SONY walkman remote protocol

[xpertvis]

Hi guys. i got old nwz A815 walkman. it still working
i checked service manual for this player, and it got RX and TX pins on connector. I think it can be use for switch song, or change volume or something like that, but i dont know, how commands it use. So is there anybody, who know how to remote player over this pins ? there are some dock stations that using this pins to switch song, so i believe that there is someone, who know, how it works. thanks.