Mitsubishi Electric Melsec QJ71LP21 Reference Manual page 75

3 SPECIFICATIONS
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(4) Loopback function (optical loop system)
In the optical loop system, the transmission path is dual-structured. When an
error occurs in a transmission path, the faulty area is disconnected by switching
the transmission path from the forward loop to the reverse loop or from the
reverse loop to the forward loop, or performing a loopback. The transmission is
continued normally between the stations that are still able to perform data
communication.
(a) When normal
The data link is performed using the forward loop (or the reverse loop).
Control station
(station No. 1)
Normal station
(station No. 2)
(b) When abnormal
1) Error in the forward loop (reverse loop)
The data link continues using the reverse loop (forward loop).
Control station
(station No. 1)
Normal station
(station No. 2)
Cable disconnection
2) When some of the stations are down
The data link continues excluding the stations that are down.
When two or more stations are down, the data link cannot be
performed with the station located between the stations that are down.
However, when there are two or more stations between the stations
that are down, the normal station with younger station number
becomes the sub-control station to continue the data link.
Loopback
Control station Normal station
(station No. 1) (station No. 6)
Normal station Normal station
(station No. 2) (station No. 3)
Loopback
Down
Normal station Normal station
(station No. 6) (station No. 5)
Reverse loop
Normal station Normal station
(station No. 3) (station No. 4)
Normal station Normal station
(station No. 6) (station No. 5)
Normal station Normal station
(station No. 3) (station No. 4)
Down
Normal station
(station No. 5)
Normal station
(station No. 4)
Communication
disabled
MELSEC-Q
Forward loop
Data flow
Down
Control station Normal station Normal station
(station No. 1) (station No. 6) (station No. 5)
Normal station Normal station Normal station
(station No. 2) (station No. 3) (station No. 4)
Sub-control
Down
station
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