KNX TP1-256 Topology

[IC_Systems]

Hello,

Could we use the new KNX TP1-256 Topology?

Could you make some explanation, and what is  your recommendation?

 

[Erwin-vd-Zwart · Accepted solution]

That depands on the products you are using, see the attachment for more details..

There has always been two conditions, a max number of devices due to transmission characteristics and the total current consumption of the devices.

Let’s start with transmission characteristics:

Since all times we learned that the maximum allowed number of KNX devices connected to a line segment is 64 for signal transmission reasons.

It is not true anymore. More devices are possible and this is now also taught in the certification training, maximum is now 256.

What is behind this?

For KNX twisted pair devices there exist two different transceiver types, which is the electronic part closest to the bus wires and managing the physical layer. You can see an example to the right.

They are called TP-64 and TP-256. TP-64 has existed from the beginning of EIB/KNX and TP-256 came some years ago.

So there are a mix on the market, some devices are TP-64 devices and new once are TP-256 devices.

They are compatible and can be used on the same KNX line segment without any problem, so nobody noticed the two types.

However, they have different transmission characteristics which affect the signal differently.

With only TP-64 devices, the maximum number of devices on one line segment is 64 and with only TP-256 devices a max of 256 devices is possible. It means, with a mix of transceivers, each TP-64 device should be counted as 4 and each TP-256 as 1 and the sum, the so called fan-in must not exceed 256.

For example as you see in the table, 63 TP-64 devices gives fan-in 252, which means there is room for one more TP-64 device or four TP-256 devices, which then will result in 67 devices.

Whether a device has TP-64 of TP-256 is not an information given or handled by ETS.

We have also not given this information in our data sheets in the past.  However, all new devices since some years has TP-256 transceivers and we now give the info.

In our product portfolio we have currently both TP-64 and TP-256 devices.

The second condition is that the total current consumption of the KNX devices must not be higher than the nominal output current of the power supply.

This must be calculated for each line segment as ever before. The maximum nominal current is as you now know 1280 mA

The typical current consumption of a KNX device today is between 7 and 18 mA,

Let’s assume an average of 13 mA which leads to a maximum of 100 devices. So out of these 100 devices, how many can be TP-64 to also fulfil the first condition? The answer is that 52 or less of these 100 devices can be TP-64:  52 TP-64 gives 208 fan-in and 48 TP-256 gives 48, which equals 256.

[IC_Systems]

Thank You @Erwin-vd-Zwart  for the valuable info.

Could you attach the full document if there is?

Also, Is there a  documents classifying the products to TP1-64 and TP1-256?

[Erwin-vd-Zwart]

No there is no full document classifying the products (at least i have never seen one), i think you need to look into the product documentation of each product.

[Erwin-vd-Zwart]

My colleague (Alexander Efremov) brought to my attention this document (see attachment), it's from 2019 but i think it's save to assume that all devices released after that are TP-256 ...

[domotiqa]

M. Osterman told me about a listing too.

Maybe he has it

[Horacio1]

Thanks for the great information!!!

Does this mean that if I just have TP-256 devices I dont need any line repeater?

[Erwin-vd-Zwart]

Correct