Rockwell Automation 1785 PLC-5 User Manual download pdf (Page 88)

Selecting Interrupt Routines

Chapter 7

7-6

Important: If the PLC-5 processor detects a fault in the fault routine

(double fault condition), the PLC-5 processor goes directly to fault mode

without completing the fault routine.

Programming a Fault Routine

If you choose to program a fault routine, first have the fault routine

examine the major fault information recorded by the PLC-5 processor and

decide whether to do the following before the PLC-5 processor

automatically goes to fault mode:

set an alarm

clear the fault

shutdown in an orderly manner

On detecting a major fault, the PLC-5 processor immediately suspends the

program file it was running and, if programmed, runs the fault routine file

once to completion. If the PLC-5 processor does not run a fault routine, or

the fault routine does not clear the fault, the PLC-5 processor automatically

switches to fault mode.

Set an Alarm

You may need an alarm to signal when a major fault occurs. Put this rung

first in your fault routine program

output

alarm

and combine it with a counter. You can also set an alarm in your fault

routine to signal when the fault routine clears a major fault.

Clearing the Fault

If you decide to clear the fault in the fault routine, place the ladder logic

for clearing the fault at the beginning of the fault routine. You can

compare the fault code with a reference.

Compare fault code with a reference—Identify the possible major faults

and then select only those your application will let you safely clear. These

are your reference fault codes.

From the fault routine, examine the major fault code that the processor

stores in S:12. Use an FSC instruction to compare the fault code to the

reference file that contains “acceptable” fault codes (word-to-file

comparison). If the processor finds a match, the FSC instruction sets the

found (.FD) bit in the specified control structure. Use a MOV instruction

to clear the fault in S:11. Then jump to the end of the fault routine to

quickly complete running the fault routine.

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Rockwell Automation 1785 PLC-5 User Manual Page 88