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package nmea
import (
"encoding/hex"
"fmt"
"strconv"
)
const (
// TypePGN is type of PGN sentence for transferring single NMEA2000 frame as NMEA0183 sentence
TypePGN = "PGN"
)
// PGN - transferring single NMEA2000 frame as NMEA0183 sentence
// https://opencpn.org/wiki/dokuwiki/lib/exe/fetch.php?media=opencpn:software:mxpgn_sentence.pdf
//
// Format: $--PGN,pppppp,aaaa,c--c*hh<CR><LF>
// Example: $MXPGN,01F112,2807,FC7FFF7FFF168012*11
type PGN struct {
BaseSentence
PGN uint32 // PGN of NMEA2000 packet
IsSend bool // is this sentence received or for sending
Priority uint8 // 0-7
Address uint8 // depending on the IsSend field this is Source Address of received packet or Destination for send packet
Data []byte // 1-8 bytes. This is single N2K frame. N2K Fast-packets should be assembled from individual frames
}
// newPGN constructor
func newPGN(s BaseSentence) (Sentence, error) {
p := NewParser(s)
p.AssertType(TypePGN)
if len(p.Fields) != 3 {
p.SetErr("fields", "invalid number of fields in sentence")
return nil, p.Err()
}
pgn, err := strconv.ParseUint(p.Fields[0], 16, 24)
if err != nil {
p.err = fmt.Errorf("nmea: %s failed to parse PGN field: %w", p.Prefix(), err)
return nil, p.Err()
}
attributes, err := strconv.ParseUint(p.Fields[1], 16, 16)
if err != nil {
p.err = fmt.Errorf("nmea: %s failed to parse attributes field: %w", p.Prefix(), err)
return nil, p.Err()
}
dataLength := int((attributes >> 8) & 0b1111) // bits 8-11
if dataLength*2 != (len(p.Fields[2])) {
p.SetErr("dlc", "data length does not match actual data length")
return nil, p.Err()
}
data, err := hex.DecodeString(p.Fields[2])
if err != nil {
p.err = fmt.Errorf("nmea: %s failed to decode data: %w", p.Prefix(), err)
return nil, p.Err()
}
return PGN{
BaseSentence: s,
PGN: uint32(pgn),
IsSend: attributes>>15 == 1, // bit 15
Priority: uint8((attributes >> 12) & 0b111), // bits 12,13,14
Address: uint8(attributes), // bits 0-7
Data: data,
}, p.Err()
}
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