5.
HALF-DUPLEX COMMUNICATIONS USING THE RS-232C INTERFACE
MELSEC-A
5.2
Buffer
Memory
Settings
The following describes the buffer settings of the AJ71C24 for doing half-
duplex communications.
Perform the following settings with the sequence program only when the
AJ71C24 READY signal is turned
ON
after the CPU
Is
reset or when the PC
CPU
is turned
ON
or
(b)
the higher byte in the mode switching designation
area (address 119H) of the AJ71C24's buffer changes to 02H when the mode
is switched.
Section 7.2 gives setting details.
(1) Communications setting using the RS-232C interface (Address
1
OFH)
Set
"1"
to do half-duplex communications.
(2) Setting of priorityhon-priority at the simultaneous transmission (Address
110H)
When the AJ71C24 and the external device begin transmitting data
simultaneously
in
half-duplex communications, designate (a) continua-
tion (priority) of the send from
the
AJ71C24, or (b) interruption (non-
priority).
Set
"0"
to designate
"
priority
".
Set
"1"
to "225" to designate
I'
non-priority
'I.
This set value is the send wait time (unit :10 msec), until data transmis-
sion starts, after the data send state is restarted.
POINT]
When an AJ71 C24
is
set to
'I
priority
'I,
the AJ71 C24 keeps on transmitting
data and ignoring received data. Even if data is transmitted from
the
external device after the AJ71C24 has started data transmission.
The external device that transmits data must execute the following
so
that
the AJ71 C24 does not ignore received data:
.Transmit response messages to start communications
.Resend data when a time out error of a response message occurs.
(3)
Setting the method of resend (Address 11 1 H)
When setting
'I
half-duplex communications
'I
+I'
non-priority
I'
according
to (1) and (2), this setting becomes valid.
As for simultaneous transmission from the external device and the
AJ71C24, when the AJ71C24 restarts the send after interruption of the
send, designate whether the interrupted message is transmitted again
from the beginning ("resend")
or
only the remaining part is transmitted
("not resend").
Set "1" to designate
I'
resend
'I.
Set
"0"
to designate
"
not resend
".
5-2
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