How does the serial data input SDI for the MAX31856 (thermocouple to digital converter) works?
From the MAX31856 datasheet https://datasheets.maximintegrated.com/en/ds/MAX31856.pdf Figure 6 SPI Multibyte Transfer, the address byte is the first byte sent with CS low, all subsequent bytes sent are for write data. And for reading data, the first byte sent with CS low is the address byte, and all subsequent bytes sent are ignored (while the data bytes are output in sequence on bytes 2, 3, 4, etc.)
So to read the temperature data, assert CS low and then transmit the read command 0Ch (binary 0000_1100) followed by three 'dummy' bytes of all 0 (the actual value doesn't matter), with CS still held low the whole time. During the 4-byte SPI transfer, the received data will be one dummy byte (during the address) and the three bytes of data, TC temperature byte 2, byte 1, and byte 0. CS must return high after the SPI transfer completes, since it is the falling edge of CS that identifies the start of the read/write command.
(full disclosure: I am an applications engineer at Maxim Integrated)