When connecting a JTAG, Serial, or Parallel between powered devices how is it that one of the devices doesn't get fried?
The devices either need a common ground reference, or there needs to be something to isolate the circuits like an optoisolator.
Most JTAG probes go for the common ground, just connecting it together, and assuming that both sides use isolated power supplies, and if they bring in an external ground reference, it will be the earth connection from the power supply.
Most development boards have an isolating power supply in a wall adapter and are not connected to the earth pin in the outlet, so they are floating.
If you connect a JTAG adapter connected to a desktop PC that uses the earth pin as ground reference, the entire system will be referenced to that.
The only configuration that is a bit difficult is if both devices use the earth connection as ground, like I have here with an FPGA card in a desktop PC that is controlled via JTAG from another desktop PC.
This is however still safe since the earth reference they have is the same, I'd have to take a really long extension cord and connect one of the PCs' supplies in another building. That is impractical, so that case is basically ignored.