KNX Guide: How do Couplers work?

If you are an engineer who designing, installing and programming a KNX smart home or building project, you will definitely have to work with couplers. In medium and large-scale construction projects, couplers play an important role in routing data and ensuring the security of the KNX system.

The KNX Coupler is essential in medium and large projects. The Coupler ensures both data routing and the security of the KNX system.

The core roles of Couplers in KNX networks include:

• Filtering: group address (multicast) and physical address (unicast) filter table

• Hop Count

• LL acknowledgement (acknowledgement mechanism)

Filtering

The filter is responsible for optimizing data on the transmission line, ensuring that packets (telegrams) are allowed to pass through the coupler or not based on the group address and physical address filter table. In the TP1-64 topology, only the backbone coupler and line coupler have a filter table, the line repeater does not. But in the TP1-256 topology, even the segment coupler has a filter table. Thanks to that, packets are managed at a deeper level and data overload on the bus is avoided.

Multicast telegrams

The coupler will check the destination address of the multicast packets, which are the group addresses. If this address is in the filter table, the packet is allowed to pass through the coupler from the Main side to the Line side or vice versa. The filter tables are created during the programming of the ETS project.

The coupler examines the packet address and decides whether packets (telegrams) are allowed to pass through the coupler based on the group address and physical address filtering table.

Unicast telegrams

For unicast packets, the destination address is the physical address. The coupler will examine the destination address and decide whether it should be 'filtered' or not depending on:

• Where did the packet come from? That is, the source address of the packet is the deciding factor in 'filtering'.

• Physical address of the coupler

For example: individual address of coupler = 1.1.0

• Unicast packet from main side has destination address = 1.1.55 --> no filtering

• Unicast packet from main side has destination address = 1.0.55 --> filter

• Unicast packet from line side has destination address = 1.1.55 --> filter

• Unicast packet from line side has destination address = 1.0.55 --> no filtering

Hop count

Hop count ensures that multicast packets do not propagate indefinitely in the KNX network even in the event of topology failures.

For multicast packets:

• Coupler and Repeater will decrease hop count each time packet is transmitted.

• Each multicast packet has a hop count sent with an initial value of 6.

• Maximum hop between two KNX devices is =6

For unicast and broadcast packets:

• Coupler and Repeater do not reduce hop count

• Unicast and broadcast packets have hop count = 7

  
  

LL acknowledgements/repetitions

• If the sending device and the packet destination device are on the same line, the destination device will directly send an acknowledgement. The packet repetition is done directly by the sending device.

• If the sending device and the destination device are not on the same line, the sending coupler will be responsible for acknowledging and the destination coupler will be responsible for repeating the packets.