Trouble with Custom Function Code: Unable to Decode Response from Slave Device

[arthurgfhack]

Hello everyone,

I am working on a project where I am communicating with a Modbus slave device using PyModbus. My specific use-case requires me to use a custom function code (65), instead of the standard Modbus function codes.

Here is the Python code I am using:

from pymodbus.client.serial import ModbusSerialClient as ModbusClient
from pymodbus.register_read_message import ReadHoldingRegistersRequest, ReadHoldingRegistersResponse
import logging


# Custom Request and Response classes
class ReadFactoryRegistersRequest(ReadHoldingRegistersRequest):
    function_code = 65  # Custom function code

class ReadFactoryRegistersResponse(ReadHoldingRegistersResponse):
    function_code = 65  # Custom function code


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

client = ModbusClient(method='rtu', port='COM5', baudrate=115200, timeout=1)
client.connect()

request = ReadFactoryRegistersRequest(slave = 1, address = 0, count = 4)
response = client.execute(request)

# Check if the response is an instance of the custom response class
if isinstance(response, ReadHoldingRegistersResponse):
    print(f"Successfully read {len(response.registers)} factory registers: {response.registers}")
else:
    
    print(f"Failed to read factory registers: {response.registers}")
    
print(response.registers)
client.close()

Issue:

When I execute this code, I get the following log output:

DEBUG:pymodbus.logging:Current transaction state - IDLE
DEBUG:pymodbus.logging:Running transaction 1
DEBUG:pymodbus.logging:SEND: 0x1 0x41 0x0 0x0 0x0 0x4 0x3c 0x6
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: 0x1 0x41 0x8 0x7 0x8 0xb 0xb8 0x3 0xe8 0x5 0xdc 0xdd 0x36
DEBUG:pymodbus.logging:Processing: 0x1 0x41 0x8 0x7 0x8 0xb 0xb8 0x3 0xe8 0x5 0xdc 0xdd 0x36
DEBUG:pymodbus.logging:Getting transaction 1
DEBUG:pymodbus.logging:Changing transaction state from "PROCESSING REPLY" to "TRANSACTION_COMPLETE"
Traceback (most recent call last):
  File "C:/pyqt6/modbus teste.py", line 29, in <module>
    print(f"Failed to read factory registers: {response.registers}")
AttributeError: 'ModbusIOException' object has no attribute 'registers'

Even though I get a response from the slave device, I am unable to decode it. The error message suggests that there is no response, which is clearly not the case as seen from the logs.

If I change the custom function_code for 3 or 4, I get the following log output:

DEBUG:pymodbus.logging:Current transaction state - IDLE
DEBUG:pymodbus.logging:Running transaction 1
DEBUG:pymodbus.logging:SEND: 0x1 0x3 0x0 0x0 0x0 0x4 0x44 0x9
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: 0x1 0x3 0x8 0x0 0x1 0xa 0x28 0xa 0x8c 0x4 0x4c 0x24 0xbd
DEBUG:pymodbus.logging:Processing: 0x1 0x3 0x8 0x0 0x1 0xa 0x28 0xa 0x8c 0x4 0x4c 0x24 0xbd
DEBUG:pymodbus.logging:Getting Frame - 0x3 0x8 0x0 0x1 0xa 0x28 0xa 0x8c 0x4 0x4c
DEBUG:pymodbus.logging:Factory Response[ReadHoldingRegistersResponse': 3]
DEBUG:pymodbus.logging:Frame advanced, resetting header!!
DEBUG:pymodbus.logging:Adding transaction 1
DEBUG:pymodbus.logging:Getting transaction 1
DEBUG:pymodbus.logging:Changing transaction state from "PROCESSING REPLY" to "TRANSACTION_COMPLETE"
Successfully read 4 factory registers: [1, 2600, 2700, 1100]
[1, 2600, 2700, 1100]

Questions:

• Has anyone encountered similar issues with custom function codes in PyModbus?
• How can I correctly implement the decoding logic for the custom function code?
• Could there be other reasons for the failure in decoding the response from the slave device?
• Later on I'll also need to write to this custom registers

Any help or pointers would be greatly appreciated. Thank you!

[janiversen]

A couple of things:

• Please use our standard function to apply logging.
• I think you miss to register your custom codes, please have a look at our examples.

To your questions:

• I use a custom function code (to login to my solar panel inverter) and it works without problems.
• The easy way is to copy a similar decoding class from the pymodbus library
• Yes, you did not register the function code
• No problem is supported (if you support it in your classes).

I think that should be help enough, otherwise ask.

[arthurgfhack]

Thanks man! You have helped a lot!

[janiversen]

my pleasure.