Schneider Optics Web Embedded Server - Manual (Page 91)

Quantum Ethernet TCP/IP Modbus Application Protocol
840 USE 115 00 Version 1.0 81
The header is 7 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 2
fields:
mb_pdu ::= { func_code[1], data[n] }
func_code[1 byte] MB 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.