Adding external HDD to Raspberry Pi

[Alex Shnitman]

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 <ori at helicontech.co.il> wrote:

>
>
> On Tue, Dec 23, 2014 at 4:19 PM, Alex Shnitman <alexta69 at 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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