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 M odbus Rea d/
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 varia bles 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 station s and ca n subsc ribe to
remote application variables independent of the location of the producer.
The communication port has only one multicas t 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 th e
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 rec eive 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.