When looking to the schematic on the back side of the unit then you will see the signal paths:
- from MIDI IN to Groove TXD and the Switch
- from Grove RXD to SAM2695 and the Switch
- from the Switch to MIDI OUT
Connecting the MIDI IN direct to the SAM2695 is not prepared.
You have to go thru Groove port to do that. It may work if you just bridge the TXD and RXT on the port with a jumper wire (don't forget the 5V supply). Otherwise you have to go thru your processor and put everything what is coming in direct to the out.