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5. HALF-DUPLEX COMMUNICATIONS USING THE RS-232C INTERFACE
MELSEC-A
5.4.1
Data transmission timing from an external device
When doing half-duplex communications, the data transmission timing from
the external device is shown by using the CD and RS signals of the AJ71 C24.
Setting the buffer memory of the AJ71C24 to "prioritylnon-priority at simul-
taneous transmission" controls the CD signal of the AJ71C24.
(I)
AJ71 C24 is set to " priority
".
. .
Resend
Data A
Data
B
External device
1
\
I
I
aJ71C24
\
Data C
IAJ71C24I
B
. -
SD (send data)
T
RS (send request)
CS (crear to send)
- .
CD (receive carrier
detect)
- .
RD (received data)
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 t o 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
(a)
starting or during data transmission to the AJ71C24,
(b)
if the DTR(ER) signal of
AJ71C24 turns OFF, interrupt data transmission until the DTR signal turns ON.
See Appendix 4.
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