1747-6.22, Backup Scanner User Manual

Switchover Considerations 8-161
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 - 10 stations, the impact on the
link due to switchover could be the loss of communication for as
much as 1 - 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: 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.
If the new primary is: the following could occur causing this result
Polling the link
Listening to the link
Message packet from another
station disrupted
Reply packet disrupted Same as above
Token pass packet disrupted
New primary processor takes
the token
New primary processor receives
a message from another station
The source station turns on error bit in its MSG instruction You
provide programming at the source station to regain
synchronization with the receiving station. You can do this by
monitoring the message instruction error bit as a condition for
retransmitting the message.
The DH+ link times out and must rebuild. (250ms watchdog timeout)
+ (50 ms x number of stations on the link).
The new primary processor has nothing in its active node table. The
DH+ link times out and must rebuild. (250 ms watchdog timeout) +
(50 ms x number of stations on the link).
The link continues working normally.
Important: In the recommended configuration, set both SLC 500
processors to the same station address. The backup
modules permit only one of the two processors to be
connected to the DH+ at a time. In this configuration,
remote PLC processors and operator stations
communicate to the processor that currently has primary
status. Note that the secondary processor can also be
accessed through the primary SLC 500 processor node
address plus one (n+1).
Publication 1747-6.22