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Post: That is interesting. The I2C pins on the GPIO header side pass through a MOSFET level shifter which converts the 3.3V Raspberry Pi I2C bus to 5V for the ADC chips. By connecting to the SDA and SCL pads on the ADC Pi you are bypassing the MOSFETs but that does mean you are passing 5V into the GPIO header on the Banana Pi. The 5V passes through a 10K resistor first so hopefully, the current is low enough not to damage the I2C bus on the Banana Pi but you may not want to leave it plugged in too long. When the Banana BPI M2 Zero I have ordered arrives I will connect the I2C port to my logic analyser and oscilloscope which should hopefully tell me why it is not communicating with the ADC Pi.