The Modicon Quantum Hot Standby with Control Expert solution is a powerful feature of NOEs, a feature that increases the reliability of your installation. Hot Standby uses a network, and using the Hot Standby feature over a network can affect the behavior of:
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browsers
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remote and local clients
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I/O scanning service
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global data service
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FTP/TFTP server
The following are factors you may encounter while using the Modicon Quantum Hot Standby with Control Expert solution.
If a browser requests a page and during the process of downloading that page an IP address switchover occurs, the browser either hangs or times out. Click the Refresh or Reload button.
Hot Standby switchover affect remote clients.
A NOE module resets under the following conditions:
Remote Connection Request during Hot Standby
: If a remote client establishes a TCP/IP connection during a Hot Standby switchover, the server closes the connection using a TCP/IP reset.
Hot Standby Switchover during Remote Connection Request
: If a remote client makes a connection request and a Hot Standby switchover occurs during the connection request, the sever rejects the TCP/IP connection by sending a reset.
Outstanding Requests
: If there is an outstanding request, the NOE module does not respond to the request, but the NOE module does reset the connection.
The NOE module does a Modbus logout if any connection has logged in.
During a switchover, the NOE module resets all client connections using a TCP/IP reset.
NOTE: When you select hold last in a 140 CPU 651 •0 coprocessor, the input data may fall back to 0 when the I/O scanner health bits are NOT OKAY. Confirm that the I/O scanner health bits are valid (OK=1) before removing the Ethernet cable to maintain hold last value.
I/O scanning provides the repetitive exchange of data with remote TCP/IP nodes I/O devices. While the PLC is running, the primary CPU NOE sends Modbus read/write, read, or write requests to remote I/O devices, and transfers data to and from the PLC memory. In the secondary controller, the I/O scanning service is stopped.
When the Hot Standby switchover occurs, the primary CPU NOE closes all connections with I/O devices by sending a TCP/IP reset. The I/O scanning service in this NOE is standby CPU.
After the switchover, the new primary CPU NOE re-establishes the connection with each I/O devices. It restarts the repetitive exchange of data with these re-connections.
The module provide the I/O scanning feature. Configure this feature with the Control Expert software.
Using either method, the configuration and transfer of data between network addresses can be done without using the MSTR/IEC function block.
NOTE:
Account for the following Ethernet I/O scanning considerations during a switchover.
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If MSTR/IEC function block is used for TCP/IP, only some of the opcode is used. Therefore, the block does not complete its transaction, and it returns error code 0•8000.
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While the NOE module is in the process of performing the transaction, a new MSTR/IEC function block may become active.
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The output states of the scanned I/Os follows the state defined in the last value option configured in the I/O scanning table of the NOE module (in Control Expert software).
These 2 states are either:
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set to 0
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hold last
With the above considerations, we recommend using switchover with Ethernet I/O scanning for less critical applications.
Global Data (Publish/Subscribe) Service
The Hot Standby primary CPU NOE is 1 station within a distribution group. Distribution groups exchange application variables. Exchanging application variables allows the system to coordinate all the stations in the distribution group. Every station publishes local application variable in a distribution group for all other stations and can subscribe to remote application variables independent of the location of the producer.
The communication port has only 1 multicast address.
In this network service, the Modicon Quantum Hot Standby with Control Expert controllers are viewed like only 1 station. The primary CPU NOE publishes the Hot Standby application variables and receives the subscription variables. The secondary NOE global data service is in a stopped state.
When the Hot Standby switchover occurs, the primary CPU NOE stops the global data service. The NOE module does not publish the local variable during a switchover. And after the switchover, the new primary CPU NOE starts to publish application variables and to receive the subscription variables.
The FTP/TFTP server is available as soon as the module receives an IP address. Any FTP/TFTP client can logon to the module. Access requires the correct user name and password. Modicon Quantum Hot Standby with Control Expert allows only 1 active FTP/TFTP client session per NOE module.
When the Hot Standby switchover occurs, the primary CPU and secondary NOEs close the FTP/TFTP connection. If you send an FTP/TFTP request during the switchover, the communication is closed.
Whenever you re-open communication, you must re-enter a user name and a password.
WARNING 