piclist 2008\07\03\214146a >
Thread: USB power in parallel
www.piclist.com/techref/power.htm?key=power
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face BY : Herbert Graf email (remove spam text)



On Thu, 2008-07-03 at 20:48 -0400, Dr Skip wrote:
> I have a few external drives that have a dual usb end for tapping more power
> for the drive by using 2 ports. In most cases, these drives won't work with
> just one port. Looking at drive specs on portable, port-powered drives like the
>   Western Digital ones, they can take 1000mA to spin up, and many take 700mA.
> With port power restricted to 500mA, it makes sense why I can hear it can't
> spin up with just one usb connection.
>
> I'm assuming the second connection to the pc is power only. How can they just
> parallel ports? Is that part of the spec, or just a hack (like making a simple
> and gate by paralleling OC outputs)?

It is absolutely a hack, and is not in spec, in the cases I've seen.

The way USB works is you must ask permission to take more then a certain
minimal amount of current. The "main" USB connection likely properly
requests a full 500mA. In all the cases I've seen the second cable
doesn't even connect to the USB data pins, but only takes power.

In a perfect world the ports share the duty, and the current is equally
divided between the two. In the real world it's likely that in many
cases one port will source more then then other.

Ignoring that, many USB ports just can't supply the full 500mA. Laptops
are bad for that, but also any unpowered hub can't supply 500mA per port
since it gets it's power from one port.

While this hack mostly works, it can be hit and miss.

TTYL

<1215135706.15401.18.camel@E2140> 7bit

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