Data Consistency Function - Mitsubishi Electric Melsec Q Series User Manual

4
FUNCTIONS

4.5 Data Consistency Function

When I/O data from DP-Slaves are read from or written to buffer memory, this function
prevents the I/O data from being separated and incorrectly mixed.
(1) I/O data consistency function
<Before the data consistency function is used>
QCPU
1
0
3 2
B A
D C
Read during
data transfer
After the data consistency function is used
QCPU
1 0
3 2
5 4
7 6
Read after completion
of data transfer
(a) The PROFIBUS-DP bus cycle and QCPU sequence scan are performed
asynchronously.
Because of this, when the QCPU reads input data in the buffer memory during
input data transfer from a DP-Slave to the buffer memory, the original data may be
divided generating inconsistency in the input data. (The same applies to output
data.)
The following shows an example of data inconsistency when data are read from
the QCPU during the input data transfer from a DP-Slave to the buffer memory.
QJ71PB92V
Buffer memory
1
0
3 2
B A
D C
Data transfer
in process
Figure 4.19 Example of Input Data Inconsistency
(b) When the data consistency function is enabled, it makes reading from the QCPU
wait until data transfer from a DP-Slave to the QJ71PB92V buffer memory (Input
data area) is completed, and the reading is executed upon completion of the data
transfer.
Alternatively, the QJ71PB92V stands by for data transfer to DP-Slaves until
writing from the QCPU to the QJ71PB92V buffer memory (Output data area) is
completed, and executes the data transfer upon completion of the writing.
QJ71PB92V
Buffer memory
1 0
3 2
5 4
7 6
Data transfer
completed
Figure 4.20 Example of Input Data Consistency
4.5 Data Consistency Function
DP-Slave
1
0
3 2
5 4
7 6
DP-Slave
1 0
3 2
5 4
7 6
: Updated area for data
transferred from DP-
Slave
: Not-updated area for data
transferred from DP-
Slave
Updated area for data
:
transferred from DP-
Slave
Not-updated area for data
:
transferred from DP-
Slave
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