Mitsubishi MELSEC-A Series Reference Manual page 245

8. PROCEDURES TO OPERATION
(2)
For optical fiber cables
For coaxial cables
Reverse loop test
(a) The reverse loop test checks the reverse loop in MELSECNET data link system
after optical fiber cables or coaxial cables are laid down.
(b) When the receive end of the reverse loop in the master station cannot receive
data sent from the send end of the reverse loop in the master station, the loop is
determined to be faulty.
Fig 8.16 Reverse loop test
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