The TCP port used is: 502.

Request Format:

Name		Length	Function
Transaction Identifier	MBAP	2	For synchronization between messages of server and client
Protocol identifier		2	0 for Modbus/TCP
Length field		2	Number of remaining bytes in this frame
Unit identifier		1	Unit address (255 if not used)
Function code	PDU	1	Function codes as in other variants
Address of first register		2	Address of the first register
Number of registers		2	Number of registers

Response:

Name		Length	Function
Transaction Identifier	MBAP	2	For synchronization between messages of server and client
Protocol identifier		2	0 for Modbus/TCP
Length field		2	Number of remaining bytes in this frame
Unit identifier		1	Unit address (255 if not used)
Number of Bytes following	PDU	1	Number of Bytes following
Data		N	Data

MBAP Header = Modbus Application Header
PDU = Protocol Data Unit

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Simply Modbus: Modbus TCP Protocol