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Thread: Monitoring and analyzing the raw data and the supported PIDs - CAN bus

  1. #1
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    Join Date
    Feb 2012
    Posts
    2

    Monitoring and analyzing the raw data and the supported PIDs - CAN bus

    Hi, I am new here!

    I am working in a personal project which consist in scanning, monitoring and analyzing the raw data of the CAN bus.

    My car is a Fiat Grande Punto Mk3 2005 (CAN 15765-4 29 bits headers and 500Kbaudio/s).

    I have already bought a scan tool, the ODBLink SX.

    I have correctly installed the drivers and software. The software I am using is STNTerm, which is very useful and simple.

    I have read the ELM327 and STN1100 data sheets, so I know most of the basic commands.

    I have done some tests and everything went alright, my problem is that I don't understand why I get two different kind of data packets.

    I mean, if I use the ST MA command (after AT H1 to activate the headers), I obtain a lot of packets, for example:
    [...]
    0C 28 A0 00 14 00 18 11 20 11
    00 1C A0 06 18 BB 00 90 18 EC B3 E4
    00 14 A0 06 18 DE 73 BB 19 0B EA 1A
    00 30 A0 02 40 00 00 00 00 01 00 09 <DATA ERROR
    02 10 A0 06 FF 04 FF 00 00 00 00 00 <DATA ERROR
    06 18 A0 01 00 2B 04 A7 28 16 71 21
    06 28 A0 01 00 21 00 80 00 30 00 20
    08 10 A0 00 00 FF 04 00
    00 1C A0 06 28 BC 28 BB 28 ED D1 76
    00 14 A0 06 28 DF 95 79 29 0C 04 5B
    00 30 A0 02 40 00 00 00 00 01 00 0A <DATA ERROR
    02 10 A0 06 FF 05 FF 00 00 00 00 00 <DATA ERROR
    06 18 A0 01 00 2B 04 A7 27 16 71 21
    06 28 A0 01 00 21 00 80 00 30 00 20
    08 10 A0 00 00 FF 08 00
    [...]

    but if I use the ELM327 command to show current data of a given PID I get a very different packets, with different headers, for example, 01 23: 18 DA F1 10 04 41 23 0D AC 00 00 00.

    I already know the structure of the packets, I just want to know why I get different kind of packets when I monitor the bus and when I do a request for a PID with the ELM327 command, for example 01 23

    I am analyzing the raw data, for example, I already know:

    0C 28 A0 00 14 00 18 11 20 11 is the date: 14:00 18/11/2011

    08 10 A0 00 00 FF 00 00
    08 10 A0 00 00 FF 04 00
    08 10 A0 00 00 FF 08 00
    08 10 A0 00 00 FF 0C 00
    These 4 packets provide information on the brakes pressure for the 4 wheels (X0, X4, X8 and XC), this pressure (X) can be 0, 2 or 6 (00, 20, or 60; 04, 24 or 64;...)

    00 1C A0 06 28 BC 28 BB 28 ED D1 76
    00 14 A0 06 28 DF 95 79 29 0C 04 5B
    I think these packets provide information about the speed of the 2nd wheel, but, why can I get the speed with 01 0D through the ELM327 and this information is in a different format than in these two packets which are shown with the ST MA command? I mean, why the same information is shown in two different ways?

    thank you! and sorry if I made ​​any grammatical mistakes, my english is not perfect

  2. #2
    Newbie
    Join Date
    Feb 2012
    Posts
    2
    Hi again!

    I already know the answer for my problem, and I am going to explain that, I hope It could be useful for anyone.

    The answer is that I was wrong, my protocol is "raw CAN", not CAN ISO-15765. So, when I monitor the bus (AT MA / ST MA) I see packets with a very particular structure: 4bytes for header and then the data, which is not the structure defined in ISO-15765.

    Otherwise, when I do a request for a specific PID, what I am obtaining are OBD packets, not CAN packets, which have a specific structure:
    18 DA F1 10 04 41 23 0D AC 00 00 00:
    18 = priority (5 bits: 00011000)
    DA = physical addresing
    F1 = Target Address
    10 = Source Address (ECU)

    41 = Mode 01: 40 + 01
    23 = PID

    0D AC 00 00 00: Data

    The OBD packets may contain the same information as raw CAN packets (speed, RPM, etc.) but expressed in a different way, much easier to understand than in raw CAN where the structure and the headers are defined by every manufacturer.

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