Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 96)

Switchover Considerations
Chapter 6
6-11
Should the 1785-BCM module transfer one or more of these values to the
secondary system and the primary system fails, the data table of the
secondary system will contain intermediate values for an indeterminate
amount of time, thus creating illegal BCD values.
Important: Because block transfer instructions operate asynchronously to
the execution of the program scan, we suggest using the condition of the
done bit (DN) of the BTR or BTW instruction as part of the logic used to
initiate the transfer when using block transfer instructions to move data
from the primary to the secondary 1785-BCM module. Figure 6.3 shows
typical rungs that use the condition of the done bit to initiate a transfer.
Refer to appendix A, “Sample Programs” to see these types of rungs used
in a program.
Figure 6.3
Typical Data Move Rungs
BTR
DN
DATA
VALID
Check Block ID Move Data
BTW
DN
Check Block ID Move Data
A —> B
B —> A
11108I
Switchover Diagnostics
The 1785-BCM backup module continuously executes an internal
hardware diagnostics routine to ensure that it is operating correctly. In the
case that a primary module detects a major fault like bad RAM memory or
bad CPU, its relays will immediately open and a switchover will
automatically occur.
Check if the 1785-BCM module is operating correctly and avoid problems
in case of an eventual switchover in the backup system by executing a
manual switchover when it is possible. If a minor fault is detected during
this manual switchover, the module will switch. In addition, it will also
turn on the switchover diagnostic bit (bit 16) of the extended status word,
indicating that the module should be replaced as soon as possible. Refer to
the section in Chapter 8, Diagnosing Faults with Bits of the Auxiliary
Diagnostics/Status Words, for further information.