Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 76)

Using the 1775ĆMX
Memory Communication Module
Chapter 4
4-29
For example, if the primary processor encountered a message instruction
containing the command:
ENABLE COMPARE 1/2/0
(assuming the proper label comment numbers exist in the sections of the
ladder program) the memory communication module would:
compare the section in the main routine of the ladder programs starting
at the first rung and ending at the rung containing the label with
comment number 1.
compare the section in the subroutine of the ladder programs starting at
the first rung and ending at the rung containing the label with comment
number 2.
does not compare the section in the fault routine of the ladder programs.
Important: Some data appearing in the ladder diagram is actually
contained in the data table. If you alter a ladder diagram (in either the
primary or backup processor) in such a way that the information in the data
table changes but the information in the program section remains
unchanged, then the program comparison function will not detect a
miscomparison. This may happen when using the following instructions:
- message send - diagnostic detect
- block transfer - timers
- file instructions - counters
- bit shifts - first-in-first-out (FIFO)
Disable Compare
The DISABLE COMPARE command lets you disable the program
comparison function. The DISABLE COMPARE command is equivalent
to the DISABLE PROG COMP in LIST.
When the primary processor executes a message instruction containing the
DISABLE COMPARE command, the memory communication module
disables the program comparison function and resets the program
comparison status bit.