A browser-based CAN bus analyzer. Plug in. One command. Analyze.

pip install canviz
canviz
# browser opens at http://localhost:8080

CAN bus tools are either expensive, locked to proprietary hardware, or require compiling from source. CANviz is different: it runs in your browser, installs with one pip command, and works with any USB CAN adapter including adapters that cost under $10.

No GUI install. No driver setup. No account. No internet connection. Whether you are debugging an ECU, commissioning a servo drive, analyzing J1939 truck traffic, or working on a robot arm over CANopen, CANviz puts every frame, signal, and protocol event in front of you immediately.

Install and Run

pip install canviz
canviz

That is it. CANviz detects your connected adapter and opens the browser automatically.

Not sure which interface to select? See the Device Guide.

What You Get

See CHANGELOG.md for full version history.

Live message table

Every frame on the bus, in real time. ID, DLC, raw bytes, frame count, rate, and last-seen timestamp. Virtual scrolling handles thousands of rows without frame loss. Tested at 2,000 fps sustained with zero drops.

DBC signal decoding

Upload a .dbc file and raw hex bytes become named signal values, shown inline in the message table. Toggle between raw and decoded view at any time.

Signal time-series plotting

Plot any decoded signal as a live time-series graph. Up to 8 signals on a shared time axis, 36,000 buffered points per signal, LTTB downsampling, 10 Hz render rate regardless of bus speed. Drag to zoom, double-click to resume live scroll, set per-signal threshold lines with breach alerts. One-click PNG export.

Multi-frame transmit with timers

Build a list of frames, each with its own independent transmission interval. Send a heartbeat at 20 Hz and a speed signal at 10 Hz simultaneously. State persists across tab switches.

Record and replay

Record to industry-standard .asc and .csv. Replay any log file with adjustable speed from 0.5x to 10x.

Bus health statistics

Always-visible status bar: frames Rx/Tx, bus load %, error frame count, bus-off events, throughput. Error frame visibility depends on hardware -- slcan firmware typically drops error frames silently; a tooltip explains this when a slcan interface is active.

CLI and headless mode

canviz monitor renders a live colour-coded table in the terminal over SSH, in CI pipelines, and on headless Raspberry Pi setups. See CLI Reference.

Protocol Decoders

CANviz includes passive decoders for J1939 and CANopen. They run entirely from captured traffic -- no extra hardware, no polling, no configuration required beyond enabling the decoder in the UI. Both decoders appear in the right-side protocol panel which sits alongside the message table and is resizable.

J1939

J1939 is the dominant protocol in trucks, agriculture, marine, and construction equipment.

• Auto-detection at 250 kbps with 29-bit extended IDs
• CAN ID decomposition -- Priority, PGN, Source Address, Destination Address on every frame
• 54 built-in PGN names -- EEC1, CCVS, DM1, ET1, AMB, VEP1, transport protocol and more
• 99 source address names -- "Engine #1", "Brakes - System Controller", "Diesel Particulate Filter Controller"
• BAM transport reassembly -- multi-packet messages (VIN, software ID, long DM payloads) shown complete
• DM1 active fault decode -- SPN number, component name, Failure Mode Indicator, occurrence count, lamp status

See J1939 Guide for full documentation and DBC-based signal decode.

CANopen (CiA 301 + CiA 402)

CANopen is the dominant protocol in robotics, industrial automation, and motion control. CiA 402 covers virtually every servo drive and motor controller.

• Auto-detection from COB-ID structure -- no bitrate assumption needed
• Nodes table -- NMT state, heartbeat interval, frame count per node
• CiA 402 drive state -- Statusword decoded to named state (Operation Enabled, Fault, Quick Stop etc.) shown inline per node
• CiA 402 Drive Control -- Quick buttons: Enable, Switch On, Shutdown, Quick Stop, Disable, Fault Reset. One-time safety notice.
• EMCY decode -- error code, error class name, error register flags, manufacturer data
• SDO log -- request/response pairs auto-paired from bus traffic with object names from a 180-entry built-in CiA 301/402 dictionary
• SDO Read / Write -- send expedited reads and writes to any live node from the panel
• NMT commands -- Operational, Pre-Op, Stop, Reset Node, Reset Comm with node selector
• Object dictionary browser -- search 180 standard objects by name or index
• PDO signal plotting -- upload a device EDS file to decode TPDO payloads to named signals that feed the Plot tab
• Read All Node Objects -- one click reads 15 standard CiA 301/402 objects from every discovered node

See CANopen Guide for full documentation, EDS requirements, drive control safety notes, and REST API reference.

Hardware

Any adapter running Candlelight firmware works plug-and-play on Windows with no driver setup:

Also supported via python-can configuration:

• slcan -- devices running slcan firmware (COM port on Windows)
• SocketCAN -- Linux, Raspberry Pi, WSL2
• PEAK PCAN-USB -- requires PEAK driver installed
• Kvaser -- supported in config, see Known Limitations
• Virtual bus -- software loopback, no hardware needed

Security Model

Browser  (your local browser tab)
    |  HTTP + WebSocket -- localhost only, never leaves your machine
Python backend  (127.0.0.1:8080)
    |  python-can
USB CAN adapter
    |
CAN Bus

The browser communicates only with a local Python process at 127.0.0.1:8080. No data leaves your machine. No cloud. No telemetry. No external connections of any kind. All USB communication happens inside the Python backend -- the browser never has direct access to your USB device or CAN bus.

The security model is the same as running any locally installed Python tool: you are trusting the code you installed via pip. If you are security-conscious, review the source on GitHub before installing.

Remote deployments: Use the default --host 127.0.0.1 binding and access via SSH port forwarding. Do not expose port 8080 to an untrusted network without a reverse proxy with authentication.

CLI Reference

canviz [OPTIONS]

  --interface   gs_usb | slcan | socketcan | virtual  (default: gs_usb)
  --channel     COM port or SocketCAN channel  (e.g. COM3, can0)
  --bitrate     CAN bitrate in bps  (default: 500000)
  --host        Host to bind to  (default: 127.0.0.1)
  --port        Port to bind to  (default: 8080)
  --headless    Start without opening a browser

Subcommands:

# Live terminal monitor -- works over SSH
canviz monitor --interface socketcan --channel can0 --dbc vehicle.dbc

# Capture frames to file
canviz capture --output trace.json --duration 60

# Decode a captured log
canviz decode --input trace.json --dbc vehicle.dbc --output decoded.csv

# API-only server, no browser
canviz serve --headless --port 8080

See the CLI Guide for full SSH workflow documentation.

REST API and WebSocket

Full interactive docs at http://localhost:8080/docs while running.

Architecture

[CAN Bus]
    |
[USB CAN Module]  ~$8-$65 depending on capability
    |
[Python Backend]
  FastAPI  python-can  cantools  aiofiles  typer  rich
    |  HTTP + WebSocket (localhost only)
[Browser UI]
  React 18  TanStack Table  TanStack Virtual  Zustand  uPlot
    |
  http://localhost:8080

Known Limitations

• USB timestamp jitter ~1 ms -- a hardware limitation of USB-connected CAN adapters. Not suitable for sub-millisecond timing analysis.
• High bus load above 2,000 fps -- untested. A server-side throttling hook is built in and can be enabled if needed.
• CAN FD -- frames with >8 byte payloads display as raw hex. Full CAN FD UI is planned.
• slcan error frames -- slcan firmware silently drops error frames before forwarding. Bus error statistics read 0% on slcan interfaces even on a degraded bus. Use gs_usb for accurate error visibility.
• Kvaser on Windows -- CANviz has full UI support. Connection fails due to a bug in python-can 4.6.1 (canIoCtlInit returning canERR_PARAM). Tracked at python-can #2051. Upgrade python-can once resolved and Kvaser will work without any CANviz changes.
• CANopen PDO decode -- requires the device EDS file. Without EDS, TPDO1 bytes 0-1 are decoded as Statusword by default assumption; other PDO bytes are raw hex.
• CANopen SDO segmented / block transfer -- only expedited SDO (1-4 byte payloads) is decoded. Segmented responses show as raw data.
• J1939 PGN database -- SAE J1939 Digital Annex is copyrighted and cannot be redistributed. The built-in 54-PGN table covers common engine and vehicle signals. Load your own DBC for full coverage.
• Replay timing -- depends on the Python asyncio scheduler, not a hardware clock.
• Browser support -- tested on Chrome. Firefox and Edge are best-effort.
• Mobile layout -- not a target. Optimised for 1080p and above.

Troubleshooting

No matching distribution found for canviz on Ubuntu / Linux

• Use pip3 install canviz or python3 -m pip install canviz. CANviz requires Python 3.10+. Ubuntu 20.04 ships Python 3.8 -- upgrade to 22.04+ or install Python 3.10 separately.
• If still doesn't work then try installing with pipx, sudo apt install pipx pipx ensurepath, then try pipx install canviz

Device shows a COM port on Windows Your adapter is running slcan firmware, not Candlelight. Use: canviz --interface slcan --channel COM3

PEAK PCAN-USB fails to connect Ensure the PEAK driver is installed. Device Manager must show the device under CAN-Hardware, not as an unknown device.

Kvaser fails to connect (canIoCtl failed -- Error in parameter) Known bug in python-can 4.6.1. See Known Limitations.

CANopen nodes not appearing Enable the decoder, then wait one heartbeat cycle (typically 250-1000 ms). If nodes have heartbeat disabled (0x1017 = 0), send an NMT Operational broadcast first.

CANopen SDO Read returns ABORT The node rejected the request. Common causes: object does not exist in that node's object dictionary (0x06020000), or the object is write-only (0x06010001). Check the abort code in the SDO log for the exact reason.

Roadmap

• Live frame table, DBC decode, filter, send, record, replay, pip install
• Signal plotting, multi-signal overlay, threshold alerts, CLI mode, bus health statistics, multi-frame transmit
• J1939 passive decoder -- PGN/SA decode, BAM reassembly, DM1 fault decode
• CANopen CiA 301 + CiA 402 -- NMT, EMCY, SDO, PDO, heartbeat, drive control
• CAN FD explicit UI support (>8 byte payloads, FDF/BRS/ESI flags)
• OBD-II over raw CAN -- ISO 15765-4, no ELM327 required
• UDS diagnostic panel -- ReadDataByIdentifier, ReadDTCInformation, ClearDTC
• Reverse engineering toolkit -- bit-flip rate heatmap, notch-filter sniffer, auto DBC generation
• Plugin API -- pip-installable community decoders and exporters

Contributing

CANviz is actively developed. The most useful contribution right now is testing hardware we have not tried -- a CANable 2.0, anything on macOS, anything on a COM port. No code required. Open an issue and tell us what happened.

Bug reports, DBC files that decode incorrectly, and code contributions are all welcome. See CONTRIBUTING.md for how to get the dev environment running.

Acknowledgements

CANviz is built on the shoulders of excellent open source work. None of this would exist without:

• python-can -- the hardware abstraction layer that makes every CAN adapter work with the same three lines of Python. The most important dependency in the stack.
• cantools -- DBC parsing and signal decoding. Rock-solid, well-maintained, handles every DBC dialect we have thrown at it.
• canopen -- CiA 301 object dictionary parsing and EDS support. Powers the CANopen PDO decode pipeline.
• FastAPI -- the backend API framework. The async WebSocket support and automatic OpenAPI docs are genuinely excellent.
• uPlot -- an absurdly fast time-series plotting library. 36,000 points per signal at 10 Hz with no jank takes real engineering skill to pull off, and Leon Sorokin pulled it off.
• TanStack Table and TanStack Virtual -- virtual scrolling thousands of CAN frames without dropping a row is not trivial. These libraries make it straightforward.
• Zustand -- minimal, fast, and does not get in the way. State management that earns its place.
• Typer and Rich -- the CLI terminal output looks good because of these two.
• pretty_j1939 -- the open J1939 PGN and SPN definitions that power the J1939 decoder's built-in name table.

Thank you to everyone who filed issues, tested hardware, and sent DBC files. Every piece of feedback made the tool better.

Support

CANviz is free and open source. If it saves you time or helps your project, consider sponsoring development on GitHub. Every sponsorship directly funds hardware for testing new adapters and protocols.

License

MIT -- see LICENSE.