Rockwell Automation 1771-PD PID MODULE (+DU) - Manual (Page 110)

Programming
Chapter 3
3Ć67
Transfer 3 - 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 1 block data, the new data will be ignored.
The sequence will toggle between transfer 2 and 3 until the error is
corrected. The module will set the loop block error bit W58 B14 high and
will continue PID control based on previous loop block data.
Transfer 4 - The processor reads the status block to determine the next
block start address.