Understanding KNX Wiring and Topology

The topology of a KNX installation is something that most people put little thought into. In smaller projects, there is very little to consider, especially as technically 256 devices could be connected to a single line. However, there are lots of reasons to separate a system into multiple lines, and in this article, we will look at some ways of doing this.  

Firstly, let's look at why you would separate a system into multiple lines: 

• Project layout: If the installation has multiple KNX panel locations, it makes sense to have a line for each area. 
• Redundancy: If an installation is spread over multiple lines, if one of those lines fails, the other lines/areas would still work.  
• Medium type: Whether you are using TP, IP, or RF, the topology of the project will need to allow for this with different line and segment types.  
• Bus traffic: Separating a project over multiple lines and filtering correctly can greatly reduce the bus traffic on each line. 

Single line system

In its simplest form, a single KNX line can consist of a KNX Power supply and a number of KNX devices. It is possible to have up to 256 devices on a KNX line, sometimes referred to as TP-256 however, power supply limitations mean that this isn’t really possible. Systems up to 2019 were TP-64, meaning you could have up to 64 devices on a line. 

Typically, a bus device will use approximately 10mA, but some devices such as the JUNG LS Touch require more power, so it is useful to check this when designing a system. You can find the required bus current in ETS under properties, or in the manual for the device.

When running cable around a building, it can be installed as a daisy chain or star wired, and it is possible to create a spur. The only restriction is a loop cannot be created at any point of the bus. It is advised to daisy chain from point to point as it makes fault finding and maintenance easier. 

It is also common to run the cable around the building or floor in a loop but only terminate one end. This provides a backup if the cable is damaged or if the line needs to be expanded. It is important to label the cable clearly if this method is used. 

KNX cable is mains rated, and due to the arrangement of the internal cores, it can be run directly next to mains cabling. In fact, it is usually much easier to do so as it can be run at the same time by the site electrician.  

A KNX device can be a maximum of 350m from a power supply, which would normally be mounted in a distribution board and the maximum length of bus cable used in a single line is 1000m.  

Image Key:
BC = Backbone coupler
LC = Line coupler
DVC = Bus device
LR = Line repeater
PS/Ch = Power supply with choke
S = Brightness sensor
RC = Routing counter

Larger systems

It is possible to expand a system beyond a single line if more devices are needed or if the layout of the project means it makes sense to do so. There are various ways to expand a system, so we will go in to these below. 

 

Line Coupler

To link two twisted pair lines together, a line coupler is used. A line coupler will act as a connection between two lines, and telegrams will either be allowed through or blocked, depending on the configuration. A line coupler will have a physical address that indicates the secondary line that it is connected to but will receive its power from the main line. The secondary line will have a KNX power supply to provide power to that line to power the devices on that line.  
When a group address is linked to devices in different lines, the group address will be added to a filter table, stored within the Line coupler.

As standard, if a line coupler receives a telegram to a group address that is stored in the filter table, then it will pass the telegram from the main line to the secondary line, or vice versa. It is also possible to configure a line coupler to pass all telegrams, or to block all telegrams if required.  

Line repeater

If more power is required on a line but there is no need for filtering, then a most line couplers can be configured as a line repeater. From a cabling and layout point of view, this is the same as a line coupler, but the difference is that all telegrams are passed freely in both directions through the line repeater, and both areas are on the same line from a topology point of view.  

IP Routers

If connecting areas using twisted pair cable is not an option, then lines can also be connected using an IP network. An IP router will be needed on each line, and these IP routers will communicate over the network with each other. This provides a great benefit of flexibility, however it means that the KNX communication is reliant on the stability of the network.  

An IP router will work in a similar way to a line coupler, and will also have a filter table, that can be configured to pass or block telegrams depending on your requirements. 

Be aware that some Visualisation servers that are not configured in ETS will sometimes require telegrams to pass through couplers that may not be in the filter table.  There are a few workarounds for this, such as using a dummy application or linking group addresses to your interface if using ETS 6.3 or newer. 

Whenever expanding system with additional TP lines, each line will need its own power supply and method to connected to the rest of the system, whether it is a line coupler, segment coupler or IP router. 

RF (Radio Frequency)

When a system needs to be expanded without adding additional cabling, KNX RF can be used. When using KNX RF, a media coupler is used between the TP and RF areas. A line can be configured as purely RF, or a segment within a TP line can be created.  
When using a dedicated RF line, the media coupler will have the ability to filter group addresses between the RF line and the rest of the installation, whereas when using an RF segment on a line, all telegrams will be free to pass between the areas.  

Further expansion - Understanding Areas

A single ‘Area’ can consist of 15 individual lines, linked by TP or IP. If there are more than 15 lines, further ‘Areas’ will be required. Expansion of areas uses the same concepts as expanding a line – A line coupler or IP router will need to be configured as an area coupler, and telegrams will be filtered depending on filter tables and configured parameters.  

Larger systems with multiple areas will require more planning and consideration when designing to system. On smaller systems, bus traffic would not be an issue, but on a larger system the filtering would need to be enabled as larger amounts of bus traffic can slow down the system and create issues. Most telegrams will not need to pass through multiple Line couplers and area couplers, but group addresses that do will need to be in the filter table.  

Backbone

When creating a new project in ETS, you will be asked to choose a Medium type for the backbone – TP or IP.   
We recommend using an IP backbone, even if you are not using IP to join the individual lines. This is because using this method means that the system can be expanded using IP in the future if needed, without having to re-address lots of devices. 

In ETS6, the medium type of a line or area is colour coded – IP is blue, and TP is green. In the image below, you can see that the Backbone is IP as it is shown in blue. The ‘Area’ is TP, so this is shown in green, and the lines are both TP. As this is a TP area, a line coupler would be used to connect the lines to the TP area.  

Understanding KNX topology is crucial for designing a reliable and scalable KNX system. While a single line system will work in lots of situations, larger or more complex installations can benefit from using line couplers, repeaters, and IP routers correctly to reduce bus traffic and ensure robust communication. 

What next?

Looking for more technical articles? Learn more about Group Addressing or browse our other Essential Guides.
Looking for help with KNX System Design? Get in touch with our team by calling 03333 079 100 or email sales@ivoryegg.co.uk