Anti collision + Multiple tag reading about rfid

[SasaKaranovic]

Reading more than one tag at the same time really depends on what standard, if any, the tags and reader utilize. For example. The 13.66 MHz ISO 14443A/B, and 15693 standards include anticollision although not all reader manufacturers actually implement it. But alternatively, low frequency Prox cards do not utilize anticollision . Anticollision is NOT typically accomplished by reading tags many times. Doing it this way is inefficient and unreliable. Almost all anticollision schemes utilize the unique serial number that most tags have. The protocol lets a reader talk to an individual tag by its serial number. Sometimes the tag is told to not talk again unless it leaves the RF field and then returns.

There are other anticollision schemes as well.

[mdxs]

For what it is worth (I'm new to Arduino and MFRC522): in my Sketch, when I present two RFID tags (PICCs) to the reader (stacked in front of the antenna), the code will read both in sequence; allowing me to deal with the first and then with the second. I'm not sure about order, but would guess that will always be random if both PICCs are presented at the same time (stacked on top of each other). Now, I was only reading the UID of these PICCs, which might have been the simplest case ;-)

As @coldkeyboard said: it depends, so I might have been lucky; so I would recommend just to experiment with your reader and your tags using the https://github.com/miguelbalboa/rfid/blob/master/examples/DumpInfo/DumpInfo.ino example Sketch. That will dump details of all the RFID tags (PICCs) you will present it. Then try stacking a few cards on top of each other and see what your Serial Monitor (Ctrl+Shft+M in Arduino IDE) will report to you.