Cat5e as USB extension cable

Question

I need an 8 meter long usb 2.0 extension cable. I've looked at commercial solutions like active USB cables, USB over Ethernet extenders etc, but I think it's a bit overkill for what I need. They're also not particularly cheap.

It's only three meter over the USB 2.0 spec anyway.

Would some leftover, shielded Cat5e work as a DIY USB extension cable (cut 8m and solder a male and female USB header on each end, using a twisted pair for each USB pin)?

My thoughts are that with the shielding and thicker wire, it would be sufficient for those few additional meters. I haven't got any female headers left so before chopping up a working usb extension cable, what are your thoughts on this?

Answer 1

No, not really. Or yes, but it might not work.

First you must ask yourself, how reliable do you want your USB connection should be, should it always work as expected, or are intermittent failures and drop-outs acceptable.

If problems are acceptable, by all means try it.

If problems are not acceptable, then you can't do it.

The length specification for USB does not come from signal being attenuated too much due to more wire, but there are timing requirements that expect the data to reach the last device within a given time, and longer wire means longer time of flight for the data.

USB differential data pair has 90 ohms impedance, and CAT5 data pair has 100 ohms impedance. The impedance difference will cause reflections in the data waveforms. And it would be extremely bad idea to use a CAT5 data pair for each USB pin, as that would break the USB differential pair into just two random wires, instead of being a twisted data pair.

So, there are reasons why USB extension cables are expensive, they will have active electronics in them to reliably do the job of extending the USB connection beyond standard wire length limits. If you want cheaper, just buy two standard but good quality 4 meter USB cables and a USB hub between them.

Answer 2

Been there, tried that, wasn't successful.

The problem with exceeding the USB distance is not necessarily one of EMI shielding, or signal degradation over cable length, but it's really one of timing and electrical propagation delay requirements as defined in the USB spec that, due to the popularity and widespread use of USB, even the cheapest of vendors adhere pretty strictly to.

For devices with loose adherence to the USB specification, a longer run may work. For most however, the devices will exhibit inconsistent, or outright unusable behaviour over a longer distance link simply due to the transmit / responses times falling outside expectations. electricity DOES take time to get from point A to point B, and, your USB devices ARE counting.

the usb.org website has some very detailed information about this if you want to really dig into the reasons, but i'm afraid it won't give you what you're after - a cheaper solution than an active USB extender.

http://www.usb.org/developers/usbfaq#cab1

THOSE work by actually placing a USB device in the middle, and managing the timing responses on the (within specification) cable on either side of the active component. so, your PC is only talking to the active extender in the middle, and the USB HDD, or whatever at the other end of the "cable" is doing the same thing. The PC and device are not actually speaking directly to eachother anymore.