>>
>>I've used hardware in the past, to set them to different pulldown currents, so that the master out-drove the slave. (or vice-versa)
>>Then the different voltage levels tell you who is talking, but you need an oscilloscope to see this.
>That thing is interesting, but i wanted to do it with a pic16f628 and a
>memory I2C, i guess is not something easy, might you capture current
>with A/D or some other parts of a PIC to try to do this ?

You'd have to be able to sample pretty fast, and doing it with external comparators would be problematic as well, due to over/undershoot, ringing, and other forms of noise on the line.

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