@Carlos-Sancho What about the above Onion links?
Have you ever read the official Onion Omega2 Documentation
Please read - at least - this text in Using the RFID & NFC Expansion:
Thanks a lot for the tips and the prompt resonses, I finally was able to see the cards being read, even without level shifters.
For starters or anyone struggling, with less experience on boards like me.
Connect to o2/o2+ via terminal perform the opkg update, then install the screen command by entering: opkg install screen on your terminal. Wait till the screen is downloaded and be configured.
Then plug wires, RDM6300 TX to o2/o2+ Rx1 and RDM6300 RX to o2/o2+ Tx1. Assuming you've plugged the 5v, GND, and the RDM6300 is already powered up (the green led is on).
To listen the RDM6300's serial type: screen /dev/ttyS1, on your terminal, screen will open like an editor on the terminal (vi/nano), blank page, swipe the cards and you'll see the hexes being read.
Finally ensure you lower down the returning power to omega with resistors as @György Farkas recommends.
Looks like your connection to Community was lost, please wait while we try to reconnect.