I have an Aera 660 wired to a GPS175 using MAP-MX via serial wire. this is one way communication from the 175 to the 660 making the 660 a MFD to the 175. But if I connect the 660 to the 175 over Bluetooth (the 175 does not need the 510 for this) then I can transfer back and forth between the 660 and 175. I asked Garmin techs at Oshkosh about this and was told that the Bluetooth (510) connection is FAA approved for two way communication while the serial connection between the Handhelds and the panel mounts are not. He said something about TSO’D equipment connected to non TSO’D equipment So what I do is use my non TSO’D IPad mini 6 with Garmin Pilot to send Flight planning to the TSO’D 175 via connects, which is transferred to the non TSO’D 660. But I do have the non TSO’D 660 is wired to my TSO’D GTR225 and the non TSO’D 660 is allowed to send frequency info to the standby position of the TSO’D GTR225. Clear as mud