Publication 1785-UM012D-EN-P - July 2005
15-2 Preparing Fault Routines
• returns to the current ladder program file if the controller can recover
from the fault
• enters fault mode if the controller cannot recover from the fault
For example, this rung includes an instruction that causes a major fault:
Understanding
Controller-Detected Major
Faults
In general:
[
A
B
C
Causes a
major fault
[
In this example, the processor runs the fault routine after detecting the fault. If the
fault routine resets the faulted bits, the processor returns to the next instruction in
the program file that follows the one that faulted and continues executing the
remainder of the rung.
If you do not program a fault routine for fault B, the processor immediately faults.
If the Controller Detects a It Sets
major fault a major fault bit and resets I/O
memory loss fault outputs in 1771-ASB remote I/O racks and/or 1771-ALX
extended-local I/O racks are set according to their last
state switch setting
The outputs remain in their last state or they are
de-energized, based on how you set the last state switch
in the I/O chassis.
IMPORTANT
In the PLC-5 controller-resident chassis, outputs are reset
regardless of the last state switch setting when one of the
following occurs:
• controller detects a major fault
• you set a status file bit to reset a local rack
• you select program or test mode
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