this post was submitted on 23 Aug 2024
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I have a TB4 eGPU chassis, it came with a short TB4 cable, I want to buy a longer one, can I use a USB4 cable interchangeably? (lemmy.ml)
submitted 2 months ago by [email protected] to c/[email protected]
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My understanding between TB4 and TB3 is that they're essentially the same, it's just that the standard of TB4 essentially mandates that the device must do all that TB3 maybe could do. Minimum bandwidth is increased and I think I read something about power delivery minimums as well. This eGPU chassis I bought came with it's own TB4 cable, which is actually the first Thunderbolt cable I've seen that specifically says "4" on it.

I assume the reason they supplied this is because, given what it does, an eGPU chassis is going to need to support some pretty bandwidth for a GPU. In my case though, I'm actually using this chassis not for a balls to the wall kick ass Graphics card, but actually to allow me to attach an old and very humble i/o card from Blackmagic. It's currently working just fine for that purpose.

Thing is, the supplied TB4 cable is pretty short and the chassis along with the ATX power supply mounted on it makes for a pretty hefty desk-space consuming setup. I'd like to move the whole setup somewhere fairly far off from the laptop to save me some precious desk space. I looked up 2m thunderbolt 4 cables which I understand is the longest distance you can get for TB4 and still maintain bandwidth and while it's not too bad, the prices are high for a cable. It occurs to me though that since I'm barely using a fraction of the available bandwidth anyway, could I use other, cheaper, long cables. USB4 comes up a lot in my search for 2m TB4 cables for example. (although they are mostly from AliExpress so don't know how good an idea it is to buy from them). If the chassis has TB4 controllers in it, as does the laptop to which it's attached, can one just put a USB4 cable between them? Are they physically different?

For that matter, since my bandwidth needs are so tiny, could I just find cheaper, longer TB3 cables?

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[–] [email protected] 0 points 2 months ago

My apologies, I missed a few of your questions at the end.

  • Yes, any USB4 rated cable can carry the TB protocol at whatever distance it’s rated for, otherwise it isn’t USB4.
  • Likewise any TB3 cable should work for your application, if it’s actually TB3 rated.
  • 3m is generally the max length I’ve seen. The theoretical limit may be higher but I suspect latency of the cable itself becomes a problem for PCIe tunneling, which is still a synchronous interface TMK.
  • The TB controllers decide whether your cable is sufficient. They will negotiate a link if they can both verify a nominal link speed/multiple. So the lower rated USB cables won’t work for PCIe tunneling, even if your ultimate bandwidth requirements are minimal.

A few things to note if you’re shopping on places like AliExpress, eBay, Amazon, etc:

  • Be wary of any long (active) TB/USB cable that’s cheaper than $30.
  • Be wary of any cable that has a small connector, like one you would use to charge your phone, because the TB/USB4 connector itself must house an active signal repeater chip, making them chunkier and/or longer than usual.
  • Similarly, the shielding requirements are fairly substantial, so the cable itself should be beefier than most USB cables.
  • IME generally, PCIe over TB/USB can sometimes just be finicky. Of the 12 or 13 cables I’ve used with various combinations of machines, enclosures, docks and risers, there are occasionally some combinations that just don’t play nice, for whatever reason.
[–] [email protected] 0 points 2 months ago

You may need an active TB3/4 cable. 30 cm is typically the longest passive cable you’ll see. Active cables are more expensive, though less so than they used to be, and can handle longer runs like 3 meters.

[–] [email protected] 0 points 2 months ago

FWIW I don't think you just lose bandwidth with longer cables, but rather it just loses sync and the cable stops working.

I have a thunderbolt 4 hub that I wanted to tuck away somewhere, so I tried several longer cables (some usb4 as you said, one actual longer TB cable) none of them worked reliably like the 30cm cable that came with it. It would sporadically lose connection for a couple of seconds before reconnecting in most cases or it just wouldn't connect at all.

Not sure if my hub or cable choices are the problem or if it really does just have to be as short as possible in some cases.