Re: ✘n2k and issue #176

[Reinhard Arlt]

Hello,

i have learned quite a bit, since i have written the n2K code for gpsd 
many years ago.

And feel, that i was (at least a bit) pushed out of this group, as "my" 
data structures for attitude are removed from gpsd.

First a few comments:

1.) A solution to respect only n2k gps and ais messages is absolutely 
the wrong way. On a boat, you want to see as much info from the nmea2000 
bus as possible, and convert them into JSON and nmea0183 messages.

2.) The source address a of a n2k device is static. It is only allowed 
to change the own address, if you lose an address claim, or your device 
gets a new address during an ISO address assignment.

On a proper working n2k bus, you can only lose the address claim, if a 
new device is added. As the address claim is broken by design (it 
violate a basic can rule), a restart of the bus and all devices is 
always a good idea, as you never know, if an other device is off bus 
afterwards!

I have no experience with the ISO bus, but i assume, that changing the 
devices on the bus will happen here more frequently.

The central problem comes form the nmea. A "normal" device on a n2k bus 
can always get the info of the devices on the bus with a dummy address 
claim. But you need a vendor id and a device id for that purpose, and 
the nmea charge you quite a bit for it. And even more worse, you will 
lose the device id, if your device is not certified after a few month.

My suggestion is: Tracking of the (j1939) address claim is not a big 
thing. And adding a timeout for devices, that do not send messages any 
longer also not.

I can write this code, but i still think, that it is not needed.

I have never seen an iso address assignment on a boat in the wild, even 
it is mentioned in the n2k standard as a possibility.


Kind regards

Reinhard

On 12/13/21 8:20 PM, Gary E. Miller wrote:
> Yo All!
>
> Any NMEA2000 folks around?  There is a new issue with the NMEA2000
> driver, and I have no idea how that driver works.
>
> The issue is:
>
>      https://gitlab.com/gpsd/gpsd/-/issues/176
>
> If you care about the NMEA2000 driver, please jump in to #176.
>
> RGDS
> GARY
> ---------------------------------------------------------------------------
> Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
>         gem@rellim.com  Tel:+1 541 382 8588
>
>             Veritas liberabit vos. -- Quid est veritas?
>      "If you can't measure it, you can't improve it." - Lord Kelvin