Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 113)

Diagnosing Faults
Chapter 6
6-9
Creating the Fault Routine
The fault routine requires two rungs; one rung as the first rung in the
main ladder program memory section (RM0) and the other as the first
rung in the fault routine ladder program memory section (RF0).
These two rungs copy diagnostic status words to storage words in the data
table status section for safe keeping until they can be analyzed. The
status area was chosen so that if an MX is being used to transfer the data
table the saved data will not be overwritten. File 9 was chosen since it
currently is not reserved by any module. (If this file should become
reserved in the future, simply pick another unused data table status file.)
Table 6.H shows the data table addresses used by the fault routine to
store the diagnostic information.
Table 6.H
Storage Words used by Fault Routine
This Address Stores:
S9:0 Memory section number
S9:1 Rung number where fault occurred
S9:2 Major fault which caused switchover
The first rung of the main routine as shown in Figure 6.5, initializes the
data table status words and the E2 memory section words. Bit S0:3/01 is
used to condition the rung since this rung need only execute once after
powering up in the RUN mode or after changing modes from PROGRAM
to RUN.
Important: This rung must be programmed before the conditional end
rung which is often programmed in a backup system.