Is it safe to connect both the DCIN and the two USB on the arduino break out?
I have a 2amp power adaptor that i use normally but would like serial access as well.
Yes - the power jack and the USB are both diode protected and there should be no issue in power both at the same time.
If you only need the USB-SERIAL then why not just use the power jack and only one USB cable - the one to J3, as J3 does not power the board, only the RS232 chip.
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