There are two types of addresses in the KNX specification:
- Physical addresses (also known as Individual addresses)
- Group addresses.
The meaning and the usage of these two types of addresses is very different and it is very important to understand the difference between the two. Also it is important to understand when a certain address type should be used.
Physical address
One of the most important aspects of the KNX network is the network topology.
But what is network topology?
What does it mean?
What does it represent?
In plain words, network topology means network configuration. The way that a certain KNX network is built.
So far so good.
Let’s go into more details of the KNX network topology.
Every KNX network is build from large number of KNX devices. Those devices are connected through one of the supported mediums:
- Twisted Pair
- Power Line
- Radio Frequency
- Internet Protocol
KNX devices together with lines of communication make the KNX network.
Every KNX device has a unique address. Address that identifies that device in the KNX network. This unique address is called Physical address or Individual address.
The Physical address of the KNX device is defined with 16 bits of information. This means that we can have a maximum of 35536 KNX devices with their unique Physical addresses.
The way that the Physical addresses of the KNX devices are organized define the topology of the KNX network. 16 bits that represent Physical address are divided into 3 parts:
- 4 bits that represent Area number
- 4 bits that represent Line number
- 16 bits that represent device number
There are a couple of special cases:
- Area number 0 is reserved for Backbone line.
- Line number 0 is reserved for Main line.
This means that we can have:
- maximum of 15 areas with Backbone line
- Maximum of 15 lines with Main line in every area
- Maximum of 256 devices in every line
Group addresses
As we have already explained, Physical addresses represent unique, unique address of the KNX device in the network. Compared to this Group addresses play completely different role. Group addresses have logic or semantic meaning in the KNX network.
Every KNX device has a certain role in the KNX network. For example, Push button sensor is used to send information in the KNX network about an event that has happened. Another example is the Dimming actuator which is used to control the lights in the house.
KNX supports multicast addressing which enables KNX devices to interact with each other. In other words, one device can send a telegram in the KNX network and every other device in the KNX network will receive this message and decide if it needs to act upon it.
From previous example: push button sensor will send the telegram that a certain event has happened and Dimming actuator will receive this telegram and act upon it. Group addresses are used for addressing these kinds of telegrams in the KNX network.
This is why I stated that Group addresses provide logical or semantical meaning. Group addresses are used to define functions in the KNX network. These functions are from real life environment and not from the KNX world.
Let me try to explain this with an example. Group address looks something like this:
Group address 0/0/1 Main light on/off
What does this means? It means that there is a group address defined with the value 0/0/1. This Group address is used to control Main light switching function in the house. Every time a push button sensor is pressed, a telegram with this Group address is sent. Dimming actuator receives the telegram and acts upon it. The Group address provides meaning or semantic to the sent telegram in the KNX network.
There are several Group address structures supported in the KNX network:
- 3 level structure
- 2 level structure
- Freely defined structure
In most cases 3 level structure for the Group addresses is used. This gives the user enough flexibility to organize the group addresses as he sees fit. For example the meaning of Group address level can be:
- Level one – function (Light switch, Light dimming, Blinds control …)
- Level two – part of the house (Base floor, 1st floor, Basement …)
- Level three – definition of the function