Mitsubishi Electric MELSEC-Q Series User Manual page 145

(5) Precautions
(a) The number of standby master stations
One standby master station can be connected in one system.
(b) Right of controlling data link of a standby master station
When a master station goes down, the right of controlling data link is automatically transferred to a standby
master station. Note that an instruction to refresh cyclic data is not issued. Use a link special relay (SB) and link
special register (SW) to issue a refresh instruction. (
After the instruction is issued, information before a master station error is detected is output to each station.
(c) Change in the parameter settings of the master station
While data link is performed by a standby master station, the parameter settings of the master station cannot
be changed.
(d) Station number setting for a standby master station
If the station number setting of the station number setting switch on a standby mater station differs from that set
in "Standby Master Station No." of the network parameter for the master station, an error (error code: B39A)
occurs in the standby master station.
If an error has occurred, reset the CPU module of the standby master station after changing the parameter
settings of the master station or changing the station number set using the station number setting switch of the
standby master station.
(e) If the terminal block of a master station is removed while the master station is
controlling data link
If the terminal block of a master station is removed without powering off the master station and reconnected,
both of the master station and standby master station will perform master operation because the right of
controlling data link is transferred to the standby master station; therefore, communications cannot be normally
performed. By powering off the master station and reconnecting the terminal block and powering on the master
station, the master station is reconnected as standby master station operation; therefore, communications can
be normally performed.
(f) The status of an LED when the right of controlling data link is transferred to a
standby master station
When the master station goes down and the standby master station starts to take in control of data link, the
ERR. LED on the standby master station flashes. The station number of the standby master station is changed
from the one set in the parameters to the station number 0, and the standby master station is regarded that it is
not connected. Data link is normally performed.
To prevent this situation, set the standby master station as an error invalid station.
(g) Number of points and range of devices to back up data using a program
The number of points and range of devices where data from a master station (station operating as a master
station) to a standby master station (station operating as a standby master station) are backed up using a
program vary depending on the system.
(h) If an error is detected in the master station in the initial status (before data link is
started)
The operation is not switched from the standby master station to the master station, and data link is not started.
(This applies only to the master station switching function and master station duplex function.)
Page 145, Section 8.2.5 (6))
CHAPTER 8 FUNCTIONS
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