Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 112)

Diagnosing Faults
Chapter 6
6-8
Diagnosing Faults with Status Word Bits
Another method of diagnosing faults is to examine the contents of status
words in the system memory. The state of individual bits, groups of bits, or
both can help you pinpoint the source of faults.
For more information about status words, see the chapter Programming
Considerations in this manual. Additionally, you will find detailed
information on status words in the publication PLC-3 Family
Programmable Controller Programming Reference Manual (publication
1775-6.4.1).
When the PLC–3 detects a major fault it looks for label 0 in the fault
routine memory section. You can enter a label with label number 0 as the
first instruction in the fault routine section and the processor will
execute a fault routine when it detects a major fault. You can determine
the cause of system failure by programming a fault routine and saving the
fault status at the time of failure.
The processor stores ladder logic fault status information in section E2 of
the system memory and major fault status in the data table status area file
0. Table 6.G lists the three memory words of interest.
Table 6.G
Diagnostic Status Words
Status Word Definition
E2.2.1.0.2.0 Decoding the two least significant bits yields the ladder logic
memory section (RM, RS, or RF)
E2.2.1.0.2.1 Rung number at which fault occurred
S0:1 System Major fault word
The following routine saves the state of these three system memory status
words to help you identify and locate faults in a ladder program. The
routine itself is short, occupying only two rungs of a ladder program.
A Logic Diagnostic Routine