Simrad SU90 - NMEA Telegram Principles; Standard NMEA 0183 Communication Parameters; NMEA Sentence Structure

SimradSU90
TheNMEA0183InterfaceStandarddeneselectricalsignalrequirements,data
transmissionprotocolandtime,andspecicsentenceformatsfora4800baudserial
databus.Eachbusmayhaveonlyonetalkerbutmanylisteners.
—NationalMarineElectronicsAssociation
FormoreinformationabouttheNationalMarineElectronicsAssociationandtheNMEA
0183standard,refertotheorganization'swebsiteathttp://www.nmea.org.
NMEAtelegramprinciples
Tomoveinformationbetweentwoelectronicunits,thedataarecollectedintelegrams.
Thecontent(protocol)ofeachtelegramisdenedbytheNMEAstandard,andseveral
telegramtypesexisttoallowdifferenttypeofdatatobedistributed.
Thephrasedatagramisalsofrequentlyusedaboutthiscommunicationmethod.
Unlessyouwishtowriteyourownsoftware,youdonotneedtoknowhowthese
telegramsaredesigned.However,wheneveryousetupequipmentinterfaces,youneed
toensurethateachsystemonyourcommunicationlineissetuptosendandreceivethe
sametelegram.Thestandardallowsonesystemtosenddata(a“talker")andseveral
otherstoreceivedatasimultaneously("listeners")onthesameline.Therefore,youmust
ensurethatallproductsreceivingdataonacommunicationlineissetuptoreceivethe
sametelegram(s)thatthetransmittingproductprovides.
StandardNMEA0183communicationparameters
ThecommunicationparametersdenedforNMEA0183are:
•Baudrate:4800bitspersecond
•Databits:8
•Parity:None
•Stopbits:One
Someinstrumentswillalsoofferotherparametersand/orchoices.
NMEAsentencestructure
Thefollowingprovidesasummaryexplanationoftheapprovedparametricsentence
structure.
$aaccc,c—c*hh<CR><LF>
1“$”:Startofsentence(Hex:24).
2aaccc:Addresseld.Thersttwocharacters(aa)identiestheTalkerID,whilethe
lastthreecharactersaretheSentenceformattermnemoniccodeidentifyingthedata
typeandthestringformatofthesuccessiveelds.
3“,”:Fielddelimiter(Hex:2C).Thischaracterstartseacheldexcepttheaddress
andchecksumelds.Ifitisfollowedbyanulleld,itisallthatremainstoindicate
nodataintheeld.
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