Alternative WP34S flashing using Arduino

[subaffo]

Hi,

I was preparing my Arduino (One) to flash a 20b but then I stopped:

the Tx,Rx levels are at 5V, not at 3.3V.

I haven't seen any resistor on those lines in the schematics.
Most of the Arduino boards are at 5V levels.

Did I miss something in the (fine) documentation of this flashing method?

Thanks to all

Fabio