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 |
| 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 |
| 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.
78 | 840 USE 115 00 Version 1.0 |