Thomas Frisch

email: [email protected]

Since i have some FS20 based components in use and under development, there was a need for an universal decoder for the FS20 protocol. Normaly i had done this as a stand alone µC solutiuon, but my intention was to dig deeper into the RTL-SDR stuff.

The concept of the OOK decoder and accessing GNU Radio samples live from Python is based in the remote thermometer script posted by Kevin Mehall. https://github.com/kevinmehall/rtlsdr-433m-sensor

This script decodes the packets used within a FS20 home automation system which uses the 868MHz SDR band for wireless communication. Typical FS20 components are:

• wireless phase control dimmer
• Switches
• remote controls
• wireless door bells
• alarm system

Features of the decoder:

• Live monitor of:
  • Housecode
  • Adresses / Subadresses
  • Command with resolution of the meaning
  • Extension Byte
  • Parity and checksum checksum

Each packet is send multiple times by the transmitter (usually 2 or 3 times). The modulation is OOK with the following symbols:

• logical 1: pulse of 600µs
• logical 0: pulse of 400µs

Each packet consists of 58 or 67 bits, depending on whether a extension byte is included or not.

Details on the protocoll can be found here: http://fhz4linux.info/tiki-index.php?page=FS20%20Protocol

Currenty only the positive signal portion is used for pulse length measurement, while the modulation varies also the follwoing low portion. To increase the robustness the detection could be improved. For me it worked fine with the simple way.

By the way, this is my first python code, so please don't be too scared. ;-)