5.
HALF-DUPLEX
COMMUNICATIONS
USING
THE RS232C INTERFACE
MELSEGA
5.4.1
Data
transmission timing
from
an
oxtemri
device
When doing halfduplex communications, the data transmission timing from
the
external device is shown by using the
CD
and
RS
signals of the
AJ71C24.
Setting the buffer memory of the
AJ71C24
to "priority/non-priority at simul-
taneous transmission" controls the
CD
signal of the
AJ71 C24.
(1) AJ71C24
is set to
I'
priority
'I.
bsond
External device
Data
A
Data
B
I
\
I
An1 C24
IAJ71C241
-
SD
(mnd data)
RS
(send request)
CS (crear to send)
-
CD (receive carrier
-
RD (received data)
detect)
i
\
Store thir data in
\
1
\
1
A171
C24 ignores
Data
B
The following steps describe the operations required for an external device at
every timing mentioned by
(1)
to
(6)
in
the above figure.
The signal names are of the signals
of
the
AJ71C24.
1)
When not transmitting data from the external device to the
AJ71C24,
turn the
CD
signal
OFF.
2)
When doing a data send, check the
RS
signal.
If
the
RS
signal is
OFF,
turn the
CD
signal
ON.
If
the
RS
signal is
ON,
wait until
it
turns
OFF.
After
the
RS
is turned
OFF,
turn the
CD
signal
ON.
3)
After turning the
CD
signal
ON,
transmit data.
4)
After completing the data send, turn
OFF
the
CD
signal
.
5)
If
the
RS
signal turns
ON
during the data send, stop the data send.
Then, turn the
CD
signal
OFF,
and perform data receive process-
ing.
(When the
AJ71 C24
and an external device start data transmission
simultaneously,
the
RS
signal turns
ON.)
6)
Retransmit all interrupted data from the external device to the
AJ71C24
after the data send from the
AJ71C24
is completed.
IREMARK)
When DTWDSR control is bing executed.
When (a) starting or during data transmission
to
the AnlC24, (b) if the DTR(ER) signal
of
AJ71C24 turns
OFF,
interrupt data transmission until the DTR signal turns
ON.
See Section 3.5.2.
5-5
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