If I connect this signal to the digital port and load the example sketch DigitalReadSignal (File>Examples>01.Basics>DigitalReadSignal), then I get a series of zeroes and ones as the output (Actual Output: 1 0 0 0 1 1 1 0 0 0 0 0 1 1 1 0 0 0 1 1 0 0 0 1 0 1). Founding this bizarre
What code? What is generating this signal? Did you connect the grounds, too?I don't find the output bizarre, if you didn't connect the grounds, or set the pin mode correctly, or a number of other possible problems.
I'd also vote for a lack of common grounds - you don't have a circuit if you don't connect ground as well, a circuit needs two wires.When you connect only one wire you are using one circuit as an antenna for the other. Typically you will pick up mains interference.
It also looks like you are trying tool okay a signal too fast for your scope, or at least the settings you have on the scope.