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5.
HALF-DUPLEX COMMUNICATIONS USING THE RS-232C INTERFACE
MELSEC-A
5.4.2
Data transmission timing from an AlSJ71C24
When doing half-duplex communications, data transmission timing from an
AlSJ71 C24 is shown by using the CD signal and RS signal of the A1 SJ71 C24.
Control the CD signal of the AlSJ71C24 by setting the buffer memory of the
AlSJ71C24 to "priority/non-priority at simultaneous transmission" for data
transmission.
(1)
AlSJ71C24 is set to
"
priority
"
Data C-1
device must store
SD (send data)
T
RS (send request)
-
CS (clear to send)
-
CD (received carrier
-
RO (received data)
The following steps describe the operation at every timing mentioned by (1)
to (6) in the above figure.
The signal names are of the signals of the AlSJ71C24.
As described in ( I ) , turn ONIOFF the RS signal of the AlSJ71C24 with the
external device and transmit data to the AlSJ71 C24.
1) When not transmitting data from the external device t o the
AlSJ71C24, turn the RS signal OFF.
a
2) When doing a data send, check the CD signal. If the CD signal
is OFF, turn the CD signal ON.
When the CD signal is ON, wait until it turns OFF. After the CD
is turned OFF, turn the RS signal ON.
3) After turning the RS signal ON, transmit data.
4)
After completing the data send, turn OFF the RS signal.
5)
If the CD signal turns ON during the data send, continue transmit-
ting data send t o the AlSJ71G24.
(This occurs when the
A1 SJ71 C24 and the external device start data transmission simul-
taneously.)
6)
Transmit all interrupted data from the external device t o the
AlSJ71C24 after data send from the AlSJ71C24 is completed
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