Rockwell Automation 1785 PLC-5 User Manual download pdf (Page 242)

Publication 1785-UM012D-EN-P - July 2005

15-4 Preparing Fault Routines

The outputs in the faulted rack remain in their last state or they are

de-energized, based on how you set the last state switch in the I/O chassis.

Outputs in the controller-resident chassis and in any non-faulted remote rack

can remain active if a remote I/O rack fault is detected. Make sure to design

your program so that the system goes to a known state in the event that

outputs in the controller-resident chassis or non-faulted remote racks are

controlled by inputs from the faulted remote I/O rack. Your program must be

able to account for the inputs remaining in their last state or the program must

monitor the rack fault status bits and reset the input image data table to make

remote inputs inactive.

Here are two programming methods you can use:

• In the very first executable instruction, the program monitors the rack

fault bits. If any faults bits are set, the program copies zeros (0) to the

faulted rack’s input image data table. The program must continually

copy zeros at the beginning of the program scan to the input image table

as long as the fault condition remains because the controller sets the

input image bits back to the last state at the end of the program scan.

• In the very first executable instruction, the program monitors the rack

fault bits. If any fault bits are set, the program sets the corresponding

inhibit bit for the faulted rack. The program must then execute a one

time copy of zeros to the faulted rack’s input image table to reset all

inputs.

Defining a Fault Routine

You can write multiple fault routine programs and store them in multiple fault

routine files, but the controller runs only one fault routine program when the

PLC-5 controller detects a major fault. You can, however, change the fault

routine program that is to be run through ladder logic. If you do not specify a

program file number, the controller immediately enters fault mode after

detecting a fault.

ATTENTION

If outputs are controlled by inputs in a different rack

and a remote I/O rack fault occurs (in the inputs rack),

the inputs are left in their last non-faulted state. The

outputs may not be properly controlled and potential

injury to personnel and damage to the machine may

result. Be sure you have recovery methods.

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Rockwell Automation 1785 PLC-5 User Manual Page 242