bacsys-pymod

Production-grade Modbus TCP / RTU driver for Python. Library + one-shot CLI.

Distribution name bacsys-pymod, import name pymod.

import pymod

with pymod.Client.tcp("10.0.0.5", 502, unit_id=1) as c:
    results = c.read([
        pymod.Holding(start=0, count=10, dtype="float32"),
        pymod.Coil(start=0, count=16),
        pymod.Holding(start=0, count=2, dtype="bit", bit_index=8),
    ])
    for r in results:
        print(r.values if r.ok else f"failed: {r.error}")

Highlights

• Modbus TCP with concurrent connections and request pipelining (TID demultiplexing).
• Modbus RTU over a serial port — sequentialized internally, safe to share across callers.
• Modbus RTU over TCP for serial-to-Ethernet gateways (Moxa-style).
• All standard function codes (FC01/02/03/04/05/06/15/16) plus a registration hook for custom FCs.
• Heterogeneous batch reads — pass a list of typed items (Holding, Input, Coil, Discrete) and the planner coalesces adjacent same-area ranges into the minimum number of Modbus requests.
• Typed values — int16, uint16, int32, uint32, float32, int64, uint64, float64, plus bit extraction from registers.
• All four PLC byte/word orderings (ABCD / CDAB / BADC / DCBA) configurable per item.
• Sync facade for Flask / scripts / REPL — pymod.Client.tcp(...) is a normal blocking object backed by an internal asyncio loop.
• Server mode — callback-based, no internal datastore. Bring your own data, use it for protocol bridging (e.g. Modbus → BACnet / MQTT).
• pymod CLI — read, write, scan, serve, plus an interactive --guide wizard.
• Python 3.11+, pure Python (one runtime dep: pyserial), mypy --strict clean.

Install

pip install bacsys-pymod

Once installed:

import pymod
print(pymod.__version__)

…and the pymod CLI is on PATH.

Quick examples

TCP — sync client (Flask, scripts, REPL)

import pymod

client = pymod.Client.tcp("127.0.0.1", 502, unit_id=1, timeout_s=0.5)
client.connect()
try:
    results = client.read([pymod.Holding(start=0, count=10, dtype="int16")])
    print(results[0].values)
finally:
    client.close()

TCP — async client

import asyncio, pymod

async def main():
    async with pymod.AsyncClient.tcp("127.0.0.1", 502, unit_id=1) as c:
        results = await c.read([pymod.Holding(start=0, count=10, dtype="int16")])
        print(results[0].values)

asyncio.run(main())

Serial RTU

client = pymod.Client.rtu("COM5", baudrate=9600, parity="N", unit_id=1)

…the rest of the API is identical to the TCP client.

Modbus TCP server (callback-based)

import asyncio, pymod

async def on_read(area, address, count):
    return store.read(area, address, count)

async def on_write(area, address, values):
    store.write(area, address, values)

async def main():
    async with pymod.Server(host="0.0.0.0", port=502,
                            on_read=on_read, on_write=on_write):
        await asyncio.Future()

asyncio.run(main())

The server has no internal datastore — the host application owns the data, the server is just a Modbus protocol terminator. Perfect for bridging Modbus to other industrial protocols.

CLI

pymod read  --host 127.0.0.1 --port 502 --area holding --start 0 --count 10 --dtype int16
pymod write --host 127.0.0.1 --port 502 --area holding --start 0 --values 1,2,3 --dtype uint16
pymod scan  --host 127.0.0.1 --port 502
pymod serve --host 0.0.0.0   --port 5020 --holding 0=100,1=200
pymod --guide       # interactive wizard

Each subcommand has detailed --help.

Documentation

Full docs at https://bacsys-pymod.readthedocs.io/.

License

MIT. See LICENSE.