8.
COMMUNICATIONS
USING
DEDICATED PROTOCOLS
MELSEC-A
(4) Transmission time through Data link systems
(a) The transmission time (Tl) for data transmission by specifying the
PC CPU number to a PC CPU on MELSECNET(I1). MELSECNET/B
not equipped with an AJ71 C24 is calculated as follows:
0
Local station
Transmission time
(Tl)
=
(Transmission delay time
A
*l
+
scan time
for station
1
loaded
with
AJ71C24)
x
2 *2
Transmission time (Ti)
=
(Transmission delay time B
*l
+
MELSECNET master station scan time)
x
2 *2
*1
See the sections of the Data Link Reference Manual that explain
corresponding data link system transmission delay times.
Transmission delay time
A
See the table which describes LRDP instruction processing
time.
Transmission delay time
B
See the table which describes RFRP instruction processing
time.
*2 Substitute
'X
3'
for
'X
2' in the above equations for the first data
communications after the power su ply to the relevant station
has been turned ON after the PC
C
$
U has been reset.
~
If fewer than 11 stations are communicating, substitute (x
1
for
x
2) for the second (and subsequent) communications.
0
Remote station
Example:
The transmission time for
9
MELSECNFT master station equipped witt
AJ71 C24 to read a local station device memory:
(Conditions: L<LS<M,
M
:
80
msec
a1
:
10
msec)
Transmission time
T1
=
(M
x
4
+
a1
x
4
+
M)
x
2
The transmission time is
880
msec. Where:
M
:
MELSECNET master station scan time
a1
:
MELSECNET master station
link
refresh time
LS
:
Link scan time
L
:
MELSECNET local station scan time
=
(80
x4
+
10x4
+
80)
x2
=
880
0
Causes of delayed transmission time (Tl)
Instructions requiring
2
scans for transmission (writing to device
'R",
etc.) need double the time derived from the equations above.
When other stations in the link are being monitored by an AGGPP,
the transmission time doubles for each station to be monitored.
The Data Link Reference Manual gives details of the data link.
-
POINT]
Under some conditions, data transmission to a PC CPU on MELSEC-
NET(II), MELSECNET/B not equipped with an AJ71C24 can cause
a
con-
siderable time delay.
This time delay can be reduced
by
carrying out all communications from
the computer to PC CPUs to stations equipped
with
an AJ71C24 (PC CPU
station number FFH) and all other data communications using the data
link
(B,
W).
8-
18
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