Rockwell Automation 1771-PD PID MODULE (+DU) - Manual (Page 188)

Application Example 2, Periodic Block
Transfer
Appendix C
CĆ4
NOTE: The pushbutton contact can be substituted with bit 15 from a
free-running timer allowing a load/enter sequence to be executed
periodically.
2. Setting the Soft Fault Reset Bit: When communication between the
PID module and processor has been interrupted (such as a
disconnected or broken I/O inteconnect cable) the module will enter
the soft fault mode of operation. After the condition which caused
the fault is corrected, either one of two possible module responses
will occur depending on the state of the soft fault reset bit W01 B10.
a. W01 B10 = 1 The module will enter the normal operating mode
immediately after the condition is corrected.
b. W01 B01 = 0 The module will remain in soft fault mode.
If the latter condition exists, the program will be attempting load/enter
sequences because bit 00 of the status monitor byte is high. The module
will not allow a load/enter sequence to occur until bit 257/10,
corresponding to module bit W01 B10, is set in the program by selector
switch 111/06. When set, the module will enter the normal operating
mode and load/enter sequence will be executed. This feature allows the
operator to decide when the module will enter the normal operating mode
after a soft fault condition has been corrected. The operator may wish to
determine the cause of the soft fault and determine current status before
returning control to the PID module.
The program also allows the analog output to be set to a programmed
value in the event of a lost of the process variable signal. If the PV signal
at analog input 1 should become lost, the module would normally hold the
last analog output value. This program examines the loss of input bit
W61 B07 and sets the bit in the master control word which allows the
value in word W05 to determine the analog output.
When the PV signal is restored, the module automatically returns to
normal operation where the analog output is determined by the PID
algorithm.
Also incorporated in the program is a provision which causes a read block
transfer to occur periodically at a user-defined time interval. This allows
loop status parameters to be reported to the PC processor as a safe-guard
to ensure that the system is operating correctly.