Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 90)

Programming Techniques
Chapter 5
5-9
Another approach for reducing the amount of data transferred across a
backup communication channel is to use Data Highway or Data Highway
Plus communication. Although you will still need a backup communication
channel to provide protection against having both processors controlling
outputs at the same time, you can use a Data Highway or Data Highway
Plus port as an additional backup communication channel.
For further information about using communication adapter modules, refer
to the 1775-KA Communication Adapter Module User’s Manual
(publication 1775-6.5.1) and the 1775-S5, -SR5 I/O Scanner
Communication Adapter Module User’s Manual (publication 1775-6.5.5).
The data table status section of PLC-3 memory contains information about
PLC-3 operation. Table 5.A lists the status information for the backup
system configuration.
Table 5.A
Data Table Status
Status Information File Word Bit
Run/Backup Bit S0 3 17
BackedĆup/notĆbackedĆup Bit S0 3 16
Communication Error Bit S0 2 10
Program Checksum S0 4 & 5 -
1775ĆMX Module Communication S4 0 -
1775ĆMX Module Program Compare S4 1 & 2 -
1775ĆMX Module System ID S4 3 -
1775ĆMX Module Backup to Primary
Transfer Data Words
S4 4 - 19 -
Run/Backup Bit
The run/backup bit (file 0, word 3, bit 17) indicates whether or not the
processor is acting as a backup processor. This bit is set in the primary
processor and reset in the backup processor.
Using Data Table Status Bits