juan2003
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Problem with Powered USB Hub. Please help!

Thu Oct 11, 2012 1:08 am

Hi,

Im a newbie with the raspberry pi (got mine yesterday) and Im planning to create a media center with it. For that I would like to attach a 500GB usb hdd that I already own.

Right now Im trying xbian but I already tried raspbmc and the same thing happened. Im using an 8GB SD for the OS, a 1.0A cellphone charger to power the pi, samsung lcd tv for display (hdmi).

For what I read I need a powered usb hub in order to power the usb hdd, so I bought one. I bought a "soyntec" powered hub with 7 usb ports (Model Nexoos 395, just for the record).

If I connect the keyboard into one of the pi's usb ports and the hub on the other (without anything plugged into the hub and without powering the hub), everything works well. If I add a 16GB pendrive in the hub (wihtout powering the hub), the pi recognizes the pendrive and works fine. If I unplug the pendrive from the hub and I plug the hub's power cord in order to get it powered (without plugging any usb device, just the hub to the pi), everything works well.

The problem comes when I try to plug anything into the hub after is powered. If I plug either the pendrive (which worked in the hub before powering the hub) or the usb hdd, the screen starts to "flicker" (black screen, back to normal for a second, black screen, and so on).

So the problem seems to be if I plug something into the hub when the hub is powered.... because if the hub is not powered, it seems to work. But if I dont power the hub I wont be able to use my hdd which is my objective.

I would really appreciate it if you could help me with this because all the uses that I thought for my pi rely on the usb hdd (media player, torrent seedbox, NAS).

Thanks in advance, and by the way excellent job with the PI, you are really inspiring the next generation of engineers. I wish I would had one back when I was on school, this is a great thing.

Juan

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penguintutor
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Re: Problem with Powered USB Hub. Please help!

Thu Oct 11, 2012 10:14 am

Sounds like a faulty hub.

Do you have another hub you can try?

juan2003
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Joined: Thu Oct 11, 2012 12:49 am

Re: Problem with Powered USB Hub. Please help!

Thu Oct 11, 2012 1:30 pm

Unfortunately I dont have another powered usb hub... This is brand new and costed me $30 :(

if I just plug the hub to the usb ports it doestn power the pi. I think that the pi have some diodes to prevent current coming in from the usb ports (I dont know anything about electronics, I read this on a forum). If I use a usb-microusb cable from the hub to the pi's power connector, the pi starts, but it flickers the same way I described before...

What I dont understand is, if the pi has something to prevent (I think they are diodes) current coming in from the usb ports (I tried this as it doesnt power the pi if I just connect the powered usb hub to the pi's usb port), why should feedback power from the powered hub affect its behavior? Because if I dont power the hub (I use it as a normal hub without powering to the electricity) the hub works fine with a usb stick.

Is there any walkaround this? Is there any way I can meassure if the usb hub is sending feedback power (maybe with a voltmeter, which pins should I check?)? If the problem is the feedback power from the hub to the pi, is there any walkaround so I can use this hub?

Thanks!

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Burngate
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Re: Problem with Powered USB Hub. Please help!

Thu Oct 11, 2012 4:22 pm

You say in the other thread that you're using HDMI. Have you tried upping the drive for that? Look in the wiki for config.txt to find out how.

There seems to be some confusion about diodes on the USB power feed. There is nothing apart from fuses on rev 1 boards, (or zero-ohm links on later ones) and nothing at all on rev 2 boards. The only diodes are for the 9512 usb hub chip to sense the power state.
So if your hub were to feed power back through the port, it could run the Pi. But you say it doesn't, which is good(ish) news.

There doesn't seem to be a way for power on the hub to affect the HDMI.

Does the Pi remain working correctly (as far as you can see) or does it continually reboot?

juan2003
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Joined: Thu Oct 11, 2012 12:49 am

Re: Problem with Powered USB Hub. Please help!

Thu Oct 11, 2012 4:45 pm

Yes Im using HDMI. What do you mean by "upping the drive for that"? Sorry, I didnt understand that.

When the screen starts flickering, I can access it through ssh and execute commands without any problems... Its not rebooting.

In case that the problem could be with the hdmi, the weird thing is that without the powered usb hub, it works fine, the image doesnt flicker. Is that possible?

Its like with the powered usb hub, the pi gets unstable. Also if I just turn off the TV and access it through ssh while its flickering, if I connect the usb hard disk to it, the pi doesnt see it.

The impression that I get is that the powered usb turn the pi unstable. Because yesterday it worked for 5 minutes as expected with this setup:

-I powered the pi with the powered usb hub using a usb-microusb cable
-I plugged the keyboard in one of the pi's usb ports
-In the other pi usb port I connected the usb hub, which I powered it (connecting its power cord to the electric grid).
-I plugged a 16GB pendrive to the hub
-I plugged my 256 GB usb HDD (its 256GB, in a previous post I said it was a 500GB disk but I was wrong)

It booted fine (xbian) and both the pendrive and the hdd were recognized and xbmc interface worked fine in my TV (through hdmi). But some thing strange happened, if I moved the keyboard (moving the cable) the video turned off, like if it was having a false contact. If I unplugged the keyboard, the video stayed of until I plugged it again... After 5 mins I restarted the pi and I couldnt get to the same scenario, it started flickering again as I mentioned in the previous posts.... So that was the best scenario, which only worked once and for 5 mins...

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Burngate
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Re: Problem with Powered USB Hub. Please help!

Thu Oct 11, 2012 5:34 pm

Very wierd.

One thought. Is the HDMI lead pushed fully home? There have been a couple of people who had that problem, thought it was in but it was not fully there.

Sorry for the confusion - I got lazy in my description!
What I meant by "upping the drive for that" is this:
The default voltage of the HDMI digital signals may be too low for the monitor to decode properly.
If so, adding a line in config.txt (or uncommenting - remove the # - the line that's there) to change that voltage may help.
see http://elinux.org/RPi_config.txt for help

juan2003
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Re: Problem with Powered USB Hub. Please help!

Thu Oct 11, 2012 5:50 pm

The problem is that without the usb hub the HDMI TV connection works just fine.

So, shouldnt be a problem with the usb hub? Because without the powered hub everything works perfect. Thats why its seems weird to me to be an issue with the hdmi connection...

About the lead pushed fully home, 100% sure it was, I checked it twiced.

Any other idea?

I was thinking in cutting 5V cable from the usb hub output to avoid power feedbacks from the usb hub to the raspberry. Do you think that could be the problem of the instability?

How can I be 100% sure of which cable to cut? how should I use my multimeter to know it?

Thanks

JustPat
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Joined: Fri Jul 27, 2012 1:47 am

Re: Problem with Powered USB Hub. Please help!

Thu Oct 11, 2012 7:50 pm

juan2003 wrote:The problem is that without the usb hub the HDMI TV connection works just fine.

So, shouldnt be a problem with the usb hub? Because without the powered hub everything works perfect. Thats why its seems weird to me to be an issue with the hdmi connection...

About the lead pushed fully home, 100% sure it was, I checked it twiced.

Any other idea?

I was thinking in cutting 5V cable from the usb hub output to avoid power feedbacks from the usb hub to the raspberry. Do you think that could be the problem of the instability?

How can I be 100% sure of which cable to cut? how should I use my multimeter to know it?

Thanks
I don't think it's a power feedback problem because there would be a greater feedback without the 500 GB drive attached than with it attached. If you want to test there are to test points on the board labeled "TP1" and "TP2" you can use. Just set your multimeter to the lowest setting above 5V (I forget, maybe 10V? I don't use multimeters often). Did you try increasing the power to HDMI as someone previously suggested? Also, as long as you're powering the pi through the micro USB port, a little backfeed from the USB port is OK. If you're determined to cut the power leads on your USB hub to you pi, they should be colored black and red. The other wires could be any color. You might want to check with the manufacturer.

Good luck

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penguintutor
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Re: Problem with Powered USB Hub. Please help!

Thu Oct 11, 2012 8:19 pm

I would not expect a good USB hub to be causing a problem so I still think it is most likely that the HUB has a fault on it. There are no diodes on the Pi to stop current flowing from the USB "output" into the Pi. There should be on the hub, but some models don't which is why back-feeding happens. On the early boards there are fuses which can trip if power is coming from the USB. They are self-resetting but I don't know how quickly it takes to reset as to whether this could be causing the flickering - eg. change in voltage if the supply changes from USB to micro-USB.

It could be just a small blip on the HDMI signal that the monitor is exaggerating by waiting for a clean HDMI signal before showing the screen again.

Have you tried powering the Pi from the USB hub (via micro-USB) compared with an external supply?

If you want to try cutting the power cable to the USB hub (cable should be cheap enough to buy online - or perhaps have a spare one from old equipment) then the wire to cut is the red one. The black is ground, green = data+ and white = data-.

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Burngate
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Re: Problem with Powered USB Hub. Please help!

Fri Oct 12, 2012 8:41 am


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