Allen-Bradley ControlNet PLC-5 User Manual page 51

You can selectively enable equivalence checking for none,
some, or all of the 6 of the equivalence check areas, with one
exception: the Program File Structure and Program File
checksum is dependent on the Data Table Structure. Thus, if
you select the Program File Structure and Program File
equivalence check, you should also select the Data Table
Structure equivalence check.
Equivalence checking is only performed during qualification.
Once a PLC-5 system passes qualification, it does not
become disqualified even if the checksums for an area are
different and the equivalence checking for that area is
enabled. For example, if the Program File Structure and
Program File equivalence check is enabled and there is a
qualified secondary, you can edit the program files on either
system without causing the secondary system to disqualify.
However, if one of the PLC-5 systems goes out of Run mode
and back into Run mode and a qualification occurs, an
equivalence check failure during qualification causes a major
fault to be declared for the qualifying system. Therefore,
when performing edits on either PLC, we recommend that
you disable equivalence checking first to avoid the condition
where a PLC-5 processor cannot become qualified due to
failing an equivalence check. Once all the edits are tested
and placed into both PLC-5 systems, you can reenable the
equivalence check.
Word 49 in the ControlNet Backup Configuration and Status
file gives you equivalence check status for a qualified backup
system. A bit is set for each equivalence check area if the
equivalence areas checksum differs between the two
processors. This word is updated every few seconds so that
you can know, if secondary qualification were to happen,
which equivalence checks would pass, and which
equivalence checks would fail.
• Complete data table crossload
If all the equivalence checks pass, a Qualification Data Table
crossload is performed. This crossload reads the data table file
values from the primary system and writes those values into the
corresponding data table files in the secondary system.
This data table crossload is performed while the qualifying
PLC-5 system is still in Program mode. It is necessary to remain
in Program mode until the data table crossload is finished in order
to prevent the qualifying system from major faulting or going into
an odd state due to uninitialized data table values. It is safer for
the qualifying PLC-5 system to first do the Data Table crossload
and then to go into Run mode. In that case, the data table values
are close to the values on the currently executing primary system.
Note: The time it takes to do the Qualification Data Table
crossload primarily depends on the amount of data table data to
be crossloaded.
Backup States
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