Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (Page 247)

Publication 1785-UM012D-EN-P - July 2005
Preparing Fault Routines 15-9
Follow these guidelines when creating fault routines:
• Store initial conditions and reset other data to achieve an orderly
start-up later.
• Monitor the shutdown of critical outputs. Use looping if needed to
extend the single fault routine scan time up to the limit of the controller
watchdog timer so your program can confirm that critical events took
place.
Changing the Fault Routine from Ladder Logic
You can change the specified fault routine from ladder logic by copying a new
fault routine file number into word 29 of the controller status file.
The following figure shows an example rung for changing the fault routine file
number.
IMPORTANT
When you clear a fault within the fault routine, it is possible
for the controller to clear the fault, return to the faulty
code, experience the same fault again and repeat the
process over and over. When this happens, the controller is
executing the faulty code and the fault routine code, but
not the main ladder program. This causes the inputs and
outputs to remain in their last state. To avoid this problem,
place additional code in the fault routine to instruct the
controller to execute the main portion of the ladder code
once the fault is cleared.
MOV
MOVE
Source
Dest
S:29
12
ATTENTION
Do not corrupt the program-file number of the fault
routine or use the same file for any other purpose. If
the file number you specify results in a non-existent
fault routine, the controller immediately enters fault
mode after detecting a fault. Unexpected machine
operation may result with damage to equipment
and/or injury to personnel.