Schneider Optics Web Embedded Server - Manual (Page 88)

Quantum Ethernet TCP/IP Modbus Application Protocol
78 840 USE 115 00 Version 1.0
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.