Thanks Henrik,

You need to poll the CTS signal of the board to know when it's able to receive commands. So these are the very basics:
1) Wait for the CTS signal to high (board busy looking for a card)
2) Wait for the CTS signal to go low (board ready to receive serial data from the PIC)
3) Send the "return card status command" ($53)
4) Receive the card status byte from the reader.
...
Up to now, I didn't made any code for a PIC. I'm trying to make it work via the Serial Communicator from MCS before I go THE step further on.

Since I bought the USB version of the board, I was assuming I would be able to, at least, read a Tag without any other connexion to the board. I'll have a look again what I need to do to get the CTS status.