Rockwell Automation 1771-PD PID MODULE (+DU) - Manual (Page 185)

Appendix
C
CĆ1
Application Example 2, Periodic Block Transfer
The periodic block transfer program allows you to keep the number of
PID block transfers to a minimum, so as not to increase the block transfer
times of other block transfer modules in the system. The example
described in this appendix is a 2-loop application that could use
continuous or periodic block transfer programming. The first loop
controls temperature, the second controls flow. Both loops operate
independently.
Following the program description, typical values of selected standard and
expanded control features are introduced using a worksheet for each
dynamic block and loop block. Worksheets simplify record keeping
associated with programming the module. Worksheet forms in appendix
A should be reproduced and used as needed.
A fluid or material is passed through a heat exchanger where the
temperature is maintained at a setpoint for loop 1 and then through a valve
where the flow is maintained at a flow setpoint for loop 2. A temperature
transducer monitors the temperature of the fluid or material as it exits the
heat exchanger in loop 1. A flow transducer monitors the flow rate as the
fluid or material leaves the flow control value in loop 2. The output of
loop 1 controls the energy permitted into the heat exchanger while the
output of loop 2 regulates the rate of flow of material or fluid. A manual
control station is electrically connected in each loop to provide manual
back-up if needed (Figure C.1).
Program for PLC-2 Family Processors
This example program shows the PID module’s stand alone operation
feature where operating parameters are transferred to the module during
an initial load/enter sequence. Thereafter, block transfers are minimized.
The status monitor byte reports module status when block transfer
instructions are not enabled. Periodically, a read block transfer and
watchdog timer instruction are enabled to verify communication with the
PID module. A parameter change, whether to the dynamic block or loop
blocks, requires a manually initiated load/enter sequence. It can be
initiated by an operator at any time.
General
Example Application