NMEA 2000 Python Library

A Python library for encoding and decoding NMEA 2000 frames. The encoding and decoding is based on the extensive canboat database. It also supports inexpensive CANBUS USB and TCP devices as gateways between your NMEA 2000 boat network and any Python code that wants to receive or send these messages. This package is the backend for the Home Assistant NMEA 2000 Integration.

Features

• Decode NMEA 2000 frames: Parse and interpret raw NMEA 2000 data.
• Encode NMEA 2000 frames: Convert structured data back into the NMEA 2000 frame format.
• Gateway clients: Send and receive NMEA 2000 data through various hardware gateways:
  • EByte — binary TCP gateways like ECAN-E01 and ECAN-W01S
  • Text — any line-based ASCII TCP gateway with auto-sensing or explicit format selection (e.g. Actisense W2K-1, Yacht Devices YDEN-02, Actisense PRO-NDC-1E2K in CAN ASCII mode)
  • Actisense BST — Actisense devices using the BST binary protocol over TCP, supporting both BST-95 (raw CAN frames) and BST-D0 (pre-assembled N2K). Compatible with the PRO-NDC-1E2K and W2K-1 in CAN Actisense mode
  • WaveShare — USB serial devices like Waveshare USB-CAN-A
  • python-can — any generic USB or SocketCAN device supported by the python-can library
  • Actisense NGT-1 (Ngt1Nmea2000Gateway), Digital Yacht iKonvert (IkonvertNmea2000Gateway) and Maretron IPG100/200 (MaretronIpgNmea2000Gateway) — using canboat's own protocol implementations, including the NGT-1 and iKonvert transmit lists (pass every PGN you will send as tx_pgns; others are refused once any is named)
• PGN-specific parsing: Every PGN in the canboat database is decoded and encoded by canboat's own Rust decoder, compiled into this package.
• Stateful decoder: The decoder supports NMEA 2000 fast messages, which are split across multiple CANBUS messages.
• CLI support: Built-in command-line interface for encoding and decoding frames.

Installation

You can install the library using pip:

pip install nmea2000

Prebuilt wheels cover Python 3.11 and later on Windows (x64), macOS (Apple silicon and Intel) and Linux (x86_64 and ARM64, glibc and musl), and the Raspberry Pi on 64-bit Raspberry Pi OS (Pi 3, 4, 5, Zero 2 W) and 32-bit Raspberry Pi OS (Pi 2 to 5, Zero 2 W). Elsewhere, including the original Pi Zero, Zero W and Pi 1, installing builds from source and needs a Rust toolchain.

Alternatively, you can clone the repository and install it locally:

git clone https://github.com/tomer-w/nmea2000.git
cd nmea2000
pip install .

Usage

Decode NMEA 2000 Frame (CLI)

To decode a frame, use the decode command followed by the frame in Actisense hex format:

nmea2000-cli decode --frame "09FF7 0FF00 3F9FDCFFFFFFFFFF"

65280 Furuno: Heave: Manufacturer Code = Furuno (bytes = "3F 07"), Reserved = 3 (bytes = "03"), Industry Code = Marine (bytes = "04"), Heave = -0.036000000000000004 (bytes = "DC"), Reserved = 65535 (bytes = "FF FF 00")

Or in JSON format:

{"PGN":65280,"id":"furunoHeave","description":"Furuno: Heave","fields":[{"id":"manufacturer_code","name":"Manufacturer Code","description":"Furuno","unit_of_measurement":"","value":"Furuno","raw_value":1855},{"id":"reserved_11","name":"Reserved","description":"","unit_of_measurement":"","value":3,"raw_value":3},{"id":"industry_code","name":"Industry Code","description":"Marine Industry","unit_of_measurement":"","value":"Marine","raw_value":4},{"id":"heave","name":"Heave","description":"","unit_of_measurement":"m","value":-0.036000000000000004,"raw_value":-36},{"id":"reserved_48","name":"Reserved","description":"","unit_of_measurement":"","value":65535,"raw_value":65535}],"source":9,"destination":255,"priority":7}

Example Code

from nmea2000.decoder import NMEA2000Decoder

# Initialize decoder
decoder = NMEA2000Decoder()

# Decode a frame
frame_str = "09FF7 0FF00 3F9FDCFFFFFFFFFF"
decoded_frame = decoder.decode(frame_str)

# Print decoded frame
print(decoded_frame)

Repeating Fields

Some PGNs (e.g. AC Input/Output Status) contain repeating field sets — for example, one set of measurements per AC line. These are exposed as a list field whose value is a list of dicts, where each dict maps field IDs to NMEA2000Field objects:

from nmea2000.decoder import NMEA2000Decoder
from nmea2000.consts import FieldTypes

decoder = NMEA2000Decoder(already_combined=True)
msg = decoder.decode(
    "2021-07-29-09:00:42.386,6,127503,1,255,20,"
    "00,01,f0,00,05,2c,01,88,13,e8,03,80,15,00,00,80,15,00,00,64"
)

for field in msg.fields:
    if field.type == FieldTypes.VARIABLE and isinstance(field.value, list):
        for idx, entry in enumerate(field.value):
            print(f"AC Line {idx}:")
            for field_id, sub_field in entry.items():
                print(f"  {sub_field.name}: {sub_field.value} {sub_field.unit_of_measurement or ''}")

Output:

AC Line 0:
  Line: Line 1
  Acceptability: Bad level
  Reserved: 15
  Voltage: 12.8 V
  Current: 30.0 A
  Frequency: 50.0 Hz
  Breaker Size: 100.0 A
  Real Power: 5504 W
  Reactive Power: 5504 VAR
  Power factor: 1.0 Cos Phi

Example reading packets using python-can

import can
from nmea2000.decoder import NMEA2000Decoder

# Initialize decoder
decoder = NMEA2000Decoder()

# Connect to CAN bus (e.g. slcan device on /dev/ttyUSB0)
bus = can.interface.Bus(interface='slcan', channel="/dev/ttyUSB0", bitrate=250000)

# Decode frames
for msg in bus:
    decoded_frame = decoder.decode(msg)

    # Print decoded frame when ready (fast data intermediate frames return None)
    if decoded_frame is not None:
        print(decoded_frame)

Simple N2KDevice example

If you want to behave like a small NMEA 2000 device instead of just reading frames, N2KDevice wraps the transport client, handles address claiming, and lets you send/receive NMEA2000Message objects directly:

import asyncio

from nmea2000.device import N2KDevice
from nmea2000.message import NMEA2000Field, NMEA2000Message


async def handle_received(message: NMEA2000Message) -> None:
    print(f"received PGN {message.PGN} from source {message.source}")


async def main() -> None:
    device = N2KDevice.for_python_can(
        interface="socketcan",
        channel="can0",
        preferred_address=25,
        model_id="Python demo device",
        manufacturer_information="nmea2000 README example",
    )
    device.set_receive_callback(handle_received)

    try:
        await device.start()
        await device.wait_ready(timeout=5)

        await device.send(
            NMEA2000Message(
                PGN=127250,
                id="vesselHeading",
                priority=2,
                source=0,  # 0 means "use the address claimed by this device"
                destination=255,
                fields=[
                    NMEA2000Field(id="sid", raw_value=0),
                    NMEA2000Field(id="heading", value=1.0),
                    NMEA2000Field(id="deviation", raw_value=0),
                    NMEA2000Field(id="variation", raw_value=0),
                    NMEA2000Field(id="reference", raw_value=0),
                    NMEA2000Field(id="reserved_58", raw_value=0),
                ],
            )
        )

        await asyncio.sleep(10)
    finally:
        await device.close()


asyncio.run(main())

Use N2KDevice.for_ebyte(...), N2KDevice.for_yacht_devices(...), N2KDevice.for_waveshare(...), or N2KDevice.for_actisense(...) if you are connecting through one of those gateways instead of python-can.

The device answers ISO requests for its own PGNs and NAKs the rest. An application that sends its own PGNs can answer requests and group functions for them:

async def on_group_function(message):  # 126208, for this device or broadcast
    if message.id != "nmeaCommandGroupFunction" or message.get_field_by_id("pgn").value != 127237:
        return False  # the device NAKs it
    for entry in message.get_field_by_id("##list##").value:
        # each value is typed by the field of 127237 it refers to:
        # a lookup's raw_value is its code, e.g. ('Heading Control', 4)
        print(entry["parameter"].value, entry["value"].value, entry["value"].raw_value)
    return True  # answered (send the acknowledgement yourself)


async def on_iso_request(message, pgn):  # PGNs the device doesn't answer itself
    return False


device.set_group_function_handler(on_group_function)
device.set_iso_request_handler(on_iso_request)

device.own_name is the 64-bit ISO NAME the device claims its address with.

Gateway Client CLI

Each gateway type has its own subcommand:

# EByte binary TCP gateway
nmea2000-cli ebyte --server 192.168.0.46 --port 8881

# Text/line-based TCP gateway with auto-sensing (W2K-1, YDEN-02, PRO-NDC-1E2K in CAN ASCII mode)
nmea2000-cli text --server 192.168.0.46 --port 8881

# Text gateway with explicit format
nmea2000-cli text --server 192.168.0.46 --port 8881 --format N2K_ASCII_RAW

# Actisense BST over TCP (PRO-NDC-1E2K / W2K-1 in CAN Actisense mode)
nmea2000-cli actisense_bst --server 192.168.0.46 --port 8881

# WaveShare USB-CAN-A serial
nmea2000-cli waveshare --port /dev/ttyUSB0

# Generic python-can adapter
nmea2000-cli can --interface slcan --channel /dev/ttyUSB0 --bitrate 250000

Use --json to output received messages as JSON (one object per line), useful for piping into other tools:

nmea2000-cli text --server 192.168.0.46 --port 8881 --json

The --json flag is available on all gateway subcommands. Use --dump_file to record raw frames to a file.

Gateway Client code

async def handle_received_data(message: NMEA2000Message):
    """User-defined callback function for received data."""
    print(f"Callback: Received {message}")

client = EByteNmea2000Gateway(ip, port)
client.set_receive_callback(handle_received_data)  # Register callback

Encode NMEA 2000 Frame (CLI)

You can also encode data into NMEA 2000 frames using the encode command:

nmea2000-cli encode --data "your_data_to_encode"

CLI Example

nmea2000-cli encode --data '{"PGN":65280,"id":"furunoHeave","description":"Furuno: Heave","fields":[{"id":"manufacturer_code","name":"Manufacturer Code","description":"Furuno","unit_of_measurement":"","value":"Furuno","raw_value":1855},{"id":"reserved_11","name":"Reserved","description":"","unit_of_measurement":"","value":3,"raw_value":3},{"id":"industry_code","name":"Industry Code","description":"Marine Industry","unit_of_measurement":"","value":"Marine","raw_value":4},{"id":"heave","name":"Heave","description":"","unit_of_measurement":"m","value":-0.036000000000000004,"raw_value":-36},{"id":"reserved_48","name":"Reserved","description":"","unit_of_measurement":"","value":65535,"raw_value":65535}],"source":9,"destination":255,"priority":7}'
Encoding frame: {"PGN":65280,"id":"furunoHeave","description":"Furuno: Heave","fields":[{"id":"manufacturer_code","name":"Manufacturer Code","description":"Furuno","unit_of_measurement":"","value":"Furuno","raw_value":1855},{"id":"reserved_11","name":"Reserved","description":"","unit_of_measurement":"","value":3,"raw_value":3},{"id":"industry_code","name":"Industry Code","description":"Marine Industry","unit_of_measurement":"","value":"Marine","raw_value":4},{"id":"heave","name":"Heave","description":"","unit_of_measurement":"m","value":-0.036000000000000004,"raw_value":-36},{"id":"reserved_48","name":"Reserved","description":"","unit_of_measurement":"","value":65535,"raw_value":65535}],"source":9,"destination":255,"priority":7}'

output:
09FF7 0FF00 3F9FDCFFFFFFFFFF

Encoder Example

from nmea2000.encoder import create_encoder
from nmea2000.input_formats import N2KFormat

# Initialize encoder
encoder = create_encoder(N2KFormat.EBYTE)

# Data to encode: vessel heading message (PGN 127250)
   message = NMEA2000Message(
        PGN=127250,
        priority=2,
        source=1,
        destination=255,
        fields=[
            NMEA2000Field(
                id="sid",
                raw_value=0,
            ),
            NMEA2000Field(
                id="heading",
                value=1, # 1 radian is 57 degrees
            ),
            NMEA2000Field(
                id="deviation",
                raw_value=0,
            ),
            NMEA2000Field(
                id="variation",
                raw_value=0,
            ),
            NMEA2000Field(
                id="reference",
                raw_value=0,
            ),
            NMEA2000Field(
                id="reserved_58",
                raw_value=0,
            )
        ]
    )

msg_bytes = encoder.encode(_generate_test_message())
print(msg_bytes)

Node-RED Integration

You can stream decoded NMEA 2000 data into Node-RED using the CLI with the --json flag and a Node-RED exec node.

Setup

1. Exec node — add an exec node and configure:

   • Command: nmea2000-cli ebyte --server 192.168.1.100 --port 8881 --json
   • Output: select "when stdout has data" so it emits a message for each line (not on process exit)
   • Use spawn mode: enable Use spawn() instead of exec()
   • Timeout: leave blank (this is a long-running process)
   • Append msg.payload: uncheck

2. JSON node — connect the exec node's first output (stdout) to a json node to parse each line into a JavaScript object.

3. Process the data — use a switch or function node to route by PGN:

   // Example function node: add topic by PGN
   msg.topic = "nmea2000/pgn/" + msg.payload.PGN;
   return msg;
   

Importable Flow

Copy and import this JSON into Node-RED (Menu → Import → Clipboard):

[
    {
        "id": "nmea2000_exec",
        "type": "exec",
        "name": "NMEA2000 Stream",
        "command": "nmea2000-cli ebyte --server 192.168.1.100 --port 8881 --json",
        "addpay": "",
        "append": "",
        "useSpawn": "true",
        "oldrc": false,
        "timer": "",
        "wires": [["nmea2000_json"], [], []]
    },
    {
        "id": "nmea2000_json",
        "type": "json",
        "name": "Parse JSON",
        "property": "payload",
        "wires": [["nmea2000_debug"]]
    },
    {
        "id": "nmea2000_debug",
        "type": "debug",
        "name": "NMEA2000 Data",
        "active": true,
        "tosidebar": true,
        "wires": []
    }
]

Each received NMEA 2000 message arrives as a parsed JSON object:

{
    "PGN": 127250,
    "id": "vesselHeading",
    "description": "Vessel Heading",
    "source": 3,
    "destination": 255,
    "priority": 2,
    "fields": [
        {
            "id": "heading",
            "name": "Heading",
            "value": 182.5,
            "unit_of_measurement": "deg"
        }
    ]
}

Tip: Replace ebyte with text, actisense_bst, waveshare, or can depending on your gateway hardware. All gateway subcommands support the --json flag.

Development

Contributions, feedback, and suggestions to improve this project are welcome. If you have ideas for new features, bug fixes, or improvements, feel free to open an issue or create a pull request. I’m always happy to collaborate and learn from the community!

Please don't hesitate to reach out with any questions, comments, or suggestions.

Setup for Development

To contribute to this library, clone the repository and install the required dependencies:

git clone https://github.com/tomer-w/nmea2000.git
cd nmea2000
pip install -e .[dev]

Running Tests

To run the tests, use:

pytest

How decoding works

Decoding, encoding, fast-packet reassembly and fragmentation, and CAN identifiers are all handled by the canboat project's Rust crate and PGN database, compiled into this package as nmea2000._canboat (the crate is in rust/). Messages are built straight into this library's own model (NMEA2000Message, NMEA2000Field, the ##list## repeating-set entries).

Units

By default values are in the units canboat.json uses (%, rpm, L, kWh, ...), as they always have been. canboat's own unit systems are available too:

NMEA2000Decoder(units="si")  # ratio, Hz, m3, J, rad, K, Pa, ...
NMEA2000Decoder(units="metric")  # deg, C, bar, %, rpm, L, ...
create_encoder("basic_string", units="si")  # values given in SI

An encoder must use the unit system its messages' values are in. The few fields where canboat.json's units differ from canboat's SI units are listed in the generated nmea2000/native_units.py.

Building from source needs a Rust toolchain; pip install -e .[dev] compiles the core with maturin.

nmea2000 follows canboat's releases: the "Sync canboat release" workflow runs weekly and opens a pull request that moves the canboat crate in rust/Cargo.toml and canboat.json to the latest release together, and regenerates nmea2000/native_units.py. To do the same by hand, set the crate's version, download that release's canboat.json asset here, run cargo update --package canboat in rust/ and then python canboat2python.py. The FieldTypes, PhysicalQuantities and ManufacturerCodes enumerations in nmea2000/consts.py are built from the crate when nmea2000 is imported; their values keep the numbering nmea2000 has always used.

Compared with the generated decoder used before, the canboat backend:

• reports fields beyond the end of a short payload, and canboat's "not available" and "out of range" sentinels, as None rather than as values read from missing bits;
• applies canboat's field offsets (e.g. AC power fields that are offset by 2,000,000,000);
• returns BINARY and dynamic field values in wire byte order, at their full field width;
• decodes a group function's VARIABLE parameter values with the type of the field they refer to ('Furuno' rather than raw bytes), and reports them as a field of that type: a lookup's raw_value is its code, a number's or a string's is its value. A value that isn't available keeps its wire bytes;
• reports an empty string field as None;
• emits only the repeating-set entries actually present in the payload, even when the count field claims more;
• reads the Manufacturer Code / Industry Code header of a 126208 Read Fields, Write Fields or their replies only when the commanded PGN is proprietary, as the standard defines it.

and its encoder:

• pads the last frame of a fast-packet to 8 bytes with 0xff, as the standard requires;
• writes fixed-length PGNs at their full length, filling unset fields with their "not available" values;
• refuses values it cannot represent (a lookup label that does not exist, a number too large for its field), raising ValueError, instead of writing truncated bits; values outside a field's nominal range that still fit are written as they are;
• writes that 126208 header exactly when the commanded PGN is proprietary, and refuses, with ValueError, a proprietary PGN without it or a standard PGN with it.

N2KDevice claims its address with canboat's ISO 11783-5 address claimer and builds its product information, heartbeat, ISO acknowledgement and PGN list messages with canboat's device module. Its persisted address is unchanged. What changes:

• address claiming follows the standard's timings (ISO 11783-5 / SAE J1939-81): the device listens for other devices' claims for 1 s, then uses its address once its claim has stood unchallenged for 250 ms. The address_claim_startup_delay and address_claim_detection_time options are deprecated and ignored, and passing either raises a FutureWarning. The old defaults waited 1 s and then 5 s, so a device is now ready about 5 s sooner;
• heartbeats are sent at priority 7 and report the CAN controller as "Error Active" (working normally) instead of not available;
• heartbeat_interval must be more than 0 and at most 65.532 s, the longest interval a heartbeat can advertise (the standard's is 60 s); anything else raises ValueError;
• an ISO Request for PGN 126464 is answered with both the Transmit and the Receive PGN lists, decoded as ##list## entries of pgn fields;
• messages the device sends are decoded like received ones: lookups carry their labels, reserved fields their wire values, and the NMEA 2000 version's raw_value is the scaled value (1.3), like every other number.

The per-PGN nmea2000.pgns.decode_pgn_<pgn> / encode_pgn_<pgn> functions remain available for existing code.

Not yet exposed from canboat

The canboat crate has more than this package exposes so far. Each item below would be a binding plus a new option or entry point, with no change to existing behaviour:

• Analyzer JSON: read and write canboat's analyzer -json records (crate feature json-input, engine/output/json.rs), for interchange with canboat, canboatjs and Signal K tooling.
• NMEA 0183 and AIS output: convert decoded messages into 0183 sentences and AIVDM (crate features nmea0183 and ais).
• Capture readers: replay SocketCAN .pcap / .pcap.gz, Navico .nif and EBL captures (io/container.rs, part of the io feature).
• SAE J1939 and Quick: decode J1939 engine and genset PGNs and Quick's 11-bit PCS bus, via a protocol option (engine/bus_protocol.rs).
• Quirks: opt-in decode-time corrections for misbehaving devices, such as GPS week-rollover dates (engine/quirk.rs).

Running the CLI Locally

To test the CLI locally, you can use the following command:

python -m nmea2000.cli decode --frame "your_hex_encoded_frame"

License

This project is licensed under the Apache 2.0 license - see the LICENSE file for details.

Acknowledgements

• This library leverages the canboat as the source for all PGN data.
• Special thanks to Rob from Smart Boat Innovations. His code was the initial inspiration for this project. Some of the code here may still be based on his latest open-source version.