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6. BIDIRECTIONAL
PROTOCOL
COMMUNICATIONS
MELSEC QnA
6.6.4
Summary of data reception from external device using bidirec-
tional protocol
The following uses an image diagram to outline the data reception from the external device using
the bidirectional protocol described in Section 6.6.3.
cpu
@
Request to Read
@
When data is sent from the external device, the QC24 stores the receive data to its
buffer memory.
@
The QC24 turns ON the Request to Read signal (X83) to the PC CPU.
@
The PC CPU uses the sequence program FROM instruction (*BIDIN instruction) to
read the receive data from the QC24.
@
At the end of data read processing, the sequence program turns
ON
the End of Read
signal ("81).
@
When the End of Read signal ("81) is turned ON, the QC24 sends a response to the
external device and turns
off
the Request to Read signal (X83) and ends one data
reception.
*
: The BlDlN instructions automatically turns the QC24 Request to Read and End of
Read signals ON and OFF by internal processing. The sequence program does not
have to turn these signals ON and OFF.
PC CPU
QC24
Sequence
Step
END
Step
0
END
Step
0
program
A
End of Read
signal
(Y81)
Request to Read
signal
(X83)
U
External devlce
6
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21
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