Sometimes a technical discussion is good and needed. I personally supported the idea of the customer to go for an UFX and told my reasons.
This thread simply became stuck, as there was no response anymore from the originator of this thread.
A simple "and what about my other questions" would have been enough to reactivate it.
To your question (when I write UFX+, it could be any primary device like UFX, UFX II, ...)
Generally to what device to choose best as master:
Best take the device as master which has the most number of channels and connect it directly to the PC.
If you change then the sample rate i.e. in your DAW, then the master and all slaves will get the correct clock automatically.
Cabling between recording interface and computer
As both devices need to be operated by TM FX, you need to connect UFX+ and UCX to the computer with the type of digital interface as you like (USB, thunderbolt, Firewire).
So you need to install the required drivers for UFX+ and UCX.
Note, there are 4 separate drivers, 2 for the UFX+ (USB/Thunderbolt), 2 for the UCX (USB/Firewire).
Digital cabling between interfaces UFX+/UCX
-From the master you need at least one digital output towards a slave so that it can receive clock. So you need i.e. ADAT in 2 directions between UFX+ and UCX a) to get the additional channels via ADAT (IN) b) to send clock to the slave device and eventually use any of its outputs
-Connect UFX+ as primary interface.
-Connect UCX to UFX+ via ADAT (both ways).
In the RME driver settings dialog:
-UFX+: set clock to internal (master)
-UCX: set clock to be received from ADAT.
The ASIO buffersize needs to be equal on both devices !
In TM FX:
-You can open a 2nd instance of TM FX. One to operate the UFX, one to operate the UCX.
-Any to Any routing works only inside of each TM FX instance. But now you can start to route any of the UCX inputs to the 8 ADAT outputs, to get i.e. the 2 additional Mic inputs and 6 further analog inputs added to the UFX.
In your case
There should be no doubt about it to connect the UFX+, which has more channels, directly to the PC.
Then the UFX+ will be set by you to master.
The UCX becomes slave.
Which connection to the PC (UFX+)
- Whether you choose thunderbolt or USB3 doesn't matter much in terms of RTT times, maybe thunderbolt has a little advantage in terms of CPU load, but tbh, I didnt see a big difference in terms of CPU load even between i.e. USB and Fw, as the RME USB driver are really good
- The thunderbolt driver is the only one which supports the pitch function of the driver, in case you need it
- It can have more advantages to use the MADIface driver (USB3/2) because then you have also access to additional recording interfaces or converters by this one driver as the MADIface driver supports a number of different devices. By this you can have direct access to each of the devices from DAW with this one ASIO driver. You only need to be careful in 2 aspects: the devices need to be clock synched and you need to set on all the same ASIO buffersize.
Optimizing / reducing cabling to PC
You could use the UCX standalone capabilities to eventually remove the USB or Firewire cable once your required routing (2 Mics and 6 analog INs through ADAT) has been set and stored to 1 of the 6 user profiles in the UCX.
But I am not 100% sure, whether you can set/change Mic gain or autoset on the device in standalone mode.
So this might be a reason to keep the USB or Fw connection of the UCX to the PC.
If there is any further question on your side, then please ask.
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub13