I liked the idea of a PT100 reader with 0.03 C resolution, and Adafruit already made the library available, so I quickly hacked support into Marlin if anyone is interested (I think there's currently a feature freeze, so I'll submit it later). Note that this only works for heater 0 (it piggybacks off MAX6675 support). 1. Get the Adafruit MAX31865 breakout board 2. Wire the board. On RAMPS 1.4,by achase79 - Firmware - Marlin