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I have built computers, and I majored in computer science in college. I am well aware that it requires a minimum of two conductors to make a complete circuit - a 'send' and 'receive,' as it were. When I said 'one line' I didn't feel it was necessary to point out the should-be-obvious common wire. Hypothetically, all the different things you would plug in could utilise the same common wire, which is why I didn't mention it. Of course, I haven't seen a schematic of the pin assigments, so I could be dead wrong.