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6. BIDIRECTIONAL PROTOCOL COMMUNICATIONS
MELSEC QnA
Bidirectional protocol handshake signals
The signal that sends the data output from the sequence program to the external device,
the signal that detects that data has arrived from the external device and enables the
sequence program to read the received data, and similar signals used when sending and
receiving data with the bidirectional protocol are called handshake input/outDut signals.
These signals are necessary in the bidirectional mode.
The following shows the handshake inputloutput signals.
PC CPU
L
Ixternal device
ixternal device
L
PC CPU
1
Signal
YnO (Request to Send)
Xn2 (Sending)
XnO (End of Transmission)
Xn3
(Request to Read Receive
Data)
Ynl
(End of Reading of Receive
Data)
Timing
Turned OFF by
the
program
h
Turned ON d y
the program
Timed
ON
by
the QC24.
Turned OFF by
1 - r
the QC24.
I
I
L
-
Turned ON by
the QC24.
1
t
Turned ON by
the program.
In this guidebook, bidirectional protocol data communications is implemented using the
AJ71 QC24 dedicated instructions.
Data transmission from PC CPU: BIDOUT instruction (Section 6.5)
Data reception from external device: BlDlN instruction (Section 6.6)
When the dedicated instructions are used, data can be sent and received without being
aware of the handshake signals and inputloutput signals above and buffer memory ad-
dress. The data send and receive programs also become simple.
6 - 9
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