I use a Behringer ADA8200 as ADAT extension for the 816. The 816 is connected via USB to the computer and the Behringer is connected via the ADAT ports. The Behringer is set to “SLAVE”, that’s it. It automaticly syncs to the 816 which is the Master.
The ins and outs show up automaticly in Cubase and in the DspFX mixer you can make the output settings.
Figuring out how to connect everything was a bit complicated for me because the manual is poorly written on this subject.
But also depending on how you will use the ins and outs. All 16 Inputs are easy, but the 8 extra outputs are more complicated.