Unable to connect to TCP Server

[abishur]

Using a Raspberry Pi Zero I installed pip3 and then install pyModbusTCP using pip3

Using the default example for a server I placed a print("Started") after server.start(), but it never prints started. If I added no_block=true then it will print "started" but in both cases I am unable to connect to the server.

Any ideas what could be going on?

[abishur]

I have also tried using the library on Ubuntu on a regular computer, and I cannot connect to the modbusTCP server either locally or remotely. I've tried installing the past 2 versions and those didn't work either.

[abishur]

After doing some testing I've discovered that if I put the actual IP address of the computer I can connect to it, but if I give localhost or 127.0.0.1 then the connection fails. For the time being I've added this bit of code to fix the issue:

from pyModbusTCP.server import ModbusServer, DataBank
import socket

s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
try:
  # doesn't have to be reachable
  s.connect(('10.255.255.255', 1))
  IP_add = s.getsockname()[0]
except Exception:
  IP = '127.0.0.1'
finally:
  s.close()

server = ModbusServer(host=IP_add,port=502,no_block=True)
server.start()

And as long as I run the script with root privileges (though it would also work without root privileges if I chose a higher port number) I can successfully connect to it. Alternatively, I could explicitly state the IP address and not need to import socket, but if the computer is getting it's ip through DHCP anytime the address changes the script would break.

[sourceperl]

You should try to use the ModbusServer class with the host parameter set to '0.0.0.0' (= listen on all host IPs).

You can also look at https://stackoverflow.com/questions/38256851/why-does-my-python-tcp-server-need-to-bind-to-0-0-0-0-and-not-localhost-or-its

[abishur]

Thank you, setting it to 0.0.0.0 did indeed allow me to connect to it (and without needing to import socket), you might want to update the example sketch to make a note of this as the documentation and all examples show 'localhost' or 127.0.0.1

[sourceperl]

You're right, I'll add a few comments.