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Thread: Group UDS wonders

  1. #1
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    Group UDS wonders

    Hello,

    I'm using a HEX-NET 0.4436.4 with VCDS 18.9.1. The car is a 2009 A4 B8 (aka 8K) with 2.0T CAEB motor. So UDS protocol for the engine.

    With my HEX-CAN+USB, I could do Group UDS, but was limited to 8 values. Without grouping, I could do 12, but the values for a single cycle were too separated in time. HEX-NET would let me use the Split UDS function, so I got that.

    But I see I remain constrained to 12 selections overall. Is that a limitation of the Audi system, or a legacy limitation of the Ross-Tech implementation? Is there a chance this might be lifted at some point? Since Group UDS can do 8 values per shot, I was hoping the split functionality was going to let me split the selections into two groups of 8.

    Regarding the Split options, "by 7, 8, 9, 10, 11". If I go 12 selections, then I assume I would split 6+6, or evenly. That's not even an option. Regardless that I can run through each option and see what it does (it appears to be x selections split by y, the first sample group is x-y entries and the second sample group is y entries), what am I missing regarding why those particular options and wording was used?

    When using Group UDS, we get results that are same time stamped. What's the feeling about how synchronized those values really are? Is the ECM still processing them in a sequential manner? So they're closer than otherwise, but will never be perfectly aligned?

    Thanks,
    Scott

  2. #2
    Benevolent Dictator Uwe's Avatar
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    Twelve values is indeed a bit of a "legacy" limit dating back to the days of three Measuring Block Groups, each consisting of four items.

    The reason the different split options exist is because some control modules will not allow more than 7 items in a request and some will allow more.

    When you group UDS requests (and the number of items in that group is below the split value), VCDS sends a single request for all the items in that group and the ECU replies with a single answer. The time at which that answer is received is the basis for the time stamps in the log. We are not privy to how ECU sequences or processes that request internally, so we are unable to say exactly how synchronized they are.

    -Uwe-
    The engineering problems are likely insurmountable. It would be like proposing to land a rocket booster section on a barge floating in the middle of the ocean.

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  4. #3
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    Thanks for the insight. Engine is the only UDS module in my car, so the 8 per group limit is all that I've known. So I would pick "by 8" under that idea. But we can't go past 12 parameters, so all the by X seems unnecessary. It's either going to be the module can handle 12 at once or you break it into two groups, as every module can handle 6+6 (assumption).

    But yeah, it would be really nice if we could bump VCDS to 16 or 20 selectable parameters. By time I pick engine speed, vehicle speed, normed engine load, gear, charge air spec, charge air actual, charge air control spec, intake temp, and term 30, there's not much room left for picking things to try and correlate in my troubleshooting.

    Thanks for your time, and for the awesome product. I've been trying to help my uncle use a Foxwell with his Mercedes, ah, the pain.

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