Hi ! Easy answer in 1word : EEPROM. If you are using the D21G we released a library on this subject on github called MEM_Fr or _en (english version) that should simplify your programmer' s life.
Here is the github URL : https://github.com/DG2RDD/eControls-MEM-library-for-Industruino-D21G-EEprom-management
Have fun !
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