Trouble Reading Modbus Data with Python Threading: Connection Issue with Modbus Slaves

Question

I am using the Python threading module and Pymodbus library to read Modbus TCP data.

import threading

from pymodbus.client.sync import ModbusTcpClient
import time
import os,time,sqlite3 as sql

class modbus_thread(threading.Thread):
    def __init__(self, row):
        super().__init__()
        self._stop_event = threading.Event()
        self.address = row[0]
        self.port = row[1]
        self.unit_id = row[2]
        self.registers = list(zip(row[3], row[4]))  # combine reg_addr and reg_nb into a list of tuples
        print(row)

    def run(self):
    client = ModbusTcpClient(self.address, self.port)
    while not self._stop_event.is_set():
        try:
            for reg_addr, reg_nb in self.registers:
                response = client.read_holding_registers(int(reg_addr), int(reg_nb), unit=self.unit_id)
                if response.isError():
                    print(f"Error reading Modbus data: {response}")
                else:
                    data = response.registers
                    print(f"Read data from Modbus: {data} {self.address}")
        except ValueError as e:
            print(f"ValueError: {str(e)}")
        except ConnectionError as e:
            print(f"ConnectionError: {str(e)}")
        except TimeoutError as e:
            print(f"TimeoutError: {str(e)}")
        except Exception as e:
            print(f"Error reading Modbus data: {str(e)}")
        
        # Sleep for 1 second before reading again
        self._stop_event.wait(1)
    
    client.close()
    print("Thread stopped.")

    def stop(self):
        self._stop_event.set()

I aim to use a multiprocessing module to run a function that reads Modbus TCP data from multiple Modbus slaves.

def modbus(queue):
    threads={}
    conn = sql.connect('database.db')
    cursor = conn.cursor()
    cursor.execute('SELECT host,port,unit_id,reg_addr,reg_nb FROM modbus_devices')
    results = cursor.fetchall()
    
    for row in results:
        t=modbus_thread(row)
        t.start()
        threads.update({row[0]:t})
        print(threads)
    
    cursor.close()
    conn.close()

Although I was able to successfully read Modbus TCP data from a local server simulator on my computer using modbus_thread, I am having trouble connecting to the Modbus slaves, even though I have confirmed there are no issues with the slave itself.