Fig. 11 Principle Cu2508 Topology; Fig. 12 Display "Auto Link Close - Beckhoff CU2508 Documentation

Product overview
The CU2508 system does not replace master implementations of Ethernet based field buses; instead, it
tunnels specified data telegrams via the Gbit connection and then sends the frames at the specified time. It
behaves transparently for the protocols fed across it, with exception of the EtherCAT protocol – in this case a
CU2508 device is visible as the first slave in the configuration. Each materially existent I/O system on the
field side must therefore match a logical master component in the controller.
Several CU2508s can be used in each TwinCAT system.
Fig. 11: Principle CU2508 topology
Downlink Port characteristics
The default setting of the CU2508 is optimized for use with EtherCAT downlinks, especially for EtherCAT IO
redundancy operation. Therefore the 8 downlinks up to and including FW11 in the factory setting had the
function to mirror incoming 100 Mbit frames back to the transmitter if the 1 GBit uplink is missing, called
"Auto Link Close".
Fig. 12: Display "Auto Link Close"
In non-EtherCAT operation this feature can be a problem and might disrupt the underlying network.
Therefore it is deactivated globally for all downlink ports starting from FW12, so that in case of an uplink loss
no return of incoming 100 Mbit frames takes place but the frames in the CU2508 "percolate". The electrical
link is not changed.
16 Version: 2.6 Real-time Ethernet port multiplier