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1. GENERAL DESCRIPTION
MELSEC-A
(b) Bidirectional communications
1)
Data communications can be initiated by a
PC CPU
Data communications can be initiated by a computer or any
PC
CPU.
Data can be transmitted from a
P C CPU
t o an external
device by using the TO instruction in the sequence program to
write data to the buffer memory.
The data send operation is completed when the response mes-
sage to the sent (received) data is received from the computer.
The result of the send (normal endlerror) is stored in the buffer
memory and can be read out.
The data received from the computer can be read with the FROM
instruction of the sequence program.
(When data is transmitted by an
AJ71C24)
2) Data length is set within the send message
PC CPU
Data length is set within the send message when the data is
transmitted to a device.
The receiving side recognizes the data length by the send mes-
sage.
(Send result)
TO instruction
XnO ON
(Send completed)
FROM instruction
Order of
I
transmission
I
AJ71 C24
The send data of the
AJ71 C24
is processed as follows
Data
length
ENQ:
.....................
Added t o the head.
Data length:
........
The send data length set in the buffermemory is transmitted
Data:
.....................
The send data stored in the buffer rnemoryis transmitted.
Sum check:
.........
Computed with the sum checking range in a message.
The data transmitted by a computer and received by an
AJ71 C24
is processed as follows.
Data send
ENQ:
.....................
Checked and removed from the received data.
Data length:
........
Stored i n the buffer memory as the received data length
Data:
.....................
Stored i n the buffer memory as the received data.
.........
Sum check:
Checked and removed from the received data.
3)
Variable communications memory area
Response recelve
I
I
Data
The user memory area can be allocated to suit the purposes and
applications of the data transmission.
Computer
Sum
check
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