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MELSEC-Q
4 FUNCTIONS
4.1.4 I/O data separation prevention function
The I/O data separation prevention function is a function to prevent the I/O data of the
PLC CPU from being unmatched with the I/O data of the slave station.
(1) I/O data separation prevention function
(a) The separation of I/O data occurs when the buffer memory is also read
from/written to the PLC CPU during the data transmission between the
buffer memory of QJ71PB92D and the slave station.
The following shows the example of the separation occurred when the data
is read from the PLC CPU during the data transmission from the slave
station to the buffer memory.
Example of separation of I/O data
1
0
3
2
B
A
D
C
B
A
D
C
1
3
0
2
5
4
7
6
1
3
0
2
PLC CPU
Read
QJ71PB92D
Buffer memory
Data
transmission
Slave station
: Area where data transmission from slave station is updated
: Area where data transmission from slave station is not yet updated
(b) When the separation prevention function is used, the reading from the PLC
CPU is waited during the data transmission from the slave station to the
buffer memory (input area ) of QJ71PB92D, and executed after the data
transmission has been completed.
Also, the data transmission to the slave station is waited until the writing
from the PLC CPU to the buffer memory (output area) of QJ71PB92D is
completed, and executed after the writing has been completed.
(c) To activate the I/O data separation prevention function, two methods are
available: a method to select the Separation Prevention in setting the auto
refresh and a method to use the dedicated instruction (BBLKRD/BBLKWR)
in the sequence program.
When a refresh is performed between the buffer memory of QJ71PB92D
and the PLC CPU using the FROM/TO instruction, the separation prevention
function cannot be used.
(d) The separation prevention function can be used when the CPU module has
the following serial No. (See section 2.5).
QCPU (Q mode): Serial No. 02092
and subsequent.
(e) When the separation prevention function is used, there is a wait for reading
from /writing to the PLC CPU and the data transmission between the CPU
and the slave station, therefore, the transmission between the PLC CPU and
the slave station is delayed.
For the transmission delay time, refer to section 6.2 Transmission delay
time.
