Rockwell Automation 1747-BSN Backup Scanner Module - Manual (Page 139)

Publication 1747-UM010B-EN-P - September 2003
Switchover Considerations 8-5
It is possible that the primary processor could have possession of the
token during a switchover from the primary to the secondary
processor. In this case, the token could be lost even though passage
of the token from one station to the next is done as quickly as
possible.
If the token is lost, all the stations on the link have an internal
watchdog timeout (250 ms) and the nodes assume the token is lost.
The other stations on the link initiate a token recovery procedure
which includes:
•
recreating the active node table
•
rebuilding the link, including the new primary processor
•
ensuring that one node gains control of the link
During this time, while the stations reconstruct the link,
communication is interrupted. The length of the interruption depends
on the number of stations involved in the link and the number of
messages received and/or transmitted. This time is typically less than
50 ms per station.
However, while the link is rebuilding, messages can collide,
increasing the amount of time needed to reconstruct the link. As a
result, on a typical DH+ link with 6 to 10 stations, the impact on the
link due to switchover could be the loss of communication for as
much as 1 to 3 seconds.
DH+ switching occurs whenever there is a transfer of control from the
primary to the secondary processor. The switchover could result from
a power failure or because of other system failures. The table below
shows what can happen to the link if the token is lost.
IMPORTANT
When a transfer of control from the primary to the
secondary processor occurs, the processor that is
now primary has the node address of n on the DH+
link. The processor that is now secondary has the
node address of n+1.