Honeywell Series A User Manual page 68

3. Fieldbus Integration With Experion System
3.4. Link and Block Schedules
1. Determine the actual mode attribute of the mode parameter. This calculation is
based on the target mode and the status attributes of input parameters.
2. Calculate the set point, if the Set Point parameter is defined for the function block.
The calculation of working set point is based on the actual mode, set point input
parameters such as cascade and remote cascade, and any backward path input status.
Also, the value of the controlled parameter, process variable, may be used for set
point tracking. The resulting set point is shown in the set point parameter.
3. Execute the control or calculation algorithm to determine the value and status of
output parameters in the forward path. The conditions that determine the status
attribute of output parameters. The value attributes of the block's input parameters
and contained parameters, the actual mode and the working set point are used in this
algorithm. Also, where defined by the block profile, some blocks may use the status
of selected inputs. In general, the calculation of actual mode and the use of actual
mode in the algorithm accounts for the status of critical inputs.
4. Calculate output parameters in the backward path. This phase applies only to output
blocks and calculation blocks designed for use in a cascade path.
TIP
A fieldbus device whose period of function block execution is an integer factor
of the macrocycle of the link will have a function block execution schedule
prepared that has the optimal shorter cycle. For example, if the control
strategy includes a CM with a 2 second period for a temperature loop, a
second CM with a 1 second period for a pressure loop, and a third CM with a
250 millisecond period for a flow loop, a 1 second macrocycle can be
downloaded to the device that contains functions blocks used in the 1 second
and 250 ms CMs.
68 Experion PKS Series A Fieldbus Interface Module User's Guide
Honeywell
R400
July 2010