RTU over IP connection possible only when other app open a port

[sforBSD]

I want read 2 input registers.

sensor > moxa 5232 in tcp server mode > ethernet > *unix shell & code below.

from pymodbus.client import ModbusTcpClient
#from pymodbus.transaction import ModbusRtuFramer as ModbusFramer
from pymodbus.framer.rtu_framer import ModbusRtuFramer

import time

def run_read_data(ip, port_num, unit_num):
    while True:
        client = ModbusTcpClient(ip, port=port_num, framer=ModbusRtuFramer)
        con = client.connect()
        while con :
            rr = client.read_input_registers(1,2, unit=unit_num)
            try:
                print(rr,rr.registers)

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

                time.sleep(2)
                client.close()
                time.sleep(2)
                break
            time.sleep(1)
        time.sleep(1)

if __name__ == "__main__":
    ip = "192.168.2.130"
    port_num = 4002
    unit_num = 0x1f
    run_read_data(ip, port_num, unit_num)

It works only when other app, ie Modbus Poll polling (the port in moxa is opened) modbus device.

Otherwise:
AttributeError: 'ModbusIOException' object has no attribute 'registers'

I've been working on this on and off for a few weeks and haven't been able to get this configuration to work.
Help plz! :)

[janiversen]

You do not write which version you use, but unit= have not been permitted for a very long time, please read the doc or follow the signature of the api.

please add a debug log, without that we cannot see what happens.

looking at the sequence you present, of course it only works when the server is started, what did you expect?