After some reading/testing I managed to make stable communication between 2 devices using I2C with FTP CAT5 copper twisted pair cable.
- Green wire – SCL
- White/green wire – GND
- Blue wire – SDA
- White/blue wire – GND
GND is connected only at one cable end, I2C bus clock is at 10Khz and I have used 10Kom pullup resistors to VCC
It works well and stable. When I decided to use other 2 pairs of the cable for power (+12V), it stopped working. I tested +12V on one pair GND on the other pair, also +12V/GND on same pair: same result, it stopped working. Whole I2C bus stopped working, other devices attached to it too.
Wonder if I can use same cable or go to safest choice – another cable for power.
Maybe overkill if it was working before, but an option is to use an I2C to Differential converter such as PCA9615, LTC4331, etc. If making the resistors smaller don't work or you need to extend the cable, consider not using I2C directly.
Not only the range will be extended but you will also have better noise immunity.