Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 60)

Using the 1775ĆMX
Memory Communication Module
Chapter 4
4-13
Enable Program Compare Command
The ENABLE PROG COMP command lets you compare the ladder
program in the backup processor with the one in the primary processor on
a word-by-word basis. This function works in conjunction with the
program compare limits selections. See Table 4.F.
Table 4.F
Program Compare
If the memory communication module: Then:
completes the program compare without
error
bit 13 is set in file 4, word 0 of the status section.
(You must reset this bit.) The memory
communication module continues the comparison
until a miscompare occurs.
encounters a miscompare, LIST will show:
L
Enable Program Compare
A
Disable Program Compare
bit 12 is set in file 4, word 0 of the status section;
the address of the rung containing the
miscomparison is put into file 4, words 1 and 2;
and bit 14 is reset in file 4, word 0.
To continue the comparison, you must reset the
status bit (bit 12 in file 4 word 0) and reselect
ENABLE PROG COMP
For a further description of status bits, see Using
Data Table Status Bits in chapter 5 of this manual.
For a program comparison to occur, the primary processor must be in run
mode or test mode. If you switch the primary processor to program mode,
the program comparison function becomes inactive. When you switch the
primary processor out of program mode, program comparison will become
active again if the selection ENABLE PROG COMP is still active in LIST.
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 the 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)