Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 14)

Backup Concepts for the PLC-5 System
Chapter 1
1-6
How the PLC-5 Backup System Works
In the PLC-5 backup configuration, one system (consisting of one PLC-5
processor, 1785-BCM module, power supply, and chassis) controls the
operation of the remote I/O. This system is referred to as the primary
system. The other system is ready to take control of the remote I/O in the
event of a fault in the primary system. This is referred to as the
secondary system. The PLC-5 backup system does not back up local I/O;
therefore, do not install I/O in the local chassis.
Data Transfer
During normal operation, the primary system sends remote input and data
table data to the secondary system so that in the event of a switchover, the
secondary system (which becomes the new primary system) has the same
data.
Remote I/O data is automatically transferred over the High-Speed Serial
Link (see Figure 1.1). This transfer is independent of the application
program.
Data table values are transferred from the primary to the secondary system
with block transfer instructions that you include in your ladder program.
You do not have to transfer data table values if not necessary for your
application. Figure 1.3 shows how data table data is transferred from the
primary to the secondary system.
Figure 1.3
Transfer of Data Table Data From the Primary to Secondary System
Primary
PLC-5 Primary
1785-BCM Secondary
1785-BCM Secondary
PLC-5
BTW BTR
Data is sent
over the
HSSL
11050I
For detailed information about data transfer from the primary to the
secondary system, refer to Chapter 5, “Operating Your PLC-5 Backup
System.”