Re: [tlug] USB Hub v Power Injector: The Mostly Unimportant Differences

[jep200404@example.com]

On Thu, 10 Jan 2013 10:47:00 +0900, "Stephen J. Turnbull" <stephen@example.com> wrote:

> jep200404@example.com writes:
> 
>  > Do you want to know the mostly unimportant differences?
> 
> ... "yes". 

This is based on my fading memory of suffering through the USB 
spec while developing USB full speed devices, 
so the following comes with an "IIRC" caveat. 

The (mostly unimportant) differences are in power monitoring 
and control. 

I am assuming that the I-O Data device is a dumb device, that 
it just jams power into the USB device as long as there is AC 
power. This power is not monitored or controlled by a USB host. 
The I-O Data device can give much more power to a USB device 
than the USB spec allows. That's a big important beneficial 
difference. 

USB hosts are supposed to monitor and control the power to 
USB devices. When a device is plugged in, the host is obliged 
to supply up to 100 mA. If the device wants to use more than 
100 mA, the device may request up to 500 mA. The USB host 
may grant or deny requests for more than 100 mA at its own 
discretion. Devices are obliged to accept "no" for an answer. 
Hosts are supposed to monitor the power actually used, 
and are supposed to turn off power to a device 
if a device uses more power than it was granted. 

I've not seen hosts monitor power actually used by devices 
and turn them off for exceeding their allotment, but I did 
not do much testing of such when I was messing with USB 
development. Sticking in a dumb USB plug just to get more 
power is against the spec, but I've never seen it fail, 
except perhaps on laptops running on battery power.