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GM-LAN not based on normal CAN packets?

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  • GM-LAN not based on normal CAN packets?

    I've been attempting to decipher my cars CAN commands - with mixed success.

    It seems that I only receive the first packet of multi packet messages via a regular can to usb adapter.

    My test case is a sniffer logging all coms on 95kbps MSCAN, and then switching on the CD player. The album name, track name etc is sent to the screen - which I can see - but only the first 8 characters of each string.

    Does anyone know why this is so?

    There are many packets I don't know the meaning of, but they don't appear to contain ascii in any form.

    I was under the impression GM-LAN was built on top of regular CAN packets =/, but now am thinking they must invalidate the CRC on subsequent packets (perhaps the CRC follows on from the first packet?) or some such rubbish.

  • #2
    what hardware are you using to sniff? I know GMLAN operates on the normal CAN and SW CAN. I'm a n00b when it comes to this stuff, but I want to start sniffing around like what you are doing.

    what vehicle and what year?

    keep up the good work.
    Former author of LinuxICE, nghost, nobdy.
    Current author of Automotive Message Broker (AMB).
    Works on Tizen IVI. Does not represent anyone or anything but himself.

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    • #3
      Thanks.

      It's an '08 Holden Astra (for Google: Vauxhall Astra/Opel Astra/Opel Corsa/Opel Vectra ), and I'm using CANUSB.

      Only started tonight, so still have a way to go... have deciphered the steering wheel controls and a few other things (will write them up tomorrow), but my CarPC design hinges on being able to decipher these text strings sent to the info screen .

      So frustrating being able to see just snippets of what's being sent to it - I'm no longer convinced that it's just the first packet though. It may be that the device is dropping some packets (although it's only 95kbaud) - considering these packets will all be sent in a big burst, I guess it's possible. Perhaps the baud rate's slightly off? Will try more tomorrow... just wish that somewhere on the net you could read how these multi-frame can packets work without paying $130 for the ISO.

      Good luck on your endeavours

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      • #4
        I am guessing you are most likely dropping can frames. GM LAN is different from standard CAN but still it will show up as a raw can message.

        Paul
        OBDPros scantool

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        • #5
          Thanks michbound - I've managed to solve the problem, which is as you thought. I've increased the "Synchronization Jump Width" (all greek to me) to its max value which is now letting me read everything. Impressed with this CanUSB, didn't realise it was so flexible when I purchased it.

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          • #6
            Originally posted by Mania View Post
            Thanks michbound - I've managed to solve the problem, which is as you thought. I've increased the "Synchronization Jump Width" (all greek to me) to its max value which is now letting me read everything. Impressed with this CanUSB, didn't realise it was so flexible when I purchased it.
            plz document what you find so us other GMLAN'ers will be able utilize your work.
            Former author of LinuxICE, nghost, nobdy.
            Current author of Automotive Message Broker (AMB).
            Works on Tizen IVI. Does not represent anyone or anything but himself.

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            • #7
              I'll write my progress in my worklog, perhaps if I solve anything substantial I'll make a dedicated thread.

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              • #8
                Ok, perhaps I've written too much... but it's all up there now.

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