From what I read this morning Apple has been placing various voltages
on the two data lines for a USB charger with more than a computer behind
it. There have apparently been about 3 different incarnations of this
scheme with each one represented by a different voltage.
Chuck Norcutt
On 1/5/2013 1:57 AM, Chris Barker wrote:
> Chuck
>
> No, sorry, I didn't, only the first few lines in which the author describes
> the bridging of the 2 ports. Thanks (and to Piers) for enlightening me on
> the existence of such protocols.
>
> They sound like sensible restrictions, rather than sinister. And I now
> remember that I had USB output to use in the car which did not charge a
> previous iPhone: I was thoroughly confused by that, but no longer :-)
>
> Chris
>
> On 4 Jan 2013, at 23:10, Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx> wrote:
>
>> I don't think you read my link. It discusses an *apparent* proprietary
>> signaling system in 12V/USB chargers that iPads recognize and Androids
>> do not. A high current charging device such as an iPad or Android
>> tablet needs to be aware of the USB connection and, if it's attached to
>> a computer, throttle itself back to the maximum current draw specified
>> for a USB port on a computer (0.5 amp IIRC). However, if the charger is
>> simply a USB connector connected to a 12V outlet in a car then the
>> device can safely draw 2 amps or more. Certain of these 12V chargers
>> will rapidly charge an iPad since the iPad somehow knows there's a high
>> current capacity automobile on the other side of the USB connector. The
>> Androids typically do not since they don't understand whatever the iPad
>> is doing to realize it's charging from a car rather than your laptop.
>> The recommended charger in the link is one that does allow an Android to
>> draw 2 amps for charging... depending on which of the 2 USB sockets it's
>> connected to.
>>
>> There's a lot of detailed engineering study in that link.
>
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