Driving an LG UltraFine 5K Monitor and a 4K Monitor with the CalDigit TS4 Dock

I’m going to old school blog and put this out there in the hopes that it hits search engines, helping someone down the road.

I wrote into CalDigit support asking if my monitors would be supported by their TS4 Dock. I love the sweet, sweet joy of a single cable between my laptop and everything else, but I’d been told that just because their marketing materials say “supports dual monitors up to 6k” via M1 Pro chips doesn’t mean the docks actually work that way.

Here’s my support request:

Hi, I need to confirm if the TS4 Dock (https://www.caldigit.com/thunderbolt-station-4/) supports dual monitors, one 5k and one 4k, using macOS on a MacBook Pro with M2 Pro.

The monitors are:

- LG UltraFine 5k (the one apple.com used to sell)
- Dell S2721QS 4k

Your marketing material implies this would be supported (dual 6K monitors support with M1 Pro), but my reading says that, for instance, dual 5k monitors are not supported.

Are my two monitors supported in a dual configuration using one cable to connect the doc to my laptop?

The response from CalDigit was rather helpful, and even offered some additional tips about my Dell monitor that have nothing to do with CalDigit or their products.

Thank you for contacting CalDigit Support. I would be happy to help.

As you may have seen on our TS4 product page, the TS4 paired with Macs is able reach up to dual 6K60Hz when paired with the Apple Studio Display or Apple Pro Display XDR. The reason for this is because these two displays use Display Stream Compression (DSC) to essentially provide a virtually lossless image while using less bandwidth.

If you take a look at the Dual 5K section on the product page or in the image provided below, you will see there’s a little note about the LG UltraFine 5K not being supported. That is because the UltraFine 5K does not use DSC.

Sorry, this is probably too small. Pika needs click to zoom.
Sorry, this is probably too small. Pika needs click to zoom.

We do not have your Dell S2721QS monitor to test with so we cannot do any testing or guarantee any results on your behalf. It is important to note that without accounting for anything else connected to the dock, specialized settings such as FreeSync, HDR etc, the LG UltraFine 5K monitor will likely use approximately 27.72Gbps out of the 40Gbps available out of the single expanded Thunderbolt 4 port from your MacBook. The S2721QS monitor will likely use approximately 15.60Gbps at 4K60Hz and 10-bit color, putting you over the 40Gbps available bandwidth.

However, these are estimates and you may be able to connect both monitors should you choose to try it. Some users have reported success running a 5K+4K monitor setup using different monitors. Intel’s recently announced Thunderbolt 5 protocol aims to address this with even greater bandwidth limits.

As a note, Dell has acknowledged a bug with the S2721QS monitors and M1 devices. We do not know if this extends to the M2 and M3 Macs. Dell states users may face flickering issues and recommend using an HDMI adapter instead of DisplayPort. Here is a link to their knowledge base article for more information: https://www.dell.com/support/kbdoc/en-ph/000185689/flickering-or-blanking-video-issue-with-dell-s2721q-qs-monitors-when-connected-to-a-m1-macbook-or-mini-via-usb-c-to-dp-adapter

I apologize for not being able to give you a definitive answer to your request. Please let me know if there’s anything else I can assist you with.

I’m going to write back asking if this is an “if it works it works” situation, or if it’s the sort of thing that could work one day, and not work another day.

[Edit for an update from Anthony, who is killing it!]

Assuming compatible hardware, if the TS4 pairs well in your setup and no issues arise after a handful of sleep, deep sleep and restart cycles, it should remain stable. Our docks do not have any Thunderbolt drivers of their own and rely on the Thunderbolt drivers provided by the host operating system, in this case macOS on your M2 Pro MacBook Pro.

If you’d still like to see if it works in your setup, we highly encourage you to purchase the TS4 from a retailer with a generous return policy. If you run into any issues with a particular piece of hardware, we’d be happy to assist you. If you have display issues from the get-go or find that your monitors refuse to wake from sleep while connected to the TS4, it’s likely there is a hardware issue that could be due to the aforementioned bandwidth exceeded limitations or something else.

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