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Thread: Low Speed GMLAN Interface

  1. #11
    VENDOR - ScanTool Vitaliy's Avatar
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    Quote Originally Posted by Hebe View Post
    I had considered doing something like what Vitality suggests and had the following problems with the concept:

    * A review of the ELM product manuals from various manufacturers did not reveal a setting for 33.33K. The settings I found seemed to exclude this speed. Are you definitively saying this speed is available on an ELM product? I had earlier in my quest called ELM and they said it did not support what I was looking at doing. Or perhaps I worded my question to them incorrectly.
    This sounds like Jim Nagy. He tends to say "no" to anything that doesn't fall within his comfort zone. I suppose it makes sense -- who needs extra support headache...

    Anyway, look at programmable parameters, specifically PP 2C and 2D.


    * Since I have been looking into this I have now generated a desire to have a good working unit for general OBDII use after this project and do not want to bugger up a perfectly good unit. I don't know about you, but my soldering skills to repeatedly solder a chip in and then back out seem to destroy pads.
    We have a reflow workstation in our lab, but I agree that if you replace the chip more than once the pads may separate from the PCB.


    * I am willing to spend a bit more for both capabilities in one product. So I am still looking around.

    I notice you limited your suggestion to the ElmScan 5 and did not mention your newer product, whose name escapes me. Would it be capable of this as well?
    OBDLink currently doesn't support setting protocols (ATSP) above 8. We plan to implement this functionality in the coming weeks.


    I thought that OBDPRO used the ELM 327 command set. That doesn't offer a way to set the 'high voltage' state (SW CAN has 3 states). You can't get to some functionality without that, so perhaps they added a superset command or something.
    Good point, I forgot that you need to have a way to set the mode bits.

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

  2. #12
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    Quote Originally Posted by jfitzpat View Post
    I thought that OBDPRO used the ELM 327 command set. That doesn't offer a way to set the 'high voltage' state (SW CAN has 3 states). You can't get to some functionality without that, so perhaps they added a superset command or something.
    -jjf
    My understanding is the 12v over voltage command is only to send "Wake-up" commands to the remaining nodes. Is there some other function the 12V does I am not aware of?

    Quote Originally Posted by Vitaliy View Post
    Anyway, look at programmable parameters, specifically PP 2C and 2D.Vitaliy
    I missed that on my first run through of the ELM 327 data sheet. Still don't fully understand it, but that is OK for now.

  3. #13
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    Quote Originally Posted by Hebe View Post
    My understanding is the 12v over voltage command is only to send "Wake-up" commands to the remaining nodes. Is there some other function the 12V does I am not aware of?
    My recollection is that the high voltage message is also involved in accessing the high-speed/low-speed GMLAN bridge. If you are a radio and want to auto adjust volume based on RPM, turn lights off on key ignition, that sort of thing.

    I didn't have anything specific in mind, I was just thinking in terms of completeness. If you are going to put in the right transceiver, it seems like it would make sense to give access to the state.

    In terms of bit rate, they call it 33.33, but everything I've seen suggests that it is 500K / 15, or 33.333333... So, if you can set the rate divisor you are probably OK. The only gotcha would be that most CAN controllers don't just take a rate, like a UART. You also have to program a bunch of segments and propagation delay stuff. I'm not sure if this is inherited from Bosch, whose controller is spectacularly complicated, or what (see Silicon Labs chips and the Bosch companion doc, or the simpler Microchip CAN Module doc to see what I mean). So, I could envision firmware not handling a previously untested super low rate correctly, but you'd have to test it to see.

    -jjf

  4. #14
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    Quote Originally Posted by jfitzpat View Post
    My recollection is that the high voltage message is also involved in accessing the high-speed/low-speed GMLAN bridge. If you are a radio and want to auto adjust volume based on RPM, turn lights off on key ignition, that sort of thing.
    In my case the Body Control Module(BCM) acts as the GMLAN bridge and I believe is just another node. In this case the BCM has a node from the low speed CAN and the High speed CAN. One piece of information that passes thru the BCM is vehicle speed, just as you mention for the radio.


    Quote Originally Posted by jfitzpat View Post
    I didn't have anything specific in mind, I was just thinking in terms of completeness. If you are going to put in the right transceiver, it seems like it would make sense to give access to the state.
    I think that would be great for any vendor to supply. Hint!!!

    Quote Originally Posted by jfitzpat View Post
    In terms of bit rate, they call it 33.33, but everything I've seen suggests that it is 500K / 15, or 33.333333... So, if you can set the rate divisor you are probably OK. The only gotcha would be that most CAN controllers don't just take a rate, like a UART. You also have to program a bunch of segments and propagation delay stuff. I'm not sure if this is inherited from Bosch, whose controller is spectacularly complicated, or what (see Silicon Labs chips and the Bosch companion doc, or the simpler Microchip CAN Module doc to see what I mean). So, I could envision firmware not handling a previously untested super low rate correctly, but you'd have to test it to see.

    -jjf
    Absolutely correct, according to my recent readings.

    My understanding, Bosch being the inventor of CAN, licenses all CAN hardware and the conditions of the license is that all products be compatible for both bus lengths and different manufacturers hardware. Every time you buy a CAN chip a royalty has already been paid to Bosch.

  5. #15
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    Quote Originally Posted by Hebe View Post
    In my case the Body Control Module(BCM) acts as the GMLAN bridge and I believe is just another node. In this case the BCM has a node from the low speed CAN and the High speed CAN. One piece of information that passes thru the BCM is vehicle speed, just as you mention for the radio.
    Now that you describe it, I don't know if the presentation I watched was giving an actual (deployed) or future/hypothetical. The gist was a scheme where dealer shifted acc's could alter what flowed through bridging automatically.

    Quote Originally Posted by Hebe View Post
    I think that would be great for any vendor to supply. Hint!!!
    We have a board we use for OEM stuff that is essentially what you seem to want. It isn't really a good fit for the high volume consumer channel, but I suppose it could conceivably be an OEM product. We have a couple of other board/module OEM items now.

    My hesitation is rather or not the firmware is at the right level for these sorts of applications. Right now it is basically SW and DW CAN passthru with basic filtering and rate control. It has no higher level protocol knowledge of GMLAN, in fact we use it on several, very different, systems. Making the host application wholly responsible for something like GMLAN is pretty demanding. Without researching it, I wouldn't be sure if certain timing requirements are hard to meet.

    Probably not with SW, it's pretty slow and very forgiving. But I'll have to think about it before I consider mentioning it to the powers that be.


    Quote Originally Posted by Hebe View Post
    Absolutely correct, according to my recent readings.
    Well, the last time I counted it has been my mis... uh, good fortune, to have worked with 7 different CAN modules, all with their own quirks and umpteen registers (to be fair, some are a lot more aggravating than others).

    -jjf

  6. #16
    E85
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    Quote Originally Posted by Hebe View Post
    My understanding is the 12v over voltage command is only to send "Wake-up" commands to the remaining nodes. Is there some other function the 12V does I am not aware of?



    I missed that on my first run through of the ELM 327 data sheet. Still don't fully understand it, but that is OK for now.
    Wake up is used when the car is not running, but one device needs to broadcast some data. i.e. on accessory or when a radio is turned on when ign is off.

    While the car is running the SW/LS CAN is fully awake and running with the speed for example as you mentioned coming across from the HS CAN, as well as most/all the other instrument cluster/DIC data.......rpm, coolant temp, fuel level, fuel consumption data, ABS status, trans gear, diagnostic status etc etc.

    If the ign is on and radio etc on, then probably the wake up state is not absolutely necessary. It is implemented as a seperate line on the driver chip.

  7. #17
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    Quote Originally Posted by E85 View Post
    If the ign is on and radio etc on, then probably the wake up state is not absolutely necessary. It is implemented as a seperate line on the driver chip.
    I played around with this a little last night in my daughter's car and it seems to me (from limited empirical testing) that it isn't as simple as just engine running/not. Right after I opened the door, some nodes were accessible without the wake up state. Later, even after the vehicle was running, the same modules were not.

    Of course, that could be conditional safety logic in the nodes themselves, but either way it surprised me. I've worked on the high speed side quite a bit, so I expected the low speed bus to be really simple and forgiving.

    I'm swamped at work now, but it's something I'd like to look more closely at when I can find the time.

    -jjf

  8. #18
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    Quote Originally Posted by jfitzpat View Post
    I played around with this a little last night in my daughter's car and it seems to me (from limited empirical testing) that it isn't as simple as just engine running/not. Right after I opened the door, some nodes were accessible without the wake up state. Later, even after the vehicle was running, the same modules were not.

    Of course, that could be conditional safety logic in the nodes themselves, but either way it surprised me. I've worked on the high speed side quite a bit, so I expected the low speed bus to be really simple and forgiving.

    I'm swamped at work now, but it's something I'd like to look more closely at when I can find the time.

    -jjf
    mmm...that is interesting. GM car obviously. Which model/year?

  9. #19
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    The modules that are on my Chevy HHR’s SW GMLAN are:
    Inflatable Restraint
    Instrument Panel
    Theft Deterrent
    Radio
    OnStar
    XM Radio
    Remote Control Door Lock & Tire Pressure
    and of course the Body Control Module.

    I can see Theft Deterrent turning off, but the others I would think should be visible at all times.

  10. #20
    E85
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    Quote Originally Posted by Hebe View Post
    The modules that are on my Chevy HHR’s SW GMLAN are:
    Inflatable Restraint
    Instrument Panel
    Theft Deterrent
    Radio
    OnStar
    XM Radio
    Remote Control Door Lock & Tire Pressure
    and of course the Body Control Module.

    I can see Theft Deterrent turning off, but the others I would think should be visible at all times.
    On thinking a little about this, maybe the modules are let return to sleep after x time to reduce CAN chatter and bandwidth use unless really needed.

    On the HS CAN all the real time data channels use around 400kbs of 500kbs available (? - 80% util), so at 33.3kbs it may get a little tight if it is all happening at once.

    A G8 for example on the LS has ~14 modules depending on options fitted:

    HVAC; Infotainment display; Bluetooth; Theft deterrent; instrument panel; Radio; Rear seat entertainment; Telematics; GPS; Seats with memory; external object detection/parking sensors; tire pressure monitoring; occupant protection and of course the BCM in the centre of it all.

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