pyfastnet

A Python library for decoding the FastNet protocol used by B&G Hydra / H2000 instruments. Feed it raw bytes from the Fastnet bus and it handles synchronisation, checksum validation, and decoding — returning structured instrument data ready for further processing.

Developed for personal use and published for general interest. Runs on Raspberry Pi, macOS, or Linux.

Quick start

pip install pyfastnet

#!/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()

Serial settings for a B&G Fastnet bus are 28,800 baud, 8 data bits, odd parity, 2 stop bits. No instruments to hand? The companion apps below ship recorded captures you can replay.

The Fastnet toolkit

Three projects stack together — pick the layer that matches where you want the data to end up:

Project	What it does	Use it when
pyfastnet (this library)	Decoder. Turns raw Fastnet bytes into named instrument channels.	You're writing your own Python and want the decoded data.
fastnet2ip	Serial → network. Broadcasts decoded data over UDP as NMEA 0183 or NMEA 2000 (over IP).	Feeding Signal K, OpenCPN, or a plotter over WiFi / Ethernet.
fastnet2n2k	Serial → physical NMEA 2000 bus. Transmits PGNs onto a CAN backbone via SocketCAN.	Wiring into a real NMEA 2000 network / chartplotter.

                          ┌─ fastnet2ip   → UDP (NMEA 0183 / NMEA 2000 over IP) → Signal K, OpenCPN, plotters
B&G Fastnet bus ─(serial)─→ pyfastnet ─┤
                          └─ fastnet2n2k → SocketCAN (NMEA 2000 PGNs)           → CAN backbone, chartplotter

pyfastnet is the engine at the bottom of the stack. If you only want the decoded data on your network or NMEA 2000 bus — including running it as an always-on systemd service — use one of the companion apps; they handle the serial port, a live data store, rate limiting, and output for you.

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",
    },
  }
}

Position (LatLon) frames

Position frames (command LatLon) appear under the "LatLon" key. The raw coordinate string (DDMM.mmm with hemisphere letters) is carried in display_text; value is None. The originating source's marker byte is preserved in channel_id (e.g. 0x47, 0x4E) but does not affect the key:

"LatLon": {
    "channel_id":   "0x4E",
    "value":        None,
    "display_text": "3352.450S15113.920E",
    "layout":       None,
}

Layout field

The layout field describes the indicator symbol shown on the physical display around the numeric value. It is the only place True/Magnetic, port/starboard sign, and similar context is carried — the raw stream contains no magnetic variation or deviation.

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

Acknowledgments

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

License

MIT — see LICENSE.