Read UID using MFRC522 card reader

I saw a card reader project the other day and I would like to try to do it, but I have some questions about the MFRC522.

  1. Does RFID read the UID of the card?
  2. If the card is already written, can it still be read? For example, access control cards.

First, you have to know that RFID to works require two components,

(I) RFID tags, which are in different types can be stickers or images.
(II) RFID reader

But read-only RFID tags UID can not be changed/modified once created.

Go back to your question,

  1. RFID can read UID?

The answer is yes only if RFID tags and RFID readers are set to the same frequency so as they can communicate.

  1. If the card is already written, can it still be read?

The answer is yes if and only if they both have the same frequency.

For example:
LF RFID tag with LF RFID reader
HF RFID tag with HF RFID reader
UHF RFID tag with UHF RFID reader

LH-Low Frequency
HF-High Frequency
UHF-Ultra-High Frequency

Remember those cards are passive devices working under electromagnetic waves/energy.

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Thank you very much for your patient reply, I seem to understand.

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