For those that that are doing it cheap (or available)

Branch of Marlin2 for OpenPnP on RAMPS by crashmatt

Unsupported. Not suitable for PR to upstream. For reference only.

• Supporting rotation axis that has no endstop switch.
• Rotation moved from extruder to I_AXIS reporting as "A" axis in gcode.
• All fans and heaters are disabled so FET outputs can be used for pumps, valves, lights etc. with GPIO direct.
• Required modification of motion do_homing_move to enable the rotation stepper without checking for endstops. This is planned to be fixed in Marlin 2 the long term.
• To enable the I_AXIS the endstops are assigned to spare X/Y axis enstop inputs. Those inputs are then inverted to make it look like the endstop is never triggered. I may be able to remove this hack.
• My configuration is homing is to the back right corner and the bed is offset to zero at the front left corner. Not ideal. Don't copy this.