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5. HALF-DUPLEX COMMUNICATIONS USING THE RS-232C INTERFACE
MELSEC-A
5.
HALF-DUPLEX COMMUNICATIONS USING THE RS-232C INTERFACE
This section explains how to do half-duplex communications using an RS-
232C interface t o connect an external device and an AlSJ71C24.
This section does not apply to full-duplex communications.
AlSJ71C24 can do half-duplex communications with an external device by
using the RS-232C interface by setting buffer memory.
While receiving data from an external device in half-duplex communications,
data is not transmitted from the AlSJ71C24 t o the external device.
The key points for doing half-duplex communications between an external
device and the AlSJ71 C24 using the RS-232C connector are as follows:
System configurations and functions
Buffer memory settings
Wiring
ONIOFF timing of the CD and RS signals of the AlSJ71C24
a
5.1
System Configurations and Functions
The following figure shows (a) the system configurations of the external
device and the PC CPU that can do half-duplex communications, and (b) the
functions of the A1 SJ71 C24
x : Unusable
*1
During data communicates, the send timing of data that a sequence
program requested to send changes due t o the on-demand function.
No-protocol
Modelbidireo-
tional Mode
Data Send
and Data
Receive
0'2
Dedicated Protocol
See Section 8.14.2.
o :
Usable
Data Communications
by
a
Command
(external device
:
Transmitted from
the External Device
1 : 1
0
The send timing also changes as mentioned i n *2 below.
Data Send from the PC
CPU
by
the On-
demand Function
0*l
*2
Send timing of data sent from the AlSJ71C24 and the external device
changes according t o the set timing of "prioritylnon-priority at the simul-
taneous transmission" set with the AlSJ71C24.
See Section 5.4.
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