>USB             Windows-centric, hard to make a non-PC based master if I
needed one

This would be the way I would look at going, using a pair of USB-Serial
adapters. AFAIK Linux is pretty advanced at using USB, especially for
generic devices like this, so making a non-PC based unit should not be a
problem. When you say PC based, I assume you mean Wintel based, as Macs use
USB as well, so to say USB is PC based is not really correct.

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