Rockwell Automation 1771-PD PID MODULE (+DU) - Manual (Page 114)

Programming
Chapter 3
3Ć71
PID module goes directly to transfer 6 which prompts the dynamic block.
Review section title Load/Enter Sequence if necessary.
The prompting is controlled by the PID module. Note that the program in
appendix B is selected for loop 1, only. To program two loops, the ladder
diagram program would not change. Only the bit selection is changed to
select the 2-loop feature. Loop 2 values would have to be entered into the
appropriate file.
Periodic Block Transfer
When the PC processor is communicating with many block transfer
modules, it may be desirable to reduce the number of continuous block
transfers in order to reduce the system scan time. This can be done by
performing block transfers periodically or only when they are required.
The technique may not apply to all applications.
The technique is based on the capability of the status monitor byte to
report the general status of the PID module without a read block transfer.
No block transfers are performed until necessary. If a fault is detected and
indicated in the status monitor byte, the dynamic/status toggle sequence is
enabled by program logic until the problem can be located and corrected.
The status monitor byte should never by used alone to determine module
status. Periodic read block transfers should be initiated to verify proper
operation. A fee-running timer should be added to the ladder diagram
program to request a read block transfer at timed intervals. In the event of
a PID module or communications failure, the status monitor byte will not
be updated to show a fault. However, the faulted module will not perform
a block transfer. The watchdog timer for the read block transfer will time
out to indicate that the PID module is no longer communicating with the
PC processor. This technique is used in the program in appendix C.
Block Transfer Timing
There are two areas of block transfer timing: module timing and system
timing. Module timing refers to the time required to process the
transferred data, perform the PID algorithm and prepare for the next block
transfer. This is typically 100 msec (110msec max.) regardless of the
number of words transferred to the PID module. Once a block transfer is
completed, another transfer will not be permitted until the next loop
update is complete. Module timing is also the loop update time referred