Depending on what features you got with your Nebula you may or may not need the parallel port cable. If you got the touch plate and or the ATL sensor it should be plugged into the second parallel port on the PC. The horizontal P port is for the cable from the controller. The vertical P port is for the accessories.
The yellow AC plug is indeed for anyone using a router rather than a 220v spindle. You can just leave it hanging.
The black cable should be attached to Earth Ground on the back of the Unity controller box. It is indeed to ground the CNC frame through the case through its plug to your house/shop ground. Not 100% sure on this as my CNC didn't come with one.
Anyone feel free to correct me if I missed something. As I said my CNC didn't come with the ground or accessory P port cable.
The yellow AC plug is indeed for anyone using a router rather than a 220v spindle. You can just leave it hanging.
The black cable should be attached to Earth Ground on the back of the Unity controller box. It is indeed to ground the CNC frame through the case through its plug to your house/shop ground. Not 100% sure on this as my CNC didn't come with one.
Anyone feel free to correct me if I missed something. As I said my CNC didn't come with the ground or accessory P port cable.