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 instrument data as Signal K paths in SI units, ready for further processing.

v3.0 changes the output format. Decoded frames are now {signalk_path: SI_value} (e.g. navigation.speedThroughWater = 3.6) instead of the v2 name-keyed {value, display_text, layout} dicts. See Output format below. The full v2-style decode is still available via FrameBuffer(project=False).

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 path, value in frame["values"].items():
                print(path, value)   # e.g. navigation.speedThroughWater 3.6
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 Signal K paths in SI units. 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. values maps Signal K paths to SI values — one canonical entry per physical quantity:

{
  "to_address":   "Entire System",
  "from_address": "Normal CPU (Wind Board in H2000)",
  "command":      "Broadcast",
  "values": {
    "environment.wind.speedApparent":     4.6,    # m/s
    "environment.wind.angleApparent":     0.419,  # radians
    "environment.wind.directionMagnetic": 1.239,  # radians
    "navigation.speedThroughWater":       3.19,   # m/s
  }
}

Units follow the Signal K spec: angles in radians, speed in m/s, distance in metres, temperature in Kelvin, pressure in Pascals. A value is a float (SI), a str enum (e.g. steering.autopilot.state → "standby"), a position object, or None when unavailable.

Redundant unit-variant channels the bus sends (feet/fathoms depth, knots wind, °F) are collapsed to one canonical path. B&G-proprietary channels with no standard Signal K path — including the pre-calibration raw sensor values — are emitted under a bandg.* namespace (e.g. bandg.wind.rawAngleApparent).

Units and the full path map

Every emitted path has a canonical SI unit, available programmatically:

from fastnet_decoder import unit_for
unit_for("navigation.speedThroughWater")   # "m/s"
unit_for("navigation.headingMagnetic")      # "rad"
unit_for("environment.water.temperature")   # "K"
unit_for("navigation.position")             # "deg"

The complete mapping — every B&G channel id/name → Signal K path + unit, the bandg.* vendor namespace, layout-driven Magnetic/True routing, and the dropped/collapsed channels — is documented in docs/v3_signalk_mapping.md.

Position

Position frames are emitted as navigation.position, a decimal-degree object (negative for S / W):

"navigation.position": {"latitude": -33.8742, "longitude": 151.2320}

True vs Magnetic

The reference is carried by the path, not a separate field: navigation.headingMagnetic vs navigation.headingTrue, environment.wind.directionMagnetic vs directionTrue, environment.current.setMagnetic vs setTrue. The decoder selects the right path from the display's indicator symbol at decode time. (The raw stream carries no magnetic variation or deviation, so it cannot convert between the two.)

Complete (rich) decode

FrameBuffer(project=False) queues the full internal decode instead of the Signal K projection. Each value is then a dict — value, display_text, layout, channel_id — keyed by human-readable channel name (the v2 format). Useful for debugging and reverse-engineering; display_text and layout are not exposed in the default Signal K output.

You can also project a single decoded frame yourself:

from fastnet_decoder import decode_frame, project
rich = decode_frame(raw_frame_bytes)      # complete decode
si = project(rich)                        # {signalk_path: SI_value}

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

License

MIT — see LICENSE.

Project links

Key dates

PyPI data
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  • Released:
Newer release available (3.1.0)

1 maintainer

PyPI data
Data sourced directly from PyPI's database.
Avatar for ghotihook from gravatar.com ghotihook

Credits

Author: Alex Salmon

License

MIT License (MIT License)

Requires

Python >=3.7

Provides Extra

test

Tags

fastnet bandg hydra h2000 marine sailing decoder

Classifiers

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