Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 10)

Choosing a Backup Configuration
Chapter 1
1-3
Primary processor - the processor that is controlling outputs. When
switchover to the backup processor occurs, the backup processor becomes
the primary processor, since now it is controlling outputs.
Stand-alone processor - a programmable controller that is not configured
for backup.
Switchover - transfer of control from the primary processor to the backup
processor.
Backup Communication
Backup communication is communication between the primary and backup
processors. Without backup communication, the backup processor receives
only input and peer-to-peer data.
Since the primary and backup processors may have started at different
times and are not synchronized, the data table values may be different in
each processor. You can reduce these differences by transferring internal
storage points via backup communication.
You can establish backup communication using:
one or more of the serial I/O channels as a backup communication
channel.
the parallel data communication of the 1775-MX memory
communication module
Data Highway or Data Highway Plus communication