Ray Fluxgate Blues Help Needed

quaelgeist2

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OK - now I got it.

I noticed the limited read-out of NMEA data by the 4000+, but I do not yet have the fully linked system that you apparently install, e.g. no radar and no full PC integration. Sounds like you control the boat from the navtable.

My PC is merely acts as a (jazzed-up) chartplotter; for redundancy reasons I am not even driving my 4000+ from the PC, but always upload the waypoints to the GPS.

I do not trust the relatively new subnotebook to be tough enough until I have usedit for at least one full season in rough weather). Also power consumption is an issue for me, the GPS runs 24hrs on its back-up batteries, the PC just 3. That said, everything worked well so far.

chris

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tome

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Jfkal

The Seatalk bridge has peculiarities and I've observed some of them, especially in the circular mode you are using it. There is also a severe limitation with the volume of data that can be handled at the slow NMEA rate of 4800 baud. This will lead to data being dropped if there are more than about 6-8 sentences per second (sentences vary in length up to 80 characters, but 480 characters per sec is the max). The GPS output alone accounts for 430 characters per second (Garmin 128) hence using nearly all the NMEA bandwidth.

I've noticed that the Seatalk bridge doesn't ignore data already on the Seatalk bus, and have discovered a 'bug' where it adds 32,768 miles to the log reading in certain circumstances. I've fed this back to Raymarine but had no success in resolving it. As a matter of interest 32,768 is 2^15 and this is very significant to any programmer. I have facilities to analyse the Seatalk bus directly and can confirm that the problem is in the bridge. If anyone wants to see what it does to the NMEA data I can post some examples which demonstrate some very peculiar behaviour.

I would advise against using the bridge in any circular mode and believe that this is almost certainly the cause of your problem.

Regards
Tom

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MainlySteam

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Tom, Joerg

You have saved me having a look into it as I had promised Joerg, because I am sure you are on the right track. I have had no problems with the bridges but not used them in circular mode (and I declare my hands on experience with them is only with 2 applications).

I do know that the Seatalk course computers from a few years back (but still current models) only placed limited GPS information (GLL only from memory) onto their NMEA output port together with other nav data from the Seatalk bus. I did not investigate as I found that the Raymarine bridge did what I needed, so that was the simple fix - but on reflection it may likely be because of the bandwidth limitations.

I am no software engineer but I would have more than a gut feeling that taking sentences off the Seatalk bus and then returning them in a circular fashion is asking for trouble, even if the bridge documentation says (and I don't recall it doing so though) it will substitute for same data already on the bus, but I would stand to be corrected on that observation. From what I recall of the bridge's innards, there was not much to them.

I am not 100% clear on the configuration that Jfkl has now, but from what I can see a first try might be to MUX the NEMA from the remote NEMA instruments, put that onto the bridge's NMEA input port, Seatalk to the course computer, and the PC off the bridge's RS232 port. But probably does not resolve any bandwidth problem - go to it you are the electronics engineer, not me (although my first degree was in fact in electronic physics, that was many, many moons ago)!

John

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jfkal

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Hi Tom, thanks for the info. Would be great if you could send me some samples to my private email address joerg.kalisch@dexteritas.com .
I did some traces and the result supports your suggestion that the bridge itself may be the culprit here. I see only one set of heading data on the NMEA side and the values are increasing. So the bridge seems to re-interpret the loop backed data and is doing something funny with them. I do not have the circuitry which let me trace the Seatalk Bus however.


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jfkal

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Solved the problem. Disconnected the Seatalk feed to the MUX and used the NMEA out of the bridge instead. Works now. Seems to be that the MUX interprets the SEATALK data wrongly or the Bridge interprets the Seatalk to NMEA converted sentences wrongly. Traced the NMEA side and the HDM values creep up. Cannot drill deeper since I do not have the circuits to trace SEATALK. But my guess it is the MUX to blame here.

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