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3. SPECIFICATIONS
MELSEC-A
3.5
Buffer Memory Applications and Allocation
a
-
The term 'buffer memory' used in this manual refers t o a memory area of an
AJ71C24 used to store the control and communications data which is trans-
mitted between an external device (e.g., a computer) and
a
PC CPU.
The buffer memory can be accessed from the sequence program by using
the FROMITO instruction.
The buffer memory can be accessed from an external device by using the
buffer memory readlwrite command (CR, CW) with dedicated protocols 1 to
4.
(1)
Buffer memory applications
There are two types of buffer memory area. One area may be used
freely by the user, but the other area has a special application.
4
(a) User area
There are four applications of t h e user area, which can be
categorized as follows.
1) Data receive area in no-protocol modelbidirectional mode
This area stores data transmitted from an external device in the
no-protocol mode or bidirectional mode.
2) No-protocol modelbidirectional mode data send area
This area stores data from the PC CPU t o be transmitted to an
external device.
3) On-demand data storage area
This area stores send data to be transmitted from the sequence
program to an external device using the on-demand function.
4) Area when using buffer memory readlwrite commands
4
This area stores data when communication is made using
protocols 1 t o 4 for buffer memory readlwrite commands
(CR,CW).
(b) Special applications area
The applications of this memory area are fixed. They are used to
determine the data communications format and to change the al-
location of the memory area for section (a) above.
When the power is turned ON or the PC CPU is reset, default values
are written to this special applications area.
Default values can be changed to suit the purposes and applica-
tions of data transmission and the specifications of the external
device. Section
7
gives details.
4
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