Daisy Chain picture from Apple TB Display to third party TB3 Display

[OhDB]

Hi,

I'm currently running my trusty MBP (late 2013) to an Apple TB Display but am investigating a Ultrawide Monitor (LG 34WL850-W) as a possible addition/replacement as I would like to run both a work PC Laptop and my MBP on the ultra wide.

I'm trying to confirm if possible/find a way to daisy chain the displays together (or drive the LG from the MBP) using a TB2 to TB3 adaptor (maybe via a TB3 Dock?) as what i currently understand is that the adaptor will not drive video from TB2 to TB3.

Is there any way to do this or do i just go from TB2 to DisplayPort or HDMI and just keep the TB3 port spare for when i eventually upgrade the MBP?

any help thankfully received!

 

[joevt]

MBP 2013 has Thunderbolt 2 which is limited to 20 Gbps, so it can connect one HBR2 (17.28 Gbps) display (e.g. 4K60 16 Gbps) or two HBR (8.64 Gbps) displays (e.g. 1440p60 7.2 Gbps) to a single Thunderbolt 2 port.

The Apple Thunderbolt Display cannot output DisplayPort from its downstream Thunderbolt 2 port (unless I'm wrong?) but that doesn't matter since the 34WL850 supports Thunderbolt - so just use an Apple Thunderbolt 2 to Thunderbolt 3 adapter with a Thunderbolt 2 cable to connect the displays together. The DisplayPort is tunnelled over Thunderbolt to a downstream Thunderbolt controller in a Thunderbolt display (in this case, the LG) or Thunderbolt dock to be converted to DisplayPort.

The Apple Thunderbolt Display needs to be detected first because otherwise the LG will connect using HBR2 (required for 3440x1440 60Hz 10bpc HDR 9.6 Gbps) leaving no bandwidth for the Apple. macOS will reserve HBR link rate for the Apple display (sufficient for 1440p 60Hz 10bpc 7.2 Gbps). Then you can connect the LG to the Apple display. It should connect using HBR to allow 8 bpc (no HDR). Maybe you can do 10 bpc using 50Hz.

As far as I know, there's no way to get HBR and HBR2 or dual HBR2 through a Thunderbolt 2 cable - Apple does some trick to get dual HBR3 (25.92 Gbps) on a Thunderbolt 3 cable (40 Gbps) - I suppose it could work for Thunderbolt 2 as well but the method is unknown.

You might be able to get the connection info (HBR or HBR2) using the AGDCDiagnose command. Use that to see what's going on with the connections (it might not work with the old AMD GPU though)

Trying to chain the displays may not work or be too much of a hassle because of the bandwidth limit of Thunderbolt 2. The MBP has a HDMI 1.4 output and the LG has an HDMI input so consider using that. Otherwise you'll have to use one Thunderbolt port per display.

With HBR2, you could do 3440x1440 75Hz 10bpc (12.1 Gbps). Actually, the MacBook Pro 2013 tech specs say the Thunderbolt ports are limited to 2560x1600. This may mean that they cannot do HBR2. In that case there would be no point to using one Thunderbolt port per display and it wouldn't matter what order the displays were connected since they would both always use HBR.

You may need a utility like SwitchResX to add or select the 3440x1440 60Hz mode.

 

[OhDB]

Thank joevt, very comprehensive answer. think i'll run direct to both monitors from the MBP.