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8. COMMUNICATIONS USING DEDICATED 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 AlSJ71C24, it is done in.byte units (1 address
=
8
bits).
The addresses specified in the computer (hexadecimal) are converted
from FROMITO instruction addresses as shown below:
-
Designated address (hexadecimal)
=
Module head address
+
[(FROMFO instruction address
x
2) converted into hexadecimal]
Example: To designate AlSD61 high-speed counter module FROMlTO
instruction address 1 (CH.1 preset value).
Specified address
=
FROMITO instruction address 1
x
2
+
Head address
82H
2H
8 0 H
The data format when the computer makes a read or write to or from the
special-function module buffer memory via the AlSJ71C24, is explained
below using the AISD61 module as an example.
AlSD61 buffer memory
AlSD61 buffer
I
i
viewed from the PC CPU
viewed from the
Address
A
Preset value
Middle bits: Lower bits
1
2
r
O H
I
Preset value
I
1
Higher b i t s b
OOH
01 H
I
Mode register
I
Upper
84H
1
01 bits H
1 -
re55
85H
I
OOH
I
l
82H
83H
-
Lower
bits
40H
Middle
bits
E2H
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.
86H
I
PC CPU or special-function module errors may occur if reading or writing
is not done correctly.
I
Mode
register
12H
I
I
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