pairing 4k monitor with LG5K via dock

[olliedog]

I'm considering switching to a dual monitor setup and would like to keep my LG5K. Based on reading past posts, it seems like there is an issue pairing 4k monitors with the LG5K through a TB3 dock.

If I'm understanding correctly, each thunderbolt port on the macbook pro receives up to two display port signals. If you run a single cable to a TB3 dock and you have the LG5K monitor connected via TB3 and a 4k monitor connected via display port, then the LG5K will only receive one display port signal.

If this is the case, would I have to connect the LG5K directly to the laptop and the 4k monitor to the dock and connect the dock to another port on the laptop?

This is for a 2019 macbook pro by the way.

 

[chikorita157]

The LG 5K doesn’t support connecting through the same Thunderbolt dock with another monitor. It needs to be connected seperately since it lacks DSC support, thus it needs two Displayport connections. Each Thunderbolt 3 bus only provides two. I own a Studio Display and it’s really the only 5K monitor that can hook up another Studio Display or a 4K or less display on the same dock with full resolution. If you hook them up on the same dock, the 5K will probably only run at 4K resolution.

The Caldigit site for the TS4 says that Dual 5K only works with a Studio Display and on M1 Pro/Max/Ultra macs only.

 

[joevt]

Actually, for Intel Macs, a Thunderbolt bus (or Thunderbolt controller) has two Thunderbolt ports. Each Thunderbolt bus can allow two DisplayPort connections, either two to one Thunderbolt port or one to each Thunderbolt port.

The LG UltraFine 5K requires two DisplayPort connections (from a single Thunderbolt port using a Thunderbolt 3 cable) to do 5K60 10bpc RGB.

The MacBook Pro 2019 is an Intel Mac, and has a Thunderbolt bus for each side of the laptop (the left side and the right side).

Therefore, the LG UltraFine 5K needs to be connected to one side of the MacBook Pro while the other side can connect another LG UltraFine 5K or two 4K displays.

https://support.apple.com/kb/SP794

The LG UltraFine 5K can be replaced with an Apple Studio Display or an Apple Pro Display XDR. Since the GPU in the MacBook Pro 2019 doesn't support DSC, those displays will use a dual DisplayPort connection also. These displays are referred to by Apple as dual link SST.

Dual link SST displays also support single link SST connection mode. If you connect a dual link SST display (5K or 6K) and a single link SST display (4K) to a Thunderbolt dock, then one of two things can happen:
If the dual link SST display is detected first, then the single link SST display will not work.
If the single link SST display is detected first, then the dual link SST display may be limited to 4K60 10bpc RGB.

 

[olliedog]

Therefore, the LG UltraFine 5K needs to be connected to one side of the MacBook Pro while the other side can connect another LG UltraFine 5K or two 4K displays.

Would it be optimal to connect the LG5K directly to one side, and then the 4k display to the other side through the TS3+ dock?

Would this connection setup negatively impact charging in any way since both the dock and LG5K can supply power separately?

 

[joevt]

The LG 5K doesn’t support connecting through the same Thunderbolt dock with another monitor. It needs to be connected seperately since it lacks DSC support, thus it needs two Displayport connections. Each Thunderbolt 3 bus only provides two. I own a Studio Display and it’s really the only 5K monitor that can hook up another Studio Display or a 4K or less display on the same dock with full resolution. If you hook them up on the same dock, the 5K will probably only run at 4K resolution.

The Caldigit site for the TS4 says that Dual 5K only works with a Studio Display and on M1 Pro/Max/Ultra macs only.

The only way to get full resolution from two 5K displays or two 6K displays connected to the same Thunderbolt port is with a Mac that has a GPU that supports DSC (and only with displays that support DSC, such as the Apple Studio Display and the Apple Pro Display XDR). The MacBook Pro 2019 doesn't support DSC so it must use the dual link SST modes of the displays for full resolution/refresh rate. The LG UltraFine 5K doesn't support DSC so it can't get full resolution and refresh rate without using dual link SST.

Of course, you can get full resolution without DSC if you use a timing with a lower refresh rate or using lower color depth. All of the displays are limited to HBR2 link rate in single link SST mode.
For single link SST with HBR2 link rate, the following may be possible:
1) 5K60 6bpc (6bpc is not easily obtained in macOS - but can be selected in Windows using Radeon software)
2) 5K47 8bpc (the Apple displays may allow up to 47Hz but I haven't seen anyone try, same with all the other possible options listed here)
3) 5K39 8bpc (this may be a limit of the LG UltraFine 5K but I've only seen one person test this)
4) 5K37 10bpc
5) 6K45 6bpc (Windows)
6) 6K34 8bpc
7) 6K27 10bpc (required for HDR)

Intel Macs can use SwitchResX or similar to add custom timings. Apple Silicon Macs don't have an easy method to add custom timings.

 

[joevt]

Would it be optimal to connect the LG5K directly to one side, and then the 4k display to the other side through the TS3+ dock?

I think this is optimal. The LG UltraFine 5K is like a Thunderbolt dock, and it would perform slightly better as a Thunderbolt dock if it wasn't connected to a Thunderbolt dock.

Would this connection setup negatively impact charging in any way since both the dock and LG5K can supply power separately?

I think the Mac will choose to be charged by the USB-C device that offers the most power. Check the Power tab of System Information.app.

 

[olliedog]

If you hook them up on the same dock, the 5K will probably only run at 4K resolution.

Is it possible to scale the LG5K display to see what it would look like at 4K?

 

[joevt]

Is it possible to scale the LG5K display to see what it would look like at 4K?

Use SwitchResX to see if there is a 4K mode that is not 5K scaled down to 4K (look for unscaled modes in the list of Current Resolutions). If the mode doesn't exist then add it as a custom resolution, 3840x2160 CVT-RB timing.
You can also try connecting the LG5K using a 10 Gbps USB-C data cable (allows only 20 Gbps Thunderbolt connection which only allows single HBR2 connection).