Async ModBus

Asynchronous (as in python async/await) modbus python 3 client library. A thin layer on on top of the venerable umodbus library providing an object oriented client API.

async_modbus is async library agnostic. You should be able to use it with asyncio, curio, trio or anyio.

It expects an object with the same interface as StreamReader and StreamWriter so you may need to write a thin wrapper if you are not using asyncio. The examples below show how to use it with curio.

Note: the modbus_for_url() relies on the connio library which relies on the asyncio event loop so you it's usage is limited to asyncio applications.

Why another modbus library?

This library is not a re-implementation of the ModBus communication protocol. You can view it instead as a complement to the umodbus library.

Here is what async_modbus provides on top of umodbus:

• Simple yet powerful object oriented API
• Convenient modbus_for_url() helper function. Type an URL and you're ready to go.
• when appropriate, numpy arrays are used. It's usage not only reduces the memory footprint and increases speed, but also makes it easy for users to efficiently reformat data.
• Compatible with the connio, sockio and serialio libraries which provide transparent socket re-connection among other features.

Installation

From within your favorite python environment type:

$ pip install async_modbus

Numpy will be used if installed. You can install it yourself, or include the optional dependency:

$ pip install async_modbus[numpy]

Library

The core of the async_modbus library consists of a modbus_for_url() function and the two classes AsyncTCPClient and AsyncRTUClient.

Here are some examples:

asyncio examples

simple TCP client

import asyncio

import numpy

from async_modbus import modbus_for_url

async def main():

    client = modbus_for_url("tcp://localhost:15020")

    values = numpy.array([1, 0, 1, 1])  # would also work with list<bool or int>
    reply = await client.write_coils(slave_id=1, starting_address=1, values=values)
    assert reply is len(values)

    reply = await client.read_coils(slave_id=1, starting_address=1, quantity=len(values))
    assert (reply == values).all()

asyncio.run(main())

RTU on local serial line with custom serial options

import asyncio

from async_modbus import modbus_for_url

async def main():

    client = modbus_for_url("serial:///dev/ttyS0", {"baudrate":19200, "parity":"E"})

    values = [1, 0, 1, 1]
    reply = await client.write_coils(slave_id=1, starting_address=1, values=values)
    assert reply is len(values)

    reply = await client.read_discrete_inputs(slave_id=1, starting_address=1, quantity=len(values))
    assert (reply == values).all()

asyncio.run(main())

RTU over remote serial line using RFC2217

import asyncio

from async_modbus import modbus_for_url

async def main():

    client = modbus_for_url("rfc2217://moxa.acme.org:6610")

    values = [1, 0, 1, 1]
    reply = await client.write_coils(slave_id=1, starting_address=1, values=values)
    assert reply is len(values)

    reply = await client.read_discrete_inputs(slave_id=1, starting_address=1, quantity=len(values))
    assert (reply == values).all()

asyncio.run(main())

asyncio TCP streams

import asyncio

import numpy

from async_modbus import AsyncTCPClient

async def main():

    reader, writer = await asyncio.open_connection('localhost', 15020)
    client = AsyncTCPClient((reader, writer))

    values = numpy.array([0, 2**15 - 1, 10, 3, 32766])
    reply = await client.write_registers(slave_id=1, starting_address=1, values=values)
    assert reply is len(values)

    reply = await client.read_holding_registers(slave_id=1, starting_address=1, quantity=len(values))
    assert (reply == values).all()

    writer.close()
    await writer.wait_closed()

asyncio.run(main())

async serial line RTU using remote raw TCP

import asyncio

import numpy

from async_modbus import AsyncRTUClient
from serial_asyncio import open_serial_connection

async def main():

    reader, writer = await open_serial_connection(url="socket://moxa.acme.org:6610")
    client = AsyncRTUClient((reader, writer))

    values = [0, 2**15 - 1, 10, 3, 32766]
    reply = await client.write_registers(slave_id=1, starting_address=1, values=values)
    assert reply is len(values)

    reply = await client.read_input_registers(slave_id=1, starting_address=1, quantity=len(values))
    assert (reply == values).all()

    writer.close()
    await writer.wait_closed()

asyncio.run(main())

curio examples

curio TCP streams

import curio
from async_modbus import AsyncTCPClient

async def main():

    sock = await curio.open_connection("0", 15020)
    client = AsyncTCPClient(sock.as_stream())

    values = [1, 0, 1, 1]
    reply = await client.write_coils(slave_id=1, starting_address=1, values=values)
    assert reply is len(values)

    reply = await client.read_coils(slave_id=1, starting_address=1, quantity=len(values))
    assert (reply == values).all()

    await sock.close()

Credits

Development Lead

• Tiago Coutinho coutinhotiago@gmail.com

Contributors

None yet. Why not be the first?

Special thanks to

• umodbus
• numpy