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4. COMMUNICATIONS USING A DEDICATED
PROTOCOL
MELSEC QnA
4.9
On-Demand Function
4.9.1
What is the on-demand function?
During dedicated protocol data communications, usually only the external device starts
data trans-
mission.
However, when the PC CPU wants to send emergency data, etc. to the external device, it can
start transmission. This function is called the "on-demand function".
The on-demand function can be used when the external device and PC CPU configuration is 1 :1.
4.9.2
On-demand function control procedure and message structure
In the ASCII mode, the on-demand function uses A compatible frames (formats 1-4) and in the
binary mode, the on-demand function uses an QnA extension frame (format 5) as the data send
frame.
The A compatible frame format can be selected with the QC24 mode switch.
The following describes the control procedure that uses the CH1 on-demand function in the ASCII
mode (A compatible frame format 1).
External device
Added by the AJ71 QC24.
J
L
-
.
AJ71 QC24
;
(CHI)
T 3
I
oz
! ! E
x
(A compatible frame format 1)
1
E
Sendl data
g
-
x D
I
0 0 F E 1 2 3 4 1 5 6 7 8
9 2
Sending (Xn2)
>
End of transmission
/ error detected
(XnOKnl)
Request to
Send (YnO)
d
rccessln
Sends only the send
data from the PC CPU.
(Word units)
9 6 q - l
1 1
Buffer memory
c/-
-
I
- 1
I
I
(Head address)
AOH
(Data length)
A?~+L 1 1
Stores the error code
_-
.
-
when a send
error
IS
generated.
Sequenttally sent in 4-bit units,
beginning from the most significant blt
When A compatible frame transmission control procedure format 2 is used, the block
No.
becomes "00".
The header and trailer added by the QC24 can be changed to data registered in a user
frame. (Buffer memory addresses A9H to ACH designate the user frame No.)
4
-
36
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