USB Power for Multiple Devices

February 22, 2005 8:02 AM

I'm setting up a new Mac Mini for someone. I needed to plug in a keyboard, a mouse, a printer, and, occassionally, a flash disk. I went with the Apple Pro keyboard because it had two USB ports. I figure, one for the mouse, and one for the flash disk. The printer could go in the other USB port on the machine itself. Well, I try plugging in the flash disk and I'm told there isn't enough power. Is this perhaps a software setting? Surely the flash disk can't use a tremendous amount of power, so can I do something to trick the system? Do I really need to buy a powered USB hub now? I might as well replace the mini and the keyboard with Airport-enabled equivalents, if I can never use both USB ports on the keyboard.
posted by odinsdream to computers & internet (18 comments total)

Er, I mean Bluetooth-enabled equivalents. Sorry.
posted by odinsdream at 8:08 AM on February 22, 2005

I have no experience with Macs, but the keyboard is presumably acting as an unpowered USB hub. In my PC experience, unpowered USB hubs are basically worthless. So yes, you may have to get a powered hub.

Actually, if the printer has its own power, which presumably it does, you could plug the flash disk in the back and the printer in the keyboard. Though that would probably suck ergo-wise.

By flash disk, do you mean like a USB thumb drive? The RAM chips in those do require a certain level of power, IIRC.
posted by selfnoise at 8:17 AM on February 22, 2005

BTW, a powered USB hub will be vastly less expensive than going to Bluetooth. And probably more useful.
posted by smackfu at 8:21 AM on February 22, 2005

You're right smackfu, the hub would be a cheap option, and might be worth it anyway, but what I'm mainly thinking about now is, what's the point of having the two USB ports on the keyboard at all if I can only use one at a time?

So, I'm thinking it must be some kind of software setting, something that checks what type of device I plugged in, and automatically assumes whether it is going to use too much power or not. Apparently the keyboard uses 200mA of the 500mA provided. Surely Mouse+Flash Disk != 300mA...?

Good idea selfnoise, I'll try plugging the printer in to the keyboard, and the mouse into the main system. Then I have a single port left on the keyboard for the flash disk, assuming it'll actually work. I'll fiddle with the combinations before I buy anything, I guess.
posted by odinsdream at 8:27 AM on February 22, 2005

I use that second USB port to plug in the camera, which runs off its battery power when I download photos from it.
posted by mcwetboy at 8:32 AM on February 22, 2005

You could use both ports if the devices on each were unpowered (I would assume). A mouse counts as unpowered, BTW.

Oh, this is interesting: (copied from http://www.macintouch.com/usbflash.html )

"All work perfectly but there is one thing a buyer should know: the 128 MB models can be plugged into a Mac keyboard, but the 256 MB models need more current and require a powered hub or direct connection to the Mac." (referring to flash drives)
posted by selfnoise at 8:33 AM on February 22, 2005

The way USB works (IIRC) is that each device has asked to report how much power it uses and powered hubs report how much power they can supply. If the sum of all power usage is in excess of what the hub can do it gets reported as an error.

Let me reiterate - the amount of power is reported not measured which means it is up to manufacturer of the part to report the right amount because the host will believe whatever it says.

Is it settable? It depends on the implementation of the device, but I wouldn't get my hopes up. You can spy on devices with the O/O Registry Explorer, if you can get a copy of it and that might tell you more about what the device is using.

If I were to guess, I would say that the manufacturer was either conservative in its estimate of power usage or the firmware on the device was copy/pasted from another project and the engineer didn't bother to change that setting.
posted by plinth at 8:34 AM on February 22, 2005

as plinth said - it's a power issue, not a software issue.

Buying a powered $20 hub will solve your problem.
posted by filmgeek at 8:38 AM on February 22, 2005

If you have a Windows system handy, Device Manager will tell you how much power the USB device draws.
posted by smackfu at 9:17 AM on February 22, 2005

I would try the printer into the keyboard suggestion as it might work for you, but just so you know, I tried a similar set-up with keyboard and printer and it didn't work too well (but it was a Windows machine, not Mac).
posted by EatenByAGrue at 9:43 AM on February 22, 2005

I'd second the advice for a powered USB hub, from my own experience. I had an iBook which had several peripherals going off of an unpowered USB hub. Eventually, I began to see problems with devices not being recognized, and even my speakers, which had their own power, were experiencing sound artifacts, etc. I replaced it, on a hunch, with a powered USB hub (you can get them quite slim and unobtrusive — they're not behemoths) and everything was hunky-dory. (I did, admittedly, have a recent experience where a USB port just seemed to suddenly go dead, but a reboot took care of that issue, and it might've been the laptop as opposed to the peripherals.)
posted by WCityMike at 10:00 AM on February 22, 2005

You might try a different usb flash drive. Some of them draw more power than others, and so some can be used off the mac keyboard and some can not. See this report at Macintouch to get an idea of the issue with recommendations for which flash drives work and which don't.
posted by andrewraff at 10:32 AM on February 22, 2005

andrewraff, I tried using the smallest USB drive available, which still caused the message to pop up. This was when I plugged it in to the keyboard along with a standard optical wheel mouse. I haven't tried plugging only the flash drive into the keyboard with nothing else attached, but I'm very, very surprised that this Tiiiiny flash drive uses more than a few mA, but I have no specifications available to prove it.
posted by odinsdream at 10:44 AM on February 22, 2005

it sounds like it'd be cheaper to plug the usb drive directly into the mac mini, rather than shell out any sort of money for a usb hub. a hub may simply mean more clutter in exchange for convenience -- i try to be minimal in my computer setups, so the less clutter the better, but to each their own.

the usb ports on a keyboard are primarily for a mouse or some other sort of tracking device -- though they do look convenient for other things. but as the keyboard requires some measure of power, and a mouse as well, there's not much left to spare when you get to the usb flash drive.

if you happen to be installing a mac cinema display, you could use one of its usb ports as well. as the MCD gets its power independent of the CPU, it has a lot more to share than the keyboard.
posted by moz at 11:51 AM on February 22, 2005

Okay, so the keyboard won't even power my small USB drive with nothing else connected. Really this bugs the hell out of me, so I went ahead and got a belkin powered 4-port USB hub. Thanks everyone. I still think there must be some way to trick the system. I'm finding it hard to believe that my measly little USB stick uses any more power than my optical mouse does.

If I ever get a mini myself, I'm going to go with the bluetooth model and wireless input devices to free up these ports. I see no practical reason to have a keyboard that can't even power a flash drive.
posted by odinsdream at 2:31 PM on February 22, 2005

Alright, more info. I've finally found a device that actually has a power rating written on it. It's a wacom pen & mouse tablet rated at DC 5V, 40mA. This works fine plugged directly into the keyboard.

This leads me further to believe that the system is checking some kind of device ID. If it's an input device, it assumes low power and allows it, otherwise, it assumes high power and turns it off. There's no way my flash disk uses more than 5V at 40mA. No way.

So, there must be a way to trick the system into mounting the disk.
posted by odinsdream at 4:29 PM on February 22, 2005

Okay maybe I spoke too soon. I checked the device using Device Manager in Windows (System Profiler in OS X also works), as smackfu suggests, and it apparently uses 200mA, which is much more than I previously thought. The bus supposedly supports 500mA, the keyboard takes 250mA, so there ought to be 250mA left, enough for running the 200mA flash disk. But, it's definitely closer than I previously thought, so I understand the problem better now.

Thanks everyone for the advice, though!
posted by odinsdream at 4:43 PM on February 22, 2005

You can try USB Prober to look at it on the Mac.
posted by plinth at 6:16 PM on February 22, 2005