Siemens SIPROTEC Manual page 38

Redundant PROFIBUS DP interface
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The described mechanism does not presuppose that the BACKUP master gives a
receipt for reading the event list entries (see above, point 3.).
It as well is possible that the BACKUP master behaves passively or that there is not a
communication with the BACKUP master.
Note:
If a BACKUP slave module becomes PRIMARY because of a redundancy switchover
and it has not got an receipt via RedCom to the event list entries which currently are
being sent to the master then the same event list entries are sent again to the master
with a new incemented sequence number.
This ensures that no event list entries are lost during a redundancy switchover.
If the BACKUP module can not find the entries in its event list which are contained in
the RedCom telegram from the PRIMARY module then the bit "Message lost" in the
"Control_I" byte of the input telegram is set.
Possible reasons for this and conditions to reset the bit are explained in chap. 1.4.1.4.
SIPROTEC PROFIBUS DP - Bus mapping 6MD662/3/4
C53000-L1840-B011-03