CP180 chart plotter not sending DTM?

[nigelmercier]

I've just bought a Standard Horizon CP180 chart plotter. I'm testing it with my PC and charting software: Navmaster.

It seems that the CP180 does not send the "DTM" NMEA sentence, this would specify the Chart Datum (e.g. WGS84). Navmaster throws up an error, and I can't turn it off.

Has anyone found a way round this problem, perhaps by a virtual COM port?

 

[wotayottie]

If the plotter is not sending that particular sentence, then nothing you do with ports will solve the problem.
But to be honest I dont understand why the PC plotter would want it anyway. I checked with the USB gps I use with my plotter and that doesnt send such a sentence. Neither dos the garmin that I have used for the same purpose.

As I understood it , all GPS work to WGS 84 or alternatively give you a choice - so your plotter should default to wgs 84. But I suppose it depends on the design so maybe the best thing is to talk to the plotter maker.

If all else fails, a Chinese usb gps is pretty cheap off ebay.

 

[nigelmercier]

[ QUOTE ]
If the plotter is not sending that particular sentence, then nothing you do with ports will solve the problem.

[/ QUOTE ]

Ah, but there is!

You can munge the data coming from the ports, but it requires a bit of messing about with the Windows Kernel-Mode Driver Framework (KMDF).

I'm sure I'm not the first with the problem, so I asked in case someone had the solution.

[Thought I'd replied to this once ...]

 

[wotayottie]

[ QUOTE ]


You can munge the data coming from the ports, but it requires a bit of messing about with the Windows Kernel-Mode Driver Framework (KMDF).



[/ QUOTE ]

Eh? I like a good munge. /forums/images/graemlins/grin.gif

Explain please . I'm about to learn something (I hope)

 

[nigelmercier]

Mung (v): Mash Until No Good - mess about with data. Also used to create strong passwords.

At one level, a COM port is handled as a "stream" of data, which is processed according to the stream handlers. For example, there will be a "line reader" handler which accepts a line of data up to an end of line marker delimiter, such as <CR><LF>. The EOL will be stripped off, and the data passed to the next higher level.

These handlers can be intercepted, and custom processing can be done. Such as replacing an unwanted NMEA sentence:

$GPGLL,5120.5091,N,00312.7769,E,090813,A*2D

with the wanted sentence:

$GPDTM,W84,,00.0000,N,00.0000,E,,W84*41


Only problem is, writing the code to do this is a bit tough!

Any clearer?

Nigel
[Diploma Computing, BSc (Computing), Geek]

 

[wotayottie]

Yes, much clearer. Thanks for that. You learn something new every day.

I originally thought your were referring to changing the port settings etc, not to writing a piece of code. Still surprised that your software requires the datum to be sent without giving an option to preset it - the chartplotting software I use has the datum preset in "configuration" and there's certainly no DTM sentence coming from the usb gps engine. Which implies that other gps sets than your cp180 would also not work with the chartplotter.

 

[nigelmercier]

Indeed, I'm nagging the software people as I write!

They insist: "We are bound by IMO regulations to report if datum is missing."

I replied "I've already ... confirmed the datum manually.
... I'm sure that IMO regulations would not require the
user to ignore a red flashing icon as a matter of course!"