modbus-connection

A small, backend-neutral Modbus connection abstraction.

The top-level modbus_connection package provides the abstract ModbusConnection, the ModbusUnit Protocol, and a tiny exception hierarchy. Two interchangeable backends implement them (tmodbus and pymodbus); the bare install pulls neither.

One physical Modbus link addresses many units (1–247). Sharing a single, internally-serialized connection across many consumers is strictly better than each opening a competing socket. This package is the connection abstraction that makes that sharing possible while keeping the backend swappable.

Install

pip install "modbus-connection[tmodbus]"    # tmodbus backend
pip install "modbus-connection[pymodbus]"   # pymodbus backend

Example

Model a device once, then construct, update, read, and write it. The optional modbus_connection.model framework maps a device's registers and coils to typed attributes and reads the whole device in as few Modbus calls as possible.

import asyncio

from modbus_connection import ModbusTcpParams
from modbus_connection.model import Component, gauge, uint32, coil
from modbus_connection.tmodbus import ModbusConnection


class Meter(Component):
    voltage = gauge(0, 0.1, unit="V")  # scaled 16-bit register
    """Grid voltage."""

    current = gauge(1, 0.1, unit="A")
    """Grid current."""

    energy = uint32(2, unit="Wh")  # 32-bit over two registers
    """Lifetime energy."""

    relay = coil(0, writable=True)
    """Load relay."""


async def main() -> None:
    conn = ModbusConnection(ModbusTcpParams(host="192.168.1.50", port=502))
    try:
        meter = Meter(conn.for_unit(1))

        await meter.async_update()  # one pooled block read
        print(meter.voltage, meter.current, meter.energy, meter.relay)

        await meter.write("relay", True)  # write a writable field
    finally:
        await conn.close()


asyncio.run(main())

Documentation

Everything else — the other transports (UDP, serial, TLS), the full field-type and read-planning reference, repeated sub-units, the SunSpec field types and model generator, the in-memory mock backend for tests, and the exception hierarchy — lives on the website:

https://home-assistant-libs.github.io/modbus-connection/

Develop

uv sync --extra tmodbus
uv run pytest

The suite runs both backends against a real in-process Modbus server (tmodbus's, over TCP, UDP, RTU-over-TCP, serial and TLS) rather than against mocks, so it covers actual framing and error responses — and running the pymodbus client against the tmodbus server checks the two implementations against each other. tests/conftest.py has the datastore and the server helpers.

Formatting/linting is ruff and type-checking is mypy, both enforced in CI. Run them locally with:

uv run mypy

Install the commit hook with prek so code is formatted on commit:

uvx prek install          # set up the git hook
uvx prek run --all-files  # format + lint everything now