STM32H725 TCP Server Stuck in Retransmission Loop – Repeated Retransmissions in LISTEN Stats

[RahulPatel]

Hi everyone,

I'm working on a project using an STM32H725 microcontroller running ThreadX and NetX Duo, and I've run into a persistent issue with the TCP server on port 502(Modbus).

Setup:

• Networking handled by NetX Duo
• TCP server bound to port 502
• listen_queue_size = 1
• The socket is created once and reused (not deleted or recreated)
• Six other TCP ports are also in use and working without issues

After running for a few minutes, Wireshark starts showing repeated [TCP Retransmission] packets being sent from the STM32, even though the socket on port 502 remains in the LISTEN state. At this point, the port stops accepting any new connections.

Why is NetX Duo sending retransmissions when the socket is in the LISTEN state?
How can I prevent the board from getting stuck and ensure it continues to accept new connections reliably?

Any insights, suggestions, or similar experiences would be greatly appreciated!

Regards, Rahul

[STackPointer64]

Hello @RahulPatel and welcome to ST community!

To assist you effectively, could you please provide a minimal version of the project that reproduces this issue? If the project is private, feel free to send it to me privately.

Best regards,