The FDSK (Factory Default Setup Key) of a Secure device is a factory key, printed on the device as a QR code, that is used when the device is first integrated into ETS.

The FDSK is printed on the label of every KNX Secure device, often as a QR code that can be scanned. When the device is added to the project, ETS asks for the FDSK to authenticate the take-over. Once the device is in operation, ETS derives and uses runtime keys stored in the ETS keyring (a password-protected file). The FDSK itself must be kept safe — losing it makes later recovery of the device significantly more complex.

Always remove or photograph the FDSK label before mounting a Secure device in its enclosure — once installed, the label may become unreachable.