Re: USB Hubs - question

In article <475F421E.6040602@xxxxxxxxxxxx>,
Sandstone <spambucket@xxxxxxxxxxxx> wrote:

Clark Martin wrote:
In article <475D80CD.9040208@xxxxxxxxxxxx>,
Sandstone <spambucket@xxxxxxxxxxxx> wrote:

G4 Powermac (digital audio) 466Mhz

If I have a powered 4-port USB Hub and

- I don't plug in the Hub itself (so that it's unpowered)


- the Hub is plugged into a Mac's USB port (obviously)


- I have multiple devices plugged into the Hub

-and -

- at most one of those devices is active at once (others powered off)

Most USB devices want their full power when plugged in and don't really
power off.

Interesting. So USB devices that draw all their power from the USB
connection don't actually power off and thus siphon off some of the
available power anyway. But if the USB device is powered externally via
a wall socket and you turn it off then theoretically it really does
power off and won't siphon off any USB power (if it's designed
properly). Is this the gist of what you're saying?


then it would seem that the sole active USB device should work just fine
since, in this scenario, it would be the same as plugging that device
directly into the Mac's USB port. Is this true?

No, it's not the same. The Hub needs power. USB allocates power in
100mA chunks up to 500mA. So the Hub gets 100mA leaving 400mA for
everything else.

When you say "the Hub needs power" are you talking about the power that
comes across its USB connection to the Mac? Or are you talking about the
power that comes from the wall socket? I guess I'm wondering whether
unplugging a powered USB Hub renders it incapable of dishing out any
power coming from the Mac's USB connection to the Hub's various ports.

The hub needs power period. If the hub has an external power supply
then it is usually large enough to power all ports at full power (500mA)
and the hub itself. If it's powered from the computer or another hub it
needs some of the power coming from the upstream device.

Maybe your point is that any USB Hub - powered or not - is going to
steal some of the USB power from the Mac's USB port for itself thus
leaving less available for the devices, and that's why it's not the same
as plugging the device directly into the Mac.

Well, it's not stealing, it's using some of the power from whatever

If the hub is solely powered by the upstream USB device the power
available to downstream devices (one or more) is reduced as opposed to
simply plugging one of those downstream devices into the upstream source.

Aside from the inconvenience of finding a place to plug in the Hub's AC
adapter, I don't know why someone would do things this way. I ask only
because I'm curious about a Hub's capabilities.

It's simple. Any USB source is limited to supplying a maximum of 500mA
of current (2.5W of power). The USB standard allocates power in 100mA
chunks. If you plug a hub into your source the hub uses up a 100mA
chunk. Even if it only requires say 10mA it is allocated 100mA. This
leaves a maximum of 400mA to all downstream devices.

Now when they designed USB they could have set it up to allocate in,
say, 10mA chunks but it still would mean the Hub is using some power.

Another thing is that when devices connected they are guaranteed a
minimum of 100mA. This is so it has enough power to run it's own
processor. Once connected it can then request additional power if
needed, and if it's available.

In case it matters I'm hoping to get a D-Link DUB-H4 High Speed USB 2.0
4-Port Hub for Christmas and I'm just wondering about things ahead of time.

If you are using low power devices (100mA) then you can hook up four of
them to a hub with no external power supply. If the total power of all
external devices is more than 400mA you are going to need an external
power supply, that's the simple approach.

I wouldn't worry about devices being powered on or not, assume they are
always powered on. Otherwise some device may foul up things by wanting
power when you don't expect it.

Clark Martin
Redwood City, CA, USA Macintosh / Internet Consulting

"I'm a designated driver on the Information Super Highway"