OSHW-OpenCapitalist-Steppernug Schematics
http://opencapitalist.org/store/steppernug-interface-board-developer-edition/ is the state of the current design. I would recommend forgoing limit switch detection for the time being and focus on Z-height control. We can use the current design and any development work will be compatible with the next rev as all it would require is I2C and firmware.