ABB AC 800M Manual page 49

Section 1 Introduction
If the primary unit fails because of an error, the backup unit resumes execution from
the last rollback point, which means the last execution unit is partially re-executed
by the backup unit. In order to re-execute a portion of the execution unit without
affecting the peripheral units (communication units on the CEX-Bus), the peripheral
units' references are also logged between rollback points. During re-execution, the
results of the peripheral units' references, which have already been executed, are
used, rather than re-executing them. The results of read operations are retrieved
from the log, and write operations pass without execution, since they have already
been executed. The peripheral units' statuses, then, are not affected by the re-
execution in any way, except for the time delay which occurs.
The RAM included in the processor unit provides an automatic double inverted
memory function for detection of arbitrary bit errors in the memory.
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The double inverted memory handling is done in hardware and without any delay to
the memory cycle time.
MAC and IP Address Handling in Redundant Configuration
In order to provide for a bumpless changeover with respect to the control network,
both the MAC and IP addresses are swapped between the initial primary and backup
CPUs. The addresses of the initial primary CPU are stored and kept as the addresses
used by the acting primary CPU. Similarly the addresses of the initial backup CPU
are stored to be used by the acting backup CPU. This means that a redundant
controller will be always identified and recognized by the same addresses regardless
of which CPU module actually acting as primary.
3BSE036351-510 A
All memory updates are written to both the primary memory and to the inverted
memory in parallel.
At every memory read cycle, the data from tho two memories is compared.
If there is a mismatch in the data a changeover is forced.
PM891/PM86x/TP830 Processor Unit – Redundancy
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