Rockwell Automation PLC-5 Fieldbus Solutions for Integrated Architecture User Manual - Manual (Page 71)

Publication 1757-UM006A-EN-P - May 2002
Integrating Fieldbus into Rockwell Automation Logix System 2-27
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.
Figure 2.9 Algorithm execution phase sequence
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 10 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.
Mode
Calculation
Set Point
Calculation
Out
Calculation
Output
Parameters In
Backward Path
Cascade
Remote Cascade
Remote Out
Back Calculation
Primary Input
Block Specific
Parameters
Primary Output
Remote Cascade Out
Back Calculation Out
Remote Out Out
Target
Mode
Actual
Mode
Set Point PV
SP & OUT
1
2
3
4