# Adding fiber optic ethernet to a circuit

I'm working on a project that involves transmitting data over very long distances (10-25km) where radio communication is unavailable, and I have been looking into the possibility of adding a fiber optic ethernet connection to the board.

The first issue I have run into is trying to figure out what specification I can or should be using. My first thought was to use 100BASE-FX sfp transceivers, and connect the other end to a TRENDnet TFC-110S60i which claims a maximum distance of 60km, however further investigation seems to suggest there is a 2km limit for 100BASE-FX connections. If my objective is to connect over these long distances, what specification should I follow?

The second issue I've run into is actually connecting a fiber optic transceiver, be it SFP or otherwise, to the controller. At this time no decision has been made for what controller will be used on the board, so I'm trying to select as generic an interface as possible. Unfortunately most chips I've found such as the Microchip KSZ8041 are only PHYs, or the Microchip KSZ8441 which uses a host bus that I am unfamiliar with how to implement. I've also found the Microchip LAN7500 which handles MAC and provides an SPI interface, but the documentation doesn't specifically say if 100BASE-FX/BX/LX/etc. is supported. Would I be able to connect this chip to an SPI socket on a circuit board, or do I need something that specifically says it supports a fiber ethernet format?

• Can you just put the TrendNet box on both ends of the long link and use twisted-pair ethernet to connect from your circuits to the TrendNET box on its end? – The Photon Feb 15 at 0:31
• Are you implying one continuous run of 25km? The power losses would likely be too great; repeaters needed. Also consider the latency at these distances... fiber is about 30% slower. At 3.9µs/km, the round-trip-time would be about 195µs. That ok? – rdtsc Feb 15 at 2:21
• 1000Base-EX reaches to 40km. – DoxyLover Feb 15 at 3:10
• More to the point, are you really going to run 25km of fiber by yourself? If not, isn't this something your network contractor ought to be able to advise on? – pjc50 Feb 18 at 13:58
• @winny Is interfacing with an SFP module as simple as putting data to the TX+ and TX- pins and reading from the RX+ and RX- pins? – Nicholas Hollander Feb 18 at 18:41