Rockwell Automation PLC-5 Fieldbus Solutions for Integrated Architecture User Manual - Manual (Page 70)

Publication 1757-UM006A-EN-P - May 2002
2-26 Integrating Fieldbus into Rockwell Automation Logix System
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 Foundation Fieldbus mandated feature.
They may run asynchronously.
The block execution time can be broken into these three phases.
1. Preprocessing — Snap-shot 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 Figure 2.9.
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.