Query about USB and extension cables

  scales 21:09 08 Jan 2011

Hi, I was given a webcam for Christmas. I plugged it into a USB port and it worked and was fine with Skype. I then tried it with the same port but with a USB extension and it didn't work. I tried several extension cables but the same nil response. I then plugged the webcam and extension into a different USB port and that worked. I then took off the extension cable and it failed on that port. I plugged it back in the original port without the extension and it worked but not with the extension. What is going on? What difference is the extension cable having and why will it work with one port and not the other and why will the second port only work with the extension cable but not without?

  Fruit Bat /\0/\ 21:18 08 Jan 2011

Its down to a combination of things

1. Max 500mA for a pair of ports (hub)
2. other devices running of same Hub
3. Loss of power per length of cable due to resistance of cable.
Constant plugging in out loads drivers for all ports and can cause conflicts i.e. device not recognised.

Generally devices are better using the ports directly off the motherboard or on a USB-PCI card
ie USB port run from a header off the port to say front ports on a PC are generally weaker than the rear ports.

I use USBdeview to unload driver from the registry and stop conflicts click here

  scales 21:31 08 Jan 2011

Thanks, USBdeview is an interesting tool. The definitions for the two ports showed up with the webcam but nothing to indicate any issues with either. Both the ports were on the front of the pc. I have now got it plugged into the back without the extension cable (and it is working fine).

  Bris 17:56 09 Jan 2011

Hi scales.

The USB specification does not allow extension cables as they exceed the allowed length, however, they are in common use and work most of the time but the longer they are and the cheaper they are the less likely they are to work and they are not guaranteed to work.
As Fruit Bat says each port has a maximum of 500ma and each time you split this the current available is reduced, eg if you plug a 4 port passive hub into a single port then each port in the hub has a capacity of 125ma so if you then plug in 4 devices requiring 200ma each you will exceed the maximum and it will stop working. The way around this is to use a powered hub.

  scales 18:13 09 Jan 2011

Thanks for the replies, but why does the one port only work if I put the extension lead on it? When I plug the webcam directly into that port without the cable it stops working, when I put the cable back in it start working again. Weird isn't it.

  Bris 18:38 09 Jan 2011

My guess is that the resistance in the extension lead is reducing the current drain such that it takes it below the threshold allowed but still enough to keep the cam working. If you get my drift.

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