Serial1 on mega not working when power by adapter

Development Tools IDE 1.x
1

Hi all -
I got 2 arduino, one nano and one Mega, talking to each other by serial. It's working fine when my laptop is usb-connected to my Mega. But when I unplug the USB cable and power the Mega with a DC adapter 12V, the serial does not seems to work.

2

can you measure if the voltages on the board are the same (powered by USB vs powered by adapter)

do other parts (leds) work as expected?

3

I will check that. It's not constant. Sometimes it works, other times it did not. I will make more tests and get back to you!

4

Are the Gnds between the boards connected when powered by 12V?

5

Yes they are. At first it was not, but I got a lot of garbage.