Why does the XW+ inverter display a F66 F69 fault during a firmware upgrade?
If the units remain connected together (in a master/slave arrangement) during the firmware update process, the units could display a fault due to the multi-unit configuration, causing an interruption of the update process.
To resolve this issue, clear the fault and disconnect everything (XANBUS, AC sync Cable) from the unit that requires the firmware upgrade. This will help rule out any communication issue due to noise and bad cables. Then connect the configuration tool directly to the single unit and proceed with the update accordingly.