Is there a trick here? re AES/EBU

I brought home a balanced cable (MIT proline digital ref) to try out. I ran it between my Theta Data Basic II and my Theta Gen 5 A......select the appropriate inputs on the Gen 5 a....&....NO SOUND.
Is there some lind of internal switch or some other legerdemain that needs to be done? I have already taken the cover off of both units and the XLR sockets seem to be connected appropriately
Thanks

jd