Rockwell Automation 1771-PD PID MODULE (+DU) - Manual (Page 162)

Application Example 1, Continuous Block
Transfer
Appendix B
BĆ3
If data in a loop block requires change at any time, a single load/enter
sequence can be initialized by closing a pushbutton switch (111/00, rung
2). The load/enter sequence is initiated automatically at module power-up
and during recovery from a power loss.
The program also allows manual control over the soft fault reset bit (W01
B10, 257/10) using a selector switch. When bit 257/10=1, the PID
module automatically return to normal operation once the cause of a soft
fault has been corrected. However, when bit 257/10=0 the PID module
will not automatically return to normal operation when the cause of the
soft fault is corrected. Return to normal operation must be manually
initiated by closing switch 111/06. Manual soft fault reset allows the
operator to determine the cause of the soft fault and review the current
process status before returning control to the PID module.
If, for some reason, the process variable signal becomes lost, the PID
module would normally hold the analog output value existing at that
moment. As an alternative, in this application the ladder diagram
program automatically sets the output to a stored value upon loss of the
process variable signal. The program examines the loss of input bit (W61
B07) and sets the output 1 bit (W01 B01). This sets the analog output to
the value stored in W05.
Indicators have been programmed to illuminate if any of the following
conditions should occur:
the manual control station is in manual mode
communication between PID module and PC processor is delayed or
lost
the PID module is powered and waiting for a load/enter sequence
the load/enter sequence failed
The ladder diagram program for PLC-2 family processors that use block
format instructions is presented on the next three pages.