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KNX TP topology explained: lines, areas and couplers

KNX TP installations are built from lines, areas and couplers. This guide explains each level, how individual addresses follow the topology, and how to plan a project that stays within the limits.

1. The line: the basic building block

A line is a group of bus devices on one KNX TP cable with its own KNX power supply. A standard line segment takes up to 64 bus devices.

When a line needs more devices, line repeaters can add segments. With up to three repeaters a line has four segments and up to 256 devices. Each segment needs its own power supply.

2. The area: up to 15 lines

Up to 15 lines connect to a main line through line couplers. A main line with its lines is called an area.

3. The backbone: up to 15 areas

Up to 15 areas connect to the backbone line through area couplers. In theory a full KNX TP installation can hold more than 57,000 devices, far more than most buildings need.

4. What couplers do

Line and area couplers do more than connect cables. They keep each line electrically separate and filter telegrams with a filter table, so a telegram only passes to another line when a device there needs it. This keeps bus traffic low on every line.

Today many projects use KNX IP routers as line or area couplers, with an Ethernet network as the fast backbone instead of a TP backbone line.

5. Individual addresses follow the topology

Every device has an individual address in the form Area.Line.Device, for example 1.2.15. The first number is the area, the second the line, the third the device on that line.

  • Area 0 is the backbone line, and line 0 in each area is its main line.
  • Device number 0 on a line is used by the coupler of that line.
  • Plan the addresses before commissioning so the ETS project matches the real cabling.

6. How to plan the topology of a project

  • Count the devices per floor or zone, and keep a reserve of free addresses on every line for later extensions.
  • Give each floor, apartment or building zone its own line, so faults and traffic stay local.
  • Use one area per building or per large wing when a project has many lines.
  • Size the power supply of every line from the bus current of its devices.
  • Check the cable rules of every line segment: up to 1,000 m of cable, up to 350 m from the power supply to a device, and up to 700 m between two devices.

7. Couplers and IP routers on KNX MORE

Related guide

How to size KNX power supplies for your project

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