Bradley Smoker PLC-5 user manual Using Ladder Logic to Recover from a Fault

15-10Preparing 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	Using fault zone programming method, you disable sections of
method	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.

Publication 1785-UM012D-EN-P - July 2005