User manual

Hot Standby
322
33002479 06 07/2008
I/O Scanning
Service
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 Unity
Pro software.
Using either method, the configuration and transfer of data between network
addresses can be done without using the MSTR/IEC function block.
UNINTENDED EQUIPMENT OPERATION - DEVICES GO TO THEIR
FALLBACK STATES DURING SWITCHOVER
Configure Ethernet output devices to their Hold Last Value fallback state whenever
possible. Output devices that support only a Set to Zero fallback state may produce
a pulse during switchover.
Failure to follow these instructions can result in death, serious injury, or
equipment damage.
WARNING