You need only one TOSLINK cable between 12Mic and UCX II.
Reason:
- the UCX II has only 1x ADAT I/O
- the 12Mic has only ADAT outputs ...
So, you can only route 8 Mic channels from 12Mic to UCX II
Better would have been to get an 802 FS or UFX II, which have 2x ADAT I/O
By this, you could have connected the 12Mic with 2 TOSLINK cables to be able to use all 12 Mic channels (at single speed).
Even better would have been to get an UFX III with MADI to connect the 12Mic via MADI.
Note, while we are at it, you would also need to buy an RME SFP (Transceiver) for the 12Mic.
When using ADAT you need to make the 12Mic to act as clock master and UCX II as slave.
Or .. you need to use WC.
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub14