How to interpret a Modbus over IP telegram in Wireshark?

FAQ #101441

In this FAQ a Modbus data exchanged between a CLIENT and a SERVER displayed with the Wireshark analyzer is explained.

The below telegrams are produced by using the SBC Modbus FBox library.


Protocol: Modbus/TCP
Used port: 502 (default for Modbus)

Modbus Client:
IP Address:

Modbus Server: (PCD3.M5540)
IP Address:
UID: 33 (Unit Identifier)


Modbus telegram request:

--> indicates what kind of REQUEST is sent to the SERVER (READ or WRITE)
--> indicates on which of the Modbus variables the request will be applied (HOLDING REGISTER, INPUT-REGISTER, DISC-INPUT, COIL)

--> indicates the address of the first Modbus variable/media which the request accesses.

word count / bit count:
--> indicates the number of Modbus variables/media which the request accesses.

byte count:
--> indicates how many bytes shall be read/written from/to the corresponding Modbus variable/media.

--> contains the required data from the request.



PG5 2.0 / Modbus

Last update: 26.05.2015 12:55

First release: 17.06.2010 14:59

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