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4
MODBUS STANDARD FUNCTIONS
4.3.9 Read file record (FC: 20) (SC: 06)
Function code
Function code
(14
)
H
4 - 15
Reads the values of one or more extended file registers.
(1) Request message format (Master → Slave)
Reference
Number
number
(0000
of bytes
(06
)
m 7
H
(H)
Reference
number
(06
)
H
: The upper limit of the file number available for the QJ71MT91 slave function
is equal to the file register size of the mounted programmable controller CPU.
(Refer to Section 7.4.4.)
(a) Specify the number of sub-requests "m" so that the PDU (protocol data unit)
size in the request message does not exceed 253 bytes.
2 + m × 7 ≤ 253
If the above condition is not satisfied, the request message is discarded.
(b) Specify the total read points of all the sub-requests, "N (n1 +
so that the PDU (protocol data unit) size in the request message does not
exceed 253 bytes.
2 + m × 2 + N × 2 ≤ 253
If the above condition is not satisfied, the slave returns an exception
response.
Number of accessible devices with one message
Data
Sub-request 1
Read head device number
File number
to FFFF
)*
(0000
H
H
H
(L)
(H)
(Number of bytes m 7)
Sub-request m
Read head device number
File number
(0000
to FFFF
)*
H
H
(H)
(L)
(Number of bytes m 7)
1 to 124 points
Read points
to 270F
)
n1
H
(0001
to 007C
H
(L)
(H)
Data
(0000
to 270F
)
H
H
(0001
(H)
(L)
(H)
MELSEC-Q
)
H
(L)
Read points
nm
to 007C
)
H
H
(L)
+ nm)"
4 - 15
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