlled through the setting of contained
parameters. Contained parameters are external 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.
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.
7.1.4

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. You 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 the bus. Instead,
their displays and interactions with devices are driven from the device descriptions.
Device descriptions may also include a set of processing routines called methods. Methods
provide a procedure for accessing and manipulating parameters within a device.
7.1.5

Instrument-specific function blocks

In addition to function blocks, Fieldbus contains two other block types to support the
function blocks. These are the resource block and the transducer block.
The resource block contains the hardware specific characteristics associated with a device.
Transducer blocks couple the function blocks to local input/output functions.
Reference Manual
Foundation Fieldbus
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