NMEA multiplexer

wolff

Dear Stampers,
I want to make a NMEA multiplexer out of some basic stamps I have. (BS2 and BS2SX)
I want the device to receive a NMEA sentence from one (of say 4 different ports) and send it out a common port. I realize the biggest limitation is the RAM size. Most NMEA sentences are rather long.
Do you think this is possible? If so could you recommend a strategy? I’m not asking you to design it, just a sentence or two to give me your take on how to skin this cat.
Thank you!
J

Dave Paton

Is this for the NEMA GPS sentences?

Folks have parsed the NEMA-0183 data from GPS receivers with various stamps before, since it's only 4800 baud. The trick would be doing it fast enough that you could receive the 4 continuous streams on 4 seperate inputs. An SX would probably be better (not a BS2sx) due to it's amazingly fast execution speed. The downside is that I think you'd need to program it in something other than basic (assembly, C, etc), as SX/B is somewhat limited compared to PBASIC.

-dave

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Bean

What do you want to happen, if two (or more) send at the same time ?

Bean.

achilles03

Most GPS units I've seen specs for can output multiple NMEA sentences per cycle (usually 1 second)... meaning you only need 1 port and no multiplexing.

Do you actually have 4 separate GPS units that function differently or are located in different locations?

wolff

I want to take GPS, engine data (oil pressure and temp), fuel and water tank data, wind and boat speed and direction data and run them to a common data bus. All data is NMEA format and if a few sentences are missed it’s not a problem.

achilles03

And this data is coming from a GPS unit, the boat's internal computer... and what else?· It really depends on the specs of everything you're interfacing.· Some devices you might be able to send a "request" for information, while others will just output 1/sec.· I'd look for a sequence that would minimize time required to get data.· Worst case scenario, it would take 4 seconds to get data atleast 1 NMEA sentence from all units.