Not sure if the FX3G supports the ethernet card i.e. ENET, however, I believe it can use the RS232-BD board, this gives you a RS232 port, it's default setting is for programming protocol but can be set for ASCII coms (if that is what the keyence uses). You set it up in the PLC parameters for example Channel 1 set for non procedural & set the baud rate, chars, stop bits etc.
You need the manual ideally to see how it works. Attached is a simple project & info on 232 coms.
Please note: this is for FX3U but it applies to all FX.