I used SPI and i2c connections together with ethernet cable. There are 8 wires, I used MOSI, MISO, SS, RST and SCLK For SPI, SDA and SCL for i2C, last wire is Ground. (I also used level shifters for Arduino Raspberry communication)
In my system, there are 4 RC-522 module, 4 Arduino. I Control RC 522 with SPI Cable and i control Arduino with i2c and yes i have 4 ethernet cables.
I use Raspberry Pi SPI0 and SPI 1, each spi have 2 slaves (RC-522), and my i2c connection has 4 arduinos (Same bus)
I have no problem with short cables. (it is about 30 cm) But when i use long cable, i got serious problems. I mean long is more than 5 mt, maybe 10 mt...
I don't know what is real problem but my RC-522 modules not work with long cables. i tried lower speed of spi 1.000.000 to 100.000, and it doesn't work. I also not sure about i2c, probably it is not working too. How can i solve this problem?