This project revolves around receiving and decoding the KNX RF signal sent from the Uponor T-55 and T-75 thermostats to the Uponor C-55 Controller. These are all legacy products which have gone out of production, but second hand versions are readily available on various marketplaces.
KNX-RF is a well defined protocol and easy to understand, the specification is publicly available here.
There are surely many ways which the KNX-RF signal can be received. This project uses a TV tuner with the RTL2832U chip in connection with the rtl_433 project.
If you're running on Windows and the tuner is not found, it is likely you need to overwrite the driver with WinUSB.
The following command is the most basic one can be used to receive KNX-RF signals using rtl_433
rtl_433 -f 868.3M -s 1024k
Notice that the command is tuned to 868.3 MHz which is for the EU region, KNX-RF may be using 433 MHz in your region.
The Uponor T-55 and T-75 thermostats sends two different signals of different sizes, one with 6 data octets and one with 8 data octets.
rtl_433 decodes the majority of the received KNX-RF signal, here is an example data string that has not yet been decoded 1144ff030074026009ec0005ff0001e800800cad7a14.
Important rtl_433 strips the CRC octets from both block 1 and block 2.
| Block 1 | Block 2 |
|---|---|
| 1144ff030074026009ec | 0005ff0001e800800cad7a14 |
Decoding the data gives us the following data
| Value | Decoded into |
|---|---|
| 11 | Length of the telegram = 17 |
| 44 | Static value that never changes |
| FF | Static value that never changes |
| 03 | RF-Info, converted into bits: 00000011, bit 6 (zero-indexed) is the battery state (0 = battery is weak, 1 = OK), so battery state = OK |
| 0074026009ec | The serial number of the thermostat. It appears that 00740 are T-75 thermostats, and 00744 are T-55 thermostats |
Receive and decode the KNX RF signal sent from the Uponor T-55 and T-75 thermostats