Rockwell Automation 1771-PD PID MODULE (+DU) - Manual (Page 112)

Programming
Chapter 3
3Ć69
Transfer 5 - The processor writes the loop 2 block to the PID module
where it is stored in a buffer.
If an error is detected in loop 2 block data, the new data will be ignored.
The sequence will toggle between transfers 4 and 5 until the error is
corrected. The module will set the loop block error bit W58 B14 high and
will continue PID control based on the previous loop block data.
Transfer 6 - The processor reads the status block to determine the next
block start address. The block contains ready bit W58 B12 set to 1 which
indicates the new data was valid.
At this point in the sequence, one of two paths may be taken. The enter
portion of the load/enter sequence can be performed immediately or the
data can be maintained in the buffer until activated.
A) Enter later:
Transfer 7 - The processor writes the dynamic block to the PID module.
The load bit W01 B06 is reset to 0 preventing a new load/enter sequence
from starting. Loop 1 and loop 2 constants remain stored in the module
buffer because the enter bit is not set.
Transfer 7 repeats as long as the enter bit remains reset.
New dynamic values to be used with the stored loop constants should be
transferred until the enter bit is transferred.
Transfer 8 - The read transfer of the status block will prompt the write
transfer of dynamic block. The ready bit W58 B12 information was valid
and the PID module is waiting for the enter command W01 B03 set to 1.
The enter bit must be set to 1 for the new loop constants to become active
as explained in part b.
B) Enter now:
Transfer 7 - The processor writes the dynamic block to the PID module
with enter bit W01 B03 set to 1. The PID module will begin control
based on the data received in the dynamic block. The loop 1 and loop 2
constants which were received and buffered become active.