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CD Changer protocol?

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  • CD Changer protocol?

    I'm looking for a document describing the protocol GM/Delco/Delphi head units use to communicate with their external CD changers. I have the pinout for my unit, and it lists a Class 2 pin, so I'm assuming that it uses Class 2 (SAE J1850 VPW, whatever they want to call it) to talk to the CD changer, but I really need the list of commands (I already have a nice module coming in the mail to handle the electrical/physical Class 2 bus). Anyone know where I can find such a doc?

    My next stop is talking to the dealer. They were very helpful in my search for the pinout (I emailed them, and they faxed me the diagrams the next day), but I figured I'd ask here first.


    (P.S. Yes, I posted almost the same thing in another thread. Sorry if it's against this BBS's rules to doublepost, but after posting I noticed the thread was from early 2004. Not likely to have many look at it, I'd say.)

  • #2

    Not easy to find, you may well have to analys it yourself and design a circuit to emulate it.

    I have analysed some protocol cheaply before by using the pins on the parallel port as an inputs. And use a software to sample the pins at a fixed rate and save them to a txt file as "1" and "0". or ASCII graphics for clarity.

    If you want it you can play back the file to see if you have succesfully captured it and the changer reacts to it. Just be sure to have a consistent sample rates. I used turbo pascal for dos back then.

    Its painful but you can atleast see whats going on in there. Do verify the signal with a scope first to see the shortest time its changes. So you can set the sample rate to 16 times or so.

    It be alittle bit trick if the bus lines are bidirectional.

    Unless you have acces to some equipment that can do that for you already.

    Good luck.


    • #3
      I fear I may have failed to make my problem clear. I already have a module that will convert the data on the class 2 bus into data on an RS232 bus for me, so I'll be able to log the data being passed around with my computer.

      What I am looking for is a translation table. I need to know what 0x86fc (or whatever comes out) actually means. I'm currently planning to just snoop everything on the bus and try to decipher it on my own, but it would just be much easier if I could just look up what "Next Track" is.

      Ah well, it wouldn't be any fun if it were easy.


      • #4
        Ah ok...

        But really it cant be having that much command? probably 16 the most? So yeah I guess you just gonna have to decode each one.

        The 16 bit long stream would most likley contains a start/stop condition, command and probably some checking.

        You already have the equipment to see the data, you are sorted then. Just a matter of being patient with it now

        Good luck.