The input to an optoisolator (i.e., an LED) is two pins, with no reference to ground. So either current is flowing through the opto's input, or not. Because the

The input to an optoisolator (i.e., an LED) is two pins, with no reference to ground. So either current is flowing through the opto's input, or not. Because they're not referenced to ground (or any voltage), the voltage on either pin relative to ground is irrelevant. (In this way, two connected MIDI devices can have different ideas of what signal ground is without causing issues.)