8.
COMMUNICATIONS USING
DEDICATED
PROTOCOLS
MELSEC-A
The conditions under which the
AJ71C24
transmission sequence is
initialized are as follows:
.The power supply is turned
ON,
the
PC CPU
is reset with the reset
0
Data communications is completed normally.
.The control code
EOT
or
CL
is
received.
.The
NAK
control code is received.
0
Durin full-duplex communications through the
RS-232C
interface,
the
C
s
signal is turned
OFF.
(The
ON/OFF
status of the
CD
signal is ignored
if
the
CD
terminal
check function
is
disabled.)
switch, or the mode of the
AJ71C24
is switched.
NULL
code transmission from the
AJfl C24
A
framing error might occur
in
the
AJ71C24
if nothing is sent from the
computer to the
AJ71C24
via the
RS-422
interface. In this case, the
AJ71 C24
sends
'OOH'
(NULL
code) to the computer. These
NULL
codes
should be ignored by the computer.
The computer should also ignore all data sent from the
AJ71C24
prior
to an
STX,
ACK,
or
NAK
code.
NAK
response from the
AJ71C24
The
NAK
response is given from the
AJ71C24
to the computer using
the dedicated protocol if an error is detected. Therefore, the
NAK
response may be output even while the computer is sending data
in
the
full-duplex communications mode.
Data link error processing
The
AJ71C24
enters the standby state (see Section
3.6
I/O
list for
programmable controller
CPU)
if
a data link error occurs during data
communications with a
PC CPU
(the
PC CPU
number being other than
FFH) on
MELSECNET(II), MELSECNET/B.
If
an error
is
detected by the computer when executing the time check,
send a clear command (EOT or
CL,
see Section
8.4.5
(1))
to initialize
the transmission sequence.
Sending a command from the computer
When sending a command from the computer to the
AJ71 C24
using the
dedicated protocol, send the command only after the data communica-
tions called
by
the preceding command is completed.
Replacing a
PC CPU
on the data link system
After starting up the
AJ71C24,
if
a
PC CPU
on the data link system is
replaced by a different model
PC CPU,
start up the
AJ71C24
again.
(First, reset
the
power supply to the self station's
PC CPU,
then reset
the
CPU,
and then switch the mode of the
AJ71 C24.)
a-4
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