Mitsubishi QJ71MB91, User Manual

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- 55 4.17 Read File Record (FC: 20) (SC: 06)
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MODBUS(R) STANDARD FUNCTIONS
4.17 Read File Record (FC: 20) (SC: 06)
Reads multiple extended file register values.
(1) Request message format (Master Slave)
* 1 The maximum file number available for the QJ71MB91 slave function is dependant on the file
register size of the mounted programmable controller CPU.( Section 7.3.2)
(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
2 + m 7 253
*2
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.
*3
2 + m 2 + N 2 253
*3
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.
Figure 4.91 Read file record (Request message)
Data
(L) (L) (L)
Sub-request 1
Data
(L) (L) (L)
Function code
Function
code
(14
H
)
Number of
bytes
m x 7
Reference
number
(06
H
)
File number
(0000
H
to FFFF
H
) *
1
Read head device number
(0000
H
to 270F
H
)
Read points
n1
(0001
H
to 007C
H
)
(Number of bytes m x 7)
(Number of bytes m x 7)
Sub-request m
Reference
number
(06
H
)
File number
(0000
H
to FFFF
H
) * 1
Read head device number
(0000
H
to 270F
H
)
Read points
nm
(0001
H
to 007C
H
)
(H) (H) (H)
(H) (H) (H)