#define _SET_OUTPUT(IO) do {DIO ## IO ## _DDR |= MASK(DIO ## IO ## _PIN); } while (0)

So, I modified the firmware to use Bluetooth (#define BLUETOOTH_SERIAL 3, #define BLUETOOTH_BAUD), compiled, and uploaded the firmware to the test Arduino Mega board via USB. 

#define EXT0_STEP_PIN ORIG_E0_STEP_PIN

#define EVENT_CHECK_FAST_KEYS(action) {}

1) Leave this as is : #define REPURPOSE_FAN_TO_COOL_EXTRUSIONS 0

#define CONFIGURATION_H

#define SDCARDDETECT -1

#define ARM_LENGTH 220

#define RXD DIO15

#define CONFIGURATION_H

  #define FAN_PIN -1  #define FEATURE_FAN_CONTROL 0#endif

#define DAVINCI 1 // "0" if not DAVINCI, "1" For DAVINCI 1.0, "2" For DAVINCI 2.0 with 1 FAN, "3" For DAVINCI 2.0 with 2 FAN, 4 for AiO (WITH NO SCANNER SUPPORT

* #define UI_DISPLAY_TYPE 5

#define EVENT_UI_OK_WIZARD(action) {}

In the configuration.h file I have set #define HAVE_HEATED_BED 0

#define ZAXIS_STEPS_PER_MM 8000

#define DRIVE_SYSTEM 0

#define ENDSTOP_X_BACK_MOVE 5

I have the following:#define EXT0_INVERSE 0

Inverse is#define EXT0_INVERSE false