Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 108)

Programming Techniques
Chapter 7
7-11
As with the timer instruction, your program should transfer the counter
accumulated value from the primary to the secondary processor at least once
after you:
start up the backup system
switch the secondary PLC-5 from PROGRAM to RUN mode
restart a repaired system
This allows the accumulated value in the secondary processor to track the
accumulated value in the primary processor.
Diagnostic, Sequencing, File Arithmetic and Logic, File Search and
Compare, File Copy and Fill Instructions
The behavior of these file instructions, which include the File Arithmetic and
Logic instructions (FAL), File Search and Compare (FSC), File Copy (COP),
File Fill (FLL), and Diagnostic (FBC and DDT), depends mainly on the type of
data you are transferring and how you transfer the data to the secondary
processor.
You should place the instruction’s control element and the data file it contains
all within the same 62 words of data. If you don’t, the 1785-BCM module
could transfer the control values responsible for control of file at a different
point than when it transfers the file. This results in a time lag in the secondary
processor between when the file is updated and when the associated control
element is updated. Thus, during switchover, it is possible that a file operation
that was running may be off by one word, a word could be operated on by the
file twice, or perhaps not even be operated at all.
FIFO and Bit Displacement Instructions
Again, the behavior of these instructions depends on the type of data you are
transferring and how you transfer the instructions to the secondary processor.
If you are transferring only selected areas of the data files which do not include
data for FIFO and bit instructions, the instructions will be executed normally.
However, if you are transferring all of the data table files over the HSSL, you
may encounter problems. For example, if you are shifting four bits of BCD
data through several words bit by bit, the data table will contain invalid data
until each shift is complete. Should the 1785-BCM module transfer one or
more of these values to the backup system and the primary system fails, the data
table of the backup system will contain intermediate values for an indefinite
amount of time, thus creating illegal BCD values.