datasheet of the CR216 chipset and some schematic in it
other el cheapo meter have began to use this CR216 too : Tianyu T21L
seems to have an possible serial port, maybe some eeprom hack may render it active ?
some seems to say it has some ressembleance with dtm0660 eeprom contents
CALIBRATION : need to be verified, be careful, from mydigit.cn google translated ...
Calibration method: hold down the function key to turn on, there is a display of a similar version number,
and then continue to press the function key, the screen appears 0, the top right corner has an automatic shutdown symbol flashing!
The standard values of each block input are calibrated according to the hold.
thks @ Daisizhou