I have my RME Fireface400 connected to my MOTU 828 via ADAT optical. The RME is set up as Master and the out from the RME ADAT OUT is going to the ADAT IN on the 828 and vice versa (that way they can talk back and forth if they get bored. Actually I did this because I thought if affected synchronization).
The RME is set to ADAT for the Clock Source and the 828 is set to ADAT Optical. The RME indicates that the clock signal is "synch." If I switch the Clock Source on the RME to Internal, the status changes to "locked," however there is no change in outcome.
HW sees the ADAT channels and I've set the headphone mixdown to go to Channels 1/2 of the ADAT (RME Channels 10 and 11). The RME mixer indicates that there is output on the RME Analog 1/2 (Main) and output on ADAT 1/2 (Headphone mixdown).
Only problem? No sound.