Modbus TCP interface get stuck

alexgooi — Tue, 14 Apr 2026 09:24:47 -0000

Dear Forum,

I'm currently experimenting with Modbus interfaces in combination with a virtual controller.

I have the following setup (Modbus_Setup.png):
I have an IPC with Debian installed on it (Debian 13). On this Debian machine I have installed a docker server and on this docker server a Codesys runtime container is deployed (Codesys_image.png).
The Modbus scanner makes connection with and IO coupler and this works well. But when I physically disconnect the ethernet connection between the IPC and the IO coupler the Modbus interface gets stuck in a state where it gets stuck in the busy state (Modbus_Stuck.png). When I plug the cable back in, the IO coupler isn't communicating until the xBusy variable is manually set to False, after this action is performed the communication goes back to normal. I have tried to set the container network to host and bridge (Network.png), but both version gives the same result (stuck scanner).

I would expect that the Modbus interface is generating a timeout after 1 sec, because this is how it is setup (Modbus_settings.png), but this is not what happens. I have thought to write a piece of code that writes xBusy to false when it is stuck but because it is an output I'm not able to write to it.

I have tried this with another PC (different CPU/motherboard) but this gives the same outcome. I tried updating to the latest version of Virtual control (4.20) but this gives the same result.
I also tried to do the same thing with an ethernet IP scanner, and this gives an error after the timeout is expired (so this works well).

I have tried to connect to the IO coupler using the Modbus_FB lib (Modbus_FB.png) the FB's are giving the correct errors and behaving as expected.

At last I have tried to use a Scanner on a Codesys controller (no IPC) and here the scanner behaves as expected (an alarm is triggered after the timeout has been expired).

The version of the Codesys components are listed in the picture Versions.png

Is there anyone that can give me a push in the right direction?

Kind regards Alex

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Modbus TCP interface get stuck