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.
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.
Best Answer
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.