pymodbus : Modbus Error: [Input/Output] No Response received from the remote slave/Unable to decode response

Question

I am trying to read holding registers from a modbus server with pyModbus but I am having issues with the response from the server.

To test that my connection is working, I tried Reading and writing with Modbus-poll and QModMaster and everything is working well.

Could you please let me know what I can do to fix this issue ? Thanks

This is my code, tested on windows 10 and vscode:

python Version : 3.12.0 pymodbus Version : 3.5.2

import time
from pymodbus.client import ModbusTcpClient

import logging

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

# Define the Modbus server's IP address and port
server_ip = "10.10.10.10"
server_port = 502

# Create a Modbus client
client = ModbusTcpClient(host=server_ip, port=server_port, timeout=10)

#[enter image description here](https://i.stack.imgur.com/2g6lI.png)
def read_registers(startAddress, count):
    # Connect to the Modbus server
    while True:
        client.connect()
        try:
            if client.connected:
                print("client connected")
                # Read holding registers
                response = client.read_holding_registers(
                    address=startAddress, count=count, slave=1
                )

            else:
                print("client not connected")

            if not response.isError():
                data = response.registers
                print(f"Read data: {data}")
            else:
                # Do stuff for error handling.
                print("Error message: {}".format(response))

        except Exception as e:
            print(f"Error: {str(e)}")

        finally:
            # Close the Modbus client
            print("closing client")

            client.close()
            time.sleep(5)


if __name__ == "__main__":
    read_registers(1000, 5)

I also tried using slave=1 and unit=1 but none of that worked.

This is the full log :

DEBUG:pymodbus.logging:Connection to Modbus server established. Socket ('10.10.10.100', 51868)
client connected
DEBUG:pymodbus.logging:Current transaction state - TRANSACTION_COMPLETE
DEBUG:pymodbus.logging:Running transaction 55
DEBUG:pymodbus.logging:SEND: 0x0 0x37 0x0 0x0 0x0 0x6 0x1 0x3 0x3 0xe8 0x0 0x5
DEBUG:pymodbus.logging:New Transaction state "SENDING"
DEBUG:pymodbus.logging:Changing transaction state from "SENDING" to "WAITING FOR REPLY"
DEBUG:pymodbus.logging:Changing transaction state from "WAITING FOR REPLY" to "PROCESSING REPLY"
DEBUG:pymodbus.logging:RECV: 0x8d 0xce 0xd0 0xbd 0x0 0xc2 0x1 0xa5 0x1a
DEBUG:pymodbus.logging:Processing: 0x8d 0xce 0xd0 0xbd 0x0 0xc2 0x1 0xa5 0x1a
DEBUG:pymodbus.logging:Frame check failed, ignoring!!
DEBUG:pymodbus.logging:Resetting frame - Current Frame in buffer - 0x8d 0xce 0xd0 0xbd 0x0 0xc2 0x1 0xa5 0x1a
DEBUG:pymodbus.logging:Getting transaction 55
DEBUG:pymodbus.logging:Changing transaction state from "PROCESSING REPLY" to "TRANSACTION_COMPLETE"
Error message: Modbus Error: [Input/Output] No Response received from the remote slave/Unable to decode response
closing client```

and This is the result using QModMaster for checking if my server is working.

QModMasterScreenshot

I also tried to see the packets with wireshark and I can see that my request is sent to the server and right after that the request is reset, not sure why.

WiresharkPackets

Answer 1

I'm also working in the same areas. And I have used the Pymodbus library as follows;

#importing the library
from pymodbus.client import ModbusTcpClient

#connecting with the IP
client = ModbusTcpClient('10.10.10.10', port=502)
client.connect()

#extracting the data
result = client.read_holding_registers(address=register, count=1, unit=1)
print(result.registers[0])

Hope this will help you!

Answer 2

IMHO there is a bug in pymodbus because:

• you can send RTU frames without problems with pymodbus
• when you receive frames using RTU framer the debug output shows some weird bytes instead of actual real response from the modbus.
• using other methods (like other programming languages) receive works without problem
• sniffing the bus itself shows correct answer which is simply misinterpreted by rtu framer of pymodbus incorrectly.

I have a modbus slave (MODBUS RTU over TCP), bus sniffer and a pymodbus application on a linux box running a very simple code:

from pymodbus import (ExceptionResponse,Framer,ModbusException,    pymodbus_apply_logging_config,)
pymodbus_apply_logging_config("DEBUG")
client = ModbusTcpClient("10.0.2.200", port=502, framer=Framer.RTU)
success = client.connect()
#client.write_registers(address=0,values=[128,129,130,131],slave=1,unit=1)
result = client.read_holding_registers(address=0, count=4, slave=1)

when I run it on the modbus there is a correct transmission (I am sniffing the bus with RS485 sniffer):

01 03 00 00 00 04 44 09 
01 03 08 00 80 00 81 00 82 00 83 C9 88 

The first line is the correct "read holding registers" RTU frame, and the second line is the correct response. The registers contain 0x0080,0x0081,0x0082 and 0x0083 respectively.

And now to debug output from pymodbus code above:

2024-02-03 10:05:22,743 DEBUG logging:103 Current transaction state - IDLE
2024-02-03 10:05:22,743 DEBUG logging:103 Running transaction 1
2024-02-03 10:05:22,743 DEBUG logging:103 SEND: 0x1 0x3 0x0 0x0 0x0 0x4 0x44 0x9
2024-02-03 10:05:22,744 DEBUG logging:103 Resetting frame - Current Frame in buffer -
2024-02-03 10:05:22,744 DEBUG logging:103 New Transaction state "SENDING"
2024-02-03 10:05:22,744 DEBUG logging:103 Changing transaction state from "SENDING" to "WAITING FOR REPLY"
2024-02-03 10:05:22,904 DEBUG logging:103 Changing transaction state from "WAITING FOR REPLY" to "PROCESSING REPLY"
2024-02-03 10:05:22,905 DEBUG logging:103 RECV: 0x8d 0xce 0xbd 0xdc 0x1
2024-02-03 10:05:22,905 DEBUG logging:103 Processing: 0x8d 0xce 0xbd 0xdc 0x1
2024-02-03 10:05:22,905 DEBUG logging:103 Getting transaction 1
2024-02-03 10:05:22,905 DEBUG logging:103 Changing transaction state from "PROCESSING REPLY" to "TRANSACTION_COMPLETE"
Modbus Error: [Input/Output] No Response received from the remote slave/Unable to decode response

as you see the critical line: RECV: 0x8d 0xce 0xbd 0xdc 0x1 shows some weird bytes 8d ce bd dc 01 instead of expected 01 03 08 00 80 00 81 00 82 00 83 C9 88

The version information for pymodbus is:

pip3 show pymodbus
Name: pymodbus
Version: 3.6.3
Summary: A fully featured modbus protocol stack in python
Home-page:
Author: Galen Collins, Jan Iversen
Author-email:
License: BSD-3-Clause
Location: /usr/local/lib/python3.11/dist-packages