Alex, I just tripped to this and will check it myself when I get back to my Omnibus…I’ve read posts that say I2C on this board is on motor pin 6 and 7 and I’ve seen it say 7 and 8 (perhaps these were on early versions of the board). Your diagram indicates SCL and SDA (I’m assuming this means they’re I2C pins, I’m far from an expert on this stuff?) are the bottom two pins on J3. This means if I’m connecting external magnetometer to the board I use those two pins?
I’ve been flummoxed for a couple of days trying to figure how to get external compass working…always trying the motor pins for the connection. Now that I’ve taken a closer look at the diagram you provide, I’m suspecting that is the error in my ways.