do we have an NMEA data expert on the forum?

[Birdseye]

As I asked, is there an NMEA data expert on the forum. I have a decent laymans understanding but thats short of what is needed.

How I ended up with this set up is a long boring story and I'll freely admit that it was a mistake. But its where I am not where I would want to be.

My boat has Raymarine ST60 ( analogue) instruments. I have added a modern B&G Zeus plotter and to get the two to talk there is a Brookhouse multiplexer converting Seatalk to NMEA 0183. I have an Easy AIS receiver inputting 0183 at 38400 and that means the B&G has to output any data at the same speed. Since all this is below deck I have a NASA repeater above deck to feed me the track / CMG etc. The GPS data for the system comes from the B&G internal GPS

Problem is that the repeater works OK on a calm day but the data jumps about all over the place if its bouncy - like 60 degree variation in the course made good.. Presumably this happens because the B&G GPS data is unsmoothed - with their system I believe that the data smoothing happens at their display units not at source like Raymarine. Certainly altering the data smoothing in the settings doesnt improve the problem though it does smooth out the data displayed on the B&G plotter. I know that the NASA is only set up for slow data but it does work at high data speeds when the sea is flat so I cant see it being a data speed issue. If it was that then the data would change less often but still randomly jump about.

Before I sold it along with my old Raymarine plotter, I had a Raymarine raystar 125 gps in circuit and the NASA display was stable.

Seems to me that there are two possible ways of sorting the issue. One is to add in an external GPS working at normal speed and feeding the repeater alone but would this have data smoothing? The other is to change the AIS to an NMEA 2000 unit and drop the baud speed for the NMEA 0183, but I am not sure about this since the GPS datya from the B&G could still be unsmoothed.

As you might imagine, the various tech help desks all blame each other before even giving the issue some serious technical thought.

Does any of this make technical sense to you?

 

[GHA]

. I know that the NASA is only set up for slow data but it does work at high data speeds when the sea is flat so I cant see it being a data speed issue.

Might be. Have you tried living without AIS for a moment and setting the B&G to output at 4800, which the NASA expects to see to check if the problem goes away?

Writing & reading at different rates can do weird things.

 

[Birdseye]

Having asked by pm if you could look at my problem I find that you already have !

All the kit is off the boat which is on the hard a distance away so I cant set up a trial right now. The problem comes up when the boat is being bounced around so it must be getting a GPS signal that reflects short term boat movements. Cant see it being just data transmission speed since smoothed dat being transmitted at 38400 would be the same info as transmitted at 4800. Problem is, Raymarine tech help tell me this morning that their data smoothing is also done at the display so the input from the Raymarine 125 external GPS must have been unsmoothed too.

Plus even when I had the Raymarine GPS in the circuit I still had the NMEA 0183 running at 38400. I think that the Raymarine GPS was wired to the repeater and the multiplexer in parallel so the repeater was getting 0183 data on speed and track at 9600 and bearing and wpt distance at 38400.

 

[Kerenza]

Try recording the data, in all circumstances, with navmon PC or equivalent, set not to ignore bad messages. This is probably the best way to identify if its erroneous, corrupt or just unsmoothed data. Look at the sentences in a text editor, they should be very similar in frequency but obviously not content. Play back from navmon into a software chartplotter and observe. Nothing better to do over Christmas?

Not conclusive, but try plotter in demo mode while sailing. If it outputs data (not all do) then in demo mode might help identify where the error is.

My money would be on the data conversion rates. But then I've never been good at gambling.

 

[GHA]

Having asked by pm if you could look at my problem I find that you already have !

So much more fun than the jobs list...

Just so happened I've a nasa clipper gps repeater sitting about waiting to get installed, so with a chart table covered in wires....

And a failed attempt at getting gps data out of a Rasp Pi and having the arduino repeat it into a separate ttl/rs232 adapter to the Nasa, gave up with that so the data goes like this, rasp Pi->usb->arduino->arduino RX0 pin->RS232 adaptor TX pin->Nasa repeater.
Convoluted route but end result should be the Nasa is seeing NMEA straight from the Pi running open plotter.

So after all that, I can't get anything out of the Nasa unless the output baud rate is set in OpenPlotter as 4800

Sorry, can't be of any more help, all sounds a bit odd.

 

[Birdseye]

It does, doesnt it. I understand your reasoning behind suggesting that I record the data flow, but the problem is not only that I cant do that until launch in April but even then I cant do it unless the boat is being thrown about. And when it is being thrown about, I'm not very interested in recording data as you can imagine.

The Zeus plotter is definitely outputting at 38400 and the NASA repeater is definitely showing plotter data getting input at that speed. So id the VHF radio for that matter.. So I dont understand your data speed issue but you are saying what NASA said to me, that the repeater wont work at 38400.

Is it possible to have a mixed data flow at both 4800 and 38400? I'm wondering if the multiplexer leaks 4800 input data through onto the 38400 output. Presumably the sentence structure is the same at both speeds. Is the sentence length time also the same ie the higher frequency achieved by shortening the time between sentences as opposed to making the sentences themselves faster

 

[MikeCC]

Is this a rerun of the thread in September? I suggested trying without AIS and setting everything to 4800 back then but didn't hear the results.
Have you confirmed what speed the multiplexer output ports are running at?
Don't think it's been mentioned but have you been getting reliable AIS on the plotter?
It would still be helpful to know the exact multiplexer model. Is it the standard or AIS enhanced version?
I may have an answer but it will depend on that.

Also, is there a DSC VHF linked in and does that show correct position info?

Technically you could program a device to send data at different speeds from sentence to sentence but the receiving device wouldn't know what to do with it. The port speed is about setting the timing so each device knows where each bit of data should be.

 

[neilf39]

A simple wiring diagram would help especially how everything goes to the multiplexer. Also what actual model is it? The NASA unit does not work at the high baud rate so the multiplexer is doing rate conversion. Does not answer about your problem though.