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For time/cost saving reasons, I want to use a USB 3 cable as a USB 2 cable. The cable will be carrying other (slow) signals, and so extra conductors are needed (which is why I can't just use a usb 2.0 cable). I know that the USB 3 cable has a differential pair meant for USB 2 signals, but for improved shielding, I want to use one of the (shielded) Superspeed pairs for the USB 2 instead. Are there any downsides to this? From what I can tell, the impedance of all the twisted pairs are 90 Ohms.

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I'm making the devices on both ends, and they'll have custom connectors, so compatibility with other stuff isn't important. To be clear, the custom connectors are not USB connectors.

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    \$\begingroup\$ In general, recycling a common connector for something non-standard, especially one as common as USB, is not a great idea. It's basically inevitable that someone will connect an actual USB device to the connector at some point, which could possibly result in damage to one device or the other. \$\endgroup\$
    – alex.forencich
    Feb 21, 2020 at 20:55
  • \$\begingroup\$ @alex.forencich I am not using USB connectors, so compatibility with other USB devices is not important \$\endgroup\$
    – BeB00
    Feb 21, 2020 at 20:58
  • \$\begingroup\$ @BeB00 Sounds like a great idea to me, SuperSpeed routing (and it applies to cable too) requirements are much harsher than USB2.0 and still carrying the same impedance so you shouldn't have any SI issue with loading USB2 on a USB3 wire pair :) \$\endgroup\$
    – eeintech
    Feb 21, 2020 at 22:05
  • \$\begingroup\$ There might also be other types of cables better suited for your application than USB cable, potentially cheaper. How many wires do you need and are USB2.0 (High-speed? Full-speed?) the fastest signals? \$\endgroup\$
    – eeintech
    Feb 21, 2020 at 22:07
  • \$\begingroup\$ @Cisco25 USB 2 is the fastest signal, and I need 8 conductors total (2 power, USB, and 4 signal). \$\endgroup\$
    – BeB00
    Feb 21, 2020 at 22:31

1 Answer 1

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It’s pretty common to leverage the USB cable for other things. One that comes to mind is the PCIe ‘bitcoin miner’ external adapter that connects one PCIe Tx/Rx lane pair on the SS set and sends clock over DP/DM.

Anyway, based on the discussion I think that just using the standard 4 wires for USB2 and Vbus/GND, and reusing the SS pairs for your other signals is perfectly fine. There’s no benefit to using a SS pair for DP/DM and I don’t recommend it, as there is the possibility of connecting your signals to the USB2 PHY.

In planning how to use the SS pairs, note that the TX pair (inbound to the device) will have DC blocking caps on the host side, while the RX ones (outbound from the device) are connected directly to the PHY.

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  • \$\begingroup\$ As I understand it, the Superspeed pair has more shielding, and is actually shielded from the power wires as well. The environment will be relatively noisy, and there may be extra noise on the power wire, so extra shielding is appreciated. The superspeed pair will not have any capacitors connected to anything, since that happens on the PCB and not on the cable. \$\endgroup\$
    – BeB00
    Feb 23, 2020 at 1:55
  • \$\begingroup\$ What I’m getting at is how to make your solution ‘goof proof’. That is, keeping USB2 as-is and mapping your nonstandard use of the SS pairs in such a way that if someone plugs your device into an actual USB3 host port, your device won’t fry the host. I know that isn’t your intent, but if you’re going to use standard cabling then it’s something to consider. \$\endgroup\$
    – hacktastical
    Feb 23, 2020 at 2:08
  • \$\begingroup\$ again though, the connectors won't be usb connectors, so I struggle to see how someone could accidentally connect this device to an actual usb host port, and if they did, it wouldn't matter which pairs it was using because the pinout is unrelated to which conductors you use for usb \$\endgroup\$
    – BeB00
    Feb 23, 2020 at 2:10
  • \$\begingroup\$ Of course you can do that - that is, use your own connectors. However, my PCIe ‘miner’ example is showing how a USB cable can be repurposed to take advantage of very-available and low-cost USB cabling and connectors. It’s one less problem you’ll have to solve. \$\endgroup\$
    – hacktastical
    Feb 23, 2020 at 3:07
  • \$\begingroup\$ The voltage here will be >5V, so I would rather not use an off the shelf usb connector and risk destroying someones USB device \$\endgroup\$
    – BeB00
    Feb 23, 2020 at 3:46

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