PLANET FT-801 - Manual (Page 9)

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If one of the optical conductors is bad (as shown in the diagram
box below), the converter with LLR function will return a no-link
condition to its link partner. With LLCF function also enabled, the
no-link condition is carried forward to the switch/hub where a trap
is generated to the management station, and the administrator
can then determine the source of the loss.
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NOTE:
LFP function is turn-on in default. This feature
can also be turned off via the side DIP-switch.
If you are familiar with the network installation
and for diagnostic purpose (i.e. check which
end is broken), you can turn it off and reset
the converter to make it take effect. Otherwise,
please remains it in the default position.