The House has a KNX bus installation, connected to a KNX IP Interface which allows home automation ‘hub’ software like openHAB or Home Assistant to control KNX devices such as lights and roller blinds. This IP Interface connection is also required for programming KNX devices using the KNX ETS software.
The Outbuildings will also have several KNX devices. The original plan was to implement these on a completely separate KNX bus installation, because:
- There is no anticipated requirement for KNX devices in the House to interact directly with KNX devices in the Outbuildings – e.g. for a KNX wall switch in the House to control lights in the Outbuildings
- The ‘Lite’ license for the KNX ETS software only allows a maximum of 20 KNX devices in each KNX ‘Project’ (but unlimited Projects) and the House already has 19 devices configured
- By making the two KNX installations separate, they would each have their own KNX ‘Project’ and each of those would have fewer than 20 devices
- The next step up from the ‘Lite’ license is the ‘Home’ license, which only allows a single Project – but that can have up to 64 devices
For convenience, a single home automation ‘hub’ will be used to manage all of the KNX devices – but that means it would need to talk to two separate KNX IP Interfaces, which is not (currently) a supported configuration for the Home Assistant KNX Integration.
There are several possible options to address this:
- Use a not-fully-supported workaround to allow Home Assistant to connect to two KNX IP Interface devices by running a second instance of the xknx software
- Use two separate instances of Home Assistant, each with their own KNX integration, connected to each other using https://github.com/custom-components/remote_homeassistant
- Revert to using openHAB (rather than Home Assistant), since openHAB does support connections to multiple KNX IP Interface devices
- Potentially using openHAB in conjunction with Home Assistant, using the openHAB <-> Home Assistant Bridge integration
- Connect the House and the Outbuildings as different ‘lines’ (or ‘line segments’) on the same KNX ‘bus’ – although then all the KNX devices need to be in a single ‘Project’, which requires a more expensive ETS license
- Potentially using a KNX ‘Line Coupler’ device and a KNX cable connecting the buildings
- Alternatively using KNX ‘IP Router‘ devices (rather than KNX ‘IP Interface‘ devices; one per building) – which then connect the two KNX ‘lines’ via the IP network connection between the buildings
- In principle, this could be achieved using the https://github.com/knxd/knxd software (running on something like a Raspberry Pi) to implement the Routing, using an IP Interface device in each building talking to knxd
Other considerations:
- The existing ETS ‘Lite’ license is for ETS v5 but ETS v6 has been available for a while (and is already up to v6.3). An upgrade to ETS v6 will be required at some point anyway.
- An upgrade from ETS v5 ‘Lite’ to ETS v6 ‘Lite’ costs €150
- An upgrade from ETS v5 ‘Lite’ to ETS v6 ‘Home’ also costs €150
- There would be a benefit in being able to add further KNX devices to the House, above the limit of 20 imposed by the ‘Lite’ license – further KNX wall switches, for example
- The KNX ‘IP Router’ devices are relatively expensive (around £195) – and Routing KNX over IP would need two of those – whereas the KNX ‘IP Interface’ devices are cheaper (around £120)
- A KNX ‘Line Coupler’ is around £140.
- These can be configured as a Repeater (like an Ethernet Bridge) to simply replicate all the KNX messages while electrically isolating the two cables
- Or they can be configured as a Coupler (more like a Network Router), with rules determining which subset of messages get passed across
- A KNX installation consisting of even 64 devices is still considered ‘small’ and is well within the limits for even a single KNX line segment
- The KNX cable runs are probably just short enough to allow this to all be cabled on one KNX line segment, but since there are separate buildings with separate mains electricity supplies (from the same grid connection and meter) it is best to keep them (electrically) separate
In the interests of keeping things simple, it seems best to consolidate everything into a single KNX Project, which means upgrading to the 64-device ‘Home’ license for ETS v6.
That leaves the question of whether to connect the buildings with a KNX cable (and a KNX Line Coupler) or whether to use an IP network connection between the buildings (which is required anyway, and will be a fibre optic cable) and two KNX IP Router devices. While KNX cable is pretty tough, and well-shielded, for an underground connection between different buildings it seems best to use something different (and less sensitive to interference) – which favours an IP connection and two KNX IP Routers.
One complication is that KNX IP Routers use a Multicast network address. This is straightforward to manage on a single LAN segment but more problematic when the traffic needs to pass through multiple IP Routers, such as are planned for the IP network connection between the two buildings.