Hosting multiple clients with freemodbus

Question

I am working on a project involving a microcontroller communicating to a PC via Modbus over TCP. My platform is an STM32F4 chip, programming in C with no RTOS. I looked around and found LwIP and Freemodbus and have had pretty good success getting them both to work. Unfortunately, I'm now running into some issues which I'm not sure how to handle.

I've noticed that if I establish connection, then lose connection (by unplugging the Ethernet cable) I will not be able to reconnect (once I've plugged back in, of course). Freemodbus only allows one client and still has the first client registered. Any new clients trying to connect are ignored. It won't drop the first client until after a specific timeout period which, as far as I can tell, is a TCP/IP standard.

My thoughts are...

2. I need a Modbus module that will handle multiple clients. The new client request after communication loss will be accepted and the first client will eventually be dropped due to the timeout.

   • How do I modify Freemodbus to handle this? Are there examples out there? I've looked into doing it myself and it appears to be a decently sized project.
   • Are there any good Modbus packages out there that handle multiple clients, are not too expensive, and easy to use? I've seen several threads about various options, but I'm not sure any of them meet exactly what I need. I've had a hard time finding any on my own. Most don't support TCP and the ones that do only support one client. Is it generally a bad idea to support multiple clients?

3. Is something wrong with how I connect to the microcontroller from my PC?

   • Why is the PC changing ports every time it tries to reconnect? If it kept the same port it used before, this wouldn't be a problem

4. Should I drop the client from Freemodbus as soon as I stop communicating?

   • This seems to go against standards but might work.

I'm leaning towards 1. Especially since I'm going to need to support multiple connections eventually anyways. Any help would be appreciated.

Thanks.

Answer 1

If you have a limit on the number of modbus clients then dropping old connections when a new one arrives is actually suggested in the modbus implementation guide (https://www.modbus.org/docs/Modbus_Messaging_Implementation_Guide_V1_0b.pdf)

Nevertheless a mechanism must be implemented in case of exceeding the number of authorized connection. In such a case we recommend to close the oldest unused connection.

It has its own problems but everything is a compromise.

Regarding supporting multiple clients...if you think about modbus/rs server - it could only ever have one master at a time. Then replace the serial cable with TCP and you see why it's not uncommon to only support one client (and of course it's easier to program). It is annoying though.

Depending on what you are doing you wont need the whole modbus protocol and implementing the parts you do need is pretty easy. Of course if you have to support absolutely everything its a different prospect. I haven't used freemodbus, or any other library appropriate to your setup, so I can't help with suggestions there.

Regarding the PC using different TCP source port each time - that is how TCP is supposed to work and no fault on your side. If it did reuse the same source port then it wouldn't help you because e.g. sequence numbers would be wrong.

Regarding dropping clients. You are allowed to drop clients though its better not to. Some clients will send a modbus command, notice the connection has failed, reconnect, but not reissue the command. That may be their problem but still nicer to not see it that often where possible. Of course things like battery life might make the calculation different.