Open-collector VS bipolar lines on I2C devices

[Demon]

Okaaaay...

So that begs the question, how do we calculate the resistor size?

The manual says 4.7K resistors, but I suspect that is an 'average'.

Robert

[Dave]

Demon, What you need to do is use an oscilloscope and look at the SDA line as well as the SCK line. However the SDA line is of more importance due to the fact that any device on the bus can drive it. Usually there is only 1 master and it generates the clock for the slaves. With the maxumum devices on the bus and the longest bus lines, opertaing at your maximum speed, make sure that the SCL and SDA lines are not distorting the waveform. If the waveform is rounded then decrease the resistance in 100 ohm increments until the waveform edges look pretty square. Be carefull not to go below about 1K ohm as that would be 5 ma. of current draw. Some devices out there can't pull down much mode than that.

Dave Purola,
N8NTA