[email protected] wrote:
Hi all,
I just bought myself another USB external hard drive and this one like the
last one I bought has at the end that plugs into the PC a double USB end.
Now I was wondering if plugging in both of these plugs affects the speed
that data is transferred? I am assuming not as it is still going through
the same cable and would just bottleneck at the other end of the cable
which just has the one plug. Has anyone much wiser than me have any idea
about this?
Cheers,
Kerry
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USB data only goes through one of the plugs, so you do not get extra
speed. The extra plug is for additional power.
The USB specification says that an external device can draw a maximum of
500mA from any USB port. The PC or hub has circuitry to restrict the
current to this amount. The dual-plug lets the external hard drive cheat
and draw up power from two USB ports, so it can draw up to 2x 500mA or
1000mA.
Going off-topic from this:
The restriction to 500 mA means that USB powered speakers can not
possibly provide more than 5V x 500mA = 2.5W of power -- this will be
2.5W RMS. So any USB-powered speaker that claims to provide more audio
power than 2.5W RMS is doing the numbers as audio manufacturers are wont
to do. In fact, even 2.5W RMS assumes a 100% efficient amplifier -- I
think you should expect around 2W (80% efficiency).
Stephen Irons
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