User guide
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
b
y
tes. 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 b
y
tes lon
g
and includes the followin
g
fields:
inv_id [2 b
y
tes] invocation id used for transaction pairin
g
proto_id [2 b
y
tes] used for intra-s
y
stem multiplexin
g
, default is 0 for
Modbus services
len [2 b
y
tes] the len field is a b
y
te count of the remainin
g
fields and
includes the dst_id and data fields
The remainder of the pdu includes two fields:
dst_idx [1 b
y
te] destination index is used for intra-s
y
stem routin
g
of
packets
(
currentl
y
not implemented
)
data [n b
y
tes] 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 b
y
te] Modbus function code
data [n b
y
tes] this field is function code dependent and usuall
y
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.