U4919DW, Startech Thunderbolt 3 Cable not working

Thanks! I had read that article but I thought my monitor was TB3 capable. I searched before buying and found this page, but didn't realize they only said PC output must support TB3, not that the monitor itself didn't support it. I wouldn't have bought this monitor if I'd realized. It seems it only supports TB3 if the cable is passive, so that it's really supporting DP Alt via USB-c. Is that correct? If so, that's tricky.

My understanding after re-reading your article (a lot more slowly this time) is that only passive TB3 cables will work for USB 3.1 Gen 1/2, and passive are limited to 0.8 m. So I'm out of luck on length if I want one cable to work as both a TB3 cable for my 2017 iMac 5k, and as the USB-c with DP Alt mode for Lenovo laptop to Dell monitor, which had been my reasoning for the higher end cable (though when I thought it was a Thunderbolt monitor). 

It looks like the iMac supports DP Alt. Apple doesn't say this of course, but others do. If so, you've saved me some $ with that cable. I had just hoped to use same cable for external GPU on Mac if I do that someday. 

Any idea whether USB4 will allow for longer cables that support 10+ + Gbps USB-C and 40 Gbps TB?

Also, could you link the Cable Matters 10' cable you mentioned? Or alternatively if there's another USB-c 3.1 Gen 2 (10 Gbps) cable that is preferably 5A and at least 6'. I'd like to get as much bandwidth as I can for future proofing and interoperability with the Mac. 

Thank you very much for those write-ups and the quick help. I actually have time to return the TB3 cable, so it's very much appreciated.

@TslaCorg  No, that isn't correct.  A display doesn't have to support Thunderbolt 3 to support DP Alt Mode.  That can be carried over regular USB-C as long as the USB-C source port supports DP Alt Mode. That is an optional capability on USB-C ports and therefore it isn't found on all laptops that have USB-C ports, but your laptop supports it.  And any PC port that supports Thunderbolt is required to support USB-C DP Alt Mode.  (Note: That does not mean that a native Thunderbolt display needs to accept video as regular USB-C DP Alt Mode from a non-Thunderbolt system.  But that doesn't apply here.)

In terms of the iMac 5K, I'm not sure what you mean about it supporting DP Alt Mode.  Are you asking if it supports DP Alt Mode to drive an additional display, or if it supports DP Alt Mode to use the iMac's own display as an external display for some other system, i.e. Target Display Mode?  To my knowledge, the iMac 5K models don't support Target Display Mode.  But if you plan to use an additional display with the iMac 5K as the source, then yes it would support DP Alt Mode over regular USB-C because that support is mandatory under the Thunderbolt spec.  Thunderbolt just allows more video bandwidth than can be achieved over regular USB-C DP Alt Mode.  I wrote an explainer post about the various operating modes of USB-C and TB3 with specific focus on the impact on possible display setups here if you feel like even more reading.

In terms of USB4, you don't have to look at a newer spec.  There's nothing in terms of existing specs that rules out longer cables.  It's just that passive cable manufacturing technology can only hit the requirements of these specs at certain lengths today.  If cable manufacturing got better, then in theory longer passive cables could be released even for USB-C carrying 3.1 Gen 2 or TB3.  But the U4919DW doesn't support 3.1 Gen 2 anyway, in fact when connected via USB-C it will only run USB 2.0 over that cable.  If you want USB 3.x speeds for the peripherals you might attach to the U4919DW's built-in USB ports, then you'll need to run a separate USB 3.0 cable from the display to the source system in addition to the USB-C cable.  That has to do with the fact that the U4919DW uses 5120x1440 resolution and only supports DP 1.2 over USB-C, which creates a limitation I described in that thread I linked above.

The Cable Matters cable I mentioned is 6 feet, not 10, and I linked it in my previous post on the word "this".  Cable Matters does make a 10-foot USB-C cable that can run 4K 60 Hz and USB 3.x Gen 2, but it only supports 60W PD.  I don't know of a USB-C cable longer than 3 feet that supports 3.1 Gen 2, 100W, and DP Alt Mode.

Thank you for the clarification. I was also not clear. What I meant to say was that when the table says that the u4919dw supports 5120x1440 60 Hz over "USB-C Thunderbolt," it is not actually supporting Thunderbolt. It's redundant in that context, since what it's really supporting is DP Alt over USB-C; which, as you said, all PCs that support Thunderbolt support DP Alt over USB-C that. My laptop already said that it supports DP via USB-C, so I don't need Dell's non-TB monitor to tell me it supports TB output. By putting Thunderbolt in the table, it would lead someone not versed in your posts to think that the monitor and any Thunderbolt cable are also supported.

As far as 5K iMac, I didn't mean Target Display. I meant your latter use case, using the iMac to output video to my u4919dw. The iMac has a beautiful display, but I use it so little now because everything is hooked up to the Dell since I use that for work and I can't be bothered to switch it back every time. It also takes up a lot of space on my main desk.

To that end, I didn't want the TB3 cable for the extra speed on the Dell, as I did understand that from your posts (USB 2.0 speed only via USB-C). I just wanted one cable that I could swap back and forth between either iMac/Dell or Lenovo/Dell and use also for iMac or MBP to external GPU or storage. In the same vein, I was just curious what USB4 might bring to the table since I'd heard USB was finally going to be getting the capabilities of TB3.

The 10' Cable Matters I was referring to was in your cable post under #1, "UPDATE." I'd be fine with 60 W, as the Lenovo only came with a 45 W power supply, and I'll now not be trying to get one all-in-one cable unless I spring for the Apple Thunderbolt Pro cable.

To contribute a bit back, it turns out it is according to what I can find, the only TB3 cable which supports both DP and USB-C 3.1 Gen 2. What is not clear and no one seems to be able to say definitively is whether it's passive or active. If the former, Apple somehow overcame the limitation on length that Intel couldn't. If the latter, it violates your 3.1 Gen 2 rule for active cables, and perhaps violates Intel's Thunderbolt specs.

@TslaCorg  I could see the appearance of Thunderbolt in the table going either way.  I can't even tell you how many times I've had people ask questions here along the lines of, "My system has Thunderbolt, does that mean I can use USB-C devices or only Thunderbolt devices?"  Yes, for anyone who knows that TB3 support on a system means DP Alt Mode is also available, it's redundant and maybe even misleading.  But for others, it's assurance that the display in question is compatible with their system.

USB4 is essentially Thunderbolt 3, but it's not EXACTLY Thunderbolt 3.  What I've read is that if a designer is building for USB4, the effort required to add Thunderbolt support is "minimal", but it's not nothing.  So if a system supports USB4, it may not be able to use TB3 peripherals.  And I haven't found a clear answer as to whether USB4 peripherals can always, optionally, or never run at their maximum performance when connected to systems that support TB3 but not USB4.

Here is that active cable.  I thought the T480s was designed for a 65W source rather than 45W, but if not and you're ok having this cable create a USB PD limitation that wouldn't otherwise exist from the display, then it should work.

In terms of Apple's specs, I would be surprised if it were a passive cable given its length, cost, and lack of any similar cable elsewhere -- unless they just found a very expensive process for making a passive cable meet TB3 requirements over that length.  The part that isn't explicitly stated is whether it can support USB 3.x Gen 2 outside of a TB3 context.  Technically TB3 only carries DP and PCIe traffic, and USB would be tunneled through the latter, but I wouldn't completely put it past Apple for them to say, "It carries USB 3.x Gen 2 traffic when connected to devices like our Pro Display XDR, which uses Thunderbolt."  But I just don't know.  I don't think it would be feasible for them to make a cable that violates TB3's spec requirements, though.  Intel owns Thunderbolt and therefore requires that any products marketed as Thunderbolt-compatible pass their testing and receive their certification.  That's partly why it's been relatively rare to find on products.  USB4 has no such "overlord" that enforces certification on all products, so the fact that most of TB3 is going to end up in USB4 should lead to much broader adoption.