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It's been pointed out many times that USB 2.0 can (more or less) safely be used with cat5/5e cables and connectors, at least as long as these requirements are met:

  • Data pins are on its own twisted pair
  • Power pins are on the separate twisted pair
  • To keep it in safest way possible, the two pairs used are the "spare" pairs of 100M line, so on RJ45 pins 4, 5, 7 and 8
  • You don't plan to mix any connection with POE ports/fed cables!!!!!
  • You avoid plugging such cable from USB host to the network devices, since some (seen on 100M hubs) might have pair pins merged together, resulting in bridging your USB power lines.
  • You really really have a good reason to use this in the first place

Now, with keeping all that in mind, I do have a good reason. However, I'm interested in knowing whether the same twisted pair topology is ok (mostly performance-wise) to be used with USB 3.0.

As USB 3.0 includes additional two natively twisted paired connections, I was thinking of using the first two pairs (normally used for ethernet) as new USB 3 StdA_SSRX−, StdA_SSRX+, StdA_SSTX− and StdA_SSTX+ pins, with old Data + and - pins used on third pair and power on fourth.

Could this decrease performance of the USB 3.0 (when used in non-excessive lengths)? What damage can be made in case of such use and plugging the host-side of the cable into Gigabit network port of the PC?

The reason I'm interested in this is, that I need to have USB 3.0 connection between two boxes, that can be disconnected and connected easily on-the-go, cable length would be between 10 to 15cm, the reason for this connector is that both boxes already have RJ45 on them and changing them for USB would mean a few troubles here and there. Please, don't start to "why not this why not that", I'm interested in an actual answer, for informative purposes rather than suggestions on how to handle my case specifically. :)

Thanks!

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    \$\begingroup\$ Maybe it will work but only because the distance is only 15 cm. Reasons why it might not work: Cat5E cable has a bandwith of 350 MHz per pair, USB3 needs 2.5 GHz per pair. Cat5E cable has a a 100 ohm characteristic impedance, USB3 needs 90 ohms. You will get signal reflections at the cables and connectors since they're not designed for USB3, this might limit the speed to USB2 speed. \$\endgroup\$ – Bimpelrekkie Apr 7 '16 at 13:14
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    \$\begingroup\$ Those are good statements and reasons indeed! Is there any way to lower these limits by using the proper cable, and only "lend" the actual RJ45 connector and RJ45 socket? At the socket, the USB 3.0 cable can be soldered right after the socket pins, for what it matter. Ps: this should be posted as an answer, I guess! \$\endgroup\$ – That Marc Apr 7 '16 at 13:37
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    \$\begingroup\$ The shorter/smaller the "not USB 3.0" the better I would say. But still the signal reflections will get you I expect. High frequency signals are simply not very forgiving in that respect. \$\endgroup\$ – Bimpelrekkie Apr 7 '16 at 13:47
  • \$\begingroup\$ The other major concern is the crosstalk. USB 3.0 has very tough requirements for crosstalk between Rx and Tx pairs, and these pairs are shielded. CAT5/5e cables use unshielded twisted pairs. \$\endgroup\$ – Ale..chenski Sep 18 '18 at 23:39
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No. USB 3.0 uses 9 cables internally.

If you use the scheme given in the question, you will effectively end up with USB2 which has its own cableset within USB3.

See USB Cable Diagram.

It might be possible to combine the two ground wires and use 2 STP pairs for the two Superspeed lines, but I found no reference of this working.

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    \$\begingroup\$ Your statement of simple "No" because of 9 cables versus 8 cables is a bit lightweight. You mentioned of combining the two ground wires but finding no reference of it working, but given the fact that these both grounds are from same source going to same destination device, I see no reason as to why this could make a difference? So, with this I, myself assume that there are8 vs 8 wires. Furthermore, the 9th pin could be simply taking its use as shield of the shielded cat cable. No? \$\endgroup\$ – That Marc Apr 7 '16 at 13:34
  • \$\begingroup\$ For a starter, Tx and Rx pairs must be shielded pairs, which is not quite common, regular cables are "UTP. So the answer is still "No" if you mean a cable longer than 10-15 cm, as @Bimpelrekkie said. \$\endgroup\$ – Ale..chenski Apr 26 '18 at 23:46

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