Eric Smith wrote:
"Michael J. Mahon" <mjmahon@aol.com> writes:The USB data lines are differential, and moderately resistant to common-mode voltages.No, not really. The differential common mode range spec, Vcm, is 0.8 to 2.5 volts for low speed and full speed. For high speed, Vhscm is -50mV to +500mV. See Table 7-7 of the USB spec (rev 2.0). Maybe you could get away with more than that in some cases, but you can't count on it.
By moderately resistant, I meant that he didn't have to worry about ground-loop currents (as he asked), not that he could float the ground by 12v! -michael NadaPong: Network game demo for Apple II computers! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool--and it's seriously underused."