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USB port idiosynchracies

Hi,

I've played with two appliances in the past week that each imposed what *seem* to be arbitrary restrictions on the use of their USB ports.

The first, an NSLU2, only allows a "thumb drive" to be used on one (particular) of the two ports ("real disks" can be used on either/both).

The second, a LinkStation, also allows only one thumb drive (plus one printer) -- though it doesn't force the thumb drive to a *particular* port.

Peeking inside the NSLU2 shows power is simply not routed to the second USB port (though it could easily be with the addition of an unpopulated ferrite bead).

I haven't taken apart the LinkStation for fear of damaging the case -- yet. :>

I don't see why this arbitrary restriction. Unless they simply haven't the extra 2-3W (worst case) to power an extra thumb drive??

Are there some other USB subtleties of which I am unaware? Or, chipset problems??

--don

Reply to
D Yuniskis
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I'm guessing the answer is "engineering compromises".

If you have a USB2 port, you are *required* to be able to source a full amp to it (worst case =3D 5W), or turn it off if it over-currents. Either of those costs money. 99% of the world is happy connecting one thumb drive, or an externally powered HDD. So there you have it.

RK

Reply to
d_s_klein

Are you sure about that figure? I thought USB2 was 5 unit loads (5x100mA) max and USB3 was 6 unit loads (6x150mA).

*And*, a device starts with 1 unit load and *negotiates* the higher power requirement. So, a *smart* "host" can handle several single unit load ("low power") devices in lieu of just one "high power" (5 unit load) device.

I.e., this "one thumb drive" restriction seems quite arbitrary as it assumes all thumb drives would be "high power" (that is assuming power requirements *are* the reason behind the "one thumb drive" restriction)

The NSLU2 has a 10W supply. Even if it used 5 of that internally that still leaves enough for two USB2 "high power" devices (2.5W each).

Dunno about the LinkStation, yet.

Reply to
D Yuniskis

While you can manage the devices' notional power limit in software by refusing to enumerate a device that would take the system over the power limit, I've not used or seen any hub chips that detect anything other than 500mA as the current limit. That is, the hardware only detects excessive high power current consumption.

Also since you can plug anything into the port and the notional 100mA isn't enforced by hardware then only powering one port does prevent the system keeling over. I suspect that that might annoy you more(!).

It also depends on what OS the NLSU2 is using. If it isn't running Linux/Windows then it is quite possible that it doesn't do power management. So, if you like, a "dumb" not "smart" host.

As RK says it is most likely a compromise.

Andrew

Reply to
Andrew Jackson

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