pyModbusTCP: which IP address should be specified on a server?

Question

I'm trying to make my Raspberry Pi 3B+ as ModbusTCP server and for testing purposes my PC as a client. I'm using pyModbusTCP module for this.

My server script (RPi) is:

from pyModbusTCP.server import ModbusServer, DataBank
from time import sleep
from random import uniform

server = ModbusServer("localhost", 502, no_block=True)

try:
    print("Start server...")
    server.start()
    print("Server is online...")
    while True:
        DataBank.set_words(51, [int(uniform(0, 100))])
        sleep(1)
except:
    print("Shutdown server...")
    server.stop()
    print("Server is offline...")

Client side script (PC) is:

from pyModbusTCP.client import ModbusClient

client = ModbusClient(host="192.168.1.122", port=502)
client.open()

192.168.1.122 - is my Raspberry Pi (Server) IP address.

As you can see - I put localhost as IP address in the server script. I tried 127.0.0.1 as well. However Clilent were not able to connect to the server and returned False after 30 seconds of waiting.

If I specify server's IP address (192.168.1.144 in my case) - server script imediately go to "Sever is offline..." part.

I suspect that I was wrong with that IP address.

Any suggestion?

Answer 1

localhost or 127.0.0.1 (same thing) are only accessible form the local host, i.e .the pi itself in this case - so using either of those two values is definitely not going to work

you need to specify the IP address of the interface you want to listen on ... 192.168.1.122 in your case -

However, if that's a DHCP assigned address you can't rely on that

Many networking type things allow you to use 0.0.0.0 to specify ALL interfaces - that may be the ideal solution

Answer 2

In my case port value needed to be equal to or more than 1024. From the Modbus Specifications and Implementations in some cases, you need a port higher than 1024.