USB Hubs - question

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Sandstone - 10 Dec 2007 18:09 GMT

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)

-and-

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

-and-

- I have multiple devices plugged into the Hub

-and -

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

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?

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.

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.

TIA

Eric

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Mamamia - 10 Dec 2007 18:16 GMT

> G4 Powermac (digital audio) 466Mhz
>
[quoted text clipped - 28 lines]
>
> Eric

You may find that devices that require more power, such as printers, may
(or may not) require a powered hub. That has been my experience, at
least. Part of this may depend on how far your device is from the hub,
and how much power is needed for any given device. It may turn into
trial and error on your part. I always use a powered hub for printers,
scanners, and the like. Of course, maybe that's just me.

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D. Kirkpatrick - 10 Dec 2007 19:57 GMT

> 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.

Your mileage may vary. ;-))

I have a Keyspan 2-USB port card in my older Mac.

I found that my HP printer (USB) works better if plugged directly into
the Keyspan card.

The 4-port hub is plugged into the other port, which is at the rear of
the computer, and now sits conveniently near th efront of the system
to make access handy. I use it to plug in an MP3 player to load or my
digital camera. I have not attempted to try to run both at once.

This hub came unpowered and if I wanted I could get a separate power
supply to power it. When not powered it seems to have enough juice
straight from the USB card for what I need.

Usually if there is not enough power available you will get some kind
or warning or error message. Or if it does not work at all try
plugging directly into the card. Sometimes it is hard to determine in
advance what will and what will not work.

I did notice a strange trait with my set up.

At one point I started having a problem with my CD tray opening on its
own when rebooting.

It seems there was some issue with the hub or card. Don't ask me how
these are related. Its just one of your typical hauntings.

In that case I had to disconnect the hub and printer and reboot and
rebuild the desktop (OS9 system) a couple of times, then all was well.
I could then add back in the printer and hub for one final reboot
without the CD tray opening on its own.

I got the fix here on one of the Mac groups.

My alternative was sacrificing a chicken to it, and that's just plain
messy.

DMK

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Clark Martin - 11 Dec 2007 01:25 GMT

> G4 Powermac (digital audio) 466Mhz
>
[quoted text clipped - 13 lines]
>
> - 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.

> 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.

> 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.
>
> 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.

The power management scheme in USB was a good thought but not so well
designed and very poorly implemented in many cases. I've seen things
like printers, which need no power, call for 500mA. And many devices
call for 500mA even when they need less.

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

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

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Sandstone - 12 Dec 2007 02:06 GMT

>>G4 Powermac (digital audio) 466Mhz
>>
[quoted text clipped - 16 lines]
> 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
[quoted text clipped - 3 lines]
> 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.

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.

>>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
[quoted text clipped - 7 lines]
> like printers, which need no power, call for 500mA. And many devices
> call for 500mA even when they need less.

Gack - didn't realize things were so bad. thanks for putting up wih my
questions - I'm not the hardware type but like to understand what's
going on anyway.

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Howard Brazee - 12 Dec 2007 17:21 GMT

Plugging my iPod into the back of an unpowered keyboard along with a
USB mouse won't work. An iPod uses enough power that it either needs
to be in a stand-alone USB port, or plugged into a powered hub.

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Clark Martin - 13 Dec 2007 02:02 GMT

> Plugging my iPod into the back of an unpowered keyboard along with a
> USB mouse won't work. An iPod uses enough power that it either needs
> to be in a stand-alone USB port, or plugged into a powered hub.

That's because the iPod wants full power in order to charge itself.

The other thing to watch out for is that the hub in keyboards are
sometimes Full Speed (12MBps) which means that synching an iPod would
be slow.

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

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

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Clark Martin - 13 Dec 2007 02:00 GMT

> >>G4 Powermac (digital audio) 466Mhz
> >>
[quoted text clipped - 23 lines]
> power off and won't siphon off any USB power (if it's designed
> properly). Is this the gist of what you're saying?

Yes

> >>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
[quoted text clipped - 9 lines]
> 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
source.

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.

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

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

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Sandstone - 14 Dec 2007 01:33 GMT

>>>>G4 Powermac (digital audio) 466Mhz
>>>>
[quoted text clipped - 86 lines]
> always powered on. Otherwise some device may foul up things by wanting
> power when you don't expect it.

Thanks Clark. You answered all my questions and I enjoyed the technical
explanations - they were what I was looking for.

Eric

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Mac Forum / General / Hardware / December 2007

USB Hubs - question