Read bytes from Modbus TCP/IP message in Big Endian byte order

Question

I'm new with modbus and I need to write python script to read TCP/IP messages from Modbus client. I need to read 4 bytes starting in address 002049 using Big-endian order. so my questions are:

1. which library should I use- pymodbus or pymodbustcp?
2. what is the best practice for creating Modbus Client- auto_open=True/False, auto_close=True/False?
3. How can I read Bytes instead of bits?
4. How can I set the bytes values in Big-endian order?

I tried to use How to read byte order endian BIG with Python? but couldn't find an answer.

---UPDATE---

how can I do it using AsyncModbusTcpClient?

Answer 1

Code to read 4 bytes (2 registers) starting at address 2049 (address 002049 in Modbus):

from pyModbusTCP.client import ModbusClient

def read_modbus_registers(ip_address, port, start_address, num_registers):
    # Create a Modbus TCP client
    client = ModbusClient(host=ip_address, port=port, auto_open=True)

    if client.is_open():
        result = client.read_holding_registers(start_address, num_registers)

        if result:
            # Combine the two registers into a single 32-bit integer (big-endian order)
            value = (result[0] << 16) | result[1]
            print(f"Value read from address {start_address}: {value}")
        else:
            print(f"Failed to read from Modbus server: {client.last_error}")
    else:
        print(f"Failed to connect to Modbus server at {ip_address}:{port}")

    # Close the Modbus TCP client
    client.close()

# Modbus TCP server configuration
modbus_ip = "your_modbus_server_ip"
modbus_port = 502  # Modbus TCP port

# Address and number of registers to read
start_address = 2049  # Address 002049 in Modbus
num_registers = 2  # Reading 4 bytes (2 registers)

# Read Modbus registers
read_modbus_registers(modbus_ip, modbus_port, start_address, num_registers)

To perform the task asynchronously, we need to use await when interacting with the client.

import asyncio
from pyModbusTCP.asyncio import AsyncModbusTcpClient

async def read_modbus_registers_async(ip_address, port, start_address, num_registers):
    # Create an async client instance
    client = AsyncModbusTcpClient(host=ip_address, port=port)

    try:
        # Connect to the Modbus server
        await client.connect()

        if client.is_connected:
            # Read registers
            result = await client.read_holding_registers(start_address, num_registers)

            if result:
                # Combine the registers
                value = (result[0] << 16) | result[1]
                print(f"Value read from address {start_address}: {value}")
            else:
                print(f"Failed to read from Modbus server: {client.last_error}")
        else:
            print(f"Failed to connect to Modbus server at {ip_address}:{port}")

    except Exception as e:
        print(f"Error: {e}")

    finally:
        # Close the Modbus TCP client
        await client.close()

# Modbus TCP server configuration
modbus_ip = "your_modbus_server_ip"
modbus_port = 502  # Modbus TCP port

# Address and number of registers to read
start_address = 2049  # Address 002049 in Modbus
num_registers = 2  # Reading 4 bytes (2 registers)

# Create an event loop and run the asynchronous function
async def main():
    await read_modbus_registers_async(modbus_ip, modbus_port, start_address, num_registers)

# Run the event loop
if __name__ == "__main__":
    loop = asyncio.get_event_loop()
    loop.run_until_complete(main())