Which 16" do you have? Which graphics? 5300M or 5500M? Which OS, Big Sur or Catalina?
The CalDigit TS3 Plus dock is an older Thunderbolt 3 dock which uses the Alpine Ridge Thunderbolt controller. The Alpine Ridge Thunderbolt controller can only be connected to a Thunderbolt port. The XDR does not have any downstream Thunderbolt ports.
Only newer Thunderbolt 3 docks with the Titan Ridge Thunderbolt controller or Thunderbolt 4 docks with the Goshen Ridge Thunderbolt controller can be connected upstream to a non-Thunderbolt USB-C port. But in that case, they will be limited to USB functionality and a single display using only two lanes of DisplayPort (instead of the usual Thunderbolt capability of two separate DisplayPort connections with 4 lanes each).
The only upstream port on the XDR is the Thunderbolt input port. That port must be connected for the display and for the downstream USB-C ports to work. You must connect the upstream Thunderbolt port of the XDR to a Thunderbolt or USB-C with DisplayPort Alt Mode port - either from a computer or from a dock.
Since you have a 5300M or 5500M, then your MacBook Pro may support DSC (except maybe 5300M doesn't support DSC, or maybe Big Sur doesn't support DSC). The XDR also supports DSC. If DSC works, then you may be able to do this setup:
MacBook Pro -> TS3 Plus -> XDR.
In that case you'll get 6K 60Hz 10bpc RGB using HBR2 (17.28 Gbps) and DSC which leaves 22 Gbps for USB from the TS3+ or XDR (which should support 5 Gbps in this case).
On the other hand, if DSC doesn't work, then you'll need to connect the XDR directly to the MacBook Pro. It will do 6K 60Hz 10bpc RGB using two HBR3 connections over Thunderbolt (39 Gbps), leaving only 1 Gbps or less for USB which might only allow 480 Mb/s USB connections (I don't remember if anyone has shown what the USB connection actually is in this case even though I've asked many times). In this case, the TS3+ needs to be connected to a different Thunderbolt port of the MacBook Pro and it will support 22 Gbps of USB from its various USB controllers. connecting USB devices to the XDR in this setup would be wasteful because they would be limited to USB 2.0 speeds (keyboard and mouse should be ok).
The reason both connection methods are limited to 22 Gbps of USB is because 22 Gbps is approximately the limit of PCIe data over Thunderbolt. The rest of the 40 Gbps is usable by DisplayPort.
Newer Thunderbolt docks with Titan Ridge or Goshen Ridge only allow 10 Gbps of USB because they only have one USB controller/port - they don't add more USB controllers because they would not be compatible with non-Thunderbolt hosts. Additional USB controllers require PCIe tunnelling. Instead, these newer docks use USB hubs for all their ports instead of USB controllers.
To discover if DSC is working, we need to examine the output of the AGDCDiagnose command.
/System/Library/Extensions/AppleGraphicsControl.kext/Contents/MacOS/AGDCDiagnose -a > AGDCDiagnose_XDR.txt 2>&1
To discover the info about USB, the following commands should be sufficient:
system_profiler SPUSBDataType > system_profiler_usb.txt
Code:
ioreg -filw0 | perl -0777 -pE '
s/^(([ |]+)\+-o .* <class .*(IOPCIDevice|AppleUSBHostController|IOUSBDevice|AppleUSBHostPort|IOUSBInterface).*), id 0x.*$/$1/mg;
s/^(([ |]*)\+-o .* <class .*), id 0x.*\n(\2[ |]{3} +)(?>\{\n)(.*\n)*?\3\}\n\3\n/$1\n/mg;
' \
> ioreg.txt
The ioreg command gets info about USB controllers/ports/devices/interfaces. A USB controller is a PCI device, so info about PCI devices is included.
zip the result using the Finder and post if you like.
