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I'm building a keyboard with an integrated KVM, using an MCU with 2 USB device interfaces.

I've been wondering what would happen if I connected the keyboard to two PCs powered from different ground potentials.

Optimistically, at least one of the two host devices will have a floating DC supply, and after initial connection (and maybe some sparks) the grounds will equalize, no problem there.

However, if both hosts DC supplies are not floating and do not match, then I'm guessing the keyboard or the cables would burn to a crisp.

Is the second scenario an actual possibility, and one that I should worry about for a consumer device? Hopefuly not, because I'm not ready to deal with the complexity that a galvanic isolator would bring.

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As per your main question, connecting two different potentials together with a wire makes current flow in the wire loop between devices.

The larger question is if it matters at all. Many products connect grounds of devices together. PC and monitor, add in a printer, and other peripherals. Connecting grounded equipment together has typically not been an issue, so connecting two PCs into a standard KVM has been already proven to work.

However, connecting non-grounded floating equipment together with grounded one or another floating equipment may have problems, which is why many devices say they should be connected together only when the mains plug is disconnected.

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