today i used another level adapter, just to be sure.
i removed the isolation material from all the test points, to do a better contact.
then i tested with multimeter each test point in order to don't try the pins where the voltage is greather than 3,5volt.
then i started the serial line at 115200bps and i tested al the test points (ground pin screwed on one of the two antenna connectors, and rx pin of the level adapter moved on each test point during boot time).
each time i power off and on the device, and on some test points i found some printed data.
on those pins i tried to change the speed, but the data is never readable.
The only connector that i wasn't able to test, is the one on the bottom, but the contacts are too small.
I'm thinking to another possible solution: may be we can reset the board but not the upgrade port driver.
By this way, the upgrade port will always be enabled, and not just at the end of the boot process.
the problem is how to find the board schematic. where is the upgrade port driver? is it embedded in the main chip?
Is it there a method to allow the pc to stay connected on the upgrade port?
p.s.:i found that on the board there is a RESET test point. shorting it to ground, the board is resetted.