Question on mixing Thunderbolt 2 and 3 devices on iMac 2020

[Gmouse]

I have an existing Thunderbolt 2 external drive used for backup and additional file room, and am getting a larger Thunderbolt 3 external RAID-1 drive for backup of the 2020 iMac. I'm going to need to hook both up to the 2020 iMac for a time, to go through and dump, then copy files I need to keep, as well as use the newer drive for backups. I also planned on hooking the new drive to my old iMac and doing a backup and copying files over, then transfer them to my new iMac.

I have purchased a Thunderbolt 2 to 3 adapter from Apple, and have Thunderbolt 2 and 3 cables already, so I assume I'll have no issues connecting either drive to either iMac?

For the iMac 2020, if you connect both Thunderbolt 2 and 3 devices at the same time (one on each iMac port, not daisy-chained) does it force lowering the bus speed to Thunderbolt 2, or each can support a different speed? I know there was some speculation on just how many buses the 2020 iMac has due to the stated ability to run two 6K monitors...

Thanks!

 

[joevt]

I have an existing Thunderbolt 2 external drive used for backup and additional file room, and am getting a larger Thunderbolt 3 external RAID-1 drive for backup of the 2020 iMac. I'm going to need to hook both up to the 2020 iMac for a time, to go through and dump, then copy files I need to keep, as well as use the newer drive for backups. I also planned on hooking the new drive to my old iMac and doing a backup and copying files over, then transfer them to my new iMac.

I have purchased a Thunderbolt 2 to 3 adapter from Apple, and have Thunderbolt 2 and 3 cables already, so I assume I'll have no issues connecting either drive to either iMac?

For the iMac 2020, if you connect both Thunderbolt 2 and 3 devices at the same time (one on each iMac port, not daisy-chained) does it force lowering the bus speed to Thunderbolt 2, or each can support a different speed? I know there was some speculation on just how many buses the 2020 iMac has due to the stated ability to run two 6K monitors...

Devices connected to one port do not affect the Thunderbolt connection speed (10, 20, 40 Gbps) of the other port.

If you want to chain multiple devices together, arrange them from fastest to slowest with the fastest connected to the iMac.

With Thunderbolt devices that use PCIe (not displays), there is very little benefit to dividing them between multiple ports unless the ports are of different buses (or the Thunderbolt controller is integrated in the CPU like Ice Lake 10th generation CPU). One benefit might be slightly less latency (devices later in a chain have higher latency).

When mixing Thunderbolt PCIe and display devices, displays should be connected to a different port of the bus so that it doesn't impact performance of PCIe devices connected to the other port of the bus. A single 3840x2160 60Hz display has a small impact on PCIe write speed of PCIe devices connected to the same port of the bus if the write speed is over 11.4 Gbps (depends on the PCIe devices and benchmark). Two 4K 60Hz displays or one 6K display (without DSC) can greatly impact PCIe write speed of PCIe devices connected to the same port of the bus (down to 4.3 Gbps for dual 3840x2160@60Hz or 4.1 Gbps for dual 4096x2304x69Hz or 1.1? Gbps for single 6016x3384x60Hz without DSC) and half the read speed (down to between 9.2 Gbps or 11.2 Gbps depending on the bandwidth of the display's timing, and the PCIe device and benchmark).

The 2020 iMac has one bus and two ports. It supports dual 6K XDR displays because the XDR display can use DSC and the iMac has 5700 XT which supports DSC. When DSC is used, a 6K display only requires a single HBR2 connection (similar to a 4K display). I don't know if it's less or more bandwidth than a 4K display but I know it fits within HBR2.

When DSC is not used, a single 6K display requires two HBR3 connections to support 6K. A single Thunderbolt bus only has two DisplayPort connections to a GPU, therefore only one such display can be used. It is the same for 5K displays that use two HBR2 connections (iMac display, LG UltraFine 5K, Dell UP2715K., ...) which allows 10 bpc. There also exist 5K displays that use a single HBR3 connection (supports only 8 bpc) such as the Iiyama Prolite.

 

[Gmouse]

Thanks for your comprehensive explanation! I was mainly worried about mixing devices on the iMac, coz I knew it only had one bus, but, will do as you suggest.