Most redundant communication cards have a third Ethernet port which is an interlink port that can be used to connect other relays or devices to HSR or PRP station bus. Interlink port should not be connected to second network, only to simple device. This port works as a redundancy box (RedBox) port for other devices. It also supports IEEE 1588 making it possible to connect the PTP grandmaster clock to the station bus using it. Additionally, single attached nodes connected to the interlink port can receive 1588 time synchronization over this port.