Thunderbolt 3 Dock (14-port): Thunderbolt 2 Adapter Limitations

Created on: April 1, 2021
Last updated: April 19, 2022

Occasionally MacSales Customers with Thunderbolt 2 Macs ask about or would like to try connecting their computer, via a 20 Gb/s Thunderbolt cable and the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter, to our 14 port Thunderbolt 3 Dock. While not as ideal as connecting a native Thunderbolt 3 host to the dock, this type of configuration can work for some people. Generally all ports should function normally but at reduced speeds in some cases.

One of the areas where Customers are more likely to encounter limitations is when connecting one or more displays to the dock. At different times OWC has conducted display tests using a variety of different host configurations. The 'Display Notes' listing below provides some general guidelines for configuring additional external displays when using the Thunderbolt 3 Dock with a Thunderbolt 2 Mac via the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter.

General Limitations

• Overall bandwidth is reduced from 40Gb/s (Thunderbolt 3) to 20Gb/s (Thunderbolt 2).
• When using the spare Thunderbolt 3 port for storage devices, Thunderbolt 3 devices that support booting should work but USB-C devices will not boot.
  
  • Note: some Thunderbolt 3 or USB-C devices (or the drives they contain) may have requirements that exceed the baseline macOS 10.12 requirement of the dock and adapter. Make sure your host OS version supports all the device types you intend to use before attempting this configuration.
• This configuration does not allow for host charging.
  • Device charging via the USB Type-A ports should continue to work assuming the port provides enough power for the device (see User Guide for more information). 
    
    

Display Notes

IMPORTANT: the stability of a given configuration can be affected by display type, the display cable, and any additional adapters that are added to the mix. As a general rule, the more adapters or variables you add, the less likely a given display configuration will function as expected. Since the starting point of this configuration is a Thunderbolt 2 host adapted to a Thunderbolt 3 Dock, this rule is more applicable than would be the case with a Thunderbolt 3 host.

The results below were confirmed with a variety of Mac host computers including but not limited to:

• 2013-2015 MacBook Pro
• 2014-2015 iMac
• 2014 Mac mini
  
  
• A variety of OS versions were used (10.13.6 to 10.5.7), depending on the host model. Note that both the Apple adapter and the OWC require at least macOS 10.12 as a minimum requirement.

For reference, these Mac models have Thunderbolt 2 ports:

https://support.apple.com/en-us/HT20173

Single Display Configurations

• Thunderbolt 3
  • In one scenario we achieved 5K @ 60Hz using the LG UltraFine 5K model sold by Apple; we have not been able to confirm that other 5K displays work, or displays that go beyond 5K resolution.
  • With displays from two different manufacturers, we confirmed that 3840x2160 @ 60Hz worked consistently.
    
    
• MDP (to DisplayPort)
  • With displays from two different manufacturers and two different brands of cable, we confirmed that 3840x2160 @ 60Hz worked consistently in all but one instance (MacBookPro11,1) where 2560x1600 @ 60Hz was the maximum achieved.
    
    
• MDP (to HDMI)
  • With displays from two different manufacturers and three different cable brands, we confirmed that 3840x2160 @ 30Hz worked consistently in most cases.
  • This display setup did not function with the MacBook15,1 but we do not understand yet the reason why.
    
    
• Overall, we believe that when using a single external display, and either a Thunderbolt 3 connection, or one of the two listed MDP connection types, most 4K displays should function normally, and in the case of HDMI connections, usually at a reduced refresh rate of 30Hz.

Dual Display Configurations

• Unfortunately, while the Thunderbolt 3 Dock should be able to function normally in other types of scenarios, we had limited success getting a typical dual display configuration to work using the same displays and cables that worked normally for single display setups. 

• Among all the hosts listed above only the iMac17,1 allowed for dual external displays, and only intermittently. In some cases a restart was required and the only working configuration we achieved was
  • Thunderbolt 3: 3840x2160 @ 30 Hz
  • MDP to DisplayPort 2560x1600 @ 30 Hz
    
    
• In past tests, we achieved some additional success by modifying the connections using additional Apple adapters and display types, but again with limited resolution and refresh rates. In general we do not advise trying to work around display limitations by adding more adapters to the mix as in our experience it is as likely to cause problems as solve them.
  
  
• Overall, while it has been shown to work in limited circumstances, we do not recommend attaching more than one display to Thunderbolt 3 Docks that are connected to Thunderbolt 2 hosts via the Apple adapter. Additional testing may be done in the future, as it is possible these limitations will be lifted with updates to macOS