Modbus TCP/IP Client to Client communication


I need to exchange data between two Modbus TCP/IP clients.
I've tested a software called ModRSsim2 and it seems to do the job, except for the fact that it writes a zero to all the writable registers when it starts...
However because this is a free software I'm not sure if I can use it for commercial purposes.
Another big concern I have relates to Cyber security. Not sure if it's safe being a free software...

Does anyone know a good and safe software I can buy to be used as a Modbus TCP/IP server and that allows at least two simultaneous clients?

Any ideas also on how to avoid the startup value of zero for the writable registers?

Thanks in advance,
You could use ModSlaveSim or Modsak (in slave mode). Modsak has the possible advantage that it can be started automatically using the '-auto' command-line option (ModSlaveSim requires you to click a button to start it). Both programs allow you to configure initial register values.

Frank O'Gorman
Wingpath Software Development
Hi Frank,

Thanks for your reply.
Got a few questions, would appreciate if you could clarify:
Are both these programs Cyber safe? And do they accept more than one client simultaneously?
Do they run on a Windows Server environment?
Can I start ModSlaveSim as a service when my Windows 2016 Server starts?

Hello Antonio

The Modbus protocol isn't "Cyber safe" (unless you use the Modbus/TCP Security Protocol, and so far as I am aware nobody is using it), so no program or device using Modbus is "Cyber safe". Modbus should only be used in a network that is isolated, either physically or by a firewall. If you need access from outside the network, you could use VPN technology to tunnel through the firewall, or use secure SNMP outside the network and use ModSnmp to provide a gateway to the Modbus devices behind the firewall.

Both programs will accept multiple clients simultaneously.

Both programs will run under Windows Server.

ModSlaveSim requires user interaction to run it, so it would not be sensible to start it as a service. Modsak can be started without user interaction, although it does have a GUI interface which might not be appropriate.


Hi Frank,

Thanks for the clarification.
I will download the trial versions to test which tool is more appropriate and quote it to my client.