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The remote device does not respond to the Read Holding Registers request.

Looking at the first capture and applying a display filter of "modbus" you can see that the Read was given transaction id 2. The transaction id allows the matching up of requests and responses and should be incremented for each new request from the client. The output of the info columns shows the transaction code and it can be observed that the the read request for the holding registers (frame 9) occurs after the first request to write registers (frame 6) and before the server could respond. The message in frame 10 is not a response to the read registers request but the response to the write registers request. In the rest of the capture there are no further responses from the server.

frame.number Info
6               Query: Trans:     1; Unit:   1, Func:  16: Write Multiple Registers
9                Query: Trans:     2; Unit:   1, Func:   3: Read Holding Registers
10           Response: Trans:     1; Unit:   1, Func:  16: Write Multiple Registers [TCP segment of a reassembled PDU]
11              Query: Trans:     3; Unit:   1, Func:  15: Write Multiple Coils
15              Query: Trans:     4; Unit:   1, Func:  16: Write Multiple Registers
18              Query: Trans:     5; Unit:   1, Func:   3: Read Holding Registers
21              Query: Trans:     6; Unit:   1, Func:  15: Write Multiple Coils
25              Query: Trans:     7; Unit:   1, Func:  16: Write Multiple Registers

The remote device does not respond to the Read Holding Registers request.

Looking at the first capture and applying a display filter of "modbus" you can see that the Read was given transaction id 2. The transaction id allows the matching up of requests and responses and should be incremented for each new request from the client. The output of the info columns shows the transaction code and it can be observed that the the read request for the holding registers (frame 9) occurs after the first request to write registers (frame 6) and before the server could respond. The message in frame 10 is not a response to the read registers request but the response to the write registers request. In the rest of the capture there are no further responses from the server.

frame.number Info
6               Query: Trans:     1; Unit:   1, Func:  16: Write Multiple Registers
9                Query: Trans:     2; Unit:   1, Func:   3: Read Holding Registers
10           Response: Trans:     1; Unit:   1, Func:  16: Write Multiple Registers [TCP segment of a reassembled PDU]
11              Query: Trans:     3; Unit:   1, Func:  15: Write Multiple Coils
15              Query: Trans:     4; Unit:   1, Func:  16: Write Multiple Registers
18              Query: Trans:     5; Unit:   1, Func:   3: Read Holding Registers
21              Query: Trans:     6; Unit:   1, Func:  15: Write Multiple Coils
25              Query: Trans:     7; Unit:   1, Func:  16: Write Multiple Registers