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Ethernet Hot Standby Solution
840 USE 106 00 January 2003 175
I/O Scanning
Service
The I/O Scanning provides the repetitive exchange of data with remote TCP/IP
nodes I/O devices. While the PLC is running the Primary NOE sends Modbus Read/
Write, read or write request to remote I/O devices, and transfer data to and from the
PLC memory. In the secondary controller, the I/O scanning service is stopped.
When the Hot Standby swap occurs, the Primary NOE closes all connections with
I/O devices by sending a TCP/IP reset. The I/O scanning service in this NOE is
standby.
After the swap, the new Primary NOE re-establishes the connection with each I/O
devices. It restarts the repetitive exchange of data with these re-connections.
Global Data
(Publish/
Subscribe)
Service
The Hot Standby NOE is one 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 one multicast address.
In this network service, the Hot Standby controllers are viewed like only one station.
The Primary 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 swap occurs, the Primary NOE stops the Global Data
service. The NOE does not publish the local variable during a swap. And after the
swap, the new Primary NOE starts to publish application variables and to receive the
subscription variables.
FTP/TFTP Server The File Transfer Protocol/Trivial File Transfer Protocol (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. Hot
Standby allows only one active FTP/TFTP client session per NOE module.
When the Hot Standby swap occurs, the Primary and Secondary NOEs 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.