In full duplex mode SPI master device simultaneously transmits data to a slave and receives data from a slave. This way only a single slave device can be engaged at one time. An SS line is used to select the particular SPI slave device.
SPI in full duplex modeIn half duplex single read mode SPI master device only receives data from a slave. This way only a single slave device can be engaged at one time. An SS line is used to select the slave device.
SPI in half duplex single read modeIn half duplex single write mode SPI master device only transmits data to a slave, and doesn't receive any data from the slave. This way simultaneous operation with several slave devices is possible.
SPI in half duplex single write mode