Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (Page 248)

Publication 1785-UM012D-EN-P - July 2005
15-10 Preparing Fault Routines
Using Ladder Logic to Recover from a Fault
If you have the appropriate fault routine and ladder logic to perform an orderly
shutdown of the system, you may want to configure an I/O rack fault as a
minor fault. You can program ladder logic in several ways to recover from an
I/O rack fault.
Ways to Recover from a Rack Fault
Method Description
User-generated major fault The program jumps to a fault routine when a remote I/O rack
fault occurs. In other words, if the status bits indicate a fault,
you program the controller to act as if a major fault occurred (i.e.,
jump to the fault routine). You then program your fault routine to
stop the process or perform an orderly shutdown of your system.
When the controller executes the end-of-file instruction for the
fault routine, a user-generated major fault is declared.
Reset input image table You monitor the status bits and, if a fault is detected, you
program the controller to act as if a minor fault occurred. After
the status bits indicate a fault, use the I/O status screen in your
programming software to inhibit the remote rack that faulted.
You then use ladder logic to set or reset critical input image
table bits according to the output requirements in the
non-faulted rack.
If you reset input image table bits, during the next I/O update,
the input bits are set again to their last valid state. To prevent
this from occurring, your program should set the inhibit bits for
the faulted rack. The global inhibit bits control the input images
on a rack by rack basis; the partial rack inhibit bits control the
input images on a 1/4-rack basis. For more information on these
global status bits, see the documentation for your programming
software.
This method requires an extensive and careful review of your
system for recovery operations. For more information on
inhibiting I/O racks, see the documentation for your
programming software.
Fault zone programming
method Using fault zone programming method, you disable sections of
your program with MCR zones. Using the status bits, you
monitor your racks; when a fault is detected, you control the
program through the rungs in your MCR zone. With this method,
outputs within the MCR zone must be non-retentive to be
de-energized when a rack fault
is detected.
For more information on MCR zone programming, see the
documentation for your programming software.