Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual - Manual (Page 93)

Selecting Interrupt Routines
Chapter 7
7-11
In general, if the processor detects a hardware fault, it sets a major fault
and resets I/O. If the processor detects a run-time error, it sets a major
fault bit and the remote I/O racks are set according to their last state
switch. Module outputs in remote racks remain in their last state or they
are de-energized, based on how you set the last state switch in the 1771
I/O chassis.
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 the Classic 1785 PLC-5 Family Programmable
Controllers Hardware Installation Manual, publication 1785-6.6.1.
Important: In the PLC-5 processor local chassis, outputs are
reset—regardless of the last state switch setting—when one of the
following occurs:
processor detects a run-time error
you set a status file bit to reset a local rack
you select program or test mode
Fault in a ProcessorĆResident Local I/O Rack
The chassis that contains the Classic PLC-5 processor is the
processor-resident local I/O chassis. If a problem occurs with the chassis
backplane, the input and output data table bits for the resident local I/O
rack are left in their last state. The processor sets a minor fault and
continues scanning the program and controlling extended-local and remote
I/O.
Your ladder program should monitor the I/O rack fault bits and take the
appropriate recovery action (covered later in this section).
ATTENTION: If a 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
personnel and machine damage may result.
Understanding
ProcessorĆDetected
Major Faults