Home Assistant on KNX and Modbus: an engineering integration guide
Home Assistant does not replace a KNX or Modbus installation -- it sits on top of it, reading and writing through a gateway while the bus keeps running its own logic. This page covers how that integration is actually wired, and what should stay bus-side.
How the integration is built
Connect via an IP interface or gateway
For KNX, the official Home Assistant integration talks to the bus through a KNX IP interface or gateway rather than connecting directly to the twisted pair. For Modbus, HA polls the target device over TCP or RTU/serial, depending on how the device exposes its registers.
Map group addresses
Each KNX datapoint HA should read or control needs its group address mapped in the integration configuration, matching the addressing scheme used in the ETS project so the two stay consistent as the installation evolves.
Configure register polling
Modbus devices such as energy meters, inverters or HVAC controllers are added register by register, with the correct data type, scaling and polling interval for each -- these details come from the device's own Modbus register documentation, not from Home Assistant.
Decide what stays bus-side
Anything that must keep working if HA is offline -- basic light switching, safety interlocks, scene recall triggered locally -- should be programmed into the bus device or ETS logic itself, not only as a Home Assistant automation.
Engineering considerations
The bus keeps running without Home Assistant
A KNX actuator programmed in ETS, or a Modbus controller with its own onboard logic, continues to operate on its own rules whether or not Home Assistant is online. That independence is the reason to keep critical logic on the bus device, and treat HA as the layer on top.
HA as a dependency, not a foundation
Any automation logic that exists only inside Home Assistant stops working if HA is down, restarting, or mid-update. That is acceptable for convenience automations and dashboards; it is not acceptable for anything safety-related or business-critical.
Network segmentation and gateway placement
Where the KNX IP interface or Modbus gateway sits in the panel and on the network affects both reliability and security -- isolate bus traffic from general LAN traffic where practical, and treat the gateway as documented panel wiring, not an afterthought.
Document which logic lives where
A commissioning record should state explicitly which functions are implemented in the bus device (ETS programming, controller firmware) and which are implemented as Home Assistant automations, so the next engineer does not have to reverse-engineer it.
KNX group address integration specifics
The HA KNX integration reads and writes group addresses; it does not replace ETS commissioning. Address changes made in ETS need to be reflected in the HA configuration, and vice versa -- there is no automatic sync between the two.
Modbus register polling specifics
Polling interval, register type and byte order have to match the device exactly, and TCP versus RTU/serial changes the wiring and addressing approach. Misconfigured polling can load the bus device unnecessarily or simply return wrong values.
What we help with
Engineering support for integrators adding Home Assistant on top of an existing or planned KNX or Modbus installation.
- Review of gateway placement and network segmentation in the panel
- KNX group address mapping cross-checked against the ETS project
- Modbus register configuration for meters, inverters and HVAC controllers
- Guidance on which logic should stay bus-side versus HA-side
- Commissioning documentation covering both bus and HA configuration
Frequently asked questions
Does the KNX bus stop working if Home Assistant crashes or is offline?
No -- KNX actuators programmed through ETS keep executing their own logic independently of Home Assistant. HA loses its visualisation and any automations that live only in HA, but the bus itself keeps running.
Should I use TCP or RTU/serial for Modbus?
It depends on the device and installation: TCP suits devices already on the network with an Ethernet port, RTU/serial suits devices wired on an RS-485 bus. Check what the specific device supports -- this is a per-project decision, not a fixed rule.
Is Home Assistant reliable enough for critical automations?
Treat it as an automation and visualisation layer, not a safety controller. Keep anything safety-related or business-critical in the bus device's own logic, and use HA for convenience automations, dashboards and cross-protocol logic.
Does adding Home Assistant replace the need for proper KNX or Modbus commissioning?
No. HA reads and writes to an already-commissioned bus; it does not configure actuators, controllers or the bus topology itself. ETS commissioning and Modbus device setup still have to be done correctly first.
What happens if the IP interface or gateway fails?
Home Assistant loses visibility and control through that gateway, but bus-side logic programmed into the devices themselves keeps running. This is exactly why critical functions should not depend solely on HA being reachable.
Planning a Home Assistant integration on top of KNX or Modbus?
Our engineers can review your gateway setup, group address mapping and register configuration, and help you decide what should stay bus-side. Get in touch to discuss your project.