Can't read coil(code 0x05) from PyModbus Simulator

[GrollGroll]

Hello! I'm really enjoying using your library(pymodbus[simulator] v3.8.6). The simulator is a big help in my work. But now I've encountered a problem: I can't write to or read coil(code 0x05) from the simulator.

My simulator configuration:

{
    "server_list": {
        "server": {
            "comm": "serial",
            "port": "com5",
            "stopbits": 1,
            "bytesize": 8,
            "parity": "E",
            "baudrate": 9600,
            "timeout": 3,
            "reconnect_delay": 2,
            "framer": "rtu",
            "identity": {
                "VendorName": "pymodbus",
                "ProductCode": "PM",
                "VendorUrl": "https://github.com/pymodbus-dev/pymodbus",
                "ProductName": "pymodbus Server",
                "ModelName": "pymodbus Server",
                "MajorMinorRevision": "3.1.0"
            }
        }
    },
    "device_list": {
        "device": {
            "setup": {
                "co size": 63000,
                "di size": 63000,
                "hr size": 63000,
                "ir size": 63000,
                "shared blocks": true,
                "type exception": true,
                "defaults": {
                    "value": {
                        "bits": true,
                        "uint16": 0,
                        "uint32": 0,
                        "float32": 0.0,
                        "string": " "
                    },
                    "action": {
                        "bits": null,
                        "uint16": "increment",
                        "uint32": "increment",
                        "float32": "increment",
                        "string": null
                    }
                }
            },
            "invalid": [],
            "write": [20480, 20481, 20484, 20485, 20486, 20489, 20490, 20494, 20496, 20503, 20736, 20739],
            "bits": [20503, 20736, 20739],
            "uint16": [
                {"addr": [20480, 20481], "value": 1234, "action": null},
                {"addr": [20484, 20485], "value": 3456, "action": null},
                {"addr": 20489, "value": 3, "action": null},
                {"addr": 20490, "value": 1, "action": null},
                {"addr": 20496, "value": 1, "action": null},
                {"addr": 20535, "value": 1, "action": null}
            ],
            "uint32": [
                {"addr": [20486, 20487], "value": 5678, "action": null},
                {"addr": 20494, "value": 7890, "action": null},
                {"addr": 20552, "value": 7890, "action": null}
            ],
            "float32": [
            ],
            "string": [
            ],
            "repeat": [
            ]
        }
    }
}

My code for writing to the coil:

from pymodbus.client import AsyncModbusSerialClient

self._modbus = AsyncModbusSerialClient(
            port=settings.port,
            baudrate=settings.baudrate,
            parity='E',
        )

async def _set_hiz_if_error(self):
        await self._write_value(
            self._modbus.write_coil, 0x5017, value=True,
        )

async def _write_value(
        self,
        write_func: Awaitable,
        value_address: int,
        value: list[int] | int | bool,
    ):
        try:
            logger.info(f'Write into register {value_address}, value {value}, function {write_func.__name__}')
            response = await write_func(value_address, value)
        except ModbusException as ex:
            raise self._device_err_type(
                f'{self._name} data write error ({ex})',
            ) from ex
        if response.isError():
            raise self._device_err_type(f'{self._name}{SEP}{MODBUS_ERROR}{SEP}{response}')

Log from simulator:

2026-03-13 15:44:37,004 INFO  base:85 Server listening.
2026-03-13 15:44:37,008 INFO  http_server:259 HTTP server started on (0.0.0.0:8081)
2026-03-13 15:44:54,681 ERROR pdu:128 Exception response 133 / 2

Log from my code:

2026-03-13 15:44:54.651 | INFO     | stw_eps_sms.periph.cmpst_param_dev.stepmotor_controller.adapter:_write_value:302 - Write into register 20503, value True, function write_coil
Exception response 133 / 0
2026-03-13 15:44:54.698 | ERROR    | stw_eps_sms.periph.devices.device:initialize:37 - stepmotor_controller: Error during device initialization: stepmotor_controller: Modbus error: ExceptionResponse(dev_id=1, transaction_id=0, address=0, count=0, bits=[], registers=[], status=1)

Since I'm writing to the coil, I added it to the "write" section. Since it's a coil, I also duplicated it in "bits." Coils i used is: 20503, 20736, 20739.

I've already tested this on a real device and everything works. So, I have no doubts about my code.

What am I doing wrong? I have no problems reading or writing registers from simulator. I would be very grateful for your help. I hope I posted in the right place.

[janiversen]

your example do not include the read_coil ??? the exception response tells that you are not allowed to write, remark in shared memory you cannot write a single coil, since the address references 16 bits.

What do you mean you cannot read coil, what happens ? I assume you get an exception response, which should tell you what went wrong.

Did you try to set "type exception": false,

Please add the pymodbus debug log, so we can see what happens (on both simulator and client side).

[GrollGroll]

Thanks for the quick reply.

your example do not include the read_coil ??

Yes, because I can neither read nor write into the coil.

remark in shared memory you cannot write a single coil, since the address references 16 bits.

Can you explain in more detail what exactly this means?

I assume you get an exception response, which should tell you what went wrong.

Yep, the attached log contains an error - Exception response 133 / 2. I understand that the error indicates that the command failed due to an invalid address. But I don't understand where I went wrong in the configuration.

Did you try to set "type exception": false

Yes, I am

Please add the pymodbus debug log

Can you please tell me how I can enable debug log on the simulator?

---

I simplified the code to better understand the problems:
My code:

import asyncio as aio
from pymodbus.client import AsyncModbusSerialClient

async def main():
    modbus = AsyncModbusSerialClient(
                port='com6',
                baudrate=9600,
                parity='E',
    )
    coil = 0x5017
    value = True

    await modbus.connect()

    print(f'try ro write into coil {coil} value: {value}')
    response = await modbus.write_coil(address=coil, value=value)
    if response.isError():
        print(f'Data write error: {response}')

    print(f'try ro read from coil {coil}')
    response = await modbus.write_coil(address=coil, value=value)
    if response.isError():
        print(f'Data write error: {response}')

    modbus.close()

if __name__ == '__main__':
    aio.run(main())

Log from my code:

try ro write into coil 20503 value: True
Exception response 133 / 0
Data write error: ExceptionResponse(dev_id=1, transaction_id=0, address=0, count=0, bits=[], registers=[], status=1)
try ro read from coil 20503
Exception response 133 / 0
Data write error: ExceptionResponse(dev_id=1, transaction_id=0, address=0, count=0, bits=[], registers=[], status=1)

Log from simulator:

2026-03-13 16:20:00,738 INFO  base:85 Server listening.
2026-03-13 16:20:00,750 INFO  http_server:259 HTTP server started on (0.0.0.0:8081)
2026-03-13 16:20:03,915 ERROR pdu:128 Exception response 133 / 2
2026-03-13 16:20:03,947 ERROR pdu:128 Exception response 133 / 2

[janiversen]

Did you note that the newest release when SimData/SimDevice making data configuration much easier.

[GrollGroll]

No sir. Didn't know about that. I guess I should try the newest release and come back here if the problem persists. Until then, should we suspend the discussion, what do you think?

[janiversen]

I think the new SimData/SimDevice is better for most parts....it does not however offer the html interface (yet to come).

I found your problem, your address is wrong.

As Documented Read/Write_coils addresses a bit NOT a register, so in shared mode you need to do

address = register_address * 16 + bit_offset

[GrollGroll]

Thanks a million!