[Elecraft] USB Cable Power Requirements

[Bill Frantz]

I have been running my SignaLink USB directly from my MacBook 
Pro for two years now with no problem. I checked the Wikipedia 
page for USB <http://en.wikipedia.org/wiki/USB> and I think it 
has an explanation:

   Power

   ...

   A unit load is defined as 100 mA in USB 2.0, and 150 mA in 
USB 3.0.
   A device may draw a maximum of 5 unit loads (500 mA) from a 
port in
   USB 2.0; 6 (900 mA) in USB 3.0. ... Every device functions initially
   as low-power but the device may request high-power and will 
get it
   if the power is available on the providing bus.

   ...

   A bus-powered hub initializes itself at 1 unit load and transitions
   to maximum unit loads after it completes hub configuration. Any
   device connected to the hub will draw 1 unit load regardless 
of the
   current draw of devices connected to other ports of the hub (i.e.
   one device connected on a four-port hub will draw only 1 unit load
   despite the fact that more unit loads are being supplied to the
   hub).

   A self-powered hub will supply maximum supported unit loads 
to any
   device connected to it. ...

The MacBook Pro USB ports act as a powered hub. The SignaLink 
probably won't work on a unpowered hub.

Cheers - Bill, AE6JV

On 12/31/12 at 8:21 AM, n1ho at yahoo.com (Bayard Coolidge) wrote:

>BUT, you should be aware that the Signalink USB *does* require an external powered
>USB hub, because it draws close to 500mA, and says so in its specs, etc.

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