2017-06-27, 08:49 PM
There has been a little discussion on NMEA checksums over the past few months, but I don't think this one has been discussed in detail:
I have an old AutoHelm ST-60 with an NMEA output. It puts out DBT and HDM strings. I have trouble integrating it with other stuff. Depth randomly shows up and I don't think HDM ever does. I can actually see the strings coming in to OpenPlotter, but they don't seem to get used anywhere.
I think this may be due to the lack of checksums, and in the case of DBT strings, the presence of ONLY feet measurement. My thought was that I could create a couple of named pipes in the filesystem, pass the strings into one, manipulate them to add checksum and the other depth measurements, then pass the result to another named pipe that goes into KPLEX/OpenPlotter.
I've written the python code to do the string manipulation and it works great on a stored file of NMEA data that I received from the ST-60. It will happily take the strings, add the appropriate depth measurements to the DBT strings, then output both the DBT and HDM strings with checksums. Where I get stuck is how to get this data from KPLEX, and how to pass the data back to KPEX. I can't seem to get the named pipes to work as expected. The only other way I can think to do it is to perhaps create a small ramdisk and read from a file there, and write to another file, occasionally flushing the data stored there so as not to overrun the disk.
Have I taken myself down a rabbit hole here? Is there some other magic that I'm missing in Node-RED or KPLEX configuration that will do what I need to do? Am I wrong about the need for checksums and the other depth metrics in the strings I'm getting?
Your thoughts would be most appreciated.
I have an old AutoHelm ST-60 with an NMEA output. It puts out DBT and HDM strings. I have trouble integrating it with other stuff. Depth randomly shows up and I don't think HDM ever does. I can actually see the strings coming in to OpenPlotter, but they don't seem to get used anywhere.
I think this may be due to the lack of checksums, and in the case of DBT strings, the presence of ONLY feet measurement. My thought was that I could create a couple of named pipes in the filesystem, pass the strings into one, manipulate them to add checksum and the other depth measurements, then pass the result to another named pipe that goes into KPLEX/OpenPlotter.
I've written the python code to do the string manipulation and it works great on a stored file of NMEA data that I received from the ST-60. It will happily take the strings, add the appropriate depth measurements to the DBT strings, then output both the DBT and HDM strings with checksums. Where I get stuck is how to get this data from KPLEX, and how to pass the data back to KPEX. I can't seem to get the named pipes to work as expected. The only other way I can think to do it is to perhaps create a small ramdisk and read from a file there, and write to another file, occasionally flushing the data stored there so as not to overrun the disk.
Have I taken myself down a rabbit hole here? Is there some other magic that I'm missing in Node-RED or KPLEX configuration that will do what I need to do? Am I wrong about the need for checksums and the other depth metrics in the strings I'm getting?
Your thoughts would be most appreciated.