Recommendation for dual 4K@60hz + downstream data setup


#1

What solution would you suggest to connect via TB3 to two 4K@60hz DisplayPort monitors, and still have a daisy-chainable outgoing TB3 port for connecting additional downstream TB3 data devices (e.g. an Apple TB3-TB2 adapter to connect a chain of legacy TB2 devices, or to newer TB3 devices like the TBT3-NVME480 ).

If I were to use the downstream TB3 port on the TBT3-UDV to connect a second 4K@60hz DP connection, I then can’t then use that port for more downstream data devices. Conversely, if I plugin a data device (like the TBT3-NVME480, or other TB3 devices without a 2nd TB3 port, like many 10Gbps Ethernet dongles), I won’d be able to connect a second DisplayPort connection.

Thunderbolt marketing makes much of claims like “one port does it all”, “dual 4K + data”, “daisy-chain up to 6 devices”, etc., but actually accomplishing those promises seem elusive.

Can you offer any recommendations? E.g., if I daisy-chained two TBT3-UDV together, could I get one 4K@60hz DP connection out of each one, and then have the downstream TB3 port of the 2nd TBT3-UDV open for connecting/daisy-chaining addition data devices?

If this wouldn’t work, any explanation as to why it wouldn’t would be great. Likewise, if there are any TB3 limitations I’m not understanding, or technical documents available which might better explain TB3 capabilities and limits which you could recommend, I appreciate that greatly.

Thanks!


#2

Hi John,

Thank you for posting!

To your core question, we haven’t tested daisy-chaining two TBT3-UDV docks as you describe (nor would such a configuration be officially supported) so we can’t speak specifically to the result or performance of such a configuration.

To your follow-up question, ultimately the issue comes down to the bandwidth available via a single Thunderbolt 3 connection to the host. In brief, a Thunderbolt 3 host has a finite amount of bandwidth with which to pass both the video and data information over a Thunderbolt 3 connection.

Depending on the number of displays connected (and their resolution and refresh rate) and the needs of other data devices connected to the host via Thunderbolt 3, Thunderbolt systems will dynamically balance the amount of bandwidth allocated to each device.

Intel has posted a technical brief on their own Thunderbolt Technology website that describes how this works here -> https://thunderbolttechnology.net/sites/default/files/HBD16235_Thunderbolt_TB_r05.pdf (source -> https://thunderbolttechnology.net/tech/solution-briefs)

That said, one method that system manufacturers have used to enhance the capabilities of their systems is to build systems with more than one Thunderbolt 3 port or in some cases more than one Thunderbolt host controller.

For example, Apple MacBook Pro 15" models with Thunderbolt 3 have two Thunderbolt host controllers, with each driving two Thunderbolt 3 ports. This results in a system with four Thunderbolt 3 ports available to allow for expansion similar to what you describe.

Thank you,

Bob
Plugable Technologies


#3

Hi Bob,

Thanks for the great info. I have a few followup questions…

The technical brief you pointed to offers some good detail on these limits, visualized pretty succinctly in this graphic:

The point also seems driven home in this text:

However, the takeway I get from this document is that “Dual 4K@60hz + Data” is indeed possible – and within TB3 standards – with the only caveat being that at worst case (i.e. max dual DP channel utilization), outbound data bandwidth could get capped at 8Gbps.

As such, doesn’t that mean the only limit to “Dual 4K@60hz + data” TB3 utilization in a device is vendor design choices?
For example, Plugable makes the TBT3-UDV which consumes (and exposes to an onboard DisplayPort connector) 1 of the 2 possible TB3 DP streams; it also has local PCIe data consumption/exposure (audio, Ethernet, and USB ports), AND offers a downstream TB3 port for connectivity for additional display and data devices. But, within TB3 standards, Plugable could have chosen to make similar a dock that consumed both TB3 DP streams and exposed them to two onboard DP connectors, and still consumed/exposed other onboard data, and offered a downstream TB3 connector for TB3 daisy-chaining.
Likewise, Plugable makes the TBT3-DP2X-83 which consumes (and exposes to two onboard DisplayPort connectors) 2 of the 2 possible TB3 DP streams, but has NO onboard PCIe data uses (audio, Ethernet, and USB ports), and offers NO downstream TB3 connectivity. But, within TB3 standards, Plugable could have chosen to make a similar device that both exposed 2 DP connections, and offered other data and downstream TB3 options.
Am I wrong that the limits of these devices are design choices, and not TB3 protocol limits? I’m especially curious since it seems like, if it is a design choice, pretty much every vendor has made those same choices. I’d wonder why: is it an engineering challenge, a cost decision, a marketing expectation, etc.?

This is true, and I indeed have both Mac and Windows TB3 computers and have seen all sorts of variation in TB3 implementation. But, while I could, say, plug a TBT3-DP2X-83 into one TB3 port on my 15" MacBook Pro (to max out that controller’s potential for exposing DP channels) and then use the adjacent TB3 port for TB3 data devices (to exploit any remaining PCIe channel bandwidth), and do the same with the ports for the controller on the other side of the laptop, that seems sub-optimal because:

  1. now you’re plugging/unplugging multiple cables when docking/undocking (first world problem, I know :wink:)
    and
  2. more seriously, lots of Windows TB3 hosts don’t have even 2, much less four ports, so those hosts are unavoidably put into either/or choices when trying using their Thunderbolt 3 potentials.

What’s more, this situation seems to go against the Thunderbolt 3 promise, again as articulated in the Intel Thunderbolt 3 Technology Brief:

It seems more like it’s instead “You can do any one of this, or a limited combination of these, with a single connector”.

Why would this functionality need to be tested and supported? Isn’t it just daisy-chaining and Thunderbolt mode selection as defined by the TB3 standards, and something any TB3 device should perform. Again, from the Intel Thunderbolt 3 Technology Brief:

These don’t sound like optional features. Does the TBT3-UDV belong to a different category of implementation than what above is described for a “Thunderbolt device”? If not, shouldn’t daisy-chaining work and be supported on the TBT3-UDV?

Thanks for any input or clarification. Even additional pointers to further technical reading would be welcome. It’s confusing because, again, the Intel Thunderbolt 3 documentation says, “All you really need to know is that if you plug your device into a Thunderbolt 3 port, everything just works”, but instead it seems like there are many ‘gotchas’ where you can’t do this, or this doesn’t work, or that’s not supported.

Thanks again for any help!


#4

Hi John,

Thanks for getting back.

While we can’t speak for other manufacturers, when it comes to our own products our design choices and usage recommendations are practical ones based on the audience being served by each product. Dual 4K@60hz + data is possible using a single TBT3-UDV when used in conjunction with one of our USB-C video cables or adapters connected to the dock’s downstream Thunderbolt 3 port, with the data elements being the dock’s USB ports, Ethernet port and audio port.

As mentioned previously, there is a finite amount of bandwidth available via a single Thunderbolt 3 connection to the host. Multiple Thunderbolt 3 devices can be daisy-chained together and connected to a single Thunderbolt 3 host port (when said devices have two Thunderbolt 3 ports that allow this practice), but ultimately the limiting factor affecting all of the connected devices’ performance will be how this bandwidth is allocated between said attached devices (depending on their individual needs).

To put that another way, while connecting two TBT3-UDV’s in a daisy-chain configuration is possible, the total amount of bandwidth available for both video and data among the two docks is still finite. In such a situation two displays will work via each unit’s built-in DisplayPort outputs. This assumes that the host system supports two displays via a single Thunderbolt 3 connection (all Apple systems support this, but not all Windows systems do).

However, the bandwidth available for the data devices spread among both dock’s USB ports, Ethernet ports, audio ports and the remaining downstream Thunderbolt 3 port of the second dock will still be the balance remaining of the original 40Gbps available from the host (minus the bandwidth used by the video streams).

For example, you mention the intention was to connect our Thunderbolt 3 SSD to the downstream TB3 port of a second TBT3-UDV dock in a daisy-chain configuration. While that is possible, we cannot guarantee the level of performance you will see (in terms of transfer speeds) in such a configuration, especially since we do not know what other devices will be connected to both of the docks. We do our best to set honest expectations prior to purchase to help prevent frustration later on, so that informed our previous statement and recommendation (Thunderbolt 3 is a great technology, but there are practical limits outside of the marketing tag lines).

Thank you,

Bob


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