Mitsubishi Electric Melsec QJ71LP21 Reference Manual page 78

3 SPECIFICATIONS
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(5) Prevention of station failure by using external power supply (Optical
loop system)
Direct power supply (24 V DC) from outside to network modules will prevent the
loopback operation. Because of this, station(s) placed between faulty stations will
not go down when more than one station go down, (The QJ71LP21S-25 is the
network module where power can be supplied from outside.)
Even if the distance between normally operating stations (1Ns2 and 1Ns4, 1Ns4
and 1Ns6) is 1 km or more, normal data link will be available
(a) Precautions for operation
If the external power supply of the network module is powered on while the
CPU module power supply is off, the network module will not normally
operate.
Power on the CPU module and the external power supply then start system
operations.
REMARKS
Even if the CPU module on the control station is powered off, the control station will
not shift to a normal station because the network module operates normally.
MELSEC-Q
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