User manual

Hot Standby
33002479 06 07/2008 319
Hot Standby
Switchover
The following table describes the manner in which NOEs coordinate the Hot
Standby switchover.
Step Action
1 NOE A is running in the primary CPU PLC and NOE B is in the secondary PLC in a
hot standby configuration.
2 NOE A detects that its PLC has changed from primary CPU to offline mode.
3 NOE A changes its HSBY state from primary CPU to offline with the same Ethernet
services running, and starts its watchdog timer (with 500 ms timeout setting). It waits
for a UDP request to swap IP addresses from NOE B.
4 NOE B detects that its PLC has changed state from secondary to primary CPU.
5 NOE B stops all its Ethernet services, sends a UDP request to NOE A for the
synchronization of the IP address swap, starts its watchdog timer (with 500 ms
timeout setting), and waits for an UDP response from NOE A.
6 Once NOE A receives the UDP request from NOE B (or after its watchdog timer
times out), it stops all its Ethernet services. If it has received a UDP request, NOE
B sends a UDP response to NOE A; if its watchdog timer has timed out, NOE B does
not send a UDP response. NOE A then swaps its IP address and starts secondary
services.
7 NOE B swaps IP addresses and starts Ethernet services as primary CPU.
8 After NOE A senses that its local controller changes state from offline to standby, it
takes the secondary IP address.
9 NOE B now becomes the primary CPU NOE.
10 NOE B opens all client connections and listens for all server connections and
reestablishes those connections.
11 NOE A listens for all server connections and reestablishes those connections.
Note: During the Hot Standby switchover, there is a loss of communication during
500 ms between the PLC and the HMI and/or Unity Pro.