So I've been working on getting UART access to the head unit in my car; I've got the schematics of the unit and found that TX3/RX3 on the Renesas SH7750 are presented externally - along with 3 "test bit" pins.
Pulling the test bits low in certain combinations enables certain modes (eval or debug). I'm happy that I am in debug mode as the unit does output on UART in this mode whereas there is nothing at all in normal operation.
Some of this is documented in a workshop manual for another headunit from the same manufacturer - but also includes a "test box" which sits between the unit and the PC (herein potentially lies the problem)
It should output "Hitanykey" and then "xanavi>"
I however only got garbage output... I've been through the baud rates and no luck so have resorted to sticking a logic analyser on it.
Any pointers or suggestions would be greatly appreciated!