Find u-boot PPC interrupt sequence

The purpose of the project is to find u-boot interrupt sequence (or if it needs some bytes enabled in a memory location in order to activate TX because it ignores everything I send over). I really have no idea. In order to activate U-Boot initially I had to write to the flash at a memory location 80 00 00 00 (from the running linux). Now U-Boot prints something but I cannot interrupt autoboot. The board is running on a mpc83xx cpu (PowerPC)