What is the purpose of the Orange and Orange+White wires (pins 5 & 6) in the Data2 pigtail cable? I see both are labelled as Grounds: when should they be connected and should they be connected to Ship's Ground, back to battery -ve?
There isn't an assigned purpose for those pins. They are fillers "just in case" they wanted to add another special port or signal to the system. Since they are not used, they are grounded. Not all grounds are created equal. There are signal grounds, Chassis/case grounds etc… In the Furuno products our chassis ground is isolated from our signal grounds. This provides for better communications and cleaner signals. As a general rule, do not connect up a ground or shield wire without being instructed to do so by the manual or your Furuno tech. Connecting it up because it says “ground” will get you into trouble. For example our NMEA cables normally have a braided shield protecting the wires. They are internally grounded within the unit. If you connect up the other end; you have just created a path of electrical flow. This can result in grounding loops and power differential issues. When in doubt ask, but NEVER connect something that you are not instructed to do by the manual or your friendly knowledgeable neighborhood Furuno tech. :cool