iPad Pro->Thunderbolt->USB-C port (Not working)

I am trying to use a thunderbolt USB-C cable to run my iPad to my spectrum monitor but the cable is not working. I then tried a USB-C to HDMI cable and that worked. Wondering why this thunderbolt cable wouldn’t be working?

Cable Link:Belkin Thunderbolt 3 5A Cable (2 m) - Apple

I’ve done a lot of digging into USB-C cables since getting my Spectrum, and it’s very clear that not all cables are created equal.

Here’s a quick brain dump of things that are applicable here:

  • There are active and passive cables.
    • Passive cables are just wires between plugs.
    • Active cables have circuitry to boost the signal. This circuitry typically needs to understand the data signal or else it won’t work.
    • The maximum length for 40Gbps passive USB 3.x cables is 0.8m.
    • The maximum length for 40Gbps passive USB4 cables is 2m.
  • There are different methods of sending the video signal.
    • The USB-C specs allow wires inside the cable to be used for DisplayPort rather than USB. A raw DisplayPort signal is sent over these wires.
      • DisplayPort alt mode was not part of the original USB 3.x spec, so a cable can support USB 3.x and not support DisplayPort alt mode.
    • Thunderbolt treats DisplayPort as just another type of data. It will intermix PCIe and DisplayPort data over the same wires.
    • The Spectrum only supports DisplayPort alt mode. It doesn’t contain the circuitry needed to decode Thunderbolt to extract the video portions of the data.
  • Active cables don’t always handle video signals well.
    • The circuitry inside the cable needs to understand the signal in order to boost it correctly. In particular, it needs to know the frequency of the signal so that it can pick out the individual bits (0 or 1) to boost.
    • An active USB 3.x cable only needs to understand the basic USB signal. It doesn’t need to understand DisplayPort alt mode or Thunderbolt.
    • An active Thunderbolt 3 cable needs to understand a basic USB signal and a Thunderbolt signal (which may or may not carry DisplayPort data). It doesn’t need to understand DisplayPort alt mode.
    • There’s often no way to tell from the packaging or specs whether an active cable supports DisplayPort alt mode.

Sadly, the best option here is to look for a USB4 / Thunderbolt 4 cable of the right length. The USB4 spec explicitly covers both Thunderbolt and DisplayPort alt mode, so active cables should be able to boost all types of signal.

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