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Quantum Ethernet TCP/IP Modbus Application Protocol

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

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

tes lon

and includes the followin

fields:

inv_id [2 b

tes] invocation id used for transaction pairin

proto_id [2 b

tes] used for intra-s

stem multiplexin

, default is 0 for

Modbus services

len [2 b

tes] the len field is a b

te count of the remainin

fields and

includes the dst_id and data fields

The remainder of the pdu includes two fields:

dst_idx [1 b

te] destination index is used for intra-s

stem routin

packets

(

currentl

not implemented

)

data [n b

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

te] Modbus function code

data [n b

tes] this field is function code dependent and usuall

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.

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