User manual

Ethernet TCP/IP Modbus Application Protocol
350
33002479 06 07/2008
Structure and
Content
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 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
serviceslen[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 codedata[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.