KNX is the open standard for intelligent building installations, where lighting, heating, solar shading and blinds are controlled via one bus cable instead of individual wires. Here is the difference between conventional and bus installation, KNX topology, the bus cable and ETS programming.
Introduction
In a traditional installation, there is a wire from each switch to each lamp: if you want to control a lamp from three locations, you pull cross-connection wires between them. In a bus installation, control and power are separate. All control buttons and all actuators are connected to a single common bus cable and "communicate" with each other using small messages (telegrams). Which button controls which lamp is determined not by the wiring, but by the programme — and can be changed without moving a single wire. KNX is the most widely used open standard for this and is established as an international standard in ISO/IEC 14543-3 (and European in the DS/EN 50090 series).
Traditional vs. bus — what is the difference?
- 01Traditional: fixed coupling. The control is in the live wire; a change requires new wires.
- 02Bus: free coupling. Operation and load are separate; functions are changed in software (ETS).
- 03Bus wins when there are many functions and connections (scenes, time control, sun/shade, integration) — traditionally it is cheapest with few, simple controls.
KNX topology
KNX is built hierarchically, so a large system can be structured and addressed uniquely. The smallest unit is the line; lines are collected into areas; areas are collected on a backbone.
| Level | Content | Boundary (TP) |
|---|---|---|
| Segment/linje | Power supply + bus units | Typically up to 64 units per segment |
| Line (extended) | Up to 4 segments via line amplifiers | Up to 256 units per line (TP1-256) |
| Area (area) | Lines connected via line couplers | Up to 15 lines per area |
| Backbone | Areas coupled via area couplers | Up to 15 areas |
Line and area couplers do two things: they couple the levels together and filter messages, so local traffic stays on the line where it belongs. It keeps the load on the bus down and makes large systems stable. (Web-verified: 64 units per segment, 4 segments → 256 per line, 15 lines per area.)
Bus cable and voltage
The common KNX media type is twisted pair (Twisted Pair, TP1) — a two-wire bus cable (typically green KNX cable) that carries BOTH power supply and communication to the devices. The bus is supplied from a KNX power supply with a nominal voltage of 29 V DC, and the devices function stably right down to approx. 21 V, so a voltage drop on the bus does not immediately cause an error. Because the cable is low voltage (SELV-like) and polarity-independent for most devices, it is quick to pull — but it must be kept separate from the 230 V installation according to the rules.
Sensors and actuators
All units on the bus are either sensors (senders) or actuators (receivers) — some are both.
- 01Sensors: pressure/switches, motion sensors, thermostats, light sensors, weather stations. They send a signal when something happens.
- 02Actuators: relay/contact outputs for lights, dimmers, motor controls for blinds/awnings, valve controls for heating. They receive telegrams and switch the actual load.
- 03System units: power supply, line-/area switches and programming interface (USB/IP).
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