Quantum Hot Standby for Control Expert
Original instructions
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The Hot Standby Solution
CAUTION
RISK OF INCREASE OF TIME TO SWAP
Whenever possible, use of a switch (not a hub) to connect the NOE modules to each other or to the network.
Schneider Electric offers switches. Contact a local sales office for more information.
Failure to follow these instructions can result in injury or equipment damage.
WARNING
UNINTENDED EQUIPMENT OPERATION
Design your application so that unmonitored modules support communication only to noncritical parts of the application.
Failure to follow these instructions can result in death, serious injury, or equipment damage.
In the hot standby solution, two identically configured PLC systems are set up to control the same application. One PLC, the primary, runs the application and updates the other secondary (standby) PLC. The standby maintains awareness of the application status but does not perform any control functions. In the event of a failure in the primary PLC, the standby PLC takes over the primary PLC responsibilities. When the PLC that has failed becomes operational again, it comes back up in the hot standby system as the new secondary PLC.
The NOEs coordinate the swapping of IP addresses. After closing both the client and the server connections, each NOE sends a swap UDP message to its peer NOE. The sending NOE then waits a specified timeout (500 ms) for the peer swap of UDP messages. Either after receiving the messages or after a timeout, the NOE changes its IP address.
NOTE: NOEs must communicate with each other in order to swap IP addresses. Schneider Electric recommends that you connect the primary and secondary NOEs to the same switch because:
  • Communication failures between the NOEs increases the time to swap.
  • Connecting 2 NOEs to the same switch minimizes the probability of a communication failure.
NOTE: Schneider Electric recommends that you use a switch (not a hub) to connect the NOEs to each other or to the network. Schneider Electric offers switches; please contact a local sales office for more information.
The NOE waits for either a change in the controller’s Hot Standby state or the swap of UDP messages. Then the NOE performs 1 of 2 Hot Standby actions.
If the NOE . . .
Then
Detects that the new Hot Standby state is either primary or standby
The NOE changes the IP address.
Receives a swap UDP message
The NOE transmits a Swap UDP message and swaps the IP address.
All client/server services (I/O scanner, global data, messaging, FTP, SNMP, and HTTP) continue to run after the switch from the old to the new primary NOE.
NOTE: Failure of an NOE module is not a condition for the primary system to leave the primary state.
Hot Standby and NOE Module Functionality
The following table identifies the Ethernet services that are available and unavailable in a hot standby solution.
Service
NOE 771 Family
I/O Scanning
Available
Global Data
Available
Modbus Messaging
Available
FTP/TFTP
Available
SNMP
Available
HTTP Server
Available
DHCP
Unavailable
NOTE: Only the 140 NOE 771 01 or 140 NOE 771 11 (TCP/IP Ethernet modules) support a Modicon Quantum Hot Standby with Unity V2.0 system.