ModbusSimulator with Python: pymodbus Guide (2026)

Why simulate Modbus in Python instead of using a GUI tool?

Three reasons engineering teams reach for pymodbus specifically: it runs headless in CI/CD without a display, register values can be scripted and version-controlled alongside test code, and a server starts/stops in milliseconds so you can spin up dozens of simulated slaves in a single test run without licensing per-instance.

Installing pymodbus

pip install pymodbus

pymodbus 3.x supports Python 3.9+ and ships both synchronous and asyncio server APIs. The examples below use the synchronous API, which is the simplest starting point for a one-off simulated slave.

Minimal Modbus TCP slave simulator

This starts a Modbus TCP server on port 502 with holding registers pre-loaded with test values — enough to point any Modbus master (SCADA, PLC, or another script) at 127.0.0.1:502 and start polling immediately:

from pymodbus.datastore import (
    ModbusSequentialDataBlock,
    ModbusSlaveContext,
    ModbusServerContext,
)
from pymodbus.server import StartTcpServer

# Pre-load holding registers 0-9 with test values
store = ModbusSlaveContext(
    di=ModbusSequentialDataBlock(0, [0] * 100),   # discrete inputs
    co=ModbusSequentialDataBlock(0, [0] * 100),   # coils
    hr=ModbusSequentialDataBlock(0, [111, 222, 333, 0, 0, 0, 0, 0, 0, 0]),  # holding registers
    ir=ModbusSequentialDataBlock(0, [0] * 100),   # input registers
)
context = ModbusServerContext(slaves=store, single=True)

StartTcpServer(context=context, address=("0.0.0.0", 502))

Any master reading holding registers 0-2 (function code 03) will get back 111, 222, 333 immediately — no PLC, no wiring, no real device required.

Updating register values while the server runs

Because store is a live Python object, you can update register values from another thread while the server is serving requests — useful for simulating a sensor value that changes over time in a test scenario:

import random, threading, time

def drift_holding_register():
    while True:
        value = random.randint(200, 250)
        store.setValues(3, 0, [value])  # function code 3 = holding register
        time.sleep(2)

threading.Thread(target=drift_holding_register, daemon=True).start()

Simulating Modbus RTU (serial) with pymodbus

Swap StartTcpServer for StartSerialServer to simulate an RTU slave over a real or virtual COM port (pair with a null-modem emulator like com0com on Windows to test without hardware):

from pymodbus.server import StartSerialServer

StartSerialServer(
    context=context,
    port="COM5",       # or /dev/ttyUSB0 on Linux
    baudrate=9600,
    parity="N",
    stopbits=1,
    bytesize=8,
)

pymodbus vs. a desktop Modbus simulator

The two aren't mutually exclusive — many teams use pymodbus in automated test suites and a desktop simulator for manual exploratory testing during development.

Related pages

How to Simulate a Modbus Slave Without a PLC · Raspberry Pi as a Modbus Slave Simulator · Modbus TCP Simulator · Modbus Slave Simulator

FAQ