Hi everyone,
I'm having a problem with MFRC522 RFID module. When I connect it via jumper cables it works perfectly every time. If I connect it over flat cable, normal single wires or Ethernet cable (ca. 0,8 m in length) it works very buggy or not at all. I tried to twist the cables, shield it with aluminum foil, I tried to add pull-ups and pull-downs, reduced clock speed with SPI.setClockDivider but it makes no difference whatsoever. Does anyone have an idea what am I doing wrong? I considered using RS422 extender ICs but my cables are really not that long.
Thanks in advance 
How long is your connecting cable? The SPI Protocol has been reported to work within 10 feet.
Less than 3 feet. I also checked my signal with oscilloscope in both cases (short jumper cables and cable) and it looks almost the same - I get some more ringing with the cable but I think it shouldn't make any difference.
Not necessary, it works now! I ordered another brand (Velleman), it just arrived. I plugged in the board whitout any changes and it works perfectly even with a longer cable! Before I used a bit cheaper Joy-It brand, I even tried it out with 2 different boards and it didn't work. I read in the ReadMe file for MFRC522 library that there are many faulty and counterfeit chips that don't work properly. So if anyone has similar problem, please consider using another board