Glen "Wiley" Wilson wrote:

snip
Just curious, if you have one of the 4 USB units, does it have enough
power at the ports to drive 4 USB-powered devices at the same time? I
have a USB-powered hard drive; it works fine on some laptops, not at
all on most of them. When I use an external power supply everything
is fine.

The USB specs say that a USB port can provide 500mA for the device that
is plugged into it. If the advertising says that the drive can take its
power from the USB port it should mean that the drive draws less than
500mA at max draw.

The engineering on the laptop should be that the conductors for each
port are adequate to the rated max and that the USB bus is adequate to
all the ports drawing the max (four ports, would be 2A).

If your drive works on some laptops but not on others, it must be
monitoring the available power or the load on the USB bus and electing
not to participate. I guess it could do that by measuring the voltage
drop on the USB bus?

Jack

--
Jack Erbes in Ellsworth, Maine, USA - jackerbes at adelphia dot net
(also receiving email at jacker at midmaine.com)