Preliminary note : I chose the MAX3232 instead of the 232 as this one is a low power version. It's pin to pin compatible but the capacitors car have lower values.
The first schema is just a development circuit to be able to spy on the protocol. Nothing is emitted but the chip converts all information passing in both directions into RS232-compatible levels. Each direction is fed to a separate serial port of the spying PC.
The second schema will be used later as the emulator. If you actually have a CD changer, a switch allows you to flip two relays between actual changer and PC emulation.
If you don't, just throw away the switch, the relays and the upper "CD changer" connector and directly link pin 11 and 12 of the MAX3232 to resp. pins 13 and 14 of the head unit connector, and SPDIF to pins 18 and 19 of this connector.
I'm planning to try and make the spy circuit in the following weeks, and in the meantime I'm looking for good software to spy and dump information from serial ports.
Any hint or source code is welcome, as well as feedback on these circuits.
*** PRELIMINARY CIRCUITS REMOVED. Edited versions available in post 197 for the spying circuit and in post 245 for the emulating circuit.***