Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 9)

Choosing a Backup Configuration
Chapter 1
1-2
Why Use a Backup System?
Backup is useful for applications in which an unexpected shutdown is
costly or otherwise undesirable. By configuring a PLC-3 or PLC-3/10
system for backup communication, you can guard against shutdowns
caused by processor faults.
Shutdowns can also be caused by power losses or faults in equipment other
than the programmable controller. For these cases, you must take other
measures to guard against shutdown.
Important: Backup does not protect you from faults caused by
programming errors or system timeouts, because the error or timeout will
also occur in the backup processor.
Backup Terminology
Some terms you should be familiar with are:
Backup communication - communication between the primary and the
backup processors.
Backup communication channel - transfers data from the primary to the
backup processor; transfers data table values from the backup to the
primary processor; and, identifies the default processor if a cable breaks. A
backup communication channel is required for backup systems using the
scanner backup configuration.
Backup mode - a mode in which a processor performs a listen-only
function (monitors inputs without controlling outputs) while remaining
ready to take control of the outputs.
Backup processor - a programmable controller which performs a
listen-only function (monitors inputs without controlling outputs). It
remains ready to take control of outputs should a fault occur in the primary
processor.
Backup system - includes the primary and backup processor. When we use
the term processor we are referring to a processor chassis, modules, cables,
and the other components needed for operation.
Channel configuration - a procedure using the LIST feature of the
processor to assign a specific operation to one of the channels on an I/O
scanner module.
Hardware configuration - the manner in which the backup processor is
installed with the primary processor.