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.