How many USB devices can be powered by a PC?

March 20, 2006 6:19 PM

How many USB devices can I power from my system without problems? If USB devices are drawing too much power, which ones are best to off-load on to a self-powered USB hub?

I'm trying to figure out a weird USB problem I'm having, and I think it might be related to having too many bus-powered devices connected. My system had a BSOD yesterday with a "USB_BUGCODE_DRIVER" error (or something similar). After a reboot, the system wouldn't start at all - it gave me a "no keyboard connected" error. Rebooting with a PS/2 keyboard plugged in, doing a clean shut down, and reconnecting my USB keyboard seems to have fixed the problem for now. The error happened while a number of devices were drawing power from the USB bus, which is why I suspect that may be the source of the problem.

Here's the system specs: Asus A7N8X motherboard, which has 4 rear USB ports, and two internal USB headers. The internal headers are plugged into a USB 1.1 card reader which has a USB 2.0 port on the front of it. I've also got a generic USB/Firewire card which gives me an additional 3 USB ports on the rear, for a total of 7 in the back, and one in the front. Running XP Professional, SP2.

Plugged into the rear ports are the following:
-USB keyboard (Microsoft Natural Keyboard 4000)
-USB mouse (Logitech Mouseman dual optical)
-USB-powered scanner (Canon LIDE 20)
-iPod dock
-Sony Clié PDA cradle
-External HDD enclosure (self-powered)
-Logitech webcam (bus-powered).

I use the front port periodically to plug in a USB keydrive. Basically everything I have plugged in draws some power from the computer itself, with the exception of the HDD enclosure. I think these devices might be collectively drawing too much power, leading to yesterday's BSOD. Any help would be appreciated.

posted by gwenzel to computers & internet (10 comments total)

This might help.
posted by SuperNova at 6:30 PM on March 20, 2006

SuperNova: I've read through that question and the answers, and although it explains some related issues, I still don't have an answer...
posted by gwenzel at 6:47 PM on March 20, 2006

What you might be having is a surge problem on power up. Of the items in that list, I'd be most suspicious of the scanner, because it's going to turn on its lamp and draw a lot of power initially (possibly more than it's really entitled do according to the spec).

The keyboard and mouse shouldn't draw much power at all. The two docks will draw a lot whenever their items are charging. So if you're inclined to get a powered hub, I'd suggest putting the scanner, the iPod dock, the PDA cradle and the webcam onto it, leaving the HD, keyboard, and mouse connected directly to your computer.
posted by Steven C. Den Beste at 7:26 PM on March 20, 2006

I don't think it's a USB power problem.

The USB host drivers on your PC do power accounting and hubs like the one integrated into your motherboard only give out as much power as devices report they need. (and even then, they limit it to 100mA per port if the device doesn't report its power requirements, like those USB fans or reading lights) In theory, your computer is supposed to pop up a warning if too much power is being drawn from a hub. Perhaps the event is being recorded but no message is being displayed, go into the Event Viewer and see if there's anything USB-related. But I suspect you won't find anything.

I would instead look for firmware updates for your motherboard and Windows driver updates. And if it happens again, write down exactly what it says and start doing some Google searches.
posted by todbot at 10:06 PM on March 20, 2006

The scanner will be a heavy-draw item but only when it's scanning; it won't have a startup surge current since the LIDE series of scanners use an arrangement with monochrome CCD and tricolour LEDs to illuminate the paper. Therefore they only draw lots of current (for the LEDs) while scanning.

I'm with todbot that it's not a power issue but more likely a driver bug. You can go into Control Panel and see how much power each thing draws I think.
posted by polyglot at 10:42 PM on March 20, 2006

I agree with todbot that it probably isn't a power problem. The USB hubs built into your motherboard check for overcurrent conditions. I suppose that you could have a flakey 5.0V coming from your power supply, but again that is unlikely.

Check for the latest BIOS and drivers at Asus. I think you have a Nvidia chip set so you might check there as well. Then make sure you have the latest drivers for all of your peripherals.
posted by JackFlash at 11:00 PM on March 20, 2006

One other thing you might check. Look in your motherboard manual to see if there are jumpers to select +5V or +5VSB as your USB power source. The first of these is the main 5V power for your board. The other is standby power that is still on when your system is powered down. It is supposed to allow a USB device to automatically power up the system when it needs service. This is not a feature that is very often used and the standby power is not very beefy. It is better to have the jumpers selected for the main power +5V.
posted by JackFlash at 11:25 PM on March 20, 2006

I'd suspect the drivers too. My own Logitech USB webcam crashes my Windows XP SP2 system every time if it's plugged in on boot up. (I don't get the same error though.) I rarely use the webcam, so I haven't bothered getting new drivers, but that seems like the likely fix.
posted by Yogurt at 11:06 AM on March 21, 2006

Thanks to everybody for their help. I've updated the drivers for my devices, and once I can dig up a floppy drive (my current system doesn't have one) I'll flash my BIOS as well. I've got an external hub now, which I'll use for the charging cradles, scanner, and webcam, as suggested by Steven C. Den Beste.

The error, thankfully, hasn't repeated itself (yet).
posted by gwenzel at 3:02 AM on March 22, 2006

OK, so it's a long time since I posted this question, but I've finally figured out the answer to the crashes - it was a bad USB drive. The drive finally went totally dead one day, and I sent away for a warranty replacement. Since the replacement drive arrived, the problem has totally gone away.

If anybody else out there on the intarweb has repeated crashes when using a USB drive, don't discount the possibility that the USB drive itself is causing the problems...
posted by gwenzel at 7:20 PM on August 13, 2006