[aprssig] $60 WinBook TW700 Tablet -- Webpage With More Info Up

AC kf4lvz at yahoo.com
Mon Feb 9 14:48:33 CST 2015


On 2015-02-09 10:49, SARTrack Admin wrote:
> On 10/02/2015 07:38, Gary D. Schwartz wrote:
>> The full size USB port is a host port, not a client port as seen on many
>> tablets.
> 
> And the requirement of a USB Host port is that it MUST be able to supply
> 5 Volt/1 Amp to a client device, which is why you never see them on a
> Tablet. I am quite surprised that they put one on this tablet. Very
> cool, but also will drain the battery very fast if (for example) a HDD
> is connected...
> 
> Bart

No, the USB specification for 1.0 and 2.0 calls for up to 500 mA of
current on a single port, not 1 Amp.  USB 3.0 calls for 900 mA.

USB 1/2 provide current in "unit loads" of 100 mA/unit with a maximum of
5 units per port.  USB 3 is 150 mA/unit with a maximum of 6 units.

This doesn't include newer specifications such as USB-PD (Power
Delivery) and USB-BC (Battery Charging) which have higher current
capabilities and specifications but they're also newer specs.

USB-On-The-Go (USB-OTG) addresses host ports for mobile devices.  It
relaxes the maximum current requirements of the standard by specifying
that it must supply a minimum of 8 mA for negotiation purposes and then
it may supply any current maximum that the manufacturer is willing to
provide (e.g. to reduce battery drain, etc.)  Events and signalling are
specified for overcurrent conditions on a USB-OTG connection.  A
peripheral can be self-powered which then eliminates the current issues
on a USB-OTG port while still providing USB host capability.  For
example, my phone has a USB-OTG port and it will let me plug in many
devices including hard drives, keyboards, flash memory sticks, serial
port adapters, etc.  The hard drive has to be self-powered but the port
is able to provide up to 1 unit of current (100 mA) to run the other
devices (most of which need less than 50 mA).



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