FP_A messages

NMEA style framing is used. Frames (messages) are in this form:

$FP,msg_type,msg_version,field3,field4,…,fieldN*CC\r\n

Where:

The NMEA style framing:
- $ – Start character (“$”, ASCII 36)
- *CC – Checksum: “*” (ASCII 42) and two digit XOR value of all payload characters in captial hexadecimal notation, for example: “FPX” = 'F' ^ 'P' ^ 'X' = 70 ^ 80 ^ 88 = 78 = 0x4e = checksum 4E
- \r\n – Sentence termination characters (CR + LF, ASCII 13 + 10)
A Fixposition identifier:
- FP – Fixposition ASCII message identifier, “FP” (ASCII 70 + 80)
Fixposition message type and version:
- msg_type (= field1) – Message type, all captial letters (ASCII 65–90)
- msg_version (= field2) – Message version, decimal number (letters 0–9, ASCII 48–57), range 1–…
Data fields (payload)
- field3,field4,…,fieldN – The structure of the message data is defined by the msg_type and version. Each field can contain all printable 7-bit ASCII characters (ASCII 32–126), excluding the reserved characters ! (ASCII 33), $ (ASCII 36), * (ASCII 42), , (ASCII 44), \ (ASCII 92), ~ (ASCII 126).
Field separators
- All fields (identifier, message type, message version, data fields) are separated by a comma (,, ASCII 44)
Null fields
- Data fields can be null, meaning their value is absent to indicate that no data is available. The data for null fields is the empty string. For example:
  - Definition: …,fieldi,fieldi+1,fieldi+2,…
  - Values: fieldi = 123, fieldi+1 = null, fieldi+2 = 456
  - Payload string: …,123,,456,…
Data field types:
- Numeric: Decimal integer number, one or more digits (0-9) and optional leading “-” sign
- Float (.x): Decimal floating point number, one or more digits (0-9) and optional leading “-” sign, with x digits fractional part separated by a dot (“.”)
- Float (x): Decimal floating point number with x significant digits, optional leading “-“, optional fractional part separated by a dot (“.”)
- String: String of allowed payload characters (but not the , field separator)