I have been trying to add the GTN 750/650 to the Islander and assign the correct power bus. Trying all the options I can only get the GTN 750/650 to switch on with the battery switch; unlike the radio/transponder and default GNS 530/430 with operate with the Avionics master switch. I have found that the XP default avionics bus (sim/cockpit/electrical/avionics_on) is permanently on (=1) independent of the master battery switch. It is not controlled by the Islander switch (afm/islander/cmd/sw