address offset with server with callback

[elsamuko]

When I run a server/client example:

#!/usr/bin/env python3

import multiprocessing as mp
import time

from pymodbus.client import ModbusTcpClient
from pymodbus.datastore import (ModbusDeviceContext, ModbusSequentialDataBlock,
                                ModbusServerContext)
from pymodbus.server import StartTcpServer


class CallbackDataBlock(ModbusSequentialDataBlock):
    def setValues(self, address, value):
        print(f"Callback from setValues with address {address}, value {value}")
        super().setValues(address, value)


def server():
    PORT = 1234
    block = CallbackDataBlock(0x00, [0]*10)
    store = ModbusDeviceContext(di=block, co=block, hr=block, ir=block)
    context = ModbusServerContext(devices=store, single=True)
    print(f"Starting server 127.0.0.1:{PORT}")
    StartTcpServer(context=context, address=("", PORT))


def client():
    with ModbusTcpClient(host='127.0.0.1', port=1234) as client:
        print("Writing to address 4")
        client.write_coil(address=4, value=True)


if __name__ == "__main__":
    p_server = mp.Process(target=server)
    p_server.start()
    time.sleep(1)

    p_client = mp.Process(target=client)
    p_client.start()
    time.sleep(1)

    p_server.terminate()
    p_server.kill()
    p_client.join()

I'd expect:

Writing to address 4
Callback from setValues with address 4, value [True]

but I'm getting a

Writing to address 4
Callback from setValues with address 5, value [True]

What could be the issue here?

[janiversen]

There are no issue, this is how ModbusSequentialDataBlock works for historical reasons.

ModbusSequentialDataBlock have a difference between "addressing" a value and "defining" the value.

The simulator as well as the upcoming datastore have addressing == defining.