[Ovmsdev] OBDII ECU Documentation, and a request for help

Mark Webb-Johnson mark at webb-johnson.net
Wed Apr 4 11:51:55 HKT 2018

In all the cars I’ve looked at, signal and chassis ground are connected (and at multiple points in the car).

As we don’t have chassis ground to the OVMS module, it is not possible to pass it on to the HUD/dongle. Best we can do is signal ground (which should be connected to chassis ground upstream).

That’s my thinking, anyway.

Regards, Mark.

> On 4 Apr 2018, at 2:07 AM, Greg D. <gregd2350 at gmail.com> wrote:
> Hi Mark,
> That's the spec I am currently using.  But, I had one question that I have not heard back on yet, that of signal ground vs chassis ground.  I have them tied together on the OBDII connector end, but wonder if we should only connect signal ground, or if they should be separate wires between both ends, or just leave the chassis ground unconnected.  I've seen it done various ways in various OBDII hacking diagrams.  Is there even a chassis ground pin on the DA26 connector?
> Functionally it probably doesn't make a lot of difference, but for RFI/EMC purposes, it may be important.  I always assigned that task to someone who knew what they were doing :); I just knew it was something to be attended to.  Without over-engineering this, what is the right treatment for chassis ground?
> 15cm cable length should be ok, as HUDs should have long enough cables to reach under the dash, but my first reaction was that it seemed a bit short.  I guess it depends on where and how the OVMSv3 module gets mounted.  The cable should be long enough to be able to to be tied back to the unit and/or its mounting situation.  Otherwise, an OBDII dongle would be forced to just hang there, potentially over or under one's feet.  Maybe 25cm would be better?
> Greg
> Mark Webb-Johnson wrote:
>> Regarding the HUD cable, I have asked the suppliers for this, according to the following specification:
>> OVMS 3 HUD expansion cable:
>> Pinout: “HUD"
>> J1962-F   DA26-M  Signal
>> 4         8       Chassis/Power GND
>> 5         8       Chassis/Power GND
>> 6         16      CAN-H (primary CAN)
>> 14        6       CAN-L (primary CAN)
>> 16        18      +12V Vehicle Power
>> Notes:
>> Require 120ohm resistor between DA26 pins 16 and 6 (this can be either at J1962 or DA26 ends).
>> J1962 pins 4 and 5 are both connected to single DA26 pin 8.
>> J1962 end should be FEMALE plug with straight cable. DA26 end should be MALE.
>> Both ends should be enclosed in shells, and cable should be 15cm in length.
>> I think that is correct. If there are any changes, please let me know.
>> Regards, Mark.
>>> On 28 Mar 2018, at 7:45 AM, Greg D. <gregd2350 at gmail.com <mailto:gregd2350 at gmail.com>> wrote:
>>> Hi Steve,
>>> Ha, didn't know that.  Thought they were all DB's...  Thanks, corrected.
>>> Greg
>>> Stephen Casner wrote:
>>>> Greg,
>>>> A nit: The connector is more correctly identified as DA-26, see
>>>> https://en.wikipedia.org/wiki/D-subminiature <https://en.wikipedia.org/wiki/D-subminiature>
>>>>                                                        -- Steve
>>>> On Tue, 27 Mar 2018, Greg D. wrote:
>>>>> Hi folks
>>>>> In a fit of prose, if not Shakespeare, I took a pass at creating the
>>>>> documentation chapter for the OBD2ECU task.  Comments, corrections, and
>>>>> general feedback is requested.  Is there anything significant missing?
>>>>> Mark, if this looks acceptable, can it be posted to the Google Doc, or
>>>>> send me an invite such that I can do so?
>>>>> Note that I don't have any reference material on the OBDII-to-DB26
>>>>> cable, other than the wiring.  If we eventually have a purchase source,
>>>>> that should get referenced, or lacking that, perhaps pointers to where
>>>>> the connectors can be purchased?  Placeholders are noted, but only
>>>>> that.  For the EEs in the group, a question:  I have both signal and
>>>>> chassis grounds connected together on the OBDII connector; should they
>>>>> be, or just signal ground?
>>>>> I think it would also be helpful to include a table of what metrics are
>>>>> available (supported) on what vehicles, as an aid to those who want to
>>>>> remap things for monitoring on an OBDII HUD or Dongle.  A glance at the
>>>>> available documentation didn't turn up anything, so I took a first pass
>>>>> at the table.  See the attached, which includes what I know of the
>>>>> Roadster's implementation (since I have one and have studied the code a
>>>>> bit).  I would rather not try to guess at the other cars.  Could the
>>>>> various vehicle authors contribute their columns to the table?  Send me
>>>>> an updated spreadsheet, and I will do the merge.
>>>>> Thanks!
>>>>> Greg
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