Mitsubishi Electronics MELSECNET/H manual Features, Network system using only QJ71LP21-25

[Features]

zExternal power supply prevents station from failing when power supply fails

(1)When the PLC system power supply is accidentally stopped, the QJ71LP21S-25 works as a repeater using the power supplied from an external power supply, which inhibits the communication disruption. While more than one station fails, the stations located between the failed ones will continue with data link. When the normal power supply is resumed, the QJ71LP21S-25 stops as a repeater and restarts its normal operation.

(a) Network system using only QJ71LP21S-25

Control station (station No.1)		Normal station (station No.2)		Normal station (station No.3)
	External		External		External
	power supply		power supply		power supply

External	External	External
power supply	power supply	power supply

When the power to normal stations (No.3 and No.5) fails, the network line is not disconnected, because of external power supply to QJ71LP21S-25.

The network line remains connected and normal station (No. 4) continues data link.

Normal station (station No.6) Normal station (station No.5) Normal station (station No.4)

(b) Network system using only QJ71LP21-25

Control station (station No.1)	Normal station (station No.2)	Normal station (station No.3)
Loopback

Loopback

When the power to normal stations (No.3 and No.5) fails, the network line is disconnected, therefore normal station (No.4) cannot continue data link.

Normal station (station No.6) Normal station (station No.5) Normal station (station No.4)

(c) Network system using both QJ71LP21-25 and QJ71LP21S-25

Control station (station No.1) Normal station (station No.2) Normal station (station No.3)

External

power supply

External power supply

Normal station (station No.6) Normal station (station No.5) Normal station (station No.4)

The station with power failure that works as a repeater does not execute data link same as a conventional system. Therefore, normal communication stations detect this station as a data link error station, and the B/W device hold the status before failure.

(2)When a module other than a network module fails, the faulty module can be replaced without the data link disruption.

(3)The QJ71LP21S-25 guards against the consequence of power failure and loopback, the link scan time of the network is kept stable.

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