Hot standby is a powerful feature in itself, and ETYs are a key part of the feature. It 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 service
You may encounter issues with each of these while using the Premium Hot Standby. Below are some possible issues and resolutions.
NOTE: The Global Data Publish/Subscribe Service is disabled in Control Expert when configuring Premium Hot Standby systems.
If a browser requests a page and during the process of downloading that page an IP address swap occurs, the browser will either hang or time out. Click the Refresh or the Reload button.
Hot Standby swaps affect remote clients.
An ETY will reset under the following conditions:
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Remote Connection Request During Hot Standby Swap
If a remote client establishes a TCP/IP connection during a Hot Standby swap, the server closes the connection using a TCP/IP reset.
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Hot Standby Swap during Remote Connection Request
If a remote client makes a connection request and a Hot Standby swap occurs during the connection request, the server rejects the TCP/IP connection by sending a reset.
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Outstanding Request
If there is an outstanding request, the ETY will not respond to the request, but will reset the connection.
The ETY will perform a Modbus logout if any connection has logged in.
During a swap, the ETY will reset all client connections using a TCP/IP reset.
I/O Scanning provides the repetitive exchange of data with remote Ethernet I/O devices. While the PLC is running, the primary ETY sends Modbus Read/Write requests to remote I/O devices and transfers data to and from the PLC memory. In the standby controller, the I/O scanning service is stopped.
When the Hot Standby swap occurs, the primary ETY closes all connections with I/ O devices by sending a TCP/IP reset. The I/O scanning service in this ETY is in Standby mode.
After the swap, the new primary ETY re-establishes the connection with each I/O device. It restarts the exchange of data with these reconnections.
The TSX ETY 4103/5103 provides the I/O scanning feature. Configure it with the Control Expert software.
If you are using the Ethernet I/O scanning service for critical applications, the following important issues regarding switchover must be considered:
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If a communication function block is used for TCP/IP, the block will not complete its transaction.
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While the ETY is in the process of performing the transaction, a new communication function block may become active.
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The PLC will use the state configured in the last value option in the I/O scanning table of the ETY module (in the Control Expert software) as the input state for the scanned Ethernet I/O devices.
These two states are one of the following:
NOTE: When the I/O Scanning service is configured in the monitored ETY, an ETY-sync link failure on the primary side will generate a switchover.
The File Transfer Protocol/Trivial File Transfer Protocol (FTP/TFTP) service is available as soon as the module receives an IP address. Any FTP/TFTP client can log on to the module. Access requires the correct user name and password. Premium Hot Standby allows only one active FTP/TFTP client session per ETY module.
When the Hot Standby swap occurs, the primary and standby ETYs close the FTP/ TFTP connection. If a user sends an FTP/TFTP request during the swap, the communication is closed.
Whenever you re-open communication, you must re-enter a user name and a password.
CAUTION 