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8.
COMMNlCATlONS,USING
DEDWq PROTOCOLS
MELSEC-A
(3)
Special-function module buffer memory
The special-function module buffer memory is comprised of 16-bit (one
word) addresses. Read and write of
the
special-function module buffer
memory is executed by
TO
and
FROM
instructions transmitted between
the PC
CPU
and special-function module.
When the computer reads from and writes to the special-function
module buffer memory via the AJ71C24,
it
is done in byte units
(1
address
=
8
bits).
The addresses specified in the computer (hexadecimal) are converted
from
FROM/TO
instruction addresses as shown below:
Designated address (hexadecimal)
=
Module head address
+
[(FROMTTO
instruction address
x
2) converted into hexadecimal]
Example: To designate AD61 high-speed counter module
FROM/TO
in-
struction address
1
(CH.l preset value).
Specified address
I
FROM/TO
instruction address
1
x
2
+
Head address
82 H 2H 80H
The data format when the computer makes a read or write to or from the
special-function module buffer memory via the AJ71 C24, is explained below
using the AD61 module as an example.
-
LaWW
bit'
4oH
83Hbit
Middle
Em
IPOINT~
The buffer memory of each special-function module has its read and write
area, read-only and write-only areas, and areas reserved for
OS
use,
which are not available to the use. See the manual for each module
before using the buffer memory.
PC
CPU
or special-function module errors may occur
if
reading or writing
is not done correctly.
8-71
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