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My girlfriend just got a new ebook reader — a jetbook — that came with an unusual USB cable. One end is a regular USB mini B head, but the other end has two plugs. Both are USB non-mini A size, but one is red. The manual gives no indication about what the red head does; is it a standard cable type? What is it for? It looks like this:

USB cable with two heads at one end, one red and one black

EDIT: @Chris, Thanks for the answer. It brings up a follow-up question, though: can/should both heads be plugged in at the same time? Or would that confuse or damage the device?

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Off Topic ... but how do you like it? i'm in the market for for an eReader myself (and i wouldn't touch anything SONY or Amazon with a barge pole). specs look promising, at least this one does TXT/RTF, the simplest of formats, a HUGE plus in my books! :) –  Molly7244 Nov 15 '09 at 1:32
    
It's okay I guess, I'm not actually a big eReader fan myself. The hard drive is small and the screen's just a regular TFT LCD, no fancy eInk here. The format flexibility is nice, and the SD card slot and USB charging are welcome. –  Pops Nov 18 '09 at 1:10

6 Answers 6

up vote 24 down vote accepted

The black connection provides power and data. The red one is power only. The second cord should be used if the black one doesn't provide enough power to power and/or charge the device. You'll see a lot of external hard drives come with the same type of plug (though usually both are black).

EDIT: To answer your second question, plug the device in using the black cable. If it runs normally and charges (assuming this has a battery), then you're fine. If, however, the device doesn't operate correctly or doesn't charge, then you will need to plug the red cord into another USB port. You wouldn't cause any harm to the device by plugging in both cables. Honestly, the red one would even be plugged into a different computer since it's just power only.

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Right, as per the USB spec, each port is limited to providing 500 mA of current per port, so if more than 500 mA is needed, you need to provide external power or use multiple ports. –  Lara Dougan Nov 15 '09 at 2:28
    
Yeah, I misread your answer the first time; somehow I thought you said the red was for charging only. Thanks for the clarification. –  Pops Nov 18 '09 at 1:00
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@Chris Thompson. I disagree that red one would be plugged into a different computer if different USB devices share single power supply which is usual case. Power modules may not be designed for this use and you can damage your power modules or e-reader... But note that this is rare case. –  gavenkoa Aug 24 '11 at 7:06

To answer your follow-up question... The red one most likely only has the charge wires connected, it does not have the data wires connected. If you think about it, it makes sense. The power level is increased but the data transmission comes only from a single source, the black wire. It would not be possible to link up the wires the way they are in this picture and provide a two port data source. You would have collision of signals on the way back (where the wires merge and a single one goes back to the mini-USB). So yea, you can definitely have both plugged in to a single computer. The device would not even probably know if the red one is plugged into a computer or if it's plugged into any other USB device that provides current.

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A USB port can only provide up to 500 mA. The device that cable came with requires more than that, so it must be connected to two USB ports at the same time to get enough power.

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I have an external hard drive with this kind of cable. If I don't plug in the red cable, I only get USB 1.1. USB 2.0 will not work unless I plug in the red half and provide the extra power.

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In most cases you won't need both cables plugged in, it's just that there are some laptops (primarily older) that have very low power outputs on the USB ports which require the ability to draw a little more current. From my experience with these devices they won't get confused, but if they work with the primary (black) cable, there isn't much of a need for the red cable. If you just have the red cable plugged in then the drive may spin up, but nothing adverse should happen.

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You don't need both cables plugged in, but it'll likely be a lot slower to charge and may default to usb 1.1 for data as well due to the lower power provided by only one cable.

My Nook does something similar, except instead of using the two cords, it has an extra long connector in the device end so that it'll only work on the nook and will use full power when you use their wall adapter. You can still plug in standard usb to it, but it takes forever to charge then.

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