Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 13)

Choosing a Backup Configuration
Chapter 1
1-6
Selecting Scanner or MX Backup
Table 1.A and Table 1.B summarize the attributes of scanner backup and
MX backup, respectively. Choose the backup configuration most suitable
for your application.
Table 1.A
Scanner Backup Attributes
Advantages: Disadvantages:
Ć No additional cost (every 1775ĆS4A, ĆSR, ĆS5,
and ĆSR5 has this feature built in).
Ć Backup communication channel is necessary
for transferring data table and the transfer is
relatively slow (8 words per 14 milliseconds.)
Ć External switch box may be required.
Ć Processor restart procedure is not flexible.
Table 1.B
MX Backup Attributes
Advantages: Disadvantages:
Ć Data table in the backup processor is virtually
identical to the one in the primary processor.
Ć No external hardware is needed.
Ć HighĆspeed data transfer from the primary to
the backup processor. (100k words per second
for continuous or 10k words per second for
oneĆtime transfers).
Ć Ability to transfer program data from the
primary processor to the backup processor.
Ć Extra features are available, such as memory
download, memory compare, backup status,
etc.
Ć Easily configurable through LIST.
Ć Simple restart procedure.
Ć More costly than scanner backup.
Ć Requires an extra slot in each processor
chassis.
Ć Increased time required for other operations
such as program scan. This increase is
approximately 10% for file intensive programs
and approximately 15% for I/O intensive
programs. You must consider this increase
when developing your program and configuring
your system.
Ć The only permissible switchover method is
automatic switchover with controllable
switchback.