3.
SPECIFICATIONS
SI,
;>
MELSEC-A
3.7
Buffer
Memory
AppllCatiOn8 and Allocation
The term 'buffer memory' used
in
this manual refers to a memory area of an
AJ71 C24 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
FROM/TO
instruction.
The buffer memory
can
be
accessed from an external device by using the buffer
memory read/write 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.
User area
There are four applications of the user area, which can be
categorized as follows.
Data receive area in no-protocol mode/bidirectional mode
This area stores data transmitted from an external device in the
no-protocol mode or bidirectional mode.
No-protocol mode/bidirectional mode data send area
This area stores data from the PC CPU to be transmitted to an
external device.
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.
Area when using buffer memory read/write commands
This area stores data when communication is made using
protocols
1
to 4 for buffer memory read/write commands
(WCW).
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, the PC CPU
is
reset or switching the
mode, 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.
3-36
.
-_I___
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