Q: what is the meaning of the ill-formed ".$PGACK,003,-1*71" sentence?
Some firmware versions (e.g. most AXN1.50) have a hardware mismatch protection function. When a user flashes a firmware intended for a module (e.g. PA6B) into another one (e.g. PA6C), then a sentence like the following
.$PGACK,003,-1*71
might occur.
Please make sure that your firmware matches your module before updating.