Publication 1785-UM012D-EN-P - July 2005
Preparing Fault Routines 15-3
To decide how to set this switch, evaluate how the machines in your process
will be affected by a fault. For example:
• how will the machine react to outputs remaining in their last state or to
outputs being automatically de-energized?
• what is each output connected to?
• will machine motion continue?
• could this cause the control of your process to become unstable?
To set this switch, see Appendix E.
Fault in a Controller-Resident or Extended-Local I/O Rack
If a problem occurs with the chassis backplane, the controller sets the
appropriate minor fault bit (S:7/0-7) and continues scanning the program and
I/O. As soon as this bit is set, the outputs for that rack are reset. However, the
controller continues normal run-time operation.
The outputs are enabled again only if the faulted rack condition is cleared. For
example, if a local I/O module faults, all outputs in that rack are reset and the
controller continues executing the program scan. Outputs will be enabled only
after the faulted module is removed.
Your ladder program should monitor the I/O rack fault bits (S:7/0-7) and take
the appropriate recovery action.
Fault in a Remote I/O Chassis
A remote I/O rack fault can be a loss of communications with the remote I/O
device or a problem with the remote I/O device itself. When the controller
detects a remote I/O rack fault, the controller sets an I/O rack fault bit in the
controller status table. The controller then continues scanning the program
and controlling I/O.
ATTENTION
If a controller-resident local I/O rack fault occurs and
you have no recovery methods, the input image table
and outputs for the faulted rack remain in their last
state. Potential injury to personnel and damage to the
machine may result.
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