HD500X has a setting for S/PDIF sample rate. Set the 18i20 to the same value; then select S/PDIF as the the synch source on the 18i20. You must check the Synch Source and Sample Rate when you swith to various DAW apps. Note that if the DAW app is not agile (switches automatically), you will have to re-check the Synch Source. Some DAWs will only allow a sample rate setting during setup of a new project/song and not allow changing thereafter; you must setup the sample rate in kind on each device including the DAW or match the DAW (thus the term synch as in synchronized sampling/timing).
-cheers
