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Baffling USB Ports

Posted on 2006-11-11
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Baffling USB Ports
I am working on a Alienware Area 51 with Abit AA8XE motherboard running Window XP Home.  This computer has 4 USB ports on the back and 4 on the front panel.  The customer has a large number of USB devices (11) that I just can’t get to all work at the same time..  The problems do not seem to be specific to which devices I have plugged in, but rather how many are plugged in.
It was all working fine and The problems started with the computer would not boot and would just hang at the first Windows XP screen.  I found that by unplugging a USB ZIP drive the problem went away. Then other problems started to crop up like one of the external USB hard drives was not recognized and came up under device manager as an unknown device.  Or suddenly a USB HUB would not be recognized or a thumb drive would not show up.  I plugged and unplugged different combinations of devices in different orders and ports and nothing really made any sense.  I would get to a certain point and then all of a sudden some devices would not get recognized and come as unknown.  It seems to me that this has to be a power supply issue, because last night I removed all devices and plugged in just 2 powered hubs.  I plugged all the devices into the hubs only and none into the computer directly and everything was recognized properly and is booting and running fine.   Does this make any sense?  I have had USB problems in the past and resolved them by removing USB hubs not adding them.  Here is the list of the USB devices: Canon Printer, Maxtor External Drive, Acom External Drive, Epson Scanner, Logitech Web Cam, Sailtek Joystick, Foot Peddles, Huey monitor color correction tool, and 2 1gig PNY thumb drives.
I would have thought that the power output available at each USB port would be isolated and not a cumulative effect.  Does anyone have any ideas here?  The main power supply is 460 watts.
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Question by:waytron
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by:waytron
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One other question.  What is the dissadvantage to having everything plugged into a hub instead of into the computer dirrectly?  IS there a speed issue?
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by:phototropic
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Two external hard drives will pull a lot of power. If the pc works fine with powered hubs, that would seem to indicate a power supply issue.
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by:waytron
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Well the external Hard drives are not USB powered, they have their own power supplies.  None of these links help with USB power calculations.
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by:phototropic
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The total wattage for all installed devices here is 355.45 watts:

http://tomshardware.co.uk/2002/10/21/inadequate_and_deceptive_product_labeling/index.html

This is a standard system with 2 USB devices. It would require a minimum 400 watt PSU.  Your machine is high-end spec and has 11 USB devices, so it seems reasonable to assume a wattage deficiency.
I do not know of a calculator that enables you to calculate specific power consumption of USB devices. I suggested the links to get an idea of what wattage ballpark your system was in...
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by:waytron
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I ran one of the power supply calculators and without any USB devices the calculator says about 358 watts.  I started having problems after I plugged in 2 USB Hard drives, 1 scanner and 1 printer.  When I plug in the next device and it does not seem to matter which device, I start getting unknown device.
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by:phototropic
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Why not try a 600 watt PSU and see what happens...?
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by:waytron
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You are right, why futz around taking guesses and just try a larger power supply.  I will do that and get back to you.
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by:neutron7
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If you get a powered USB hub and put the most power hungy devices on it it will probably clear it up. USB power from the motherboard is Limited so no matter how big a PSU you have the amount of power available to USB ports will be limited by the USB electronics.
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by:phototropic
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neutron7,

Yes, the amount of power available to USB devices should be limited to 500mA. However, the point here is that a 460 watt PSU may not be able to deal with the demands of a high-end system AND 11 seperate USB devices. Waytron said in his original question that the problem went away when he used two powered hubs, so I guess the issue is that his customer does not want to do that...

For a highly technical overview of the USB architecture, see here:

 http://www.beyondlogic.org/usbnutshell/usb2.htm

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garycase earned 250 total points
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I didn't see any indication that the customer was opposed to using powered hubs ... and that is clearly the solution here.   Although the USB specification allows each USB port to source up to 500ma, it is not uncommon to have problems when a large number of devices are drawing near the maximum specified power ==> and with an unpowered (i.e. USB-powered) hub is used it's easy to draw more than the specified 500ma (since multiple devices are on the hub, and each "thinks" it can draw 500ma ... even though the TOTAL for the hub should not exceed 500ma if it's powered by the USB port it's plugged in to).

Clearly the solution here is simple:  use powered hubs to reduce the power draw on the system.   A larger power supply MAY help (although I think it's unlikely) ... but a powered hub DOES help (and, as already noted, resolves the problem).
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by:garycase
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... and it's also obvious (since there are 8 USB ports and 11 devices) that there has to be at least one hub involved => if that hub is USB-powered then it's almost certain that it's exceeding the 500ma draw for the port it's using.   In that case a larger power supply will NOT help ==> no matter how much power is available, the USB controller cannot provide more than 500ma per port.
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by:phototropic
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So is the problem here that
1) When a non-powered hub (or device) demands more than 500mA the system stops responding appropriately;  or
2) When all the USB ports are in use, the total power demand (including all other devices within the pc) is greater than the PSU can supply?
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by:garycase
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Almost certainly #1.   Even 11 devices at 500ma only draw a total of 27.5 watts.   The 5.5A load on the 5v rail is also unlikely to stress that voltage output ... a 460w power supply almost certainly allows at least 20A, and more likely more, of 5v draw.   The 12v rail (not used with USB devices) is almost always the most critical voltage output in terms of stressing the power supply.
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by:waytron
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It is not that the customer does not want to use powered hubs.  And yes the hub that was on the system was powered.  It is just a matter of principal,  he has 8 ports on the computer and would like to use them.  Also, I would think that there would be some kind of disadvantage or slow down having 2 or more USB hard drives on a single hub vs. directly connected to the mother board?  If he can't then it seems like something is wrong with the system.  It worked before then why does it not work now?  Is the power supply dying or is something else wrong?
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by:WallD
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I have had a similar problem inthe past, we found the best soloution to be a new usb card. this cured powerbissues and speed though itwas in fact an upgrade from usb 1 to which probabaly improved the speed.

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by:garycase
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It's not uncommon to see USB issues when the connected devices are drawing close to the maximum specified power.   Think of it like running your car's engine at red line all the time -- within the spec, but don't be surprised if you have problems.   8 ports ... 8 devices drawing near 500ma each ==> I'm not at all surprised that this eventually caused problems.

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by:waytron
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I have not heard back from the customer as to whether or not he wants me to continue to pursue this issue or just leave it be with the 2 hubs.

However, it still seems to me that there has to be a disadvantage to running all these devices through hubs, especially the hard drives?
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by:garycase
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There's no disadvantage to using powered hubs ... and a BIG advantage => the power draw on the USB controller is significantly reduced, since the hubs provide managed power to the attached devices.

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by:waytron
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So you are telling me that if I have 2 or 3 USB hard drives connected to one hub, there would be no difference in speed vs plugging them into their own port on the motherboard?  Even I was writing from one USB Drive to the other?
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by:garycase
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There are some devices that don't work well with hubs (due to the slight timing modification imposed by the repeater), but most work fine.   I've not seen any problems with hubs except when trying to cascade them (hubs attached to hubs) => which I don't recommend.     As long as the device works through the hub, there's no impact on the speed => remember that the 480Mb transfer is the total bus capability, not a "per port" speed.

Easy enough to test if you'd like:  just connect two hard drives to 2 ports on the PC; and transfer a LARGE (1GB or so) file between them, carefully timing the transfer.   Then delete the copied file; "safely remove" the drives; attach them both to a hub; and repeat the transfer, timing it again.   While there may be a slight difference (that would be likely even if you left them attached to the same same two ports you started with), there won't be any appreciable difference.
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by:Ric Tutero
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The system board MIGHT still be supplying power to the devices... could there be any emulation or power management options in the BIOS?

there are also power management options in Windows within the USB root hubs' properties (in device manager) where you can stop windows from powering the devices on/off.

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by:waytron
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Well my customer has decied to stick with just using the Hubs for now and not look any further into the problem.  So I guess I will never know why?  So I will just split the points.  Thank guys
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