@ScruffR - So I went and looked at the Arduino Uno R3 schematic and I did not see any resistors on SDA/SCL. Are you referring to the internal Pull-ups?
Thesen might be enlightning discussions
https://electronics.stackexchange.com/questions/102611/what-happens-if-i-omit-the-pullup-resistors-on-i2c-lines
http://forum.arduino.cc/index.php?topic=297059.0
https://forum.arduino.cc/index.php?topic=369003.0
Thank you for the info. Apparently the built-in pull-up resistors on Arduinos provide enough resistance to sort of make I2C work. Not the same as having real 4.7K to Vcc. Iām curious whether the the STM32 in the Photon has buit-in pull-ups also.
@Mattster, see my response on another thread:
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