User manual

Ethernet TCP/IP Modbus Application Protocol
346
33002479 06 07/2008
Modbus Application Protocol PDU
Overview The following information describes the structure and content of the Modbus
Application Protocol PDU.
Description 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 fields listed in the following table:
The remainder of the pdu includes two fields:
The service portion of the Modbus Application Protocol, called mb_pdu, contains
two fields:
mb_pdu::={func_code[1], data[n]}
The following table describes the fields in mb_pdu.:
The size and content of the data field are dependent on the value of the function
code.
Field Description
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 it includes the
dst_id and data fields
Field Description
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 it is defined
below
Field Description
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