Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 16)

Publication 1785-UM024B-EN-P - January 2003
1-2 Backup Concepts for the ControlNet PLC-5 Backup System
Basic System Architecture The following illustration describes the various components of the
basic ControlNet PLC-5 hot backup system:
How the Backup System
Works In a ControlNet PLC-5 hot backup system, one PLC-5 programmable
controller, referred to as the primary programmable controller,
controls the operation of the ControlNet I/O. The other programmable
controller, referred to as secondary programmable controller, is set up
to take control of the ControlNet I/O in the event of a fault in the
primary programmable controller.
The ControlNet network is wired to both PLC-5 programmable
controllers, as well as to all ControlNet I/O in your system. Using
ControlNet, the programmable controllers communicate (or
“handshake”) with each other and exchange status information on
their operational states. If the primary programmable controller faults,
or is otherwise unable to continue control of the outputs, the
secondary programmable controller assumes control and becomes the
primary programmable controller.
Both PLC-5 programmable controllers are actively connected to the
ControlNet I/O, receive all input data sent by the I/O adapters, and
use that data as they execute their program scans. Output data is sent
via ControlNet from both programmable controllers. However, only
data from the primary programmable controller is used by the I/O
adapter(s).
With the ControlNet PLC-5 backup system there are several features to
consider that, though common to many backup systems, are uniquely
implemented in this particular system. Understanding these features
and their functions will help you design and implement your backup
applications.
TM
TM