How to interpret a Modbus over IP telegram in Wireshark?

FAQ #101441

 

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

Modbus Client: (PCD3.M3330)
IP Address: 172.16.1.45

Modbus Server: (PCD3.M5540)
IP Address: 172.16.1.46
UID: 33 (Unit Identifier)




Modbus telegram request:

function(code):
--> 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)

reference:
--> 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.

Data:
--> contains the required data from the request.