Hi Mattias,

I will try to clarify my question:

RME's MADI Info Center states that:
(...)"MADI is unidirectional, providing a 'point-to-point' interface between a source and a destination. The MADI format is fully specified in the AES-10 standard document.(...)."

The manual says that:
(...)"Technically every single MADI channel includes several additional bits, containing various information
(Channel Status). RME use the usually unused User Bit of channel 56 (channel 28 in
96k Frame mode), to transmit MIDI data invisibly within MADI, ensuring full compatibility.(...)

But I could not find anywhere clearly stated that you can send the MIDI data (from a HDSPe MADI card, for instance) throw it's INPUT MADI port (where it is receiving audio data), using only one cable for the whole process. (p.s. I don't have the equipment to test it by myself yet). I hope this clarify my question.

No, you can remote with one connection. The way back is needed to verify if the unit did what you wanted to do. You can even use our MIDI Remote software this way - one way...

Please read your first post again. You did not write why you need to send MIDI data FROM the 648 TO another unit. You explicitely wanted to remote control the 648 itself.