all the data is optically coupled with an opto coupler at the recieving end. there is no transfer in actual energy except for a 5v supply and a grounding rail. only three of the five pins in the midi cable are used. i've never had problems with disconnecting midi cables from activated units.That's a really good question, and in a scientific sense I really can't say what happens electrically when a midi cable is plugged or unplugged while the power is on.