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Thread: J1708 obdii

  1. #11
    Constant Bitrate
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    Quote Originally Posted by Vitaliy View Post
    The ISO driver uses a 2N3904 which can sink up to 200 mA.
    FWIW, the problem is that you are sinking, but not sourcing (in ISO protocols you were sourcing through the 500-600 ohm resistor to 12V).

    For the hack to work you have to count on a fairly narrow differential threshold on the receivers, biased termination, and sourcing to come from the termination circuits on the nodes. In other words, instead of sourcing the 485 bus, you are 'sinking' the termination.

    Now that I think about it, you are at least quasi open collector, though you can't meet the settle time thing in the spec because you don't have a way to drive, only sink during actual transmission.

    If the setup is simple and everything else uses a simple RS-485 driver, like an SN75176, I could see the kludge often working, albeit with dramatically lower line reliability. However, in quite a few setups I've seen equipment using 'Fail Safe' RS-485 drivers. These generally would detect the non-driven line as an error. Even if you tried to get fancy, and used the transistor from the L line to pull the other line to GND when transmitting, they would generally sense that as a short. You could probably hack to external J1850 circuit to provide differential IO, but at that point, why not just use a tranceiver in an 8 pin DIP...

    Quote Originally Posted by Vitaliy View Post
    If there was enough demand, we could throw together a J1708 interface that used a "real" RS-485 transceiver. But AFAIK that market is already oversaturated, and is disappearing as older vehicles are being retired.
    Yeah, we have a board we use in OEM stuff that is both CAN and RS-485, but the demand always seemed to small to us to warrant a commercial product as well.

    -jjf

  2. #12
    Newbie funnynypd's Avatar
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    SAE J1708 is different than J1939. And there are also J1708-bus arbitration which is kind of similar to J1939, multi-masters on the same network, but it is made in software, the J1939 using CAN hardware arbitration. There are some overhead on software to fully implementing the SAE J1708 protocols.

    We currently have both SAE J1708 simulator and SAE J1708 interpreter released.
    An Au J1708 simulator:

    The following is a typical J1708 topology:

  3. #13
    Newbie funnynypd's Avatar
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    J1708 uses the RS485 transceiver, but the hardware is slightly different, please search the following reference for "What's so special about J1708/J1587". (A web link here may violate the forum rule)

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