pyfastnet

Python library for decoding the FastNet protocol used by B&G Hydra/H2000 instruments. Developed for personal use and published for general interest.

Purpose

Feed a raw byte stream from a FastNet bus into the library. It handles synchronisation, checksum validation, and decoding — returning structured instrument data ready for further processing.

Installation

pip install pyfastnet

Example usage

#!/usr/bin/env python3
import serial
from fastnet_decoder import FrameBuffer

fb = FrameBuffer()

ser = serial.Serial(
    port="/dev/ttyUSB0",
    baudrate=28800,
    bytesize=serial.EIGHTBITS,
    stopbits=serial.STOPBITS_TWO,
    parity=serial.PARITY_ODD,
    timeout=0.1,
)

try:
    while True:
        data = ser.read(256)
        if not data:
            continue
        fb.add_to_buffer(data)
        fb.get_complete_frames()
        while not fb.frame_queue.empty():
            frame = fb.frame_queue.get()
            for channel, decoded in frame["values"].items():
                print(channel, decoded)
finally:
    ser.close()

Output format

Each decoded frame is a dict with to_address, from_address, command, and values. Each entry in values is keyed by channel name:

{
  "to_address":   "Entire System",
  "from_address": "Normal CPU (Wind Board in H2000)",
  "command":      "Broadcast",
  "values": {
    "Apparent Wind Speed (Knots)": {
      "channel_id":   "0x4D",
      "value":        7.0,
      "display_text": "7.0",
      "layout":       None,
    },
    "Apparent Wind Angle": {
      "channel_id":   "0x51",
      "value":        -6.0,
      "display_text": "-6.0",
      "layout":       "-[data]",
    },
    "True Wind Direction": {
      "channel_id":   "0x6D",
      "value":        213.0,
      "display_text": "213.0°M",
      "layout":       "°M",
    },
  }
}

Layout field

The layout field describes the indicator symbol shown on the physical display around the numeric value:

layout	Meaning	Sign
None	No indicator symbol	positive
"[data]="	= after value (starboard)	positive
"=[data]"	= before value (port)	negative
"[data]-"	- after value (starboard)	positive
"-[data]"	- before value (port)	negative
"H[data]"	H prefix — heading	positive
"°M"	Magnetic bearing suffix	positive
"u[data]"	u prefix — upwind (VMG)	positive
"d[data]"	d prefix — downwind (VMG)	positive
"L[data]"	L before — leeway port	negative
"[data]L"	L after — AP compass target	positive
"[data]°C"	Celsius suffix	positive
"[data]°F"	Fahrenheit suffix	positive
"[data]z" / "z[data]"	Dog-leg symbol — AP off course	positive
"TBC"	Symbol seen but not yet identified	positive

Debug API

from fastnet_decoder import set_log_level
import logging
set_log_level(logging.DEBUG)

fb.get_buffer_size()      # bytes currently in buffer
fb.get_buffer_contents()  # hex string of buffer contents

Companion apps

• fastnet2ip — reads FastNet from serial, broadcasts NMEA 0183 via UDP
• fastnet2ip_n2k — reads FastNet from serial, broadcasts NMEA 2000 via UDP

Both run on Raspberry Pi, macOS, or Linux.

Acknowledgments

• trlafleur — background research
• Oppedijk — protocol documentation
• timmathews — C++ reference implementation