Allen-Bradley ControlNet PLC-5 User Manual page 49

• Equivalence checks
The PLC-5 ControlNet backup system does not download the
entire memory from the primary system to the secondary system;
there are certain data table values that are crossloaded. Areas
such as the program files and data table structures may be
different between the primary and secondary system. In some
applications, this is desirable. However, in most applications,
you must know for certain whether the primary and secondary
systems are identical, or whether they may differ in certain areas.
Equivalence checking gives you the option of enforcing a
qualification major fault if the systems differ. (Refer to Chapter 4
for descriptions of specific major faults.)
You can implement qualification equivalence checking by
calculating checksums for different memory areas of the PLC-5
system. These checksums are calculated on both the primary and
qualifying secondary system, and are passed from the primary
system to the qualifying system before equivalence checking is
performed. If you enable configuration checking for a particular
area, and the primary checksum is different than the qualifying
checksum, qualification fails and the qualifying system declares a
major fault.
There are 6 different areas in which equivalence checking can be
performed:
- Program File Structure and Program Files - This checksum is
calculated for all the Program files, with the notable
exception of SFC files. This checksum continues to be
calculated when the backup system is qualified. This allows
status to be available regarding whether the Program file
checksums differ for a qualified backup system.
- Data Table Structure - This checksum is only calculated when
the PLC-5 system goes through qualification. Since the Data
Table structure cannot change when in Run mode, this
checksum does not change after qualification. The Data
Table Structure checksum includes the data table file number,
the data table file type, and the size of the data table file. The
Data Table Structure checksum does not include the data
table file privilege information or the values of the data table
file elements.
- ControlNet I/O Map Table - This checksum is only calculated
when the PLC-5 system goes through qualification. Since the
ControlNet I/O map table cannot change when the PLC-5
system is in Run mode, this checksum does not change after
qualification. The ControlNet I/O Map Table checksum only
includes the scheduled redundant connections. Thus, as long
as the scheduled redundant connections are the same between
the backup PLC-5 systems, the ControlNet I/O Map Table
checksums will match.
Backup States
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