Rockwell Automation 1775-S5_SR5,D17756.5.5 User Manual PLC-3 FAMILY I/0 - Manual (Page 56)

I/O Communication
Chapter 3
3-8
Figure 3.6
Communication Flow Between Primary and Backup Controllers on a
Backup Communication Channel
input file
output file
output file
input file
Each input file must be large enough to store the data from the
corresponding output file. Otherwise, a backup communication minor fault occurs.
Processor Memory
Primary System
15413
Processor Memory
Backup System
A backup communication channel also prevents contention over the I/O
channel should the backup cable break or become disconnected. If such a
situation occurs, the system with the highest resident number takes control
over the I/O. You assign the resident and partner numbers (see chapter 2).
Installation of peer-to-peer and backup communication channels is
described in the PLC-3 Family Controller Installation and Operations
Manual (publication 1775-6.7.1) and PLC-3 Programmable Controller
Backup Systems Manual (Publication 1775-6.3.1).
Programming Backup Communication
Backup communication channels operate in the same manner as
peer-to-peer communication channels with these considerations:
In using a backup communication channel, you can send information
back and forth between the primary and backup systems. Since
information moves both ways, you can monitor the run/backup bit (data
table status section, file 0, word 3, bit 17) to determine whether to use
the received data.
The input file in the backup system must be equal to or larger than the
output file in the primary system. Otherwise a backup communication
minor fault occurs.