Adding external HDD to Raspberry Pi

[Ori Idan]

The signal is supposed to be differential, however there are some devices
that take each line  with the ground as reference and only then subtract it.

On Thu, Dec 25, 2014 at 11:59 AM, Alex Shnitman <alexta69 at gmail.com> wrote:

> 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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