Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 90)

Switchover Considerations
Chapter 6
6-5
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 this time 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
Data Highway Plus link with 6 – 10 stations, the impact on the link due to
switchover could be the loss of communication for as much as 1 – 3
seconds.
Data Highway Plus 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. Table 6.A
shows what can happen to the link if the token is lost.
Important: Note that 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.