If DIN is high when reading data, which means writing in 0xFF, this implies that data are written to the CONVERSION register since 0xFF is an effective data for the MAX1231. However, when reading data, there is no need to write data to the chip, so it is necessary to ensure that no data is written into the MAX1231 during reading. This would require the DIN to send ineffective data. Since 0x00 is the ineffective data for the MAX1231, when reading data, please ensure that DIN is 0x00, rather than pulling it high (i.e. 0xFF).