I wanted to put some protection on my PI's pins and chose the zener voltage protection one, as described here. I used TZX3V3A zener diodes and 330 ohms resistors. Like this :
Then I tried to connect an external IC with I2C but it failed. However, when I remove the zener diodes it works perfectly.
I also tested my diodes separately with higher voltage and they seem to do their job nicely.
Any ideas why it won't work ? Or some more things I could try ?