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If the user changes the sequence of execution order for function blocks in a schedule,
the ORDERINCM parameters of the involved function blocks are appropriately
adjusted.
Publication of each output is scheduled immediately after execution of the function
block that produces the value, considering inter-publication delays and potential
conflicts.
Blocks publish, if their output is connected to an input in another device or the FIM.
No unneeded time delay is allowed in the default link data transfer schedule.
The macrocycle is the least common multiple of the execution periods of all the CMs
involved in the link data transfer schedule.
Note that users will be able to add extra time before and after the execution of a fieldbus
function block through the Pre-execution delay (PREXEGAP) and the Post execution
delay (POSTEXEGAP) parameters that appear on the block configuration forms in
Control Builder. This means users can adjust the schedule to achieve such actions as
forcing multiple AI blocks to execute at the same time (even though they must publish
serially).
Function block execution schedule
The function block execution schedule is derived from the portion of the link schedule
that deals with starting the execution of each function block or FB_START indications.
The link schedule provides only those entries that pertain to the blocks residing in a given
fieldbus device. While device function blocks may be synchronized to the link schedule,
it is not a Fieldbus Foundation mandated feature. They may run asynchronously.
The block execution time can be broken into these three phases.
1. Preprocessing - Snap of parameter values
2. Execution - Function block outputs are determined
3. Postprocessing - Block output values, alarm and associated trend parameters are
updated.
Since input parameter values used by a function block must not change during execution,
a copy of the input parameter values is captured or snapped at the beginning of
execution. Also, since block outputs to other blocks must be time coincident, the output
values are only updated at the completion of the function block execution. The block
algorithm execution phase is always executed in the following ordered sequence as
shown in the following figure.
R400
Experion PKS Series A Fieldbus Interface Module User's Guide
July 2010
3. Fieldbus Integration With Experion System
Honeywell
3.4. Link and Block Schedules
67
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