The principle
Beckhoff TwinCAT cable redundancy is designed to compensate for the failure of a communication cable section in the EtherCAT system. A ring topology, which normally is operated in both directions, is therefore used. Both branches can nevertheless still be reached if the ring is interrupted at some point.
A second network port is used for ring closure at the EtherCAT Master IPC. Both cyclic and acyclic frames are sent simultaneously through both ports, and are transported through the system.
- In the absence of any fault, all the EtherCAT slaves are reached in the forward direction from the primary port. This means that they are processed, since the EtherCAT Slave Controller (ESC) is only passed through in the forward sense.
- When there is no fault, all the EtherCAT slaves are reached from the secondary port in the reverse direction - the data in the "redundancy" frame is therefore not changed.
In each case, the EtherCAT frames arrive, possibly modified, at the other port, and are combined again by the EtherCAT Master. If, as a result of a broken cable, the redundancy comes into play, it is then unimportant whether an EtherCAT slave is reached from the primary or redundancy port.
Assuming that, when the redundancy comes into play (due to a damaged cable, damaged plug or electromagnetic interference) the two Ethernet frames are not, by coincidence, both directly affected, then redundant operation continues without interruption or loss of data.
The supplement is single-fault tolerant, i.e. communication with the slaves can continue if the cable is interrupted in one place. When the communication is restored the original communication direction is restored. If the communication is interrupted in more than one places, all connections have to be restored before another fault may occur.
It is also possible to start up the system under redundancy conditions.
Due to the nature of this principle, a closed ring topology is most suited to redundant cable operation. Depending on the operating conditions, it is also possible to use a connection point other than the last one for the redundant connection to the controller, see Fig. Special solution for cable redundancy.
Due to the synchronization mechanism for Distributed Clocks, the CU2508 port multiplier must be used for a combination of cable redundancy with Distributed Clocks slaves.
 | Failure of an EtherCAT slave Note the following if the "Cable Redundancy" supplement is to be used to enable communication with all remaining devices in the event of an EtherCAT device failure (e.g. EK1100 coupler or EPxxxx box).
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