I was trying for one month to make multiple **RC522 rfid readers** work together on SPI on an arduino yun, one works fine, but if **I plug 4**, each with a specific slave select, **only one, sometimes 2, sometimes none are working**.

The library i use is `RFID.h` (i think the miguelbalboa one) , but I had the same results with `MFRC522.h` wich have a specific `readUidMultiReader` so I dont think my code is at fault.

Searching for this issue, I found [this question][1] wich is basically my issue, as I suspect [the readers I bought][2] are not supporting multidrop. [These readers][3] dont work either.


I read about solutions using [multiplexers][4] for SPI lines, tristate buffers on miso lines and/or [diodes][5] but these are workarounds, and there should be a more straightforward solution.

Also I tested with **no resistors** most of the time, but tried putting 10kΩ from all MISO to 3v3, no change, as i really dont understand what their use are, even if I feel they could be mandatory.

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*I am using a yun for the builtin ethernet, because I tried nanos with ethernet shields and again the shields are ignoring the Slave select and thus not working with spi multidrop, so each time I plugged the shield on nano, even not using it, rfid stopped working.*


  [1]: https://arduino.stackexchange.com/questions/18313/issue-sharing-miso-with-multiple-rc522-rfid-readers
  [2]: https://www.amazon.fr/AZDelivery-lecteur-Arduino-Raspberry-d%C3%A9marrage/dp/B074S9FZC5/ref=sr_1_2
  [3]: https://www.gotronic.fr/art-module-rfid-13-56-mhz-vma405-25477.htm
  [4]: https://github.com/miguelbalboa/rfid/issues/191#issuecomment-242631153
  [5]: https://github.com/miguelbalboa/rfid/issues/290#issuecomment-549559010