Mitsubishi Electric MELSEC Q Series Reference Manual page 254

7 APPLICATION FUNCTIONS
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The following describes each item.
(1) Continuation of controlling remote I/O stations when multiplexed
remote master station fails
(a) By mounting the master modules of the multiplexed remote master station
and multiplexed remote sub-master station to different programmable
controller CPUs, the multiplexed remote sub-master station automatically
controls the remote I/O stations if the multiplexed remote master station
fails (switching of master operation).
POINT
The transient transmission executed when the master operation station is switched
or returns to the system may be unsuccessfully completed. Execute the transient
transmission again if it is unsuccessfully completed.
(b) Since the sub-master operating station smoothly continues the control of
the remote I/O stations when the master operation is switched, it always
receives cyclic transmission data (M station
from the remote I/O stations if the master operating station is operating
normally.
Also, the sub-master operating station can execute the link dedicated
instructions that perform read to the remote I/O stations (REMFR, READ).
The link dedicated instructions for performing write (REMTO, WRITE)
cannot be executed from the sub-master operating station as the execution
from the master operating station has priority. If executed by the sub-
master operating station, such instructions result in normal completion
without processing (data not written). (An error will not occur.)
REMARKS
When a link dedicated instruction (REMTO or REMFR) for remote I/O stations is
executed by mistake between the master station and sub-master station, the
execution station shows the following operation status.
Instruction Execution station
Master operation station
REMFR
Sub-master operation station
Master operation station
REMTO
Sub-master operation station
(c) To smoothly continue the control of the remote I/O stations when master
operation is switched, the data must be always transferred between the
multiplexed remote master station and multiplexed remote sub-master
station.
Data transfer reflects the condition statuses of outputs from the multiplexed
remote master station to remote I/O stations such as the output Y and data
output on the remote sub-master station.
Also, data must be transferred from the multiplexed remote sub-master
station before the faulty multiplexed remote master station returns to
system and switches to master operation.
(Information is transferred by cyclic transmission as performed on a PLC to
PLC network.)
R station: X, B, W) sent
Target station
Sub-master operation station An error has occurred
Master operation station
Sub-master operation station An error has occurred
Master operation station
MELSEC-Q
Operation status of the
execution station
An error has occurred
Normal end without processing
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