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MELSEC-Q
4 MODBUS
R
STANDARD FUNCTIONS
4.3.9 Read file record (FC: 20) (SC: 06)
Reads the values of one or more extended file registers.
Number of accessible devices with one message
1 to 124 points
(1) Request message format (Master Slave)
Number
of bytes
m 7
Data
File number
(H) (L)
Function code
Function code
Read head device number
(H) (L) (H) (L)
Sub-request 1
Reference
number
Read points
n1
Data
File number
(H) (L)
Read head device number
(H) (L) (H) (L)
Sub-request m
Reference
number
Read points
nm
(Number of bytes m 7)
(Number of bytes m 7)
(14
H
)
(0000
H
to FFFF
H
)*
(0000
H
to 270F
H
)
(0001
H
to 007C
H
)
(06
H
)
(0000
H
to FFFF
H
)*
(0000
H
to 270F
H
)
(0001
H
to 007C
H
)
(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 +
+ nm)" 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.
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