Mindboards

I have seen several circuits showing I2C devices only needing two input wires.

Surely there also has to be a common earth wire between the sending and receiving devices?

Or does the design assume both devices are using a common power supply?

They do need a common reference voltage, and ground works very well (so yes, common ground is almost a requirement). A common supply is not necessary, but often easiest to design.

Ok,

Thanks.