Ok it sorta works! Motors move correctly (after flipping some stepper connectors), but homing does not yet work. I made three commits:
One commit to add the pins for the RAMBo board: https://github.com/johnboiles/grbl-Mega-5X/commit/091a5ad65905d409e3900f9ff33c8b2b8b446fbd
One commit to add support for digipots: https://github.com/johnboiles/grbl-Mega-5X/commit/598eff46d0c8957fc9b53c9f7470bcfe28387ef2
One (janky) commit to add basic microsteps: https://github.com/johnboiles/grbl-Mega-5X/commit/dca34c9a26c09a684c8a5943b582ff9130f37bf1
These are alltogether in my mpcnc-rambo-support branch if anyone wants to try them out. Please feel free to leave comments on the commits on GitHub if you are able to review my code.
Next up, I need to get homing working!