Couplers contribute to KNX networks in 3 ways:
- filtering: based on Group Addresses via filter tables (multicast) and Individual Addresses (unicast)
- Hop Count
- LL acknowledgement
The filtering optimizes the traffic on the bus, i.e. couplers make sure that telegrams will only appear there where they are useful. The filtering is done by couplers only, not by repeaters.
The destination address of each multicast telegram will be checked. The destination address of a multicast telegram is a Group Address. If the Group Address of a telegram is listed in the filter table of the coupler then this telegram will be copied from the primary side to the secondary side or vice versa.
The filter table of a coupler is defined by ETS and depends on the configuration of the ETS project.
The destination address of each unicast telegram will be checked. The destination address of a multicast telegram is an Individual Address.
Whether a unicast telegram is filtered depends on:
- where the telegrams comes from
- the Individual Address of the coupler
Individual Address of the coupler = 1.1.0
- unicast telegram from primary side with destination address = 1.1.55 -> not filtered
- unicast telegram from primary side with destination address = 1.0.55 -> filtered
- unicast telegram from secondary side with destination address = 1.1.55 -> filtered
- unicast telegram from secondary side with destination address = 1.0.55 -> not filtered
The Hop Count makes sure that multicast telegrams will never travel endlessly throughout a KNX network even when topological mistakes would be made.
- couplers and repeaters decrement the Hop Count of multicast telegrams
- multicast telegrams are sent with Hop Count = 6
- the maximum number of hops between two devices = 6
- couplers and repeaters do not decrement the Hop Count of unicast and broadcast telegrams
- unicast and broadcast telegrams are sent with Hop Count = 7
- if sender and destination device are in the same line then LL acknowledgements are sent by the destination device directly, possible repetitions are sent by the sender directly
- if however sender and destination device are not in the same line, then the coupler of the line of the sender will take care of the LL acknowledgements, the coupler of the line of the destination device will take care of possible repetitions