Modbus TCP connection only sends 113655 Bytes of data before timing out.


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Dear all,

I am new to both the world of MODBUS and to LAbview. I am in the process of creating a Labview interface which will send a MODBUS command (fc15) to a WAGO (750-881) controller with intervals of 10ms.

The command sent is correct as when working, the the DO's of the WAGO respond correctly, however, the program only works up until 113655 bytes of data have been sent. After this no more messages are sent to the controller. (TCP timeout error occurs in Labview).

I have already tried to decrease the frequency of the messages sent (to every 20, 100 and 200 ms) along with increasing the TCP connection and TCP write timeout values from 10ms to 100 ms. Neither of these adaptations have had any effect on the problem.

I am really stumped as to why the program works fine up until 113655 Bytes of data have been sent (each MODBUS command is 15 Bytes so that's 7577 MODBUS commands) and then simply stops.

Any help that could be provided as to why the messages stop, (the reason for the TCP connection error) would be greatly appreciated. I am unsure if the error is due to the WAGO, the MODBUS commands or the Labview VI, so any debugging tips on how to narrow down the possible cause would also be appreciated.
Sound like a memory leak. Is it possible Labview is keeping each and every response in memory instead of each response over writing the previous response in the same memory.
Can you install Wireshark to track TCP/IP messages to see if Labview really sends that last message and see that the other program doesn't answer/answers wrong?