We have a new white paper ! This one discusses IO expanders: http://neo900.org/stuff/papers/iox.pdf An IO expander is a chip that connects to the CPU (in our case via I2C) and provides a number of IO ports. Neo900 will need IO expanders because the CPU doesn't have quite enough pins for all the signals. As an added bonus, IO expanders can also lower the number of signals that have to pass between UPPER and LOWER. This specific version is at http://neo900.org/stuff/papers/iox-20160622.pdf The source can be found at https://neo900.org/git/?p=misc;a=tree;f=iox - Werner