Virtual Slave Modbus RTU

[R-a-f-a-e-l-s]

I'm trying to simulate a Modbus RTU slave through a COM port using a USB/RS485 converter. The problem is that the slave only sends zeros instead of the simulated data.

**# Code
from pymodbus.server import StartSerialServer
from pymodbus.datastore import ModbusSequentialDataBlock, ModbusSlaveContext, ModbusServerContext

import logging
logging.basicConfig()
log = logging.getLogger()
log.setLevel(logging.DEBUG)

def run_single_port_multi_slave_server():
slaves = {}
for i in range(0, 17): # Crear 16 esclavos con IDs del 1 al 16
slave_id = i
# Inicializar los registros 40001-40008 para cada esclavo
initial_values = [slave_id * 10 + j for j in range(10)]
store = ModbusSlaveContext(
hr=ModbusSequentialDataBlock(0, initial_values)
)
slaves[slave_id] = store

# Crear el contexto del servidor con los múltiples esclavos
context = ModbusServerContext(slaves=slaves, single=False)

# Configuración del puerto serie (todos los esclavos en el mismo puerto)
settings = {
    'port': 'COM9',
    'baudrate': 19200,
    'bytesize': 8,
    'parity': 'N',
    'stopbits': 1
}


try:
    # Iniciar el servidor Modbus RTU
    StartSerialServer(context, **settings)
    print(f"Servidor Modbus RTU simulando 16 esclavos (IDs 1-16) en el puerto {settings['port']}...")
    print("Cada esclavo responderá a su unidad ID en el mismo puerto.")
except Exception as e:
    print(f"Error al iniciar el servidor Modbus RTU: {e}")
    print(f"Asegúrate de que el puerto {settings['port']} existe y no está en uso.")

if name == "main":
run_single_port_multi_slave_server()**

Output:

DEBUG:asyncio:Using proactor: IocpProactor
DEBUG:pymodbus.logging:Awaiting connections server_listener
INFO:pymodbus.logging:Server listening.
DEBUG:pymodbus.logging:Connected to server
DEBUG:pymodbus.logging:recv: 0x1 0x3 0x0 0x0 0x0 0x8 0x44 0xc old_data: addr=None
DEBUG:pymodbus.logging:Processing: 0x1 0x3 0x0 0x0 0x0 0x8 0x44 0xc
DEBUG:pymodbus.logging:decoded PDU function_code(3 sub -1) -> ReadHoldingRegistersRequest(dev_id=0, transaction_id=0, address=0, count=8, bits=[], registers=[], status=1)
DEBUG:pymodbus.logging:Frame advanced, resetting header!!
DEBUG:pymodbus.logging:getValues: fc-[3] address-1: count-8
DEBUG:pymodbus.logging:send: 0x1 0x3 0x10 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0xe4 0x59
DEBUG:pymodbus.logging:recv: 0x2 0x3 0x0 0x0 0x0 0x8 0x44 0x3f old_data: addr=None
DEBUG:pymodbus.logging:Processing: 0x2 0x3 0x0 0x0 0x0 0x8 0x44 0x3f
DEBUG:pymodbus.logging:decoded PDU function_code(3 sub -1) -> ReadHoldingRegistersRequest(dev_id=0, transaction_id=0, address=0, count=8, bits=[], registers=[], status=1)
DEBUG:pymodbus.logging:Frame advanced, resetting header!!
DEBUG:pymodbus.logging:getValues: fc-[3] address-1: count-8
DEBUG:pymodbus.logging:send: 0x2 0x3 0x10 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0xa0 0x1d

pymodbus
Version: 3.9.2

Windows 10 Pro

[janiversen]

slaves cannot have id=0 that is reserved for broadcast.

Apart from that the code looks ok, after just a short glance.

As you can see in our examples the sequential data block is tested with the value 17 (more details in the test harness) so I am not sure why you get zeroes back, did you try with just one slave?

[R-a-f-a-e-l-s]

slaves cannot have id=0 that is reserved for broadcast.

Apart from that the code looks ok, after just a short glance.

As you can see in our examples the sequential data block is tested with the value 17 (more details in the test harness) so I am not sure why you get zeroes back, did you try with just one slave?

Thanks for the note! Just to clarify, I'm not actually using slave id=0—that’s what’s puzzling me.

I also tried running the test with just a single slave, but I'm still getting all zeroes in return. I’ll double-check again in case I missed something subtle, but at this point it seems like there might be something else going on.

Appreciate any further suggestions you might have!

[janiversen]

if you look at the examples:

server_async.py, you will find it defines a sequential datastore that contains

ModbusSequentialDataBlock(0x00, [17] * 100)

and in client_async.py you will find:

    rr = await client.read_holding_registers(4, count=2, slave=1)
    assert rr.registers[0] == 17
    assert rr.registers[1] == 17

so you see there are examples that controls that data get back, these examples are run as part of every pull request (with a single slave), so I am quite sure that works.

your code contains for i in range(0, 17)so you define device_id 0-16.

[janiversen]

Did you solve your problem?

[R-a-f-a-e-l-s]

Yes, I solved it, thank a lot

My final code:

from pymodbus.server import StartSerialServer
from pymodbus.datastore import ModbusSequentialDataBlock, ModbusSlaveContext, ModbusServerContext

import logging
logging.basicConfig()
log = logging.getLogger()
log.setLevel(logging.DEBUG)

def run_single_port_multi_slave_server():
slaves = {}
for i in range(1, 16): # Crear 16 esclavos con IDs del 1 al 16
slave_id = i
# Inicializar los registros 40001-40008 para cada esclavo
#initial_values = [slave_id * 10 + j for j in range(8)]
#store = ModbusSlaveContext(
#hr=ModbusSequentialDataBlock(0, initial_values)
#)
store = ModbusSlaveContext(
di=ModbusSequentialDataBlock(0, [0]*100),
co=ModbusSequentialDataBlock(0, [0]*100),
hr=ModbusSequentialDataBlock(0, [0]*100),
ir=ModbusSequentialDataBlock(0, [0]*100))
slaves[slave_id] = store

# Crear el contexto del servidor con los múltiples esclavos
slaves[15].setValues(3, 0, [1, 2, 3,4,5,6,7,8])
context = ModbusServerContext(slaves=slaves, single=False)

# Configuración del puerto serie (todos los esclavos en el mismo puerto)
settings = {
    'port': 'COM12',
    'baudrate': 19200,
    'bytesize': 8,
    'parity': 'N',
    'stopbits': 1
}

try:
    # Iniciar el servidor Modbus RTU
    StartSerialServer(context, **settings)
    print(f"Servidor Modbus RTU simulando 16 esclavos (IDs 1-16) en el puerto {settings['port']}...")
    print("Cada esclavo responderá a su unidad ID en el mismo puerto.")
except Exception as e:
    print(f"Error al iniciar el servidor Modbus RTU: {e}")
    print(f"Asegúrate de que el puerto {settings['port']} existe y no está en uso.")

if name == "main":
run_single_port_multi_slave_server()