Thanks for the help, the advice you gave me i'm going to check up on that, its awesome!
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Note each bit is driven to either LOW (ie, 0V) or HIGH (ie, 3.3V). 'Z' high impedance value is not required, so I don't understand how/why that originally came into the picture.
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I dont really think this board was actually doing the right thing here, it would be cool to have such a state. Maybe only for audio? You are right tho, it doesnt have to be bidirectional.
I had a rail going at 0,05%, using R2R, found the busses on my GPIO connector drive pretty well, the voltages are pretty consistent along the bus. "opportunism I guess".