Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 10)

Chapter
1
1-1
Backup Concepts for the PLC-5 System
This chapter describes concepts for using a backup system with your
programmable controller. In addition, this chapter provides an overview of
a PLC-5 processor backup system that uses 1785-BCM backup
communication modules and a system that also uses 1785-BEM backup
expansion modules.
The objective of any redundant system (backup system) is to improve the
amount of up-time of a machine or process by ensuring consistent
availability of that machine, and by reducing costs associated with
equipment failure. By using this backup system, you can guard your
application against shutdowns caused by the programmable controller.
ATTENTION: Backup does not protect you from faults caused
by programming errors or system timeouts because such an
error or timeout will also occur in the secondary processor.
The backup option is used where you must transfer the control of the
process to a secondary system, without thereby interrupting the
machine/process operation.
To guard against system shutdown, a backup system must provide:
equipment with exceptional reliability
automatic fault isolation
minimal disturbance of the process when switching from the primary to
the secondary system
Chapter Objectives
Why Use a Backup System?