The USB port on the Classic is referenced to battery negative so in a positive ground system and using a pc that is grounded to the ac mains there could be a short from battery - to + through the pc and Classic. Care needs to be taken on Positive ground systems to use a laptop on battery power and if it has metal on the case not to have it setting on a grounded metal frame or table. Feel free to give us a shout on the phone or Email if you have any questions on the USB on positive ground. I have also cut and pasted our warning that we ship on the menu map with each Classic now.
WARNING ! The Classic's USB
port is NOT isolated from
battery negative. This is
typically only an issue on
positive ground systems or
systems with a tripped ground
fault protection device. Care
must be taken that a
computer connected to the
Classic's USB port is either
isolated from ground and the
Classic's negative or that the
computer's USB negative is
common with the Classic's
negative.