Schneider Optics Modicon Quantum Ethernet manual Modbus Application Protocol PDU, Is defined below

Quantum Ethernet TCP/IP Modbus Application Protocol

C.1.1 Modbus Application Protocol PDU

The Modbus Application Protocol PDU, mbap_pdu, is received at TCP port number

502.The current maximum size of the mbap_pdu for this class of services is 256 bytes. The structure and content of the mbap_pdu is defined to be:

mbap_pdu ::={ inv_id[2], proto_id[2], len[2],dst_idx[1], data=mb_pdu }

The header is seven bytes long and includes the following fields:

inv_id	[2 bytes] invocation id used for transaction pairing
proto_id	[2 bytes] used for intra-system multiplexing, default is 0 for
	Modbus services
len	[2 bytes] the len field is a byte count of the remaining fields and
	includes the dst_id and data fields

The remainder of the pdu includes two fields:

dst_idx	[1 byte] destination index is used for intra-system routing of
	packets (currently not implemented)
data	[n bytes] this is the service portion of the Modbus pdu, mb_pdu
	and is defined below

The service portion of the Modbus Application Protocol, called mb_pdu, contains two fields:

mb_pdu ::={ func_code[1], data[n] }

func_code[1 byte] Modbus function code

data	[n bytes] this field is function code dependent and usually
	contains information such as variable references, variable
	counts and data offsets

The size and content of the data field are dependent on the value of the function code.

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