modbus-connection 2.0.1

A small, backend-neutral Modbus connection abstraction (pymodbus / tmodbus).

modbus-connection

A small, backend-neutral Modbus connection abstraction.

The top-level modbus_connection package is a pure interface — the ModbusConnection / ModbusUnit Protocols, the shared WordOrder type, and a tiny exception hierarchy. It imports no Modbus library and no Home Assistant, so consumers can type against it without committing to a backend.

Two interchangeable backends implement that interface:

Backend	Module	Extra
pymodbus	modbus_connection.pymodbus	[pymodbus]
tmodbus	modbus_connection.tmodbus	[tmodbus]

The bare install pulls neither backend.

Why

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: the Protocol never changes when the backend does.

Design

• A connection is transient and owner-held. A backend connect function returns a live, already-connected instance — there is no connect() on the object.
• Requests are serialized per connection — but by the backend library, not by this wrapper: pymodbus's transaction manager and tmodbus's smart transport each hold a lock for the full request/response cycle, so concurrent unit calls on one connection can't interleave.
• The connection does not self-reconnect. On a drop it fires on_connection_lost (best-effort) and stops; recreating it is the owner's job.
• Consumers receive a ModbusUnit (via connection.for_unit(unit_id)), a stateless per-unit handle with no lifecycle methods. Every method raises on failure — it never returns None.
• The full 19-function-code Modbus surface is exposed as raw register/coil I/O. A backend that cannot implement a code raises NotImplementedError. Datatype and word/byte-order decoding lives one layer up, in modbus_connection.decode / .encode and the modbus_connection.model device framework.

Install

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

Use

import asyncio
from modbus_connection.pymodbus import connect_tcp


async def main() -> None:
    conn = await connect_tcp("192.168.1.50", port=502)
    try:
        unit = conn.for_unit(1)
        from modbus_connection.decode import decode_int16, decode_float32

        outside_temp = decode_int16(await unit.read_holding_registers(9, 1))
        flow_setpoint = decode_float32(await unit.read_holding_registers(40, 2))
        pump_on = (await unit.read_coils(56, 1))[0]
        print(outside_temp, flow_setpoint, pump_on)
    finally:
        await conn.close()


asyncio.run(main())

Swapping to tmodbus is a one-line import change:

from modbus_connection.tmodbus import connect_tcp

Exceptions

Both backends raise the same neutral types:

• ModbusError — base class.
• ModbusConnectionError — link down / not connected / transport failure.
• ModbusTimeoutError — request sent, no valid response in time.
• ModbusExceptionError — device returned a Modbus exception response (.exception_code carries the raw code).

Device modelling (modbus_connection.model)

An optional, backend-neutral framework for mapping a device's registers and coils to typed Python attributes and reading the whole device — or one sub-system — in as few Modbus calls as possible. It talks only to a ModbusUnit, so it runs over any backend (or the mock).

from modbus_connection.model import Component, gauge, uint32, coil

class Meter(Component):
    voltage = gauge(0, 0.1, unit="V")        # scaled 16-bit
    current = gauge(1, 0.1, unit="A")
    energy = uint32(2, unit="Wh")            # 32-bit over two registers
    relay = coil(0, writable=True)

meter = Meter(unit)
await meter.async_update()                   # one block read
meter.voltage                                # float | None
await meter.write("relay", True)

Generic field types ship here — integer, gauge, raw_register, uint32 / int32 / uint64 / int64, float32 / float64, string, scaled_sum, enum / flags (map to an IntEnum / IntFlag), and coil (plus an optional nan sentinel, word_order, and a level_coil write-unlock). The SunSpec module modbus_connection.model.sunspec adds the same types pre-wired with their "unimplemented" sentinels, plus the address types (ipaddr / ipv6addr / eui48).

Shaping that neither covers — composing or transforming a value, packed dates/times — is left to the consumer via a private field + a @property, so static typing stays exact. For example, prefixing a version register with a hard-coded model name:

from modbus_connection.model import Component, string

class Controller(Component):
    _firmware = string(10, 4)  # 4 registers of ASCII, e.g. "1.23"

    @property
    def model(self) -> str | None:
        firmware = self._firmware
        return f"TROVIS 5576 ({firmware})" if firmware is not None else None

Each component can refresh independently and has its own update listeners (one Home Assistant entity per component). To refresh several components that share a unit in one consolidated set of reads, group them in a ComponentGroup and call async_update() on it:

group = ComponentGroup(unit, [water_heater, circuit_1, circuit_2, circuit_3])
await group.async_update()  # one pooled set of reads; each component notified

The ComponentGroup builds its pooled read plan from the components' static layout on the first update and reuses it on every later poll. The component list, their fields, and the ranges are read once and cached — mutating them after the first update is not supported; build a new ComponentGroup (or Component) instead.

Readable address ranges

Reads are pooled into block reads — addresses close together are fetched in one call. By default the planner merges anything within a small gap, which assumes every address in between is readable. Many devices only answer reads inside specific ranges, and a read that crosses a gap is rejected.

Declare the device's readable ranges and the planner merges only within a range, never across a boundary, and still clips each read to the addresses actually used. Set them as a class attribute (shared by every instance) or per instance. A ComponentGroup reads the ranges off its components, so every component in a group must declare the same ranges (it raises otherwise):

class Thermostat(Component):
    # (low, high) inclusive. The device answers 0–6 and 9–40 but nothing in
    # between, so 7–8 are never read and a 0..40 block is split at the gap.
    register_ranges = ((0, 6), (9, 40))
    coil_ranges = ((0, 15),)

    model = integer(0)
    outside = gauge(9, 0.1, unit="°C")

group = ComponentGroup(unit, [thermostat])  # ranges come from the components
await group.async_update()

Leave them as the default None for devices with a contiguous map (plain gap-based planning).

Register spaces (holding vs input)

A component's register fields default to the holding space (FC03). For a read-only sub-system whose data lives in input registers (FC04), set register_space = "input" on the component — fields and factories are unchanged:

class Sensors(Component):
    register_space = "input"
    flow_temp = gauge(5, 0.1, unit="°C")   # read with FC04

Input and holding are separate address spaces (input 507 ≠ holding 507), so the planner never merges them into one read, and register_ranges applies within the component's own space. A ComponentGroup may mix input and holding components: it reads each space with its own block reads, and components only need matching register_ranges with others in the same space. Input registers are physically read-only, so writing a field on an "input" component raises.

Testing

An in-memory mock backend ships as a pytest plugin (auto-registered via an entry point — no conftest wiring). It implements the same Protocols, so code typed against ModbusUnit runs against it unchanged.

async def test_reads_setpoint(mock_modbus_unit):
    mock_modbus_unit.holding[40] = 1234            # single value
    mock_modbus_unit.holding[2] = [0x0001, 0x86A0]  # list -> consecutive registers
    mock_modbus_unit.holding[9] = lambda: 7         # callable -> evaluated per read

    assert await mock_modbus_unit.read_holding_registers(40, 1) == [1234]
    assert await mock_modbus_unit.read_holding_registers(2, 2) == [0x0001, 0x86A0]

Reads resolve against the per-space stores (holding, input, coils, discrete_inputs); writes mutate them and fire on_write callbacks, so a test can react to a write by mocking other registers:

def test_command_sets_ready(mock_modbus_unit):
    def respond(event):
        if event.address == 0:          # a command was written
            mock_modbus_unit.holding[100] = 1   # device flips its "ready" flag
    mock_modbus_unit.on_write(respond)

To simulate a device rejecting a write, arm fail_write. The next write covering that address raises the given error before the store is touched, so the value is left unchanged and on_write callbacks don't fire. register_type defaults to "holding" (use "coil" for coil writes — the two tables are independent); pass None to clear.

async def test_write_rejected(mock_modbus_unit):
    mock_modbus_unit.holding[40] = 7
    mock_modbus_unit.fail_write(40, ModbusExceptionError(3))  # illegal data value
    with pytest.raises(ModbusExceptionError):
        await mock_modbus_unit.write_register(40, 99)
    assert await mock_modbus_unit.read_holding_registers(40, 1) == [7]  # unchanged

    mock_modbus_unit.fail_write(40, None)                     # clear it
    await mock_modbus_unit.write_register(40, 99)             # now succeeds

To simulate a read failing, give the register a callable that raises — it's evaluated on every read:

def boom():
    raise ModbusExceptionError(2)       # illegal data address
mock_modbus_unit.holding[9] = boom

Fixtures: mock_modbus_connection (a MockModbusConnection) and mock_modbus_unit (its unit 1). MockModbusConnection / MockModbusUnit are also importable from modbus_connection.mock for direct construction.

Develop

uv sync --extra pymodbus
uv run pytest

Formatting/linting is ruff, enforced in CI. 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