10.
COMMUNICATIONS
IN
THE
BIDIRECTIONAL
.,
MODE
I*a
.
--m!m,
,/I,
'."',
I
MELSEC-A
NAK
code
Transmitting
NAK
from an
AJ7l C24
to a computer
The
NAK
response is given from an
AJ71C24
to a computer
if
an
error is detected.
Therefore, the
NAK
response might be given while the computer is
transmitting data
if
communications is made
in
the full-duplex
mode.
An
AJ71 C24
ignores the designated length of received data
if
it
detects an error while receiving data.
If
the data length is incor-
rect, the data received is ignored until the
ENQ
code is received.
I
(b) Transmitting
NAK
from a computer to an
AJ71C24
To transmit the
NAK
from a computer to an
AJ71C24,
transmit a
2-byte error code following the
NAK
code.
If
the
NAK
code is received as the response, execute error processing
according to the error code received directly after the
NAK
code.
The error codes related to the bidirectional mode communications are
described in Section
11.2.
Time-out check by a computer
If
a
time-out check is made for data transmitted from a computer send
area to an
AJ71 C24
receive area
in
the bidirectional mode, the time-out
check time to be set must be longer than the value shown below.
(Maximum scan time of the
PC CPU
x
2)
+
100
msec
NULL
code send from an
AJ71 C24
A
framing error might occur at the
AJ71C24
if
nothing is transmitted
from a computer to an
AJ71 C24
via the
RS-422
interface. In this case,
the
AJ71C24
sends
'OOH'
(NULL
code) to the computer receive area.
These
NULL
codes should be ignored by the computer.
The computer should
also
ignore all data transmitted from the
AJ71 C24
prior to an
ENQ, ACK,
or
NAK
code.
Using a
FROM/TO
instruction to access the
AJ71 C24
Using
a
FROM/TO
instruction to access the
AJ71C24
should not be
done unless absolutely necessary. This is because when data is sent
from the
AJ71 C24
to an external device at the same time as a
FROM/TO
instruction from a
PC CPU
is executed to the
AJ71C24,
the
FROM/TO
instruction has priority and is executed first. Therefore, the data send
time from the
AJ71 C24
to
the external device is delayed by the process-
ing time of the
FROM/TO
instruction.
Data
bit
setting
If
a checksum is communicated during data communications with a
computer, set the data bit (set with the
SW12
switch) to eight bits.
Section
4.3.2
gives details about switch
SWl2.
10-11
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..
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