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Thread: Speed on ODBII reading -> output

  1. #1
    Maximum Bitrate Crinos's Avatar
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    Speed on ODBII reading -> output

    I see many virtual gauges for both speed and RPM, but the videos I have seen usually show some latency and very often peak spikes.

    Is there a way to get this 100% accurate? With 0% fail...
    If not, I realy do not see the big picture of having a ODBII scanner, reader in my car, since I can get pretty much everything I want out from the IBUS. (BMW)

  2. #2
    Variable Bitrate
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    Readout speed depends on
    a) the protocol used
    b) how many ecu's respond to that protocol
    Obviously a Iso 9141-2 protocol running at 10.400 baud is a lot slower than a Can protocol running at 500K baud.
    Also the timeouts between messages are greater when using Iso protocols.
    For instance it is very normal for a car to wait 25 - 30 mili seconds after receiving the checksum of a command, before sending the answer. The second ecu will even wait a little bit longer, say about 30 - 35 mili seconds.
    Then the scantool has to wait for 55 miliseconds after receiving the checksum of the last message from the last ecu, before it can send another command.
    These delays will slow down the transfer rate significantly.
    Bmw's normally have 1 ecu for the engine, except some 12 cylinders, who have 3 ecu's for the engine, which all respond. When you have an automatic gearbox, that ecu will also respond.

  3. #3
    Maximum Bitrate Crinos's Avatar
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    Thank you for a detailed answer!

    Then it's clear for me, that I do not need any ODBII devices/programs... Atleast not for the moment

  4. #4
    VENDOR - ScanTool Vitaliy's Avatar
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    Quote Originally Posted by Crinos View Post
    Thank you for a detailed answer!

    Then it's clear for me, that I do not need any ODBII devices/programs... Atleast not for the moment
    What would you say be an acceptable refresh rate (in samples per second)?

    With some tweaking, it is possible to get reasonably fast (~10 samples/sec), even on ISO.

    Vitaliy
    OBDLink MX: world's smallest, fastest, most advanced OBD/Bluetooth adapter with SW and MS CAN support. Read the review to learn more.
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  5. #5
    Maximum Bitrate Crinos's Avatar
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    Quote Originally Posted by Vitaliy View Post
    What would you say be an acceptable refresh rate (in samples per second)?

    With some tweaking, it is possible to get reasonably fast (~10 samples/sec), even on ISO.

    Vitaliy
    10 samples / sec sound fast enough, but I realy do not have a clue

    For me, it should be equal to stock, so that I could mount a LCD infront of stock dashgauges and use my custom screen...

    This way I could go with a big RPM and tiny speed while driving active, and vice versa while driving normal.
    And active road assistance while driving to a GPS location.

  6. #6
    Variable Bitrate
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    10 samples per second for RPM is too slow, at least in my opinion.
    It'll do for 3rd and higher gears, but not in 1st and 2nd, there you will hit the limiter.
    Besides, 10 samples is when you only read 1 (ONE) item at a time.
    When you need both Rpm and Vehicle speed you will get 5 to 6 samples a second.

  7. #7
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    FWIW, I empirically checked the samplerate with my new OBDPros USB device, and was getting a little over 20 samples per second. My OBDKey Bluetooth would do between five and seven. I find that 20 samples per second is actually a pretty large number.

    Gary (-;
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  8. #8
    Maximum Bitrate Crinos's Avatar
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    wonder of what rate the stock system would report... if even possible to compare them

  9. #9
    Variable Bitrate
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    Refresh rate on stock systems depends on the protocol used and the baudrate.
    Exanple 1: Bmw diesel at 250 baud. Internal there are 2 ecu's. So, first ecu 1 asks ecu 2 some things. Then ecu 1 responds with a 40 mili delay between each character.
    Then the scanner has to echo each character. At 250 baud 1 character takes 40 mili.
    Reading codes and some data (8 items) takes up to 10 seconds.
    Example 2: Opel Iso-81 and Iso-82 protocols at 9600 baud.
    Scanner requests data and ecu repeatedly sends everything at full speed.
    Packets are 40 - 80 bytes, which gives you 20 - 10 packets every second.
    But hey, stock system means only 1 ecu responds, not maximum 8 as specified in OBD2 and EOBD (Sae J1979).

  10. #10
    VENDOR - ScanTool Vitaliy's Avatar
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    Quote Originally Posted by chunkyks View Post
    FWIW, I empirically checked the samplerate with my new OBDPros USB device, and was getting a little over 20 samples per second. My OBDKey Bluetooth would do between five and seven. I find that 20 samples per second is actually a pretty large number.

    Gary (-;
    On what protocol?
    OBDLink MX: world's smallest, fastest, most advanced OBD/Bluetooth adapter with SW and MS CAN support. Read the review to learn more.
    Need to look up a diagnostic trouble code? Try the most up-to-date, free DTCsearch.com!

    You cannot send me a private message using this forum. Use my email instead: vitaliy[@]scantool.net.

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