#6 Problem ModBus TCP over Internet



I use "ModBus simulator" that work very well!

But I have this problem:
• LAN with:
o Router/Switch with ADSL IP fixed public ( – Sample Only)
o PC1 with IP
o PC2 with IP
o PC With ModBus Simulator (Slave) with IP
• When I use “ModScan32” (or other software Master) installed in PC1 or PC2 reading data from PLC all works fine!!!
• After that I have “opened” the TCP port 502 on Router routing that on Local IP
• But if I try to access data passing through internet (simply changing IP in “Connect” of "ModScan32" writing that associated to ADSL connection) the connection don’t go!
• The port 502 is correctly open (I try this using a “PFPortChecker” utility)
• My Router is Dlink DSL-G604T
• Any idea?

Thanks for all and Good Christmas


  • Conrad Braam

    Conrad Braam - 2010-04-18
    • assigned_to: nobody --> zaphodikus
    • status: open --> open-accepted
  • Conrad Braam

    Conrad Braam - 2010-04-18

    Oh WOW - sorry for not getting back to you sooner.
    OK, I am not the Firewalling WAN/LAN guru at all, but we can try troubleshoot, because I have done home to server connections of my simulator before over web.
    you want to firstly unblock port 501 and 502, BUT!!!!! big big BUT, the moment you do so , you will get attacked, its the same port used my some part of Microsoft exchange, hence common attack - but you not worried about MS Exchange server, since you have not got one. Next you want to check that remote router is also open same port, then try again.

    Let me know how it went.


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