Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 116)

Diagnosing Faults
Chapter 6
6-12
Keep the following points in mind when you use the fault routine in your
ladder program.
the rung in the main routine must be executed in both the primary and
backup processors
you must make certain that the set value in the watchdog timer
configuration is large enough to include the execution of this fault
routine along with the normal program scan.
any logic which follows the RET instruction in the fault routine
section will not be executed.
Interpreting the Saved Diagnostic Words
Status word S9:0 stores the user program section number in bits 0 and 1.
Table 6.I shows how to interpret these bits. The other bits which may be
set in this word can be ignored.
Table 6.I
Program Section Bits
Two Least Significant
Bits in S9:0
Ladder Program Section where Fault
Occurred
00 Fault not in ladder program
01 Main routine (RM)
10 Subroutine (RS)
11 Fault Routine (RF)
Status word S9:1 saves the rung number which caused the major fault to
occur. By changing the data display on your programming device to
decimal format you can identify the rung number.
Not all faults are the result of ladder diagram execution. Certain faults,
such as watchdog timeout or backplane fault will not record anything
in S9:0 or S9:1.
Status word S9:2 saves the major fault as recorded by the processor when
the system failed. Figure 6.7 identifies the meaning of the different bits
used to represent the possible major faults. For further definition of the
major fault word see the publication PLC–3 Family Programmable
Controller Programming Reference Manual (publication 1775–6.4.1).