led-ticker-telnet

Watch your led-ticker sign live in any ANSI-capable terminal — the same content that drives your LED panels streams over TCP as true-color ANSI escape sequences.

Every client sees the current frame the moment swap() fires, at panel cadence. Open two terminals, disconnect one, start another — the sign keeps rendering without interruption.

Prerequisites

• A working led-ticker install (core ≥ 2.2).
• An ANSI-capable terminal (any modern terminal on macOS, Linux, or Windows Terminal). True-color support (24-bit) recommended; most terminals released after 2017 support it.
• telnet or nc on the connecting machine (both are standard on macOS and most Linux distros).

Install

This plugin auto-registers via the led_ticker.plugins entry point — once the package is installed, no [plugins] config change is needed.

Into a containerized led-ticker (recommended): add the package to your config/requirements-plugins.txt and rebuild:

led-ticker-telnet

# in your led-ticker checkout
docker compose up -d --build

Standalone (a venv that already has led-ticker):

pip install led-ticker-telnet

See the led-ticker Plugins docs for the constraint-based install the Docker image uses.

Select the backend

In your config/config.toml, set the backend field under [display]:

[display]
backend = "telnet.telnet"
rows = 16
cols = 32
chain_length = 5
brightness = 80
default_scale = 1

The backend name is telnet.telnet — the first telnet is the plugin namespace (from the entry-point registration), and the second names the backend within that namespace.

All other [display] fields (rows, cols, chain_length, default_scale, etc.) work exactly as they do with the rgbmatrix backend. Your widgets, sections, and transitions config is unchanged.

Connect

Once led-ticker starts with the telnet backend, connect from any machine on the same network:

telnet <host> 2300  # default; see Configuration to change

nc <host> 2300  # default; see Configuration to change

Replace <host> with the hostname or IP of the machine running led-ticker. When connecting locally: telnet localhost 2300.

The terminal renders one text row per two canvas rows, using the Unicode UPPER HALF BLOCK character (▀) with the top pixel as the foreground color and the bottom pixel as the background color. This doubles the vertical pixel density in most terminals.

Disconnecting cleanly (Ctrl-C or closing the terminal window) is safe — led-ticker logs the departure and continues running.

Configuration via environment variables

The telnet backend reads two environment variables at startup:

Set them in your shell or in Docker Compose:

# shell
export LED_TICKER_TELNET_PORT=3000

# docker compose (compose.yaml)
environment:
  LED_TICKER_TELNET_PORT: "3000"
  # LED_TICKER_TELNET_HOST: "0.0.0.0"   # bind address; default is all interfaces

Known limitations

brightness is a no-op. The [display] brightness field is required by led-ticker's backend protocol but has no effect on ANSI output — adjust brightness/contrast in your terminal instead.

pixel_mapper_config is accepted but ignored. The telnet backend accepts the pixel_mapper_config argument (required by led-ticker's backend constructor convention) but does not apply any chain-folding or U-mapper remapping — it renders the logical canvas directly to the terminal. Physical-panel layout concerns (serpentine chains, rotation) are a hardware-level concern irrelevant to a terminal preview.

TOML [display] sub-keys are not readable by plugin backends today. The port and host are therefore environment-only — there is no port = 2300 or host = "0.0.0.0" key in the [display] block.

A future led-ticker-core [display.<backend>] → from_config(cls, cfg) mechanism would close this: plugins could define their own sub-table and receive a typed config object at startup. Port and host would then be first-class TOML knobs alongside brightness, rows, and the rest. This is the key usability gap to resolve in a follow-up core change.

Behavior under error conditions

• Bind failure (port in use, insufficient permissions): the backend logs a warning and continues without a server. The sign renders normally; clients simply cannot connect until the conflict is resolved and led-ticker restarts.
• Slow or stalled client: swap() writes to the client without waiting for a drain, so the render loop never blocks. Bytes a client hasn't read accumulate in that connection's in-process write buffer; to keep a stalled client (suspended nc, terminal under Ctrl-S flow control, congested link) from growing that buffer without bound, swap() checks each client's write-buffer size and skips the write for any client over a ~4 MiB cap — that frame is dropped for that client, which recovers automatically once it starts reading again. The render loop and other clients are unaffected.
• Client disconnect mid-session: detected on the next swap() write; the client is pruned and rendering continues.

Development

cd plugins/telnet
uv sync --extra dev      # resolves led-ticker-core from PyPI
uv run pytest -q
uv run ruff check src tests
uv run pyright src

The plugin imports only the public led_ticker.plugin surface — tests/test_import_purity.py enforces it.

Headless smoke / profiling image

Dockerfile.smoke runs led-ticker fully headless (no rgbmatrix, no hardware) with this backend and the bundled smoke config — handy for a quick visual check or profiling the render path on any machine:

cd plugins/telnet
make smoke-run            # build + run, then: telnet localhost 2300
make smoke-stop           # stop + remove the container

Profile a different engine/Python or change the port:

docker build -f Dockerfile.smoke --build-arg CORE_VERSION=2.3.0 -t led-ticker-telnet-smoke .
docker run -d -p 3000:3000 -e LED_TICKER_TELNET_PORT=3000 led-ticker-telnet-smoke

Links

• led-ticker — the core project
• Docs site · Plugin system
• Plugin authoring guide