I can upload the test script for the D21G and it works while plugged in via the USB.
But when I apply external power it does not boot up the top board and LCD.
verified with multimeter, and also verified FUSE.
Please check if your code is waiting for the Serial Monitor to connect. If it has a line " while (!SerialUSB) " the sketch will block there until you connect the USB and open the Serial Monitor.
Are you connecting 12/24V to V+/V-? Do you see 4 green LEDs light up inside the INDIO? 3 on the baseboard and 1 on the topboard.
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