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8.
COMMUNICATIONS USING DEDICATED PROTOCOLS
MELSEC-A
(4) Transmission time through MELSECNET
(a) The transmission time (TI) for data transmission by specifying the
PC CPU number to a PC CPU o n MELSECNET not equipped with
(
an AJ71C24 is calculated as follows:
Local station
Transmission time (Ti)
=
(LRDP instruction processing time
+
scan
time for station 1 loaded with AJ71C24)
x
2
Remote station
Transmission time (TI)
=
(RFRP instruction processing time
+
MELSECNET master station scan time)
x
2
Substitute "3" for the factor "2"
in
the equations above for the first
data communications after the power supply is turned ON or for the
relevant station after the PC CPU has been reset.
If no more than 10 stations are communicating, use a factor of "1"
for the second (and subsequent) communications.
Causes of delayed transmission time (TI)
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.
Example:
The transmission time for
a
MELSECNET master station eauiDDed with
.
. .
AJ71 C24 to read a local station device memory:
(Conditions: L<LS<M, M : 80 msec a1
:
10 msec)
Transmission time T I
=
(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
Under some conditions, data transmission to a PC CPU on MELSECNET
not equipped with an AJ71C24 can cause a considerable 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 MEL-
SECNET data link (B, W).
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