fastnet2n2k 0.1.3

Bridge a B&G Fastnet instrument stream onto an NMEA2000 CAN bus

fastnet2n2k — B&G Fastnet → NMEA2000 bridge

Reads a B&G Fastnet instrument stream (live serial or a captured hex file), decodes it with pyfastnet, maps the channels to NMEA2000 PGNs and transmits them onto a CAN bus. Built for the M5Stack CoreMP135 but runs on any Linux box with a SocketCAN interface.

The CoreMP135 runs Linux on an STM32MP135 and exposes its two FDCAN interfaces (SIT1051T transceivers) as the SocketCAN netdevs can0 (FDCAN1, PE3/PE10) and can1 (FDCAN2, PG0/PE0). NMEA2000 runs at 250 kbit/s with 29-bit extended IDs. Message encoding, CAN-ID construction, fast-packet framing and ISO address claiming are handled by the nmea2000 library (canboat- based) on top of python-can's socketcan backend.

What it sends

Each Fastnet channel is mapped to the matching PGN and emitted only when the channel updates (with a 0.05 s minimum interval and a 5 s maximum re-broadcast), so when the instruments go quiet the output stops and consumers time the data out.

Data	PGN	Priority	Notes
Heading	127250	2	T/M reference taken from the Fastnet display layout (°M/°T)
Apparent / True wind, TWD	130306	2	knots→m/s, deg→rad; reference per layout
Boat speed	128259	2	knots→m/s
Depth	128267	3	metres
COG/SOG	129026	2	prefers True COG, falls back to Magnetic
Attitude (heel/trim)	127257	3	signed value passed through unchanged
Rudder, Leeway, Rate of turn	127245 / 128000 / 127251	2 / 4 / 2
Distance log, XTE	128275 / 129283	6 / 3	NM→m
Position	129025	2
Sea / air temperature	130312	5	°C/°F→Kelvin
Barometric pressure	130314	5	mbar→Pa
Tidal set & drift	129291	3	set deg→rad, drift kn→m/s; reference per layout

The Priority column lists each PGN's NMEA2000 standard CAN priority (0 = highest, 7 = lowest). These are the values used by default — when --n2k-priority is not given, every frame keeps the per-PGN priority shown above. Passing --n2k-priority N overrides the whole column, forcing all frames to a single priority N.

Units are converted to NMEA2000 SI. Sign comes straight from pyfastnet's decoded value. True/Magnetic is read from the pyfastnet layout field (the only place it exists); a bearing whose layout can't be resolved is skipped, never guessed. The B&G proprietary raw PGNs (65280–65282) are deferred (manufacturer-specific layout).

WiFi gateways: if you feed a WiFi NMEA2000 gateway downstream, configure it for unicast UDP, not broadcast — WiFi broadcast is unacknowledged and silently drops frames even at low rates.

Install

Recommended — install the CLI in its own isolated environment with pipx:

pipx install fastnet2n2k
fastnet2n2k --serial /dev/ttyUSB0 --channel can0

This puts a fastnet2n2k command on your PATH. python -m fastnet2n2k ... also works once installed.

From source (development)

git clone https://github.com/ghotihook/fastnet2n2k.git
cd fastnet2n2k
python3 -m venv .venv
source .venv/bin/activate
pip install -e .

Bring up the CAN bus

Once per boot (250 kbit/s for NMEA2000). restart-ms 100 makes the controller auto-recover from a bus-off instead of staying down:

sudo ip link set can0 up type can bitrate 250000 restart-ms 100   # FDCAN1
# sudo ip link set can1 up type can bitrate 250000 restart-ms 100 # FDCAN2
ip -details link show can0    # want: state ERROR-ACTIVE, bitrate 250000

If can0 shows BUS-OFF, fix that before expecting output to land — a CAN frame needs at least one other node on the wire to acknowledge it (check termination ≈60 Ω, common ground, and CAN-H/CAN-L not swapped).

Run

Activate the venv first (source .venv/bin/activate), then:

# replay a captured Fastnet hex file (safest first test)
python -m fastnet2n2k --file capture.txt --channel can0

# live from the Fastnet bus
python -m fastnet2n2k --serial /dev/ttyUSB0 --channel can0

Find your serial adapter with ls /dev/ttyUSB* /dev/ttyACM* /dev/ttyS*. Stop with Ctrl-C.

Options:

Option	Default	Meaning
--serial DEV / --file PATH	—	input source (one is required)
--channel	can0	SocketCAN interface
--n2k-priority	per-PGN standard	override CAN priority (0–7, 0=highest) for all transmitted frames; if omitted, each PGN keeps its standard priority (see the PGN table above)
--unique	from hostname	device NAME unique number
--live-data	off	print the live channel table to the console once per second
--log-level	INFO	DEBUG / INFO / WARNING / ERROR

CAN failure handling: the device reconnects automatically. If can0 isn't up at start it waits (logging retries) rather than exiting; if the bus drops or goes bus-off mid-run, sends fail quietly (logged at most every 5 s) and resume once it recovers — the bridge keeps running. Use --log-level DEBUG to see connection/retry detail.

Verify

Watch the raw frames on the board with can-utils (sudo apt install can-utils):

candump -ta can0

You should see 29-bit frames appear as instruments update — heading, wind, depth, speed, etc. — and stop when they go quiet. On a connected chartplotter / analyzer the device appears in the device list after its ISO address claim (PGN 60928).

Test the whole chain with a loopback (no instruments needed)

With can0 and can1 wired together (CAN-H↔CAN-H, CAN-L↔CAN-L, one 120 Ω terminator), can1 provides the ACK so can0 can transmit:

sudo ip link set can1 up type can bitrate 250000 restart-ms 100
candump -ta can1                                              # terminal 1
python -m fastnet2n2k --file capture.txt --channel can0      # terminal 2

Run at boot (systemd)

A unit file is provided in fastnet2n2k.service. Edit the paths, user and --serial device to match your install, then:

Set ExecStart to match how you installed it:

• pipx: ExecStart=/home/<user>/.local/bin/fastnet2n2k --serial /dev/ttySTM3 --channel can0
• source venv: ExecStart=/home/<user>/fastnet2n2k/.venv/bin/python -m fastnet2n2k --serial /dev/ttySTM3 --channel can0

sudo cp fastnet2n2k.service /etc/systemd/system/
sudo systemctl daemon-reload
sudo systemctl enable --now fastnet2n2k.service
journalctl -u fastnet2n2k.service -f      # watch logs

The unit brings can0 up itself (with restart-ms 100) before starting, and restarts the bridge on failure.

Tests

source .venv/bin/activate
pip install pytest
python -m pytest tests/ -q

The suite drives the mapping with pyfastnet's bundled capture files and round-trips the resulting NMEA2000 messages to assert PGNs, unit conversions, T/M references, sign passthrough, the send throttle, and the full file→decode→send pipeline.

---

Sender POC (nmea2000_poc.py)

A standalone minimal proof-of-concept that transmits a single NMEA2000 PGN (127250 Vessel Heading) onto the bus — useful for smoke-testing a CAN link independently of the Fastnet pipeline.

python nmea2000_poc.py --channel can0 --heading 90 --once   # single frame
python nmea2000_poc.py --channel can0 --heading 90          # ~10 Hz loop

Options: --channel (default can0), --heading degrees, --ref true|magnetic, --rate Hz, --once.

Desk testing without a bus (virtual CAN)

sudo modprobe vcan
sudo ip link add dev vcan0 type vcan
sudo ip link set up vcan0
candump vcan0                                                 # terminal 1
python nmea2000_poc.py --channel vcan0 --heading 90 --once   # terminal 2