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Old 15-05-2024, 18:55   #1
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RMB Exceeds NMEA Legal Sentence Length

When a route has been activated in my OpenCPN (ver 5.8.4) it is generating the following sentence:

$ECRMB,A,3.492,L,Chaffe,Palmer,4121.651,S,17448.84 0,E,4.534,163.370,-0.025,V,A*49

This is 81 characters and is being rejected by my NMEA4WIFI multiplexer. (The NMEA standard states a maximum sentence length of 80 characters including the initial "!" or "$" but excluding terminating line delimiters). I suspect this bug was introduced in ver. 4.99.0 when the FAA Mode field was added to ensure compatibility with NMEA Ver 2.3.

Would it be possible to enable the RMB Latitude and Longitude precision to be edited in the Connections Options - as is currently possible with the APB bearing precision? Setting precision to 2 decimal places would solve the problem.



Note: the "NMEA Revealed" RMB example shows latitude and longitude fields with 2 decimal places:



Thanking you in anticipation,

Geoff
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Old 15-05-2024, 23:54   #2
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Re: RMB Exceeds NMEA Legal Sentence Length

It would certainly be possible to generate a variation of the RMB sentence using JavaScript. I have a script running now that sends its own generated RMB sentence.

Wait and see if others offer an off-the-shelf solution (NMEAconverter?) and, if not, follow me up.
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Old 15-05-2024, 23:59   #3
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Re: RMB Exceeds NMEA Legal Sentence Length

Thoughts:
We could restrict lat/lon precision to two decimals but I've a bad feeling about that. Some may need as good accuracy as possible?

Now we truncate WPs name to max six characters. What about instead decrease that to five for (EC)RMB to AP?
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Old 16-05-2024, 02:16   #4
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Re: RMB Exceeds NMEA Legal Sentence Length

Quote:
Originally Posted by Hakan View Post
Thoughts:
We could restrict lat/lon precision to two decimals but I've a bad feeling about that. Some may need as good accuracy as possible?

Now we truncate WPs name to max six characters. What about instead decrease that to five for (EC)RMB to AP?
I agree not to reduce precision to below 3 decimal places.
Better use the precision set in APB.

The majority of other devices accept Sentences much longer than 80 chars despite the formal limit. I think it a shame to curtail them.

In my older script for syncing to iNavX I have to do work to fix up the truncation done already. So I replace the truncated names with my proper rather long names.

I vote that any truncation be a user option so that users with more forgiving devices are not clobbered.
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Old 16-05-2024, 02:26   #5
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Re: RMB Exceeds NMEA Legal Sentence Length

Quote:
Originally Posted by Antipole View Post
users with more forgiving devices are not clobbered.
It's not about that. The standard says 80 characters, as gchead says, so we should not exceed that.
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Old 16-05-2024, 02:41   #6
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Re: RMB Exceeds NMEA Legal Sentence Length

Quote:
Originally Posted by Hakan View Post
It's not about that. The standard says 80 characters, as gchead says, so we should not exceed that.
Then an awful lot of users are going to be clobbered. At minimum make enforcement an option.
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Old 16-05-2024, 03:01   #7
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Re: RMB Exceeds NMEA Legal Sentence Length

I agree.

Setting the number of decimal places should ideally be a user option.

As it currently is with APB bearing precision in the Connections Options window:

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Old 16-05-2024, 23:46   #8
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Re: RMB Exceeds NMEA Legal Sentence Length

gchead..
I think we can solve this by additionally reducing WPs name truncate. If we see a RMB sentence sized >80 we can decrease truncating one step until sentence size is <=80.

A temporary test for you now would be to in the config file in [Settings] add a property "MaxWaypointNameLength=5". Default value is 6.
See an example below.


Code:
[Settings]
------
NMEAAPBPrecision=4
TalkerIdText=EC
MaxWaypointNameLength=5
ShowTrackPointTime=1
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Old 17-05-2024, 02:21   #9
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Re: RMB Exceeds NMEA Legal Sentence Length

Thanks Hakan,
I'll try it out on the boat in the morning.

BTW - Just noticed this:
https://opencpn.org/flyspray/index.p...s&task_id=2812
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Old 17-05-2024, 06:10   #10
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Re: RMB Exceeds NMEA Legal Sentence Length

If the temporary config file solution works for you I think the flyspray note is considered solved and could be closed?
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Old 17-05-2024, 20:05   #11
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Re: RMB Exceeds NMEA Legal Sentence Length

After editing the ini as suggested by Hakan, the waypoint names are now truncated to 5 characters and the RMB sentence length now is back within the 80 character legal limit:

$ECRMB,A,2.427,L,Falco,Steep,4119.468,S,17450.778, E,3.498,128.064,-0.146,V,A*5D

RMBs are now all passing through the multiplexer without problems. Great!

If there is any future possibility of users being able to set the number of decimal places of the lat/lon on the generated RMB sentences that would still be nice.
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Old 28-05-2024, 12:11   #12
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Re: RMB Exceeds NMEA Legal Sentence Length

gchead..
Now is the change to prevent (EC)RMB to AP from being more than 82 characters incorporated in git master for the next OCPN v 5.10.
When it's released and you have updated you can take out the "MaxWaypointNameLength" adaption.

Btw: The NMEA0183 standard says:
"Maximum sentence length, including the $ and <CR><LF> is 82 bytes."
But I assume your multiplexer talks about a sentence without CR LF?
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