PLANET FT-1105A - Manual (Page 14)

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Chapter 8
Loop Back Reply
The FT-1105A / FT-1205A supports auto loop back reply while
receive loop back test packets on fiber port. Enable this function
to tell the converter to reply fiber signals request from Web
Smart media converter, such as PLANET FT-90x. You can use this
to check whether the converter can communicate with partner
converter. Please refer to the following figure.

Notice :
For FT-1105A only, while enable the Loop Back
Reply function, the Port-2 (TP) must be link up.