I’ve been playing with the Arduino and Raspberry Pi trying to get i2c working between them. The difference between 3.3v and 5v logic levels may be why so far I’ve had no success. So today I soldered some header pins onto the PICnDuino and connected that to the Arduino instead. Both operate at 5v so that ruled that problem out.
Anyway, initially no luck until I joined the ground pins together, then a lovely blinking light appeared at the end of the tunnel. Result! If I can transfer this success to the Pi then I’ll be well chuffed. However I’m not sure a two-way serial cable will work through my level converter board, which uses a couple of resistors for one direction and a transistor going the other. I’ll keep at it.