c
c
L.
P
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r
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8.
COMMUNICATIONS USING DEDICATED PROTOCOLS
MELSECIA
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.
tf
the PC CPU has emergency data to transmit to the computer, the on-
demand function
is
used.
1
prog-)
PC CPU
TO
instruction
Dedicated
protocols
1 to
4
I
POINTJ
This function is available only when there is a
1:l
ratio
of
computers to
PC CPUS.
.
8.1 4.1
On-ckmurd
handshake
signal
and
buffer
memory
(1)
On-demand handshake signal
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 com-
pleted.
It
acts as an interlock to prevent on-demand requests being
made simultaneously.
Handshako
Signal
SigMi
furnod ONlOFF
by
Doscriplion
During
execution
of
ordemand
function
OFF
:
traMmkd0ncompMad
Xn3*
A
J7
1
C24
ON
:
trafwn~und.my
"n" in Xn3 is determined by the slot location of the AJ71C24.
(2) Buffer memory used by the on-demand function
Addreu
Doscriplion
N.IW
Area to specify head
bytheTOinrtrucbondthe~~program.
buffer memory
The
hod
ddrw
ofthe
data
stored
in
the
Mbr
mmgr
1
OQH
to
be
truwnbd
p/
Um
orrdsmrrnd
function
is
spockd
address in on-demand
I
loAH
1
length
Area
to
spwify
The length of the data to
be
transmittd by the on-
demand function is specified by the PC CPU TO in-
struction of the seauence Drwram.
I
The AJ71 C24 writes a '1' to this address
if
a traw
mission error occurs during on-demand data trans-
mission.
10CH
Ondemand error
storage area
0
:
No
error
1
:
Error
8
-
105
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