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MELSEC-Q
4 MODBUS
R
STANDARD FUNCTIONS
4.3.4 Read input registers (FC: 04)
Reads the values of one or more input registers.
Number of accessible devices with one message
1 to 125 points
(1) Request message format (Master Slave)
Head input register number
Data
Read points
(H)
(L)
(H)
(L)
Function code
Function code
(0000
H
to FFFF
H
)
(04
H
)
(0001
H
to 007D
H
)
(2) Response message format (Slave Master)
(When completed normally)
Data
(H)
Function code
Function code
Device data
1
Device data
n
(Number of read bytes n 2)
*1:
(L)
(H)
(L)
Number of
read bytes
n 2*1
(04
H
)
For example, when n = 4, the number of
read bytes is 4 2 = 8 bytes.
(When completed with an error)
Data
Function code
Function code
(84
H
)
Exception
code*
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.
