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Hello I have develop a electronic card (client) for connect to a commercial controler(server) in modbus TCP. Can you tell me why the server don't answer, to modbus tcp, my ip is Thanks

asked 16 Jan '14, 05:45

gregoire's gravatar image

accept rate: 100%

I have found the problem, i have not the same source port between the 7 (SYN) and 8(modbus) frame , but wireshark have not detect the problem!


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answered 16 Jan '14, 08:19

gregoire's gravatar image

accept rate: 100%

That's a very odd TCP client.

As Wireshark is a packet analyzer not a Modbus master\slave, it doesn't particularly care that the "conversation" isn't proceeding using the same TCP stream as it just dissects the data seen on the wire. In particular consider a capture made after the start of the initial part of the conversation, Wireshark should still be able to dissect the traffic it has seen. Note that this isn't true for all protocols.

You can set Wireshark to display the ports used as columns and that may have lead to you (and me) spotting this earlier.

(16 Jan '14, 09:18) grahamb ♦

There seems to be some sort of connection issue between the client and the server. The server never responds to the client Modbus\TCP messages, and a long time later sends dup ACKS to the client SYN.

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answered 16 Jan '14, 06:48

grahamb's gravatar image

grahamb ♦
accept rate: 22%

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question asked: 16 Jan '14, 05:45

question was seen: 3,149 times

last updated: 16 Jan '14, 09:18

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