How can I control serial devices by single dongle

[labov]

I'm trying to control two slaves by connecting them to a serial to 485 communication dongle. However, only one device is properly connected, and the other devices, although the signals are transmitted, do not complete the process correctly, resulting in a "connection lost" related exception handler.

When I set the retry to 0, the process operates correctly, but when I set the retry to 2, it causes a malfunction which is sending the signal three times.

I would like to inquire whether additional options are required when connecting multiple devices.
I would appreciate it if you could provide a code example for controlling multiple slaves.

OS : MacOS 14.1.2
pymodbus version : 3.5.4, 3.5.2

from pymodbus import pymodbus_apply_logging_config
from pymodbus.client import AsyncModbusSerialClient
from pymodbus.transaction import ModbusRtuFramer

from typing import List, Optional
import sys
import logging
import platform
import serial.tools.list_ports
from os.path import dirname
import asyncio

sys.path.insert(0, dirname(dirname(__file__)))  # ../


def find_modbus_path(vid: int, pid: int) -> Optional[str]:
    """Find a device path by VID (Vendor ID) and PID (Product ID)."""
    try:
        ports = list(serial.tools.list_ports.comports())
        for port in ports:
            if port.vid == vid and port.pid == pid:
                if platform.system() == "Darwin":
                    # Replace 'cu' with 'tty' in the device path.
                    device_path = port.device.replace("/dev/cu.", "/dev/tty.")
                elif platform.system() == "Windows":
                    device_path = port.device
                elif platform.system() == "Linux":
                    device_path = port.device
                else:
                    raise NotImplementedError(f"OS : {platform.system()} modbus device not found")
                logging.info(f"Modbus device path found: {device_path}")
                return device_path
            else:
                logging.info(f"Not matched vid:{port.vid}, pid:{port.pid}, device:{port.device}")
        else:
            raise NotImplementedError(f"OS : {platform.system()} modbus device path")
    except Exception as e:
        logging.error(f"Error: {e}")
        return None


pymodbus_apply_logging_config(logging.DEBUG)


async def main():
    vender_id = 6790
    product_id = 29987
    device_port = find_modbus_path(vender_id, product_id)

    client = AsyncModbusSerialClient(
        port=device_port,
        framer=ModbusRtuFramer,
        baudrate=57600,
        bytesize=8,
        parity="N",
        stopbits=1,
        timeout=0.2,
        retries=1,
    )

    await client.connect()
    print("slave 40")
    try:
        await client.write_registers(address=3, values=[3, 0, 2 * 2 + 1, 1], slave=40)
    except Exception as e:
        print(e)

    print("slave 41")
    try:
        await client.write_registers(address=3, values=[3, 0, 2 * 2 + 1, 1], slave=41)
    except Exception as e:
        print(e)

    await asyncio.sleep(0.2)
    print("slave 40")
    try:
        await client.write_registers(address=3, values=[3, 0, 2 * 2 + 1, 1], slave=40)
    except Exception as e:
        print(e)

    print("slave 41")
    try:
        await client.write_registers(address=3, values=[3, 0, 2 * 2 + 1, 1], slave=41)
    except Exception as e:
        print(e)


if __name__ == "__main__":
    asyncio.run(main())

Excpetion print

Modbus Error: [Input/Output] ERROR: No response received after 1 retries # ver3.5.2
'NoneType' object has no attribute 'close' # ver 3.5.4

Debug log

slave 40
2023-12-06 16:23:43,684 DEBUG logging:102 send: 0x28 0x10 0x0 0x3 0x0 0x4 0x8 0x0 0x3 0x0 0x0 0x0 0x5 0x0 0x1 0xb9 0xa1
DEBUG:pymodbus.logging:send: 0x28 0x10 0x0 0x3 0x0 0x4 0x8 0x0 0x3 0x0 0x0 0x0 0x5 0x0 0x1 0xb9 0xa1
2023-12-06 16:23:43,686 DEBUG logging:102 Adding transaction 40
DEBUG:pymodbus.logging:Adding transaction 40
2023-12-06 16:23:43,823 DEBUG logging:102 recv: 0x28 0x10 0x0 0x3 0x0 0x4 0x36 0x33 old_data:  addr=None
DEBUG:pymodbus.logging:recv: 0x28 0x10 0x0 0x3 0x0 0x4 0x36 0x33 old_data:  addr=None
2023-12-06 16:23:43,823 DEBUG logging:102 Processing: 0x28 0x10 0x0 0x3 0x0 0x4 0x36 0x33
DEBUG:pymodbus.logging:Processing: 0x28 0x10 0x0 0x3 0x0 0x4 0x36 0x33
2023-12-06 16:23:43,823 DEBUG logging:102 Getting Frame - 0x10 0x0 0x3 0x0 0x4
DEBUG:pymodbus.logging:Getting Frame - 0x10 0x0 0x3 0x0 0x4
2023-12-06 16:23:43,824 DEBUG logging:102 Factory Response[WriteMultipleRegistersResponse': 16]
DEBUG:pymodbus.logging:Factory Response[WriteMultipleRegistersResponse': 16]
2023-12-06 16:23:43,824 DEBUG logging:102 Frame advanced, resetting header!!
DEBUG:pymodbus.logging:Frame advanced, resetting header!!
2023-12-06 16:23:43,824 DEBUG logging:102 Getting transaction 40
DEBUG:pymodbus.logging:Getting transaction 40
slave 41
2023-12-06 16:23:43,825 DEBUG logging:102 send: 0x29 0x10 0x0 0x3 0x0 0x4 0x8 0x0 0x3 0x0 0x0 0x0 0x5 0x0 0x1 0x78 0xa1
DEBUG:pymodbus.logging:send: 0x29 0x10 0x0 0x3 0x0 0x4 0x8 0x0 0x3 0x0 0x0 0x0 0x5 0x0 0x1 0x78 0xa1
2023-12-06 16:23:43,825 DEBUG logging:102 Adding transaction 41
DEBUG:pymodbus.logging:Adding transaction 41
2023-12-06 16:23:44,026 DEBUG logging:102 send: 0x29 0x10 0x0 0x3 0x0 0x4 0x8 0x0 0x3 0x0 0x0 0x0 0x5 0x0 0x1 0x78 0xa1
DEBUG:pymodbus.logging:send: 0x29 0x10 0x0 0x3 0x0 0x4 0x8 0x0 0x3 0x0 0x0 0x0 0x5 0x0 0x1 0x78 0xa1
2023-12-06 16:23:44,027 DEBUG logging:102 Adding transaction 41
DEBUG:pymodbus.logging:Adding transaction 41
2023-12-06 16:23:44,229 DEBUG logging:102 Connection lost comm due to Server not responding
DEBUG:pymodbus.logging:Connection lost comm due to Server not responding
2023-12-06 16:23:44,230 DEBUG logging:102 Getting transaction 41
DEBUG:pymodbus.logging:Getting transaction 41
Modbus Error: [Input/Output] ERROR: No response received after 1 retries
2023-12-06 16:23:44,233 DEBUG logging:102 Wait comm 100.0 ms before reconnecting.
DEBUG:pymodbus.logging:Wait comm 100.0 ms before reconnecting.

[janiversen]

NO, you can connect a max of 254 devices on a serial line, however you need to make the rs-485 cabling correct

• ensure all devices uses the same polarity of the cable
• have a final resistor termination
  some usb converters are very sensitive to how the cabling is done.

You address the different devices with the slave= parameter, but of course you need to ensure the devices are correctly configured to respond to that address.

It is normally recommended to try and read a value first, as the number of possible problem are at lot less, when writing you have security and formatting problem on top of communication problems.