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4
MODBUS STANDARD FUNCTIONS
4.17 Read File Record (FC: 20) (SC: 06)
Reads multiple extended file register values.
(1) Request message format (Master
Function code
Function
Number of
code
bytes
(14
)
m x 7
H
4
- 55
4.17 Read File Record (FC: 20) (SC: 06)
Reference
File number
number
(0000
to FFFF
) *
H
H
(06
)
H
(L)
(H)
Reference
File number
number
(0000
H
(06
)
H
(H)
Figure 4.91 Read file record (Request message)
* 1 The maximum file number available for the QJ71MB91 slave function is dependant on the file
register size of the mounted programmable controller CPU.(
(a) Number of sub-requests, m
Specify the number of sub-requests, m, so that the protocol data unit size of the
request message will not exceed 253 bytes.
2 + m
7
253
If the above condition is not satisfied, the request message is discarded.
* 2 When the frame mode is ASCII mode, it is 506 bytes.
(b) Read points of each sub-request
Specify the total points N (n1+...+nm) so that the protocol data unit size of the
response message will not exceed 253 bytes.
2 + m
2 + N
If the above condition is not satisfied, the slave returns an exception response.
* 3 When the frame mode is ASCII mode, it is 506 bytes.
Slave)
Data
Sub-request
1
Read head device number
1
(0000
to 270F
)
H
H
(H)
(L)
(Number of bytes m x 7)
Data
Sub-request m
Read head device number
to FFFF
) *
1
(0000
to 270F
H
H
(L)
(H)
(Number of bytes m x 7)
*2
*3
2
253
Read points
n1
(0001
to 007C
)
H
H
(L)
(H)
Read points
)
nm
H
(0001
to 007C
)
H
H
(L)
(H)
(L)
Section 7.3.2)
*2
*3
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