The bus & topology
KNX devices share a single communication medium called the bus. Understanding the bus and how it is structured (the topology) is the foundation for everything else — group addresses, ETS, and the project structure KNX Clarity shows you all sit on top of it.
The bus
In the most common form, KNX TP (twisted pair), the bus is a single green two-core cable that is daisy-chained from device to device. That one cable carries both power and data:
- Devices draw their operating power from the bus (a dedicated bus power supply, typically 30 V DC, feeds each line).
- Data is modulated on top of that same pair.
This is why a KNX wiring diagram looks so sparse compared to conventional electrical installation: one bus cable visits every switch, sensor, and actuator.
Other media exist and interoperate on the same logical bus:
| Medium | What it is |
|---|---|
| KNX TP | Twisted pair — the dominant wired medium. |
| KNX IP | KNX over Ethernet/IP (KNXnet/IP). |
| KNX RF | Wireless KNX (radio frequency). |
| KNX PL | Powerline — data over the mains (now rare). |
Decentralised intelligence
There is no central controller. Every KNX device contains its own microprocessor and the part of the logic it needs. A push-button knows it should send an "on" command; an actuator knows it should switch its relay when it receives one.
The practical consequences:
- Robustness. One failed device does not take down the building.
- No "brain" to lose. There is no PLC whose failure or obsolescence orphans the installation.
- The configuration is the system. Because behaviour is spread
across every device's parameters and links, the engineering data
(the
.knxproj) is the only complete description of how the building behaves. Losing it is losing the building's logic — the reason KNX Clarity versions every ETS upload.
Topology: areas, lines, devices
A KNX network is organised as a strict hierarchy so it scales from a flat to a campus without the bus collapsing under traffic.
- A line is one bus segment with its own power supply. It can hold up to ~64 devices (more with line repeaters).
- An area groups up to 15 lines, joined by line couplers to an area line.
- The backbone joins up to 15 areas via backbone couplers.
Couplers are the gatekeepers between segments. They filter traffic so a telegram only crosses into another line if a device there actually needs it. This filtering is what lets a large building stay responsive — and it is configured in ETS, not on the device.
The individual address
Every device has exactly one individual address (also called physical address) in the form:
area . line . device e.g. 1.2.14
1.2.14 = area 1, line 2, device 14. This address is unique per
installation and is used for engineering and diagnostics — not
for normal operation. Day-to-day communication uses
group addresses instead.
KNX Clarity does not show you areas and lines as a wiring tree — it shows the building view (floors → rooms → devices, see Rooms & devices), because that is what owners and service staff think in. The topology and individual addresses still live in the parsed ETS data; the building view is a friendlier projection of the same devices.