Modbus Slave Key ((hot)) May 2026

Here’s a key piece of code in Python using the pymodbus library. This creates a slave (server) that holds simulated register data and responds to Modbus requests.

from pymodbus.server import StartTcpServer from pymodbus.device import ModbusDeviceIdentification from pymodbus.datastore import ModbusSlaveContext, ModbusServerContext from pymodbus.datastore import ModbusSequentialDataBlock Default size: 0x00 to 0xFF (256 addresses each) store = ModbusSlaveContext( di=ModbusSequentialDataBlock(0, [0] * 100), # Discrete Inputs (read-only) co=ModbusSequentialDataBlock(0, [0] * 100), # Coils (read-write) hr=ModbusSequentialDataBlock(0, [0] * 100), # Holding Registers (read-write) ir=ModbusSequentialDataBlock(0, [0] * 100) # Input Registers (read-only) ) Set initial test values (example) store.setValues(3, 0, [123, 456, 789]) # Holding register addresses 0,1,2 = 123,456,789 Wrap in server context (slave ID 1) context = ModbusServerContext(slaves=store, single=True) Device identification (optional) identity = ModbusDeviceIdentification() identity.VendorName = 'Modbus Slave Example' identity.ProductCode = 'PYM' identity.VendorUrl = 'https://github.com/pymodbus-dev/pymodbus' identity.ProductName = 'PyModbus Slave' identity.ModelName = 'PyModbus Slave' identity.MajorMinorRevision = '1.0' Start TCP Modbus server on port 5020 (requires admin for 502) print("Modbus Slave running on port 5020...") StartTcpServer(context=context, identity=identity, address=("0.0.0.0", 5020)) modbus slave key