engeloded:
When i'm sending data from the mega to the due: tx on mega blink and rx on due blinks.
When i'm sending data from the due to the mega: only the tx light on the due blink.
Tx and Rx are only linked to the primary hardware serial. The programming port. Those lights are actually controlled by the helper chip which makes the programming port work. If you are seeing Tx flash then it means you sent something other than what you thought you were sending to Serial1.