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8.
COMMUNICATIONS USING DEDICATED PROTOCOLS
MELSEC-A
8.14
On-demand Function
The on-demand function is used when the PC CPU has data to transmit to
the computer. In this case, the PC CPU specifies the buffer memory area in
which the data to be transmitted is stored and then starts transmission.
During data transmission between the computer and PC CPU using dedi-
cated protocols 1 to 4, communications is normally initiated by the computer.
If the PC CPU has emergency data to transmit to the computer, the on-
demand function is used.
AJ71 C24
(Sequence
I
This function is available only when there is a 1 : l ratio of computers to
PC CPUS.
I
8.14.1
On-demand handshake signal and buffer memory
B
(1)
On-demand handshake signal
*
"n" in Xn3 is determined by the slot location of the AJ71C24.
(2)
Buffer memory used by the on-demand function
The on-demand handshake signal turns ON when the PC CPU transmits
a data send request to the computer to start transmission, and turns
OFF when transmission of the data specified by the AJ71C24 is corn-
pleted. It acts as an interlock to prevent on-demand requests being
made simultaneously.
The length of the data t o be transmitted by the on-
FAH
1
% :
"""
demand f u n d i o n is specified by the PC CPU T O in-
struction of the sequence program.
S i g n a l Turned ON/OFF
b y
AJ71C24
Handshake S i g n a l
Xn3*
Address
109H
Description
During execution of on-demand function
ON : transmission underway
OFF
: transmission completed
I
1 : Error
Name
Area t o specify head
address in on-demand
buffer memory
I
10CH
Description
The head address of the data stored in the buffer memory
to be transmitted by the on-demand function is specified
by the TO instruction of the Sequence program.
On-demand
error
storage area
The AJ71C24 writes a "1' to this address if
a
trans-
mission
error
occurs during on-demand data trans-
mission.
0 : No error
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