Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 91)

Programming Techniques
Chapter 5
5-10
BackedĆup/NotĆbackedĆup Bit
The backed-up/not-backed-up bit (file 0, word 3, bit 16) is set when option
switch 2 on the number one scanner module is in the down position. The
status of this bit is influenced by the switchover configuration.
The backed-up/not-backed-up bit is:
set in the primary processor when using the automatic switchover with
no switchback configuration
set in both the primary and backup processors when using the automatic
switchover with controllable switchback configuration
reset when using any other switchover configuration
Communication Error Bit
The communication error bit (status file 0, word 2, bit 10) indicates the
presence of a communication error.
This bit is set when:
either the primary or backup processor detects an error on a backup
communication channel
the primary processor detects an error on a peer-to-peer communication
channel
the designated input file is too small for the data received on either a
peer-to-peer or backup communication channel
Program Checksum
The checksum provides a preliminary indication of whether or not the
programs are exactly the same in the primary and backup processors.
When the checksums do not agree, the programs are definitely different.
Just because the checksums agree, however, does not guarantee that the
programs are identical. Because of this, you should not use the checksum
as the only verification that the programs are identical. You may use it as
an additional check, for example, after you load a program from tape.
A 16-bit checksum is generated for the application program if you created
words 4 and 5 in file 0 of the data table status section. You create these
words by using the create command or by referencing them in an
application program instruction.