SCK - This is the SPI Clock pin, its an input to the chipSDO - this is the Serial Data Out / Microcontroller In Sensor Out pin, for data sent from the MAX31865 to your processorSDI - this is the Serial Data In / Microcontroller Out Sensor In pin, for data sent from your processor to the MAX31865CS - this is the Chip Select pin, drop it low to start an SPI transaction. Its an input to the chip