It’s very unusual that you can’t get anything to work, because I’m using a relatively cheap TB3 cable and it’s working fine. (MacBook Pro 2019, 15.6", Radeon 560X)
The only things of note on my setup:
- I flashed my firmware to 102R875 immediately after receiving the Spectrum (via a Windows laptop).
- I’m currently running firmware 104.
- I have my Spectrum’s USB-C bandwidth priority set to high refresh rate.
I think 102R875 contained an update for the USB hub, so it might affect things. However, it requires a Windows laptop and a working USB connection (e.g. using an old USB-B printer cable) to perform the update.
Quick explainer on the technical side of this.
As noted above, Thunderbolt 3/4 cables (and high speed USB-C cables) have 4 pairs of wires for carrying high-speed data. They also have a separate pair of wires that are dedicated to USB 2.0 data.
Putting the Spectrum in high USB data speed mode splits the 4 lanes in half: 2 are used for USB data and 2 are used for DisplayPort. This limits the bandwidth available, therefore limiting the possible resolutions and refresh rates.
Putting the Spectrum in high refresh rate mode uses all 4 of the high speed lanes for DisplayPort data. Only the USB 2.0 wiring is used for USB data, which is why the USB speed is limited to 480Mbps. This is plenty for things like keyboards and mice, but may struggle with flash drives and network adapters.