Half Duplex SPI allows all units to send to each other using MISO or MOSI. The challenge is that if 2 units try sending one might pull high and another low so you get a short-cut. The same goes if more than one tries to become Master and run the clock. The diagram above solves that by introducing small resistors on SPI. And this is recommended for Half Duplex SPI. This is one of the changes I will need to add to all my Hat’s.