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MELSEC-Q
4 MODBUS
R
STANDARD FUNCTIONS
4.3 PDU Formats by Functions
This section explains the PDU (protocol data unit) formats of the MODBUS
R
standard
functions.
(1) Precautions for specifying device number in message
When specifying a device number in a message, specify "(device number) - 1".
However, it does not apply to the file number and device number specified for
Read/Write file record.
(Example) When the status of input 32 (100032) is read with Read discrete
inputs (FC: 02)
Head input number
Data
Read points
(H)
(L)
(H)
(L)
Function code
Function code
02
H
001F
H
0001
H
Specify 31 (001F
H
) for the head input number
to read the input 32 (100032) status.
The device number stored in the response message is "(actually read/written
device number) - 1".
(2) Overview of request/response message format given in this section
(a) Request/response message format diagram
The following indicates the overview of the request/response message
format diagram given in Section 4.3.1 to 4.3.12.
Head coil number
Data
Read points
(H)
(L)
(H)
(L)
Function code
Function code
(01
H
)
Area name
Frame contents
[Request message format]
Setting range
[Response message format]
Value stored in response message
1 byte
(8 bits)
1 byte
(8 bits)
When 2 bytes comprise 1 data, the high-
order byte (8 bits) are represented as
(H) and the low-order byte (8 bits) as (L).
(0001
H
to 07D0
H
)
(0000
H
to FFFF
H
)
(b) Response message format
The format of a response message issued by the slave to the master differs
depending on whether the processing in the slave has been completed
normally or not.
In Section 4.3.1 to 4.3.12, the formats for normal completion and error
completion are provided.
