When a Powered USB Hub Is Not Enough

One of the questions I received recently was a surprise... Gary Karasick, who subscribes to my newsletter, wrote to ask me about a problem he was having...with his iMac:

I am having difficulties maintaining connectivity with y new iMac. I have many external USB devices and instead of reaching around the back of my iMac, I have used a powered USB 7 Port hub. Sometimes I get a machine from my machine saying there isn't enough power to support all of the devices. How can this be if I am using a hub with an external power source? What can I do so that I can continue getting the benefit of the easily accessible hub?

I wrote back to Gary to say that I don't use iMac or any Apple Mac products, so I really could not answer questions specific to the Mac.

However, this problem wasn't directly an iMac issue...it was a problem with an external USB hub.

I suspected that his problem was that either his external power supply for the hub didn't provide adequate power to it, or that he was connecting too much load on the powered USB hub.

A lot of the time, we connect a USB cable from an external powered device, such as a printer, scanner or external hard drive to our computer. The device gets its power from its connection to the building's power system (whether you call that "a wall socket" or "the power mains." Either way, it's using the USB connectionn to communicate data, but not to power the device.

However, USB connections are also capable of providing power. When you connect a USB cable or a USB hub to one of the USB connectors built into your computer, you're making a connection that can provide power as well as send and receive signals.

Examples of devices that are powered by your USB connections are keyboards, mice, flash drives, and some portable external drives. Sometimes, you'll even find external devices that make not one, but two connections, to your computer in order to get enough power to drive them. External CD/DVD writer/players for notebooks and netbooks are tiny things, but it takes plenty of power to drive the electronics and power the motors to rotate the disks andn move the read & write heads.

The problem comes when you need more power than the computer or hub can provide. The USB 2.0 specification sets a maximum load (in amps) that a device is allowed to take from the connection. The computer's hardware and the USB driver are set to monitor the load — and to protect the hardware if the load is too much.

I have that problem sometimes with my Dell Inspiron 8600 and my portable external hard drive (a Western Digital Passport 160GB model). Sometimes, when I plug the drive in, the Dell (or Windows XP) will trip the power to the USB port and give me a popup warning that the load was too much. I've even had it do that while I was in the middle of reading files.

I think this particular problem is specific to my I8600, as the same external drive has not caused the probem on any other computer.

You can have this problem when you connect too many unpowered devices (that is, devices that are going to get their power from a USB hub) to a USB hub and connect the hub to one of your computer's USB ports.

The normal fix is to get a powered USB hub. With this kind of hub, you get a little transformer pack that you plug into your electrical outlet and then you plug the connector from the transformer into the USB hub.

So, Gary has the problem, even though he's using a powered hub.

It could be that the transformer isn't working, either having gone bad or perhaps he's got it connected to a power controller that he's managed to switch off.

It probably hasn't gone bad. If he's turned off the power, he's probably figured that out and turned it back on.

More likely, he's plugged too many devices that want too much power from the hub — and that want more than his hub can take from the transformer. We're back to the maximum load problem.

Fortunately, the feature that makes USB hubs even possible is that the USB specification allows for daisy-chaining of USB devices to other USB devices.

Gary's fix is probably to get another powered USB hub, connect its power to it and connect it to the powered USB hub he already has. Then, he could split his USB devices between the two powered USBB hubs.

Copyright © 2009 Terry A. Stockdale. All rights reserved.