I have a new D21G Board and i have throubles connecting to it via USB. I've bought it in 2019 but i'm startin to play with it only now.
If i connect the board without supply voltage (only with the 5v supplied by the usb port) the module works correctly and i can connect to the com port, but the ethernet module doesn't work (as seem that get the supply voltage from the supply line).
If i first connect the supply voltage, then i connect the usb cable (as per correct procedure), windows 10 tell me that i've connected an unknown usb device, and it can't install the drivers.
I've installed the D21G drivers suggested.
If i press the reset button S1 on the D21G board while all is connected, the device driver show ok on device management, but i can't connect to the serial port.
Forget it! Solved, had only to restart windows...
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