[nitrogen]

If I remember right, some very low cost ways of converting AC to DC for small electronics leave those electronics at high voltage relative to ground. So the device itself sees +5V relative to its "ground", but the device "ground" might actually be at, say, 115V and the supply at 120 to make 5V. Thus the UART pins might also be at high absolute voltage. This is fine for devices that are fully isolated.

[janekm]

The "POW" device is one example of a device which has GND at live potential, so should not be connected to anything else while plugged into the mains.

Not that I would recommend plugging any of them into the mains with the case removed, or while plugged into other wiring, to be sure!

[amelius]

> The "POW" device is one example of a device which has GND at live potential

Is that even allowed by the FCC?

[detaro]

Yes, that's a quite common design. It might be forbidden if you expose that outside the device, but in electronics internal to the device you can do that no problem.

[amelius]

It sounds to me like the ground level is exposed outside the device in this case.

[detaro]

Unless I'm missing something I don't think so. The case has mains voltage in and switched mains voltage out only, to touch the low-voltage bits with the weird ground level you have to crack open the case. Which is kind of encouraged by open design that's easily hackable, but something that falls under "well, if you do that you're supposed to know what you are doing".