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USB Power Concerns, Confusion

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April 29, 2011 8:56:30 PM

Need help with USB power questions

I thought that, in addition to providing extra USB ports, externally powered USB hubs provided a "solution" to the system power supply/board not providing sufficient power to support multiple devices. Is this incorrect?

My situation (at the moment) is this:

I have a system with a 550W power supply, with 4 USB 2.0 ports off the back of the motherboard and 2 more USB

2.0 ports off the front.

I have an externally powered, 7 port USB 2.0 hub connected to one of the 4 rear ports;

I have a 2nd externally powered 7 port USB 2.0 hub connected to another of the 4 rear ports.

I have a Hewlett Packard multi-function printer/scanner/fax connected to the 3rd of the 4 rear ports.

The 4th rear port is empty

I have a wireless mouse "dongle" connected to one of the front ports


My problem (at the moment) is this:

Nothing connected to either of the external 7 ports hubs is being seen by the system.

By way of additional explanation:

All devices formerly (or normally) connected to either of the external hubs worked just fine ... until ... I added this
multifunction printer/scanner/fax to the system. At first, I added it to one of the external hubs ... and neither it, nor
any of the other things connected to either hub was seen ... including the mouse (very frustrating).

I rectified the mouse problem by 1) unplugging the printer from the hub I had put it on. Rebooted. Everything
returned to normal. (whew)

I then installed the printer into one of the 2 empty ports on the rear of the motherboard. Rebooted. Printer "seen" by
the system, no mouse (again, very frustrating)

I then installed the mouse "dongle" into one of the 2 front USB ports. Rebooted. Mouse is back (whew). Printer seen.
Two external hubs both show power (power light lit up), but no devices (a couple of memory sticks) were seen.

What I *think* is happening here is that the printer, even though it too is externally powered, is somehow taking too much USB power (is there even such a thing?) from the rear ports, effectively killing them. If the printer is connected to either of the HUBS, even though they are externally powered, it's not getting enough juice from them.

I'm confused, and can't explain it sufficiently to myself that I can come up with a solution.

Any help would be more than greatly appreciated, and will earn a (virtual) 6 pack of the drink of your choice ... unless you
happen to be Kevin the guy who lives next door, in which case the 6 pack WON'T be virtual.

Thanks.
a b ) Power supply
April 29, 2011 11:23:06 PM

Many USB devices cannot work thru a hub.

You could test your theory by simply unplugging the printer and then plugging something into one of the hubs. Well something that works with a hub.

Are these hub ports USB1 or 2?
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April 30, 2011 12:33:08 AM

Thanks Popatim

As I noted in the original post ...

I rectified the mouse problem by 1) unplugging the printer from the hub I had put it on. Rebooted. Everything
returned to normal. (whew)

Also, as stated initially, both hubs are USB2

It appears that regardless of where the printer is plugged into, the hubs don't work. If the printer's not plugged in, the hubs do work.

For the moment, assuming it *is* a power problem ... what's the solution? Why don't the externally powered hubs appear to be sending all that juice down the rails?
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a c 91 ) Power supply
April 30, 2011 1:54:56 PM

USB hubs are intended for low power devices (lower data traffic too) such as keyboards, mouse, flash drives.

Printers, monitors, and such, need a direct connection to the front or rear USB ports because of the data traffic and the power.

So, even though external USB devices have their own power source, it is not just a power problem; data transmission too.
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April 30, 2011 2:20:24 PM

Ubrales ... thanks ... that makes sense.

But (I know, there's *always* a but) if the printer *is* connected (which at the moment, is what it is) to a rear USB port, as you've said, then why am I still experiencing a loss of both external hubs w/ power?

Would it help if I daisy chained one of the hubs to the other ... of course, the 1st one will still need to be connected to one of the onboard ports?

Although minor, I have also had similar problems when my webcam and/or iPhone connections are plugged in @ boot time ... are those also high(er) power devices?

Do you have pointers to pages/sites that are good USB reference sources?

I'm sorry ... I don't mean this to appear that I'm either doubting the responses (I'm not, they're excellent) or trying to beat a dead horse ... I genuinely want to understand this better.

Thank you, and other responders as well ... with sincerity.
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a c 91 ) Power supply
April 30, 2011 2:42:23 PM

It is not a good idea to 'daisy chain' USB hubs.

It is not just a power issue; you must also consider the data transmission too. Multiplexing has its limitations.

Since you want to read up on USB, here are some sources:
http://en.wikipedia.org/wiki/Universal_Serial_Bus
http://computer.howstuffworks.com/usb.htm (Watch the video)

One more thing. Plug in the external power into the USB device first; then plug in the USB connector into the computer USB port. This is to prevent the device from trying to draw excessive power from the USB port.
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