Instruction Manual




540User
0808‐01Rev.B
Page63
GNAV
7430‐
Manual

PayloadLength
Thepayloadlengthisalwaysaonebyteunsignedcharacterwitharangeof0‐255.Thepayloadlengthbyteisthe
length(inbytes)ofthe<variable length payload>portionofthepacketONLY,anddoesnotincludetheCRC.
Payload
Thepayloadisofvariablelengthbasedonthepackettype.
16‐BitCRC‐CCITT
Packetsendwitha16‐bitCRC‐CCITTcalculatedontheentirepacketexcludingthe0x5555headerandtheCRCfield
itself.Adiscussionofthe16‐bitCRC‐CCITTandsamplecodeforimplementingthecomputationoftheCRCis
includedattheendofthisdocument.This16‐bitCRCstandardismaintainedbytheInternational
Union(ITU).
Telecommunication
 Width:16bits
 Polynomial:0x1021
 Initialvalue:0xFFFF
ORisperformedonthefinalvalue.
 NoX
Referto
AppendixB.SamplePacket—ParserCodeforsamplecodethatimplementsthe16‐bitCRCalgorithm.
MessagingOverview
ThefollowingtablesummarizesthemessagesavailablewiththeGNAV540unit.Packettypesareassignedmostly
usingtheASCIImnemonicsdefinedaboveandareindicatedinthesummarytablebelowandinthedetailedsections
foreachcommand.Thepayloadbyte‐lengthisoftenrelatedtootherdataelementsinthepacketasdefinedinthe
tablebelow.Thereferencedvariablesaredefinedinthefollowingsections.
OutputmessagesaresentfromtheGNAV540inertialsystemtotheusersystemasaresultofapollrequestora
continuouspacketoutputsetting.InputmessagesaresentfromtheusersystemtotheGNAV540inertialsystem
andwillresultinanassociatedReplyMessageorNAKmessage.Replymessagestypicallyhavethesame
<2byte
ackettype(U2)>
p

astheinputmessagethatevokeditbutwithadifferentpayload.
Table14MessageTable
ASCII
Mnemonic
<2‐bytepacket
ype(U2)>
t
<payload
byte‐length
U1)>
(
Description Type
Available
unctions
F
LinkTest
  
PK 0x504B 0
PingCommand
andResponse
Input/ReplyMessage ALL
CH 0x4348 N
EchoCommand
andResponse
Input/ReplyMessage ALL
InteractiveCo mmands
GP 0x4750 2
GetPacket
Request
InputMessage ALL
AR 0x4152 0 AlgorithmReset Input/ReplyMessage VG,AHRS,NAV
SR 0x5352 0 SoftwareReset Input/ReplyMessage ALL
NAK 0x1515 2 ErrorResponse ReplyMessage ALL