Rockwell Automation 1785 PLC-5 User Manual download pdf (Page 85)

Selecting Interrupt Routines

Chapter 7

7-3

You can write a fault routine that the processor runs when it detects a

major fault. For example, if your program file becomes corrupted, you can

tell the processor to interrupt the current program, run your fault routine

and then continue processing the original program.

This section shows you how to set and write a fault routine and how to

protect your processor from powering up in run mode after a power loss.

Responses to a Major Fault

When the processor detects a major fault, the processor immediately

interrupts the current program. If a fault routine exists (i.e., specified in

S:29 as a fault routine), the processor runs that fault routine program for

recoverable faults. Then, depending on the type of fault, the processor:

returns to the current ladder program file if the processor can recover

from the fault

enters fault mode if the processor cannot recover from the fault

For example, the rung in Figure 7.3 includes an instruction that causes

a major fault.

Figure 7.3

Sample

Ladder Logic for a Fault

][

Causes a

major fault

In the example in Figure 7.3, the processor runs the fault routine after

detecting the fault. If the fault routine resets the faulted bits, the processor

returns to the next instruction in the program file that follows the one that

faulted and outputs on the remainder of the rung.

If you do not program a fault routine for fault B, the processor

immediately faults.

The bits in word 11 of the processor status file indicate the type of major

fault. See Table 7.G to determine whether a fault is recoverable.

Writing a Fault Routine

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Rockwell Automation 1785 PLC-5 User Manual Page 85