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4
MODBUS STANDARD FUNCTIONS
4.3.4 Read input registers (FC: 04)
4 - 9
Reads the values of one or more input registers.
(1) Request message format (Master → Slave)
Function code
Function code
Head input register number
(04
)
(0000
H
(H)
(2) Response message format (Slave → Master)
(When completed normally)
Function code
Function code
Number of
read bytes
(04
)
H
n 2*1
(When completed with an error)
Function code
Function code
Exception
(84
)
H
2: When completed with an error, the exception code and error code are stored
into the buffer memory.
Refer to Section 11.3 for the storage location, confirmation method and
more details.
Number of accessible devices with one message
Data
to FFFF
)
(0001
H
H
(L)
(H)
Device data
1
(H)
Data
code*
1 to 125 points
Read points
to 007D
)
H
H
(L)
Data
(L)
(Number of read bytes n 2)
*1:
For example, when n = 4, the number of
read bytes is 4
MELSEC-Q
Device data
n
(H)
(L)
2 = 8 bytes.
4 - 9
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