Isn't the USB signal differential? I understand that this means that the signal is encoded by the difference between the two data pins (which are connected by a twisted cable pair) rather than by referencing to ground. If that's the case, the ground shouldn't matter for the signal transmission.
On Tue, Dec 23, 2014 at 9:46 PM, Ori Idan <o...@helicontech.co.il> wrote: > > > On Tue, Dec 23, 2014 at 4:19 PM, Alex Shnitman <alext...@gmail.com> wrote: > >> Power on the Raspberry Pi is a tricky business. I think on the B+ they've >> made it easier to deal with but it still cannot pass through a lot of >> current. So a powered hub is necessary, as previous posters mentioned. >> >> It does indeed backfeed into the Pi. I used thin stripes of electrical >> tape to cover the outer two contacts within the USB connector of the cable >> that connects the Pi to the hub, and that took care of that problem. Also, >> I found out that my hub (a cheap piece bought from DX), even when >> externally powered, would draw some current from the Pi during load. So the >> electrical tape took care of that too. >> > > I find it strange that it actually work since if you isolated both the > power and the ground pins, the data pins are left floated. That might work > but cause noise problems. So if you do want to isolate, isolate only the > power pin. > > >> If you have a good power supply for the hub you can power your Pi from >> the hub itself with an additional cable (i.e. the hub will be connected to >> the Pi twice, once with a USB A-B cable like any hub, and once with a USB A >> - MicroUSB B, from one of the hub's ports into the Pi's power supply port). >> This removes the need for a second power supply. >> >> One problem that I faced, though, was that a wifi dongle connected to the >> hub was being disconnected and reconnected every few minutes. I plugged it >> directly into the Pi itself and it works flawlessly. I still don't know why >> that problem happened; after all, they both are on the same power supply >> (the hub's). Other devices on this hub (the Pi itself as well as other >> stuff) seem to work fine. Maybe the hub's power supply is noisy and the Pi >> filters it when it passes it through. No idea. >> >> Alex >> >> > Ori Idan > >
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