NMEAconverter is a utility that can parse incoming NMEA sentences/data, adjust and/or make calculations with the data, and then output a new NMEA sentence to have that adjusted data appear in OpenCPN. Examples include calculating true wind speed/angle from apparent wind data and boat speed information, changing units (degrees C to F or vice-versa, knots to km, etc.), or correcting malformed sentences created by some equipment where the sentence would otherwise be ignored by OpenCPN or displayed improperly.

Please note that there is no Toolbar Icon. NmeaConverter works from the Preferences button of the plug-in. Options|Plug-ins|NMEAConvertor –> Preferences

To use the plugin a basic understanding of NMEA sentences is required. Please see the NMEA Sentences Page for details on NMEA sentences and the fields each sentence contains. The basics of an NMEA sentence are:

Example $GPRMB,field1,field2,…field15*<0xChecksum><0x0D><0x0A>

The basic format of a conversion command is:

NewSentence = $IDYYY where ID is a talker ID you would like to use to denote sentences from the converter. The examples in this manual generally use XX as the talker ID for newly created sentences. YYY is the NMEA sentence identifier you would like to send. A simple conversion example - this sample takes the 5th and 6th fields of an MDA sentence and transmits a new MTA sentence:

Arguments can take the form of constants (numeric or text), values from an incoming NMEA sentence, or calculations using either or both constants and incoming data. In the simplest form an argument is an incoming NMEA sentence and field number - $GPRMC4 will get the fourth value in any RMC sentence received by OpenCPN from talker GP. Use the NMEA Sentences Page to figure out what sentence you want to send, what data fields are required, and how the fields are configured. You must build a complete version of the sentence you want to send, and put the right data/fields from the original source in the right places for the converter to work properly. A number of detailed examples are provided below for reference.

When inserting argument fields the wildcard “?” can be used to tell the converter to use data with any content where the “?” appears. The wildcard “??” at the front of an argument will tell the converter to use data from any talker sending the requested sentence (e.g. $??HDG3 will retrieve the third field from any HDG sentence regardless of sender). Be careful when using wildcards and the talker ID, especially when correcting malformed sentences. If you create a conversion that outputs the same sentence as any field used in the input (e.g. $XXHDG, $??HDG3,…..) you will create a loop that will crash the plugin (and OpenCPN). Every time the plugin evaluates it will see its own creation, because of the wildcard it will reevaluate, …

Once a conversion is defined you can select whether to send the calculated sentence on a fixed timing (every x seconds) or only when the incoming data is updated.

You do not need to add the end delimiter (*) or the checksum in your converter definitions - the converter will calculate the checksum and add the delimiter and checksum automatically.

Depending on the data you are manipulating you may want to go into the OpenCPN Connections Tab and tell OpenCPN to ignore the original data. If you are making unit conversions, for instance, and do not ignore the original data you will end up with a display that flashes between the original value and the converted value. Setting the data to ignore will not affect the converter, the data will still be received there, it simply prevents the display of the data in OpenCPN.

For Reference from the NMEA Sentences Page, sentences that can be displayed in OpenCPN include:

Install the plugin in accordance with the Plugin Installation Instructions.

Then open the Preferences Dialog to set up your conversions.

If you see something like above please enlarge the nmeaSendObjectDlg window so it displays the “OK” and “Cancel” buttons.

This is a completely useless sentence that is sent every second to OpenCPN. Check also the NMEAdebug window in the picture above, and note that a checksum is added to the sentence. It is easy to change this one into something like:

Just enter this line into the NMEAConverter > Preference > New > Edit box and Save it. No VDR file is needed. That will fake a GPS position into OpenCPN, but still not very useful.

The real power of this plugin is that you can change the data of the sent sentence by cherry picking from incoming sentences. The reason I wrote this plugin was I wanted Dashboard to show the air temperature. To get the data, Dashboard needs an $xxMTA sentence, however I had a Airmar PB200 unit that is sending $xxMDA sentences.

An example: We have from the windset,

MDA- Meteorological (Obsolete)

and want: MDA- Air temperature

Note that the data needed is in fields 5 and 6 of the MDA sentence. (11.3 degrees Celsius)

Setting the format to

will give an output sentence:

Where $WIMDA5 is replaced by the 5th field of the $WIMDA sentence. etc.

As seen above, the $XXMTA sentence is sent each time a $WIMDA sentence is received and read by Dashboard.

For testing use VDR_pi or VDRplayer and run

WIMDA+OCMDA.txt from this zip

But what if I was living in the US and preferred the reading to be in degrees Fahrenheit? Just change the format to:

The value is recalculated and displayed as degrees Fahrenheit.

Use the same file for testing use VDR_pi or VDRplayer and run WIMDA+OCMDA.txt from this zip

In NMEAconverter create a HDG (compass) sentence from your COG. Possible use when the autopilot is old and does not receive heading (true or magnetic).

Note: Since computation of the drift and surface current requires HDG, it is understood that while using Tactics_pi value for Current speed and direction will not be useful. All you will see is the speed difference between SOG and STW, which would force Current to be the vector component normal to the keel. That means the “current” display will always be directly from bow or stern, in other words the component of the current along the centerline of the boat, assuming your STW(speed through water) is calibrated correctly.

See CF Tactics_pi Post #22.

The NMEA file being run by VDR_pi is

vdr2 - copie.xls

Tactics_pi and in particular, NMEAconverter_pi is using

Remember that setting HDT to COG will give you no drift angle at all (except maybe a small portion via the manual heel input), so you will not see any real current I suggest to set a fixed heel angle to 0 (see screenshot above), to avoid side effects with manual heel input at very low speeds and small speed differences between SOG and STW.

Ratsea has fixed an incorrect Raymarine ST60 conversion.

Now it is possible to use wildcards in the identifiers. So you could make it talkerid independent by using something similar to

and whatever character on the ? position will cause a hit.

Even more important, you might use it for special transducers sending the something like

sentence.

Note that field 1 is part of the identifier. As the comma is the NMEA separator, you can not use it as an identifier string, but by using a wildcard it works.

For example

will work. Cruiser Forum Post

Using wildcards is also useful when the data to be matched is in the data payload rather than the header. In the example below, we have engine data coming from the NMEA stream which contains the correct sender transducer ID, however is not the format required by the Engine Dashboard plug-in. Here is the original NMEA sentences:

[code] Engine RPM $ERRPM,E,0,2324.00,,A*69 Oil Pressure, Coolant Temp, and Engine Hours $ERXDR,P,4.04,B,ENGOILP0,C,75.25,C,ENGTEMP0,G,2024 .1,,ENGHRS0*01 Battery0 (Starter) Voltage and Shunt Current $ERXDR,U,13.75,V,BATVOLT0,I,0.8,A,BATCURR0*4D Battery1 (House) Voltage $ERXDR,U,13.27,V,BATVOLT1*14 [/code]

You can see that the two battery voltages are distinguished using different transducer IDs, BATVOLT0 and BATVOLT1

The following NMEA Converter plug-in code converts to sentences so that the Engine Dashboard can read them. For the sentence that contains multiple data elements, we can us the wildcard “$ERXDR?P2” to match on the particular sentence that has the data we need.

To get battery voltage and current, we need to make liberal use of the wildcard to match out to the “I” in order to distinguish from the BATVOLT1 sentence. this gets us the battery0 voltage and the DC shunt current.

For battery1, I have not figured out how to convert this because the only character that distinguishes it from battery0 is the “1” at the end.

[code] RPM $IIXDR,T,$ERRPM3,R,MAIN ENGINE OIL PRESSURE $IIXDR,P,$ERXDR?P2*100000,P,MAIN ENGINE COOLANT TEMP $IIXDR,C,$ERXDR?P6,C,MAIN ENGINE HOURS $IIXDR,G,$ERXDR?P10,H,MAIN HOUSE BATTERY VOLTAGE $IIXDR,U,$ERXDR?U??????????????????I2,V,HOUS DC AMPERAGE $IIXDR,U,$ERXDR?U??????????????????I6,A,STRT [/code]

See this CF #341 Post and ones preceding.

Using convert C to F for temp ( $XXMTA, 9/5*$AIMTW5 + 32, F) with send after all updates variables checked.

You are using an ..MTW sentence. Format according my info:

$–MTW,x.x,C_hh

Used new formula provided above.. Still Crashed. I removed the

$Dummy

entry that came with NMEAConverter_pi install. OpenCPN no longer crashes with NMEAConverter_pi enabled with a NMEA stream present..and as provided for above

is showing the “Air Temp” in F in Dashboard on O. “I can never thank the volunteers enough for all these wonderful tools.”

Cruiser Forum Post

The problem with the incoming sentence:

Which in this case, is a magnetic heading of 113 degrees followed by a bad checksum.

Using the NMEA converter sentence, to delete the bad checksum and then add a new checksum after a forced comma,

Which gets converted to a new sentence of $IIHDM,113, *50 with a new checksum. This assigns the first field of the NMEA sentence from $APHDM (113) to a new virtual source of $IIHDM

Tim advises:

Dsanduril reports: “The TMQ checksum includes the '$' delimiter character at the start of the sentence. Per NMEA standard the '$' delimiter at the start of the sentence and the '*' delimiter at the end of the sentence should not be included in the checksum. Checksum should be calculated by XOR of all characters between the delimiters. [Suggesting that the TMQ AP needs a firmware update to correct the error.”

From the Nmea Standard:

The game SailAway sends realtime NMEA streams that can be used with plotter software, routing software, etc. The game developers have elected to send the RMC and GLL sentences with the “V” validity character, which indicates the data is invalid. OpenCPN rejects the data. An “A” character is required to show the data is valid.

NMEAconverter can easily adjust these sentences and recalculate the checksum so that the data is accepted in OpenCPN.

Thanks to Jurien72 and Rasbats CForum SOG/COG Problem

If you have an useful format formula please feel free to add it here.

Another future possibility is to do a comparison lookup of a boat’s Polar data and send a special NMEA sentence of the comparative result to the Dashboard using a special dedicated performance instrument, for example.

For calculations the following functions are supported: