Rockwell Automation 1771-PD PID MODULE (+DU) - Manual (Page 109)

Programming
Chapter 3
3Ć66
Load/Enter Sequence
The load/enter sequence permits selected word and bit data to be
transferred to the PID module in a manner that protects the data from
accidental change which could adversely affect the controlled process.
The load/enter sequence is used during normal operation when loop 1 and
loop 2 constants and bits B15, B14, B12, B11 in the master control word
W01 must be changed.
The load/enter sequence is similar to the power-up load/enter sequence.
However, the power-up bit is not examined, and the enter bit can be
delayed. During initialization, the exact sequence described in section
titled Power-Up Load/Enter Sequence must be followed. Thereafter, it is
possible to transfer loop constants in a load/enter sequence before they are
needed for a change to the control process. Loop constants can be stored
indefinitely in the PID module’s buffer until the enter bit is transferred,
completing the load/enter sequence.
NOTE: Dynamic block data can be transferred to the PID module when
needed by a write block transfer. Or, dynamic block data can be
continuously transferred to the PID module in the dynamic/status toggle
sequence. Dynamic block values are implemented immediately upon
transfer. In contrast, loop 1 and loop 2 constants require a load/enter
sequence and are buffered in the PID module until the enter bit is set.
The load/enter sequence is shown in Figure 3.22 and is similar to the
power-up load/enter sequence outlined in section titled Power-Up
Load/Enter Sequence.
Transfer 1 - The load/enter sequence is initiated by writing the dynamic
block data to the PID module. The load bit W01 B06 is set to 1. The
values transferred in this block are used immediately by the PID module.
The bits W01 B15, B14, B12, B11 which can redefine the module
configuration are not acted upon until the enter portion of the load/enter
sequence is performed.
If an error is detected in the dynamic block data, the new data will be
ignored. The PID module will set the dynamic block error bit W58 B15
high and will continue PID control based on previous dynamic block data.
Transfer 2 - The processor read the status block to determine the next
block start address.