Connection drops and stays disconnected

[ricklambrechts]

Hi, thanks for this nice addon to read the live data of my Growatt MOD inverter with APX batteries. Few days ago I installed it, adding the addon was really easy.

Sometimes the connection to the inverter is lost and does not comeback again without restarting homeassistant (same restart as after plugin update).

I have setup the connection through USB/Serial (RS485-to-USB adapter) with a generic USB to RS485 adapter with the CH340 chip and set the interval to 30 seconds.

Wondering if this is caused by the USB adapter that I use or is there something we could update in the plugin?

This is visible in the activity section till I restarted home assistant.

Logs:

[33m2026-02-09 06:00:10.099 WARNING (Recorder) [homeassistant.components.sensor.recorder] Entity sensor.growatt_grid_import_energy_today from integration growatt_modbus has state class total_increasing, but its state is not strictly increasing. Triggered by state 2.1 (previous state: 2.2) with last_updated set to 2026-02-09T04:59:49.122874+00:00. Please create a bug report at https://github.com/0xAHA/Growatt_ModbusTCP/issues[0m
[33m2026-02-09 06:00:10.100 WARNING (Recorder) [homeassistant.components.sensor.recorder] Entity sensor.growatt_grid_import_energy_total from integration growatt_modbus has state class total_increasing, but its state is not strictly increasing. Triggered by state 46.8 (previous state: 46.9) with last_updated set to 2026-02-09T04:59:49.122976+00:00. Please create a bug report at https://github.com/0xAHA/Growatt_ModbusTCP/issues[0m
[31m2026-02-09 06:43:12.765 ERROR (SyncWorker_3) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:43:12.766 WARNING (SyncWorker_3) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 1/3)[0m
[31m2026-02-09 06:43:15.775 ERROR (SyncWorker_3) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:43:15.776 WARNING (SyncWorker_3) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 2/3)[0m
[31m2026-02-09 06:43:18.785 ERROR (SyncWorker_3) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:43:18.785 WARNING (SyncWorker_3) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 3/3)[0m
[31m2026-02-09 06:48:12.755 ERROR (SyncWorker_0) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:48:12.756 WARNING (SyncWorker_0) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 1/3)[0m
[31m2026-02-09 06:48:15.764 ERROR (SyncWorker_0) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:48:15.765 WARNING (SyncWorker_0) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 2/3)[0m
[31m2026-02-09 06:48:18.773 ERROR (SyncWorker_0) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:48:18.774 WARNING (SyncWorker_0) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 3/3)[0m
[31m2026-02-09 06:49:21.755 ERROR (SyncWorker_9) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:49:21.755 WARNING (SyncWorker_9) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 1/3)[0m
[31m2026-02-09 06:49:24.765 ERROR (SyncWorker_9) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:49:24.765 WARNING (SyncWorker_9) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 2/3)[0m
[31m2026-02-09 06:49:27.774 ERROR (SyncWorker_9) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:49:27.774 WARNING (SyncWorker_9) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 3/3)[0m
[31m2026-02-09 06:49:57.755 ERROR (SyncWorker_4) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:49:57.755 WARNING (SyncWorker_4) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 1/3)[0m
[33m2026-02-09 06:50:00.764 WARNING (SyncWorker_4) [custom_components.growatt_modbus.growatt_modbus] [MOD TL3-XH Series@/dev/ttyUSB1] 5 consecutive read failures detected - backing off to safe polling interval (1s) to prevent communication errors. This may indicate serial connection issues, low baudrate, or device processing limitations.[0m
[31m2026-02-09 06:50:00.764 ERROR (SyncWorker_4) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:50:00.764 WARNING (SyncWorker_4) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 2/3)[0m
[31m2026-02-09 06:50:03.772 ERROR (SyncWorker_4) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:50:03.773 WARNING (SyncWorker_4) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 3/3)[0m
[31m2026-02-09 06:51:06.755 ERROR (SyncWorker_8) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:51:06.755 WARNING (SyncWorker_8) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 1/3)[0m
[31m2026-02-09 06:51:09.765 ERROR (SyncWorker_8) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:51:09.765 WARNING (SyncWorker_8) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 2/3)[0m
[31m2026-02-09 06:51:12.775 ERROR (SyncWorker_8) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:51:12.775 WARNING (SyncWorker_8) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 3/3)[0m
[31m2026-02-09 06:52:15.755 ERROR (SyncWorker_9) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:52:15.756 WARNING (SyncWorker_9) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 1/3)[0m
[31m2026-02-09 06:52:18.765 ERROR (SyncWorker_9) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:52:18.765 WARNING (SyncWorker_9) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 2/3)[0m
[31m2026-02-09 06:52:21.774 ERROR (SyncWorker_9) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:52:21.775 WARNING (SyncWorker_9) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 3/3)[0m
[31m2026-02-09 06:53:24.755 ERROR (SyncWorker_3) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:53:24.755 WARNING (SyncWorker_3) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 1/3)[0m
[31m2026-02-09 06:53:27.764 ERROR (SyncWorker_3) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:53:27.765 WARNING (SyncWorker_3) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 2/3)[0m
[31m2026-02-09 06:53:30.773 ERROR (SyncWorker_3) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:53:30.773 WARNING (SyncWorker_3) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 3/3)[0m
[31m2026-02-09 06:54:00.755 ERROR (SyncWorker_10) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:54:00.755 WARNING (SyncWorker_10) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 1/3)[0m
[33m2026-02-09 06:54:03.763 WARNING (SyncWorker_10) [custom_components.growatt_modbus.growatt_modbus] [MOD TL3-XH Series@/dev/ttyUSB1] 5 consecutive read failures detected - backing off to safe polling interval (1s) to prevent communication errors. This may indicate serial connection issues, low baudrate, or device processing limitations.[0m
[31m2026-02-09 06:54:03.763 ERROR (SyncWorker_10) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:54:03.763 WARNING (SyncWorker_10) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 2/3)[0m
[31m2026-02-09 06:54:06.772 ERROR (SyncWorker_10) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:54:06.772 WARNING (SyncWorker_10) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 3/3)[0m
[31m2026-02-09 06:54:36.755 ERROR (SyncWorker_5) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:54:36.755 WARNING (SyncWorker_5) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 1/3)[0m
[31m2026-02-09 06:54:39.764 ERROR (SyncWorker_5) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:54:39.764 WARNING (SyncWorker_5) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 2/3)[0m
[31m2026-02-09 06:54:42.773 ERROR (SyncWorker_5) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:54:42.773 WARNING (SyncWorker_5) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 3/3)[0m
[31m2026-02-09 06:55:12.754 ERROR (SyncWorker_1) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:55:12.755 WARNING (SyncWorker_1) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 1/3)[0m
[31m2026-02-09 06:55:15.764 ERROR (SyncWorker_1) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:55:15.764 WARNING (SyncWorker_1) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 2/3)[0m
[31m2026-02-09 06:55:18.773 ERROR (SyncWorker_1) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:55:18.773 WARNING (SyncWorker_1) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 3/3)[0m
[31m2026-02-09 06:55:48.755 ERROR (SyncWorker_0) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:55:48.755 WARNING (SyncWorker_0) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 1/3)[0m
[31m2026-02-09 06:55:51.764 ERROR (SyncWorker_0) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:55:51.764 WARNING (SyncWorker_0) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 2/3)[0m
[31m2026-02-09 06:55:54.773 ERROR (SyncWorker_0) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 06:55:54.773 WARNING (SyncWorker_0) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 3/3)[0m
[31m2026-02-09 07:00:54.755 ERROR (SyncWorker_10) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 07:00:54.755 WARNING (SyncWorker_10) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 1/3)[0m
[31m2026-02-09 07:00:57.764 ERROR (SyncWorker_10) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 07:00:57.764 WARNING (SyncWorker_10) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 2/3)[0m
[31m2026-02-09 07:01:00.772 ERROR (SyncWorker_10) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 07:01:00.773 WARNING (SyncWorker_10) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 3/3)[0m
[31m2026-02-09 07:06:00.755 ERROR (SyncWorker_4) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 07:06:00.755 WARNING (SyncWorker_4) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 1/3)[0m
[31m2026-02-09 07:06:03.765 ERROR (SyncWorker_4) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 07:06:03.765 WARNING (SyncWorker_4) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 2/3)[0m
[31m2026-02-09 07:06:06.774 ERROR (SyncWorker_4) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 07:06:06.774 WARNING (SyncWorker_4) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 3/3)[0m
[31m2026-02-09 07:11:06.755 ERROR (SyncWorker_8) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 07:11:06.756 WARNING (SyncWorker_8) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 1/3)[0m
[31m2026-02-09 07:11:09.765 ERROR (SyncWorker_8) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 07:11:09.765 WARNING (SyncWorker_8) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 2/3)[0m
[31m2026-02-09 07:11:12.774 ERROR (SyncWorker_8) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 07:11:12.774 WARNING (SyncWorker_8) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 3/3)[0m
[31m2026-02-09 07:16:12.755 ERROR (SyncWorker_9) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 07:16:12.756 WARNING (SyncWorker_9) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 1/3)[0m
[31m2026-02-09 07:16:15.765 ERROR (SyncWorker_9) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 07:16:15.765 WARNING (SyncWorker_9) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 2/3)[0m
[31m2026-02-09 07:16:18.774 ERROR (SyncWorker_9) [custom_components.growatt_modbus.growatt_modbus] Failed to read base input register block[0m
[33m2026-02-09 07:16:18.775 WARNING (SyncWorker_9) [custom_components.growatt_modbus.coordinator] Read returned None (attempt 3/3)[0m

[ricklambrechts]

Some new information, I think this is because the Inverter and the battery are in standby because there is no sun and the battery is "empty".

Will see tomorrow morning if everything keeps working.

In the ShinePhone I still see all the load of the house that is reported by the smart meter that is connected to the inverter.

I would expect that through the modbus the load is available. But maybe it is not?

Just for reference some screenshots.

I see the "Growatt Load" still shows values from the smart meter but sometimes it disconnects and does not show any load while there is load.

Growatt Solar:

Growatt Load:

Growatt Battery:

Growatt Grid:

[0xAHA]

The logs suggest that the inverter is in fact off because it's not powered. Usually it would remain on without solar because it's powered by the battery but as you suggested the battery is also empty.
I may be wrong but I'm not sure if MOD is able to remain powered by the grid or not (I know my MIN unit doesn't).

Can you attempt to poll it via a separate modbus tool when the integration isn't able to get a response?

It seems odd that ShinePhone shows continual load info though. Or is your meter also connected to ShinePhone/growatt separately??

The integration is designed to back off the polling period at night to 5 minute intervals (or more specifically, when it loses comms to the inverter) and should come back to life once the sun is up and the inverter is back online and responding.
So yes it's odd that yours needs a reload of the integration to start responding.

[ricklambrechts]

It is weird. I have a ShineLAN-X that I configured to my internal IP that runs Grott and Grott pushes the data to the Growatt servers.

The MOD inverter remain power by the grid and the green leaf flashes. The ShineLAN module keeps reading data and pushes the data to my Grott instance and that same data is pushed to Growatt and visible in the ShinePhone app and online.

Register scan while showing it is disconnected:

growatt_register_scan_20260210_133137.csv

Showing disconnected from this morning:

Solar power visible at Growatt:

Energy Trend at server.growatt.com:

[0xAHA]

Ok that makes sense, so we know the inverter is on permanently.

While it is still unavailable in the integration …. In this order, can you:

1. Use the developer tools Read Register to try and read a register from the inverter. Eg input register 0 is the status.

2. Use a separate modbus tool like ModbusPoll to see if you can connect outside of HA. This is probably only a valid test if your usb adapter and HA run on a vm etc.

3. Unplug/power cycle the usb adapter. It may be locked up and not responding properly.

Can you also send info on the usb device settings?

[ricklambrechts]

Thanks. Will look at this.

I think I found relevant information. I see the following errors and after those errors, home assistant will restart and after that the connection works again. I will try to update the logic around the sockets a bit and see it that changes things.

[31m2026-02-11 05:12:23.085 ERROR (MainThread) [homeassistant] Error doing job: socket.accept() out of system resource (task: None)
Traceback (most recent call last):
  File "/usr/local/lib/python3.13/asyncio/selector_events.py", line 178, in _accept_connection
  File "/usr/local/lib/python3.13/socket.py", line 295, in accept
OSError: [Errno 24] No file descriptors available�[0m
[31m2026-02-11 05:12:23.086 ERROR (MainThread) [homeassistant] Fatal error '[Errno 24] No file descriptors available' raised in event loop, shutting it down�[0m

[0xAHA]

Ok good info.
It looks like the usb serial connection may be flaky and the integration isn't properly disconnecting at the next poll.
This leaves open but dead connections which are the file descriptors the error mentions.
Eventually there's enough of these that the OS runs out of

V0.4.5 is released which aims to resolve this by properly checking and closing previous connections.

[ricklambrechts]

Thanks, installed the update. Fully rebooted the system. I can confirm the the modbus connection works at night when no solar and no battery power is used, at that moment it is in "Waiting" mode. So the connection should "always" works, thats nice.

I have downloaded modpoll for testing the connection for the next time it gets disconnected.

Just for reference downloading modpoll and running the modpoll.

wget https://www.modbusdriver.com/downloads/modpoll.tgz
tar xzf modpoll.tgz

modpoll/x86_64-linux-gnu/modpoll -4 2 -m rtu /dev/ttyUSB1

Also created a simple modbus poll script with the help of ChatGPT.
Command to run the script: python modbus_rtu_test.py --port /dev/ttyUSB1 --baud 9600 --unit 1 --address 0 --count 1 --retries 25.

#!/usr/bin/env python3
"""
Simple Modbus RTU connection tester (modpoll-like).

Examples:
  # Similar spirit to: modpoll -4 2 -m rtu /dev/ttyUSB1
  python3 modbus_rtu_test.py --port /dev/ttyUSB1 --unit 1 --rs485-delay-ms 2

  # Read holding register address 0 (0-based), count 2
  python3 modbus_rtu_test.py --port /dev/ttyUSB1 --unit 1 --address 0 --count 2

Notes on addressing:
  - Many tools/documentation refer to 1-based register numbers (e.g. 40001).
  - pymodbus uses 0-based addresses in the API.
"""

from __future__ import annotations

import argparse
import errno
import sys
import time
from typing import Callable, Optional

# --- pymodbus imports (support slight layout differences) ---
try:
    # Newer pymodbus
    from pymodbus import FramerType
except Exception:
    # Some installs expose it here
    from pymodbus.framer import FramerType  # type: ignore

try:
    # Common in pymodbus 3.x
    from pymodbus.client import ModbusSerialClient
except Exception:
    # Fallback for some layouts
    import pymodbus.client as ModbusClient  # type: ignore
    ModbusSerialClient = ModbusClient.ModbusSerialClient  # type: ignore


def connect_with_retry(
    client_factory: Callable[[], "ModbusSerialClient"],
    attempts: int = 12,
    delay_s: float = 0.4,
    backoff: float = 1.25,
) -> "ModbusSerialClient":
    """
    Repeatedly try to create/connect a ModbusSerialClient.
    Specifically handles pyserial exclusive-lock errors like:
      [Errno 11] Could not exclusively lock port ... Resource temporarily unavailable
    """
    last_exc: Optional[BaseException] = None
    d = delay_s

    for i in range(1, attempts + 1):
        try:
            client = client_factory()
            if client.connect():
                return client
            last_exc = RuntimeError("client.connect() returned False")
        except OSError as e:
            last_exc = e
            # Typical when port is locked/busy: EAGAIN (11). Sometimes EACCES (13).
            if getattr(e, "errno", None) not in (errno.EAGAIN, errno.EACCES):
                raise
        except Exception as e:
            last_exc = e

        print(f"Port open/connect failed (attempt {i}/{attempts}). Retrying in {d:.2f}s... ({last_exc})")
        time.sleep(d)
        d *= backoff

    raise RuntimeError(f"Failed to open/connect after {attempts} attempts: {last_exc}")


def enable_rs485_mode_if_requested(client: "ModbusSerialClient", delay_ms: float) -> None:
    """
    Best-effort RS-485 RTS toggle setup using pyserial.
    This approximates modpoll '-4 <ms>' behavior (timing isn't a perfect 1:1 mapping).
    """
    try:
        import serial.rs485  # pyserial

        delay_s = delay_ms / 1000.0
        # client.socket is the underlying pyserial Serial instance after connect()
        client.socket.rs485_mode = serial.rs485.RS485Settings(
            rts_level_for_tx=True,
            rts_level_for_rx=False,
            delay_before_tx=delay_s,
            delay_before_rx=delay_s,
        )
        print(f"RS-485 mode enabled (~{delay_ms} ms delays before TX and before RX).")
    except Exception as e:
        print(f"WARNING: Could not enable RS-485 mode via pyserial: {e}")


def read_holding_registers_compat(client: "ModbusSerialClient", address: int, count: int, unit: int):
    """
    pymodbus has used different kwarg names for the unit id across versions.
    Try in order: device_id (new), slave (some 3.x docs), unit (older).
    """
    for kw in ("device_id", "slave", "unit"):
        try:
            return client.read_holding_registers(address=address, count=count, **{kw: unit})
        except TypeError:
            continue
    # If no kw worked, try without (may still fail if unit is required)
    return client.read_holding_registers(address=address, count=count)


def main() -> int:
    p = argparse.ArgumentParser(description="Simple Modbus RTU connection tester")
    p.add_argument("--port", default="/dev/ttyUSB1", help="Serial device, e.g. /dev/ttyUSB1")
    p.add_argument("--unit", type=int, default=1, help="Slave/unit id (default: 1)")

    p.add_argument("--baud", type=int, default=19200, help="Baudrate (modpoll default often 19200)")
    p.add_argument("--parity", default="E", choices=["N", "E", "O"], help="Parity: N/E/O")
    p.add_argument("--stopbits", type=int, default=1, choices=[1, 2], help="Stop bits")
    p.add_argument("--bytesize", type=int, default=8, choices=[7, 8], help="Data bits")
    p.add_argument("--timeout", type=float, default=1.0, help="Timeout seconds")

    p.add_argument("--address", type=int, default=0, help="0-based holding register address")
    p.add_argument("--count", type=int, default=1, help="Number of holding registers to read")

    p.add_argument("--retries", type=int, default=2, help="Read retries if request fails")
    p.add_argument("--port-open-attempts", type=int, default=12, help="Attempts to open locked/busy port")
    p.add_argument("--port-open-delay", type=float, default=0.4, help="Initial delay between open attempts (s)")
    p.add_argument("--port-open-backoff", type=float, default=1.25, help="Backoff multiplier between attempts")

    p.add_argument(
        "--rs485-delay-ms",
        type=float,
        default=None,
        help="Enable RS-485 RTS toggle and set approx delay (ms) around TX/RX",
    )

    args = p.parse_args()

    print(
        f"Modbus RTU test: port={args.port} baud={args.baud} parity={args.parity} "
        f"stopbits={args.stopbits} bytesize={args.bytesize} unit={args.unit} "
        f"read holding addr={args.address} count={args.count}"
    )

    def make_client() -> "ModbusSerialClient":
        # For pymodbus 3.x: use framer=FramerType.RTU (instead of method="rtu")
        return ModbusSerialClient(
            args.port,
            framer=FramerType.RTU,
            baudrate=args.baud,
            parity=args.parity,
            stopbits=args.stopbits,
            bytesize=args.bytesize,
            timeout=args.timeout,
        )

    client = connect_with_retry(
        make_client,
        attempts=args.port_open_attempts,
        delay_s=args.port_open_delay,
        backoff=args.port_open_backoff,
    )

    try:
        if args.rs485_delay_ms is not None:
            enable_rs485_mode_if_requested(client, args.rs485_delay_ms)

        # Perform read with retries
        for attempt in range(1, args.retries + 2):
            try:
                t0 = time.time()
                rr = read_holding_registers_compat(client, args.address, args.count, args.unit)
                dt_ms = (time.time() - t0) * 1000.0

                if rr is None:
                    print(f"Attempt {attempt}: No response (rr is None).")
                elif getattr(rr, "isError", lambda: True)():
                    print(f"Attempt {attempt}: Modbus error response: {rr}  ({dt_ms:.1f} ms)")
                else:
                    regs = getattr(rr, "registers", None)
                    print(f"OK ({dt_ms:.1f} ms): holding[{args.address}..{args.address + args.count - 1}] = {regs}")
                    return 0

            except Exception as e:
                print(f"Attempt {attempt}: Exception: {type(e).__name__}: {e}")

            time.sleep(0.2)

        print("FAILED: no successful read.")
        return 1

    finally:
        try:
            client.close()
        except Exception:
            pass


if __name__ == "__main__":
    sys.exit(main())

It is possible to create a small venv with the needed packages.

python -m venv venv
. venv/bin/activate
pip install pymodbus pyserial